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Ultrastrong Exciton–Photon Coupling in Broadband Solar Absorbers
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Physical Insights into Light Interacting with Matter

Ultrastrong Exciton–Photon Coupling in Broadband Solar Absorbers
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  • Clara Bujalance
    Clara Bujalance
    Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
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  • Victoria Esteso
    Victoria Esteso
    Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
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  • Laura Caliò
    Laura Caliò
    Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
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  • Giulia Lavarda
    Giulia Lavarda
    Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
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  • Tomás Torres
    Tomás Torres
    Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    IMDEA Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain
    Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain
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    Orcidhttps://orcid.org/0000-0001-9335-6935
  • Johannes Feist
    Johannes Feist
    Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
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    Orcidhttps://orcid.org/0000-0002-7972-0646
  • Francisco José García-Vidal
    Francisco José García-Vidal
    Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
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    Orcidhttps://orcid.org/0000-0003-4354-0982
  • Giovanni Bottari*
    Giovanni Bottari
    Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    IMDEA Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain
    Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain
    *Email [email protected]
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    Orcidhttps://orcid.org/0000-0001-6141-7027
  • Hernán Míguez*
    Hernán Míguez
    Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
    *Email [email protected]
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    Orcidhttps://orcid.org/0000-0003-2925-6360
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The Journal of Physical Chemistry Letters

Cite this: J. Phys. Chem. Lett. 2021, 12, 43, 10706–10712
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https://doi.org/10.1021/acs.jpclett.1c02898
Published October 28, 2021

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Abstract

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The recent development of organic polaritonic solar cells, in which sunlight absorbers and photon modes of a resonator are hybridized as a result of their strong coupling, has revealed the potential this interaction offers to control and enhance the performance of these devices. In this approach, the photovoltaic cell is built in such a way that it also behaves as an optical cavity supporting spectrally well-defined resonances, which match the broad absorption bands of the dyes employed. Herein we focus on the experimental and theoretical analysis of the specific spectral and angular optical absorption characteristics of a broadband light harvester, namely a subphthalocyanine, when operating in the ultrastrong coupling regime. We discuss the implications of having a broad distribution of oscillator strengths and demonstrate that rational design of the layered structure is needed to optimize both the spectral and the angular response of the sunlight harvester dye.

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An organic compound strongly coupled to an optical cavity (1) can intensely absorb light at frequencies for which its intrinsic absorption is practically none (2) or emit light at spectral ranges at which the uncoupled system barely shows optical activity. (3) These effects arise as a result of the reconfiguration of the electronic structure of the molecules and the optical modes of the cavity caused by their strong coupling. (4−6) In this regime, the eigenmodes of the ensemble must be described as hybrid light–matter states, also known as polaritons. Following an intense activity focused on the description of its fundamental aspects, this phenomenon has been put into practice aiming at developing a polariton-based technology (7) that takes advantage of the possibilities that the new electronic and optical properties of hybrid light–matter states open. Hence, its potential has been proven in fields like lasing, (8−10) photodetection, (11) catalysis, (12,13) photochemistry, (14−16) and, from an even broader perspective, in synthetic chemistry, where novel reaction pathways are being explored. (16−19)

Recently, the spectral control over the polaritonic absorption of strongly coupled organic compounds has been used to reduce photon energy losses in organic solar cells by effectively diminishing the bandgap of the absorber while, at the same time, decreasing the electron driving force and hence charge transfer losses. (20) For this purpose, a layered structure made of subphthalocyanine (SubPc)-based thin films, each one performing a different function (electron donor or acceptor materials), was sandwiched between two metal contacts, which played the role of both electrical contacts and mirrors. One specific characteristic of the strong-coupling configuration employed in solar cells is the inhomogeneous character of the electronic transitions involved in the coupling: the absorption bands involved are either excitonic Q-bands, characteristic of porphyrinoids and described by Gouterman’s model, (21,22) which results from the convolution of several HOMO–LUMO transitions, or charge transfer (CT) bands, which are usually even broader. Recently, we showed that the specific characteristics of CT bands of a SubPc made them more prone to yield weak coupling when interacting with optical cavity modes, while the more intense and narrow SubPc Q-bands tend to favor the formation of polaritons, characteristic of strong coupling. (23) However, how to deal with the design of a polaritonic light harvesting system based on a broadband absorber operating under the ultrastrong coupling regime is still an open question, a few relevant examples being found in the literature. (2,11,24−27) The equations commonly employed to predict the response of a polaritonic system assume narrow electronic transitions with a well-defined spectral position and oscillator strength, while solar dyes are inherently inhomogeneous broadband absorbers. Another relevant aspect for energy conversion applications is how the unwanted parasitic absorption that occurs in the metallic mirror/contacts is affected by the polaritonic interaction.

In this work, we analyze the absorption properties of a broadband solar molecular absorber, a perfluorinated SubPc substituted with an ethynylaniline moiety in its axial position (thereafter referred to as SubPc-Et, Figure 1a), (28) embedded in an optical cavity and undergoing ultrastrong exciton–photon coupling. To do so, we have built resonators that resemble the characteristics of recently reported photovoltaic devices (20) in which strong light–matter interactions have opened new routes to control their performance. We address the question of dealing with an inhomogeneous distribution of oscillators instead of a well-defined single electronic transition and identify the best way to define reference values for the optical and excitonic transitions that partake in the hybrid light–matter polaritonic modes. Moreover, we find that unlike most other absorption control or enhancement approaches based on photonic resonances, ultrastrong exciton–photon coupling may be designed to yield an almost flat angular response, which, as we prove, results from the balance of the opposite trends shown by the S and P polarization modes. Furthermore, modeling allows us to quantify the parasitic absorption occurring at the metallic contacts and confirm that their angular and spectral dispersion resembles that of polaritonic modes.

Figure 1

Figure 1. (a) Chemical structure of the SubPc derivative used in this work, attained by anchoring an ethynylaniline ligand to the boron atom of a perfluorinated SubPc. Synthetic details are available in ref (28). (b) Schematic representation of the structure of the Fabry–Pérot resonators prepared for this work. (c) Absorptance spectra (colored solid lines) of resonators built varying the cavity thickness, namely, 100 nm (green), 115 nm (yellow), 130 nm (blue), 150 nm (red), and 163 nm (purple). The calculated normal incidence reflectance spectra of the corresponding underlying optical cavities are plotted following the same color code (dashed lines). For the sake of comparison, the absorptance of a bare SubPc-Et film is also shown (solid light gray line).

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A set of metallic optical resonators embedding films of different thickness made of SubPc-Et molecules were prepared following a previously reported procedure. (23) In brief, the cavities consist in a layer of SubPc-Et sandwiched between two poly(vinyl alcohol) (PVA) thin films (18–25 nm), which are in turn coated by two silver mirrors (35 nm). The architecture of the resonators built for this work is drawn in Figures 1b. Bare SubPc-Et films deposited on glass display an intense Q-band absorption in the visible region. In Figure 1c, we show a representative normal incidence absorptance of a 94 nm thick SubPc-Et film (light gray line). Once integrated in a metallic optical cavity, the absorptance of the ensemble presents characteristic polaritonic traits, with two well-defined absorption maxima separated by an energy gap. Colored solid lines in Figure 1c correspond to the spectra of optical cavities of different thickness (determined by the widths of the SubPc-Et and the two surrounding PVA films), namely, 100 nm (green), 115 nm (yellow), 130 nm (blue), 150 nm (red), and 163 nm (purple). The corresponding SubPc-Et thickness in each case is 50, 71, 94, 106, 113, and 146 nm. The calculated normal incidence reflectance spectra of the corresponding underlying (not absorbing and with effective refractive index n = 1.56, a choice that will be justified below) optical cavities are plotted following the same color code (dashed lines in Figure 1c), the first-order resonant photon modes being readily identified as well-defined minima.

The splitting of the absorption spectrum reflects the opening of an energy gap as a result of the anticrossing of the upper and lower polaritonic branches. This can be seen in the energy dispersion relations plotted in Figures 2a,b, which corresponds to a resonator with thickness of 130 nm (similar results are attained for all remaining optical cavities and are included in the Supporting Information; see Figures S1–S4). These maps are obtained from angle and polarization dependence measurements of both reflectance (R, Figures 2c,d) and transmittance (T, Figures 2e,f), from which absorptance (A = 1 – R – T) is attained. A double-goniometer configuration (Universal Measurement Accessory attached to a Cary5000 spectrophotometer) was used for this characterization, in which sample and detector can be independently rotated, and hence arbitrary incident and collection angles may be selected. A, R, and T intensity maps are plotted versus both the photon energy (y-axis) and the parallel component of the wavevector k∥ (x-axis, k=k∥+k⊥) following a standard representation. The absorption gap opening arises from the anticrossing of the upper and lower polaritonic energy branches, and its width is given by ℏΩR, where ℏ is the reduced Planck constant and ΩR is the Rabi frequency, which stands for the rate at which energy is exchanged between the photonic mode and the electronic transition. The exciton–photon coupling strength may be estimated through the ratio ΩR/ω0, (29) which compares the polariton energy gap with the intrinsic material transition energy (ω0). Polaritonic gaps for the set of resonators are estimated by measuring the energy jump at the k∥ point where the cavity mode, ωcav, matches the frequency of the electronic transition, ω0. In the case of the cavity displayed in Figure 2, this occurs for k∥ ≈ 0, where the cavity resonance dispersion curve (white dotted line) intersects the horizontal line, indicating the spectral position of the electronic transition (red dotted line). This yields ℏΩ̵R ≈ 750 meV, corresponding to ΩR/ω0 ≈ 0.38, well above the threshold usually set to consider an exciton–photon system to be in the ultrastrong coupling regime (ΩR/ω0 > 0.2). (29) Note that films of SubPc-Et present a stronger Q-band absorption (i.e., the electronic transitions involved have larger oscillator strengths) and are of higher optical quality (i.e., they show more uniformity and less scattering) than those made of the SubPc molecules we employed to study the different coupling between CT and Q-bands in a previous work. (23) In that case, the analysis of the coupling between the Q-band and the cavity mode yielded ΩR/ω0 ≈ 0.036, an order of magnitude smaller than in the present case.

Figure 2

Figure 2. Dispersion curves attained from the angular dependence measurements of the absorptance (a, b), reflectance (c, d), and transmittance (e, f) of an optical resonator made of a 94 nm thick SubPc-Et film (total cavity thickness 130 nm, after adding the thickness of the PVA leveling layers). Left and right panels correspond respectively to S-polarized (or transversal electric, TE, i.e., electric field E perpendicular to the plane of incidence) and P-polarized incident light (or transversal magnetic, TM, with E contained in the plane of incidence). The white dashed line corresponds to the underlying cavity first-order mode dispersion, while the red dashed horizontal line indicates the spectral position of the SubPc-Et absorption outside the cavity taken as reference.

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It is worth comparing our results with previous ones for other organic metallic cavities. Kéna-Cohen and co-workers recently demonstrated a coupling ratio of ΩR/ω0 ≈ 0.48 (11) for a ultrastrongly coupled mixed phthalocyanine/C60 film as well as a remarkable ΩR/ω0 ≈ 0.62 for a polymethine dye. (26) Also, Sanvitto, Gigli, and co-workers achieved ΩR/ω0 ≈ 0.60 for a squaraine-filled microcavity. (2) These values are among the highest reported for organic materials, in both cases deposited by evaporation techniques, embedded in Fabry–Pérot cavities. The strongly coupled SubPc film herein presented holds the highest ΩR/ω0 for a solution processed organic material.

To rationalize the main spectral features observed for the different cavities herein analyzed, a model is required. The spectral positions at which the upper (ω+) and lower (ω–) polaritonic absorption peaks occur can be approximated by the following expression, which results from the diagonalization of the Jaynes–Cummings Hamiltonian: (6)

ω±=12(ωcav+ω0)±g2+δ24
(1)
where g is the coupling constant and δ is the detuning between ωcav and ω0, δ = ωcav – ω0. Although eq 1 describes a strongly coupled system of two modes with well-defined frequencies, it has been theoretically proven (30) and experimentally confirmed (2,25) that a similar description holds for broadband absorbers. However, using this model for an inhomogeneously broadened absorption peak requires an adequate approximation of both ωcav and ω0. To do so, we have determined the optical constants of the bare SubPc-Et film deposited on a quartz substrate. We have employed the method developed by Forouhi and Bloomer, (31) based on the fitting of the experimental reflectance and transmittance measured at different incidence angles and polarizations. This method considers a series of Lorentz oscillators, each one of them characterized by an oscillator strength and width, which account for the different electronic transitions contributing to the absorption band under analysis. In Figure 3a, we plot the experimental absorptance of the SubPc-Et film (blue line) and the best fitting (red dashed line) as well as the corresponding oscillator curves employed for the fitting (light gray lines). This deconvolution is arbitrary insofar as the theoretical study of the electronic structure and optical properties of this specific SubPc has not been reported, and the number of electronic transitions involved in the formation of the Q-band varies as a function of the axial and peripheral ligands present in the molecule. In any case, this empirical approach allows us to establish a criterion to define a reference excitonic transition frequency (i.e., ω0), which we set as the average value of the spectral position of the oscillator maxima, weighted by the area of each peak. This value (λ0 = 557 nm, ω0 = 2.23 eV) is indicated with a vertical dark gray dash-dotted line in Figure 3a. Also, both real and imaginary components, n(λ) and κ(λ), of the refractive index of the SubPc-Et film attained by using the Forouhi–Bloomer model are displayed in Figure 3b. From these curves, the representative effective (constant) refractive index of the underlying, nonabsorbing, optical cavity is attained by averaging the real part of the refractive index of the SubPc-Et film in the region of interest (400 nm < λ < 800 nm). This value, n = 1.56, is highlighted with a horizontal dark gray dash-dotted line in Figure 3b, and it is the one we use to calculate ωcav. Once we have rationally estimated ω0 and ωcav, we are in a position to compare our data with the expected theoretical dispersion given by eq 1. Figure 3c shows both the experimental (red and blue circles) and theoretical (solid lines) dependence of the spectral position of the upper (ω+, red colored) and lower (ω–, blue colored) polaritonic branches versus the degree of detuning δ, estimated from the above-mentioned values of ω0 and ωcav. It should be noticed that this choice of ω0 and ωcav allows for the best correspondence between theory and experiment. Other options were tested and showed a larger discrepancy between both data sets. Each pair of scatters corresponds to the spectral position of the upper and lower polaritonic branches extracted from the absorptance spectra of each cavity measured at normal incidence.

Figure 3

Figure 3. (a) Experimental (blue line) and fitted (red dashed line) absorptance of a SubPc-Et film. The contributions from the different oscillators considered for the fitting are plotted as light gray lines. The vertical dark gray dash-dotted line corresponds to the average value of the spectral position of the oscillator maxima, weighted by the area of each peak, which we take as ω0. (b) The real (solid line) and imaginary (dashed line) parts of the refractive index, n and k, estimated from the fitting of the optical properties of the SubPc-Et film. The horizontal dark gray dash-dotted line indicates the average value of n in the region of interest (400 nm < λ < 800 nm), which we take as the refractive index of the underlying, nonabsorbing, optical medium. (c) Experimental (scatter graphs) and theoretical (solid lines) dependence of the spectral position of the upper (ω+, red colored) and lower (ω–, blue colored) polaritonic branches versus the degree of detuning δ, estimated from the above-mentioned values of ω0 and ωcav.

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The fact that metallic mirrors are used to make the optical cavity implies that parasitic absorption (i.e., nonproductive from the point of view of light harvesting) may be present. To analyze how significant this effect is in an ultrastrongly coupled light harvester, we follow a classical electrodynamics approach, described in detail in ref (3) and based on the transfer matrix method. This allows us to discriminate the absorption that occurs at each layer in the assembly. In Figures 4a and 4b, we show the calculated spectral and spatial profiles of the light electric field intensity, |E(r)|2, and the normalized absorptance per unit length, δA, respectively, for the same cavity chosen as illustrative example in Figure 2. These intensity maps have been attained considering a plane wave impinging normally on the resonators (k∥= 0). They permit not only visualizing the first-order resonant mode splitting, which is at the origin of the polariton absorption energy gap, but also estimating the absorptance spectra of each layer in the assembly. This is represented in Figure 4c. Note that the actual absorption taking place in the SubPc-Et film (purple solid line) is slightly smaller than the total one (red solid line) due to the presence of optical losses in the top (dark gray line) and bottom (light gray line) silver mirrors. Total parasitic absorptance is also plotted (black solid line). A comparison between the areas below these curves indicates that the losses in the metallic mirrors amount for 20.1% of the total absorption of the device. A similar analysis performed at different incidence angles allows us to obtain the energy dispersion of both the SubPc-Et (productive) and the metals (parasitic) absorptance, which are plotted as intensity maps in Figures 4d and 4e (results for nonpolarized light, estimated by averaging the experimental results attained for the S and P polarizations at each incidence angle). As expected, the absorption occurring at the metal layers follows the same dispersion than the polaritonic branches, since incident light can only couple to the cavity via the polaritonic modes. A rigorous analysis of the effect of the coupling (be it weak, strong, or ultrastrong) of a light harvesting material to metallic cavities should always include an estimation of contributions of the mirrors to the absorption. In this case, as it can be concluded from the comparison of Figures 2a and 2b and Figure 4e, optical losses in the mirrors only modify quantitatively, and by a small amount, the experimental polaritonic energy dispersion attained.

Figure 4

Figure 4. Calculated spatial and spectral profiles of (a) the electric field intensity |E(r)|2 and (b) the absorbed luminous power PA for a SubPc-Et cavity of thickness 130 nm, at normal incidence. (c) Discriminated absorptance at normal incidence of each layer in the cavity, namely, the SubPc-Et film (purple line) and top (dark gray) and bottom (light gray) silver mirrors. Total optical loss spectrum in the metals (black) and the full absorptance of the cavity (red) are also plotted. Intensity maps in (d) and (e) show the energy dispersion of both the SubPc-Et (productive) and the metals (parasitic) absorptance of nonpolarized light.

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Next, we evaluate the specific features of solar absorption in an ultrastrongly coupled polaritonic light harvester. To do so, we calculated the solar spectrum weighted absorptance (SSWA) that results from multiplying our experimental data by the (normalized) AM1.5 solar irradiance spectrum standard, which accounts for sunlight rays impinging on the earth’s surface and traveling through an air mass equivalent to that of 1.5 atm. Also, we calculate the normalized wavelength integrated SSWA at each angle of incidence using the expression

∫SSWA(θ)=∫400800A(λ,θ)AM1.5(λ)dλ∫400800AM1.5(λ)dλ
(2)
The results are displayed in Figure 5, where we plot the SSWA attained at different angles (Figures 5a–c) and the ∫SSWA(θ) (Figures 5d–f) for S-polarized (a, d), P-polarized (b, e), and nonpolarized (c, f) light. Note that even when the integrated absorption estimated for S- and P-polarized incident light shows a significant angular dependence, the results for nonpolarized light present an almost flat response for both productive and parasitic absorptions, as shown in Figure 5f. This low dispersion is the result of the balance between the opposite trends observed for the sunlight absorption intensities displayed by S and P polarizations. The same analysis of ∫SSWA(θ) is performed in the remaining optical cavities (Figures S5–S8), finding that the larger the absolute value of δ, the more significant the angular dependence for nonpolarized light (see Figure S9, in which this effect can be readily seen). The flat response of ultrastrongly coupled configurations has interesting consequences from the point of view of the application of these structures in light-to-energy conversion devices, as one of the main drawbacks when photonic designs are implemented in solar cells, photodetectors, or light-emitting devices is the introduction of unwanted strong angular dependences that may limit their utility.

Figure 5

Figure 5. (a–c) Solar spectrum (AM1.5) weighted absorptance, SSWA, of a metallic optical cavity embedding a 94 nm thick SubPc-Et film sandwiched between two PVA thin films. Results are shown for (a) S-polarized, (b) P-polarized, and (c) nonpolarized light incident at angles of 6° (purple line), 26° (blue), 46° (light blue), and 66° (green) with respect to the cavity surface normal. The normalized AM1.5 spectrum and the absorptance of a bare SubPc-Et film are also plotted (red and gray solid lines, respectively). (d–f) Wavelength-integrated SSWA, ∫SSWA, versus angle of incidence of sunlight for each one of the layers in the cavity, namely, the SubPc-Et film (purple line), top mirror (dark gray dashed line), and bottom mirror (light gray dashed line). Total integrated parasitic absorption (black dashed line) and full integrated absorption of the cavity (red solid line) are also plotted. Results are shown for (d) S-polarized, (e) P-polarized, and (f) nonpolarized light.

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In summary, we have performed a systematic analysis of the light harvesting properties of a canonical broadband solar molecular absorber of interest in photovoltaic devices, namely a subphthalocyanine, when it is ultrastrongly coupled to an optical cavity. We have tackled the problem of dealing with a broadband absorber and proposed a way to attain the parameters that label the optical and excitonic transitions involved in the polaritonic interaction. We demonstrate that this approach allows employing standard models to describe and design the exciton–photon polariton modes of the resonator. Finally, we have studied in detail the spectral and angular response of the resonator acting as a sunlight harvester, showing that light harvesting metallic cavities can be designed in a such a way that both productive and parasitic absorption present an almost dispersionless response when irradiated with nonpolarized light, a result of the balance between the opposite behaviors displayed by S and P polarizations. We believe our results are of relevance for further development of polaritonic light harvesting devices (solar cells and photodetectors).

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.1c02898.

  • Dispersion curves attained from the angular dependence measurements of the absorptance, reflectance and transmittance, and wavelength integrated SSWA versus angle of incidence of sunlight for each one of the layers comprising the optical resonator for the cavity thicknesses considered in the text (PDF)

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Author Information

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  • Corresponding Authors
    • Giovanni Bottari - Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain;  IMDEA Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain;  Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Orcidhttps://orcid.org/0000-0001-6141-7027;  Email: [email protected]
    • Hernán Míguez - Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain;  Orcidhttps://orcid.org/0000-0003-2925-6360;  Email: [email protected]
  • Authors
    • Clara Bujalance - Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
    • Victoria Esteso - Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
    • Laura Caliò - Multifunctional Optical Materials Group, Institute of Materials Science of Sevilla, Consejo Superior de Investigaciones Científicas−Universidad de Sevilla (CSIC-US), Américo Vespucio 49, 41092 Sevilla, Spain
    • Giulia Lavarda - Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
    • Tomás Torres - Departamento de Química Orgánica, Universidad Autónoma de Madrid, 28049 Madrid, Spain;  IMDEA Nanociencia, Campus de Cantoblanco, 28049 Madrid, Spain;  Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Orcidhttps://orcid.org/0000-0001-9335-6935
    • Johannes Feist - Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Orcidhttps://orcid.org/0000-0002-7972-0646
    • Francisco José García-Vidal - Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid, 28049 Madrid, Spain;  Orcidhttps://orcid.org/0000-0003-4354-0982
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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Funding for this work was provided by the Spanish “Ministerio de Ciencia, Innovación y Universidades (MCIU)” through AEI/FEDER(UE) projects PID2020-116593RB-I00 and PID2020-116490GBI00 as well as through projects EXPLORA FIS2017-91018-EXP, PCI2018-093145 (QuantERA program, EC), CEX2018-000805-M (María de Maeztu Programme for Units of Excellence in R&D),SEV2016-0686 (Severo Ochoa Programme for Centres of Excellence in R&D), and MODE-Fotovoltaica (Materiales Orgánicos Disruptivos para Energía Fotovoltaica) (RED2018-102815-T). V.E. thanks La Caixa Foundation (ID 100010434) for funding of her PhD (fellowship LCF/BQ/ES15/10360025). L.C. thanks Junta de Andalucía and the European Regional Development Funds program (EU-FEDER) for financial support (DOC_00220). This work was partially funded also by the European Research Council through Grant ERC-2016-StG-714870.

References

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    Nature (London) (1998), 395 (6697), 53-55CODEN: NATUAS; ISSN:0028-0836. (Macmillan Magazines)
    The modification and control of exciton-photon interactions in semiconductors is of both fundamental and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect were obsd. in planar microcavity structures in inorg. semiconductor quantum wells and bulk layers. But org. semiconductor microcavities were studied only in the weak-coupling regime. An org. semiconductor microcavity that operates in the strong-coupling regime is reported. Characteristic mixing is seen of the exciton and photon modes (anti-crossing), and a room-temp. vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorg. semiconductors. The results may lead to new structures and device concepts incorporating hybrid states of org. and inorg. excitons, and suggest that polariton lasing may be possible.
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  2. 2
    Gambino, S.; Mazzeo, M.; Genco, A.; Di Stefano, O.; Savasta, S.; Patanè, S.; Ballarini, D.; Mangione, F.; Lerario, G.; Sanvitto, D. Exploring Light-Matter Interaction Phenomena under Ultrastrong Coupling Regime. ACS Photonics 2014, 1 (10), 1042– 1048,  DOI: 10.1021/ph500266d
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    2
    Exploring Light-Matter Interaction Phenomena under Ultrastrong Coupling Regime
    Gambino, Salvatore; Mazzeo, Marco; Genco, Armando; Di Stefano, Omar; Savasta, Salvatore; Patane, Salvatore; Ballarini, Dario; Mangione, Federica; Lerario, Giovanni; Sanvitto, Daniele; Gigli, Giuseppe
    ACS Photonics (2014), 1 (10), 1042-1048CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    Exciton-polaritons are bosonic quasiparticles that arise from the normal mode splitting of photons in a microcavity and excitons in a semiconductor material. One of the most intriguing extensions of such a light-matter interaction is the so-called ultrastrong coupling regime. It is achieved when the Rabi frequency (ΩR, the energy exchange rate between the emitter and the resonant photonic mode) reaches a considerable fraction of the emitter transition frequency, ω0. A Rabi energy splitting (2ℏΩR) of 1.12 eV is reported, and values of the coupling ratio (2ΩR/ω0) ≤0.6-fold the material band gap in org. semiconductor microcavities and ≤0.5-fold in monolithic heterostructure org. light-emitting diodes working at room temp. are recorded. With such a large coupling strength it is possible to undress the exciton homogeneous linewidth from its inhomogeneous broadening, which allows for an unprecedented narrow emission line (below the cavity finesse) for such org. LEDs. The latter can be exploited for the realization of novel monochromatic sources and near-IR org. emitting devices.
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  3. 3
    Wang, S.; Chervy, T.; George, J.; Hutchison, J. A.; Genet, C.; Ebbesen, T. W. Quantum Yield of Polariton Emission from Hybrid Light-Matter States. J. Phys. Chem. Lett. 2014, 5 (8), 1433– 1439,  DOI: 10.1021/jz5004439
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    Quantum Yield of Polariton Emission from Hybrid Light-Matter States
    Wang, Shaojun; Chervy, Thibault; George, Jino; Hutchison, James A.; Genet, Cyriaque; Ebbesen, Thomas W.
    Journal of Physical Chemistry Letters (2014), 5 (8), 1433-1439CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The efficiency of light-matter strong coupling is tuned by precisely varying the spatial position of a thin layer of cyanine dye J-aggregates in Fabry-Perot microcavities, and their photophys. properties are detd. Placing the layer at the cavity field max. affords an interaction energy (Rabi splitting) of 503 meV, a 62% increase over that obsd. if the aggregates are simply spread evenly through the cavity, placing the system in the ultrastrong coupling regime. The fluorescence quantum yield of the lowest polaritonic state P- integrated over k-space is ∼10-2. The same value can be deduced from the 1.4 ps lifetime of P- measured by femtosecond transient absorption spectroscopy and the calcd. radiative decay rate const. Thus, the polariton decay is dominated by nonradiative processes, in contrast with what might be expected from the small effective mass of the polaritons. These findings provide a deeper understanding of hybrid light-mol. states and have implications for the modification of mol. and material properties by strong coupling.
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  4. 4
    Galego, J.; Garcia-Vidal, F. J.; Feist, J. Cavity-Induced Modifications of Molecular Structure in the Strong-Coupling Regime. Phys. Rev. X 2015, 5 (4), 041022,  DOI: 10.1103/PhysRevX.5.041022
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    Cavity-induced modifications of molecular structure in the strong-coupling regime
    Galego, Javier; Garcia-Vidal, Francisco J.; Feist, Johannes
    Physical Review X (2015), 5 (4), 041022/1-041022/14CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
    In most theor. descriptions of collective strong coupling of org. mols. to a cavity mode, the mols. are modeled as simple two-level systems. This picture fails to describe the rich structure provided by their internal rovibrational (nuclear) degrees of freedom. We investigate a first-principles model that fully takes into account both electronic and nuclear degrees of freedom, allowing an exploration of the phenomenon of strong coupling from an entirely new perspective. First, we demonstrate the limitations of applicability of the Born-Oppenheimer approxn. in strongly coupled mol.-cavity structures. For the case of two mols., we also show how dark states, which within the two-level picture are effectively decoupled from the cavity, are indeed affected by the formation of collective strong coupling. Finally, we discuss ground-state modifications in the ultrastrong-coupling regime and show that some mol. observables are affected by the collective coupling strength, while others depend only on the single-mol. coupling const.
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  5. 5
    Törma, P.; Barnes, W. L. Strong Coupling between Surface Plasmon Polaritons and Emitters: A Review. Rep. Prog. Phys. 2015, 78 (1), 013901,  DOI: 10.1088/0034-4885/78/1/013901
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    5
    Strong coupling between surface plasmon polaritons and emitters: a review
    Torma P; Barnes W L
    Reports on progress in physics. Physical Society (Great Britain) (2015), 78 (1), 013901 ISSN:.
    In this review we look at the concepts and state-of-the-art concerning the strong coupling of surface plasmon-polariton modes to states associated with quantum emitters such as excitons in J-aggregates, dye molecules and quantum dots. We explore the phenomenon of strong coupling with reference to a number of examples involving electromagnetic fields and matter. We then provide a concise description of the relevant background physics of surface plasmon polaritons. An extensive overview of the historical background and a detailed discussion of more recent relevant experimental advances concerning strong coupling between surface plasmon polaritons and quantum emitters is then presented. Three conceptual frameworks are then discussed and compared in depth: classical, semi-classical and fully quantum mechanical; these theoretical frameworks will have relevance to strong coupling beyond that involving surface plasmon polaritons. We conclude our review with a perspective on the future of this rapidly emerging field, one we are sure will grow to encompass more intriguing physics and will develop in scope to be of relevance to other areas of science.
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  6. 6
    Ebbesen, T. W. Hybrid Light-Matter States in a Molecular and Material Science Perspective. Acc. Chem. Res. 2016, 49 (11), 2403– 2412,  DOI: 10.1021/acs.accounts.6b00295
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    6
    Hybrid Light-Matter States in a Molecular and Material Science Perspective
    Ebbesen, Thomas W.
    Accounts of Chemical Research (2016), 49 (11), 2403-2412CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. The notion that light and matter states can be hybridized the way s and p orbitals are mixed is a concept that is not familiar to most chemists and material scientists. Yet it has much potential for mol. and material sciences that is just beginning to be explored. For instance, it has already been demonstrated that the rate and yield of chem. reactions can be modified and that the cond. of org. semiconductors and nonradiative energy transfer can be enhanced through the hybridization of electronic transitions. The hybridization is not limited to electronic transitions; it can be applied for instance to vibrational transitions to selectively perturb a given bond, opening new possibilities to change the chem. reactivity landscape and to use it as a tool in (bio)mol. science and spectroscopy. Such results are not only the consequence of the new eigenstates and energies generated by the hybridization. The hybrid light-matter states also have unusual properties: they can be delocalized over a very large no. of mols. (up to ca. 105), and they become dispersive or momentum-sensitive. Importantly, the hybridization occurs even in the absence of light because it is the zero-point energies of the mol. and optical transitions that generate the new light-matter states. The present work is not a review but rather an Account from the author's point of view that first introduces the reader to the underlying concepts and details of the features of hybrid light-matter states. It is shown that light-matter hybridization is quite easy to achieve: all that is needed is to place mols. or a material in a resonant optical cavity (e.g., between two parallel mirrors) under the right conditions. For vibrational strong coupling, microfluidic IR cells can be used to study the consequences for chem. in the liq. phase. Examples of modified properties are given to demonstrate the full potential for the mol. and material sciences. Finally an outlook of future directions for this emerging subject is given.
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  7. 7
    Sanvitto, D.; Kéna-Cohen, S. The Road towards Polaritonic Devices. Nat. Mater. 2016, 15 (10), 1061– 1073,  DOI: 10.1038/nmat4668
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    7
    The road towards polaritonic devices
    Sanvitto, Daniele; Kena-Cohen, Stephane
    Nature Materials (2016), 15 (10), 1061-1073CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
    A review. Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton-polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient 4-wave mixing to superfluidity in the condensed state. These quasiparticles possess strong intrinsic nonlinearities, while keeping most characteristics of the underlying photons. Here the authors review the most important features of exciton-polaritons in microcavities, with a particular emphasis on the emerging technol. applications, the use of new materials for room-temp. operation, and the possibility of exploiting polaritons for quantum computation and simulation.
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  8. 8
    Weihs, G.; Deng, H.; Snoke, D.; Yamamoto, Y. Polariton Lasing in a Microcavity. Phys. Status Solidi Appl. Res. 2004, 201 (4), 625– 632,  DOI: 10.1002/pssa.200304061
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    Polariton lasing in a microcavity
    Weihs, Gregor; Deng, Hui; Snoke, David; Yamamoto, Yoshihisa
    Physica Status Solidi A: Applied Research (2004), 201 (4), 625-632CODEN: PSSABA; ISSN:0031-8965. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review is given of the recent observation of exciton-polariton lasing, and new time-resolved spectra, imaging and angular dispersion data are discussed with regards to a possible Bose-Einstein condensate of polaritons. A brief outlook on the activities towards a clearer observation of polariton lasing is given.
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  9. 9
    Christopoulos, S.; Von Högersthal, G. B. H.; Grundy, A. J. D.; Lagoudakis, P. G.; Kavokin, A. V.; Baumberg, J. J.; Christmann, G.; Butté, R.; Feltin, E.; Carlin Room-Temperature Polariton Lasing in Semiconductor Microcavities. Phys. Rev. Lett. 2007, 98 (12), 1– 4,  DOI: 10.1103/PhysRevLett.98.126405
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    There is no corresponding record for this reference.
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    Kéna-Cohen, S.; Forrest, S. R. Room-Temperature Polariton Lasing in an Organic Single-Crystal Microcavity. Nat. Photonics 2010, 4 (6), 371– 375,  DOI: 10.1038/nphoton.2010.86
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    10
    Room-temperature polariton lasing in an organic single-crystal microcavity
    Kena-Cohen, S.; Forrest, S. R.
    Nature Photonics (2010), 4 (6), 371-375CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
    The optical properties of org. semiconductors are almost exclusively described using the Frenkel exciton picture. In this description, the strong Coulombic interaction between an excited electron and the charged vacancy it leaves behind (a hole) is automatically taken into account. If, in an optical microcavity, the exciton-photon interaction is strong compared to the excitonic and photonic decay rates, a second quasiparticle, the microcavity polariton, must be introduced to properly account for this coupling. Coherent, laser-like emission from polaritons has been predicted to occur when the ground-state occupancy of polaritons 〈ngs〉, reaches 1 (ref. ). This process, known as polariton lasing, can occur at thresholds much lower than required for conventional lasing. Polaritons in org. semiconductors are highly stable at room temp., but to our knowledge, there has as yet been no report of nonlinear emission from these structures. Here, we demonstrate polariton lasing at room temp. in an org. microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielec. mirrors.
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  11. 11
    Eizner, E.; Brodeur, J.; Barachati, F.; Sridharan, A.; Kéna-Cohen, S. Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light-Matter Coupling. ACS Photonics 2018, 5 (7), 2921– 2927,  DOI: 10.1021/acsphotonics.8b00254
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    11
    Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light-Matter Coupling
    Eizner, Elad; Brodeur, Julien; Barachati, Fabio; Sridharan, Aravindan; Kena-Cohen, Stephane
    ACS Photonics (2018), 5 (7), 2921-2927CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    In org. photodiodes (OPDs), light is absorbed by excitons that dissoc. to generate photocurrent. Here, we demonstrate a novel type of OPD in which light is absorbed by polaritons, hybrid light-matter states. We demonstrate polariton OPDs operating in the ultrastrong coupling regime at visible and IR wavelengths. These devices can be engineered to show narrow responsivity with a very weak angle-dependence. More importantly, they can be tuned to operate in a spectral range outside that of the bare exciton absorption. Remarkably, we show that the responsivity of a polariton OPD can be pushed to near-IR wavelengths, where few org. absorbers are available, with external quantum efficiencies exceeding those of our control OPD.
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    Campos-Gonzalez-Angulo, J. A.; Ribeiro, R. F.; Yuen-Zhou, J. Resonant Catalysis of Thermally Activated Chemical Reactions with Vibrational Polaritons. Nat. Commun. 2019, 10 (1), 1– 8,  DOI: 10.1038/s41467-019-12636-1
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    12
    Resonant catalysis of thermally activated chemical reactions with vibrational polaritons
    Campos-Gonzalez-Angulo, Jorge A.; Ribeiro, Raphael F.; Yuen-Zhou, Joel
    Nature Communications (2019), 10 (1), 1-8CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Interaction between light and matter results in new quantum states whose energetics can modify chem. kinetics. In the regime of ensemble vibrational strong coupling (VSC), a macroscopic no. N of mol. transitions couple to each resonant cavity mode, yielding two hybrid light-matter (polariton) modes and a reservoir of N - 1 dark states whose chem. dynamics are essentially those of the bare mols. This fact is seemingly in opposition to the recently reported modification of thermally activated ground electronic state reactions under VSC. Here we provide a VSC Marcus-Levich-Jortner electron transfer model that potentially addresses this paradox: although entropy favors the transit through dark-state channels, the chem. kinetics can be dictated by a few polaritonic channels with smaller activation energies. The effects of catalytic VSC are maximal at light-matter resonance, in agreement with exptl. observations.
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    Casey, S. R.; Sparks, J. R. Vibrational Strong Coupling of Organometallic Complexes. J. Phys. Chem. C 2016, 120 (49), 28138– 28143,  DOI: 10.1021/acs.jpcc.6b10493
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    13
    Vibrational Strong Coupling of Organometallic Complexes
    Casey, Shaelyn R.; Sparks, Justin R.
    Journal of Physical Chemistry C (2016), 120 (49), 28138-28143CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Strong coupling of optical and mol. vibrational states to form polariton states is a promising route towards the modification of mol. bond properties without changing the constituent atoms or formal bonding structure. Vibrational strong coupling of the ligands of aq. organometallic complex ions to microfluidic Fabry-Perot cavity modes is demonstrated for the first time using the ferrocyanide ion as a model system. The complex can exhibit tunable strong vibrational coupling while dissolved in soln. at moderate concns., with a lower limit of ∼15 mM, due to its large molar absorptivity and narrow resonance bandwidth. Combining the exquisite fluid control of microfluidic devices with the ability to modify ligand bond properties of transition metal complexes via vibrational strong coupling may lead to novel methods for the examn. of catalytic reaction mechanisms and provide a new means for tailoring catalyst mols.
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  14. 14
    Galego, J.; Garcia-Vidal, F. J.; Feist, J. Suppressing Photochemical Reactions with Quantized Light Fields. Nat. Commun. 2016, 7, 1– 6,  DOI: 10.1038/ncomms13841
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    Hutchison, J. A.; Schwartz, T.; Genet, C.; Devaux, E.; Ebbesen, T. W. Modifying Chemical Landscapes by Coupling to Vacuum Fields. Angew. Chem., Int. Ed. 2012, 51 (7), 1592– 1596,  DOI: 10.1002/anie.201107033
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    15
    Modifying Chemical Landscapes by Coupling to Vacuum Fields
    Hutchison, James A.; Schwartz, Tal; Genet, Cyriaque; Devaux, Eloise; Ebbesen, Thomas W.
    Angewandte Chemie, International Edition (2012), 51 (7), 1592-1596, S1592/1-S1592/3CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Modifying chem. landscapes by coupling to vacuum fields is discussed.
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    Mandal, A.; Huo, P. Investigating New Reactivities Enabled by Polariton Photochemistry. J. Phys. Chem. Lett. 2019, 10 (18), 5519– 5529,  DOI: 10.1021/acs.jpclett.9b01599
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    16
    Investigating new reactivities enabled by polariton photochemistry
    Mandal, Arkajit; Huo, Pengfei
    Journal of Physical Chemistry Letters (2019), 10 (18), 5519-5529CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    We perform quantum dynamics simulations to investigate new chem. reactivities enabled by cavity quantum electrodynamics. The quantum light-matter interactions between the mol. and the quantized radiation mode inside an optical cavity create a set of hybridized electronic-photonic states, so-called polaritons. The polaritonic states adapt the curvatures from both the ground and the excited electronic states, opening up new possibilities to control photochem. reactions by exploiting intrinsic quantum behaviors of light-matter interactions. With quantum dynamics simulations, we demonstrate that the selectivity of a model photoisomerization reaction can be controlled by tuning the photon frequency of the cavity mode or the light-matter coupling strength, providing new ways to manipulate chem. reactions via the light-matter interaction. We further investigate collective quantum effects enabled by coupling the quantized radiation mode to multiple mols. Our results suggest that in the resonance case, a photon is recycled among mols. to enable multiple excited state reactions, thus effectively functioning as a catalyst. In the nonresonance case, mols. emit and absorb virtual photons to initiate excited state reactions through fundamental quantum electrodynamics processes. These results from quantum dynamics simulations reveal basic principles of polariton photochem. as well as promising reactivities that take advantage of intrinsic quantum behaviors of photons.
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    Herrera, F.; Spano, F. C. Cavity-Controlled Chemistry in Molecular Ensembles. Phys. Rev. Lett. 2016, 116 (23), 1– 5,  DOI: 10.1103/PhysRevLett.116.238301
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    Feist, J.; Galego, J.; Garcia-Vidal, F. J. Polaritonic Chemistry with Organic Molecules. ACS Photonics 2018, 5 (1), 205– 216,  DOI: 10.1021/acsphotonics.7b00680
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    18
    Polaritonic Chemistry with Organic Molecules
    Feist, Johannes; Galego, Javier; Garcia-Vidal, Francisco J.
    ACS Photonics (2018), 5 (1), 205-216CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
    A review. The authors present an overview of the general concepts of polaritonic chem. with org. mols., i.e., the manipulation of chem. structure that can be achieved through strong coupling between confined light modes and org. mols. Strong coupling and the assocd. formation of polaritons, hybrid light-matter excitations, lead to energy shifts in such systems that can amt. to a large fraction of the uncoupled transition energy. This has recently been shown to significantly alter the chem. structure of the coupled mols., which opens the possibility to manipulate and control reactions. The authors discuss the current state of theory for describing these changes and present several applications, with a particular focus on the collective effects obsd. when many mols. are involved in strong coupling.
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  19. 19
    Thomas, A.; Lethuillier-Karl, L.; Nagarajan, K.; Vergauwe, R. M. A.; George, J.; Chervy, T.; Shalabney, A.; Devaux, E.; Genet, C.; Moran, J.; Ebbesen, T. W. Tilting a Ground-State Reactivity Landscape by Vibrational Strong Coupling. Science 2019, 363 (6427), 615– 619,  DOI: 10.1126/science.aau7742
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    19
    Tilting a ground-state reactivity landscape by vibrational strong coupling
    Thomas, A.; Lethuillier-Karl, L.; Nagarajan, K.; Vergauwe, R. M. A.; George, J.; Chervy, T.; Shalabney, A.; Devaux, E.; Genet, C.; Moran, J.; Ebbesen, T. W.
    Science (Washington, DC, United States) (2019), 363 (6427), 615-619CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Many chem. methods have been developed to favor a particular product in transformations of compds. that have two or more reactive sites. We explored a different approach to site selectivity using vibrational strong coupling (VSC) between a reactant and the vacuum field of a microfluidic optical cavity. Specifically, we studied the reactivity of a compd. bearing two possible silyl bond cleavage sites-Si-C and Si-O, resp.-as a function of VSC of three distinct vibrational modes in the dark. The results show that VSC can indeed tilt the reactivity landscape to favor one product over the other. Thermodn. parameters reveal the presence of a large activation barrier and substantial changes to the activation entropy, confirming the modified chem. landscape under strong coupling.
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  20. 20
    Nikolis, V. C.; Mischok, A.; Siegmund, B.; Kublitski, J.; Jia, X.; Benduhn, J.; Hörmann, U.; Neher, D.; Gather, M. C.; Spoltore, D. Strong Light-Matter Coupling for Reduced Photon Energy Losses in Organic Photovoltaics. Nat. Commun. 2019, 10 (1), 3706,  DOI: 10.1038/s41467-019-11717-5
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    20
    Strong light-matter coupling for reduced photon energy losses in organic photovoltaics
    Nikolis Vasileios C; Siegmund Bernhard; Kublitski Jonas; Jia Xiangkun; Benduhn Johannes; Spoltore Donato; Vandewal Koen; Nikolis Vasileios C; Siegmund Bernhard; Mischok Andreas; Gather Malte C; Hormann Ulrich; Neher Dieter; Vandewal Koen
    Nature communications (2019), 10 (1), 3706 ISSN:.
    Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.
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    cf. CA 55, 20615d. A quant. attempt is made to apply a 4-orbital model, described earlier (loc. cit.), to porphyrin mols. The model is a combination of L.C.A.O.-M.O. and a simplified treatment of configuration interaction. The parameters for the latter are detd. on Zn tetraphenylporphine. Orbital and transition energies and oscillator strengths are detd. for the reduced porphyrins: dihydroporphine, tetrahydroporphine, hexahydroporphine, and octahydroporphine. Qual. agreement with the observed spectra is obtained. Similar calcns. are made for phlorin-6-dihydrochloride. Spectra calcns. are given for the metal salts of mono-, di-, and triazaporphine, tetrabenzoporphine, tetrabenzotetraazaporphine (or phthalocyanine), and monobenzo-, 2 dibenzo-, and tribenzotetraazaporphine. The calcns. are in fair agreement with observed spectra. Quant. applications of the model are limited to regions of small perturbations. The application of the model to the calcn. of chem. and magnetic properties is discussed.
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    A 1,1,4,4-tetracyanobuta-1,3-diene (TCBD)-aniline moiety has been introduced, for the first time, at the axial position of two subphthalocyanines (SubPcs) peripherally substituted with hydrogen (H12SubPc) or fluorine atoms (F12SubPc). Single-crystal x-ray anal. of both SubPc-TCBD-aniline systems showed that each conjugate is a racemic mixt. of two atropisomers resulting from the almost orthogonal geometry adopted by the axial TCBD unit, which were sepd. by chiral HPLC. Remarkably, the single-crystal x-ray structure of one atropisomer of each SubPc-TCBD-aniline conjugate has been solved allowing to unambiguously assign the atropisomers' abs. configuration, something, to the best of our knowledge, unprecedented in TCBD-based conjugates. Moreover, the physicochem. properties of both SubPc-TCBD-aniline racemates have been investigated by using a wide range of electrochem. as well as steady state and time-resolved spectroscopic techniques. Remarkably, each of the two SubPc-TCBD-aniline presents a unique photophys. feature never obsd. before in SubPc chem. As a matter of fact, H12SubPc-TCBD-aniline showed significant ground state charge transfer interactions between the H12SubPc macrocycle and the electron-withdrawing TCBD unit directly attached at its axial position. In contrast, F12SubPc-TCBD-aniline gave rise to an intense, broad emission, which red-shifts upon increasing the solvent polarity and stems from an excited complex (i.e., an exciplex). Such exciplex emission, which has also no precedent in TCBD chem., results from intramol. interactions in the excited state between the electron-rich aniline and the F12SubPc π-surface, two mol. fragments kept in spatial proximity by the "unique" 3-dimensional geometry adopted by the F12SubPc-TCBD-aniline. Complementary transient absorption studies were carried out on both SubPc-TCBD-aniline derivs. showing the occurrence, in both cases, of photoinduced charge sepn. and corroborating the formation of the aforementioned intramol. exciplex in terms of a radical-ion pair stabilized through-space.
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  • Abstract

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    Figure 1

    Figure 1. (a) Chemical structure of the SubPc derivative used in this work, attained by anchoring an ethynylaniline ligand to the boron atom of a perfluorinated SubPc. Synthetic details are available in ref (28). (b) Schematic representation of the structure of the Fabry–Pérot resonators prepared for this work. (c) Absorptance spectra (colored solid lines) of resonators built varying the cavity thickness, namely, 100 nm (green), 115 nm (yellow), 130 nm (blue), 150 nm (red), and 163 nm (purple). The calculated normal incidence reflectance spectra of the corresponding underlying optical cavities are plotted following the same color code (dashed lines). For the sake of comparison, the absorptance of a bare SubPc-Et film is also shown (solid light gray line).

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    Figure 2

    Figure 2. Dispersion curves attained from the angular dependence measurements of the absorptance (a, b), reflectance (c, d), and transmittance (e, f) of an optical resonator made of a 94 nm thick SubPc-Et film (total cavity thickness 130 nm, after adding the thickness of the PVA leveling layers). Left and right panels correspond respectively to S-polarized (or transversal electric, TE, i.e., electric field E perpendicular to the plane of incidence) and P-polarized incident light (or transversal magnetic, TM, with E contained in the plane of incidence). The white dashed line corresponds to the underlying cavity first-order mode dispersion, while the red dashed horizontal line indicates the spectral position of the SubPc-Et absorption outside the cavity taken as reference.

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    Figure 3

    Figure 3. (a) Experimental (blue line) and fitted (red dashed line) absorptance of a SubPc-Et film. The contributions from the different oscillators considered for the fitting are plotted as light gray lines. The vertical dark gray dash-dotted line corresponds to the average value of the spectral position of the oscillator maxima, weighted by the area of each peak, which we take as ω0. (b) The real (solid line) and imaginary (dashed line) parts of the refractive index, n and k, estimated from the fitting of the optical properties of the SubPc-Et film. The horizontal dark gray dash-dotted line indicates the average value of n in the region of interest (400 nm < λ < 800 nm), which we take as the refractive index of the underlying, nonabsorbing, optical medium. (c) Experimental (scatter graphs) and theoretical (solid lines) dependence of the spectral position of the upper (ω+, red colored) and lower (ω–, blue colored) polaritonic branches versus the degree of detuning δ, estimated from the above-mentioned values of ω0 and ωcav.

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    Figure 4

    Figure 4. Calculated spatial and spectral profiles of (a) the electric field intensity |E(r)|2 and (b) the absorbed luminous power PA for a SubPc-Et cavity of thickness 130 nm, at normal incidence. (c) Discriminated absorptance at normal incidence of each layer in the cavity, namely, the SubPc-Et film (purple line) and top (dark gray) and bottom (light gray) silver mirrors. Total optical loss spectrum in the metals (black) and the full absorptance of the cavity (red) are also plotted. Intensity maps in (d) and (e) show the energy dispersion of both the SubPc-Et (productive) and the metals (parasitic) absorptance of nonpolarized light.

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    Figure 5

    Figure 5. (a–c) Solar spectrum (AM1.5) weighted absorptance, SSWA, of a metallic optical cavity embedding a 94 nm thick SubPc-Et film sandwiched between two PVA thin films. Results are shown for (a) S-polarized, (b) P-polarized, and (c) nonpolarized light incident at angles of 6° (purple line), 26° (blue), 46° (light blue), and 66° (green) with respect to the cavity surface normal. The normalized AM1.5 spectrum and the absorptance of a bare SubPc-Et film are also plotted (red and gray solid lines, respectively). (d–f) Wavelength-integrated SSWA, ∫SSWA, versus angle of incidence of sunlight for each one of the layers in the cavity, namely, the SubPc-Et film (purple line), top mirror (dark gray dashed line), and bottom mirror (light gray dashed line). Total integrated parasitic absorption (black dashed line) and full integrated absorption of the cavity (red solid line) are also plotted. Results are shown for (d) S-polarized, (e) P-polarized, and (f) nonpolarized light.

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  • References

    This article references 31 other publications.

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      Wang, S.; Chervy, T.; George, J.; Hutchison, J. A.; Genet, C.; Ebbesen, T. W. Quantum Yield of Polariton Emission from Hybrid Light-Matter States. J. Phys. Chem. Lett. 2014, 5 (8), 1433– 1439,  DOI: 10.1021/jz5004439
      3
      Quantum Yield of Polariton Emission from Hybrid Light-Matter States
      Wang, Shaojun; Chervy, Thibault; George, Jino; Hutchison, James A.; Genet, Cyriaque; Ebbesen, Thomas W.
      Journal of Physical Chemistry Letters (2014), 5 (8), 1433-1439CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The efficiency of light-matter strong coupling is tuned by precisely varying the spatial position of a thin layer of cyanine dye J-aggregates in Fabry-Perot microcavities, and their photophys. properties are detd. Placing the layer at the cavity field max. affords an interaction energy (Rabi splitting) of 503 meV, a 62% increase over that obsd. if the aggregates are simply spread evenly through the cavity, placing the system in the ultrastrong coupling regime. The fluorescence quantum yield of the lowest polaritonic state P- integrated over k-space is ∼10-2. The same value can be deduced from the 1.4 ps lifetime of P- measured by femtosecond transient absorption spectroscopy and the calcd. radiative decay rate const. Thus, the polariton decay is dominated by nonradiative processes, in contrast with what might be expected from the small effective mass of the polaritons. These findings provide a deeper understanding of hybrid light-mol. states and have implications for the modification of mol. and material properties by strong coupling.
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    4. 4
      Galego, J.; Garcia-Vidal, F. J.; Feist, J. Cavity-Induced Modifications of Molecular Structure in the Strong-Coupling Regime. Phys. Rev. X 2015, 5 (4), 041022,  DOI: 10.1103/PhysRevX.5.041022
      4
      Cavity-induced modifications of molecular structure in the strong-coupling regime
      Galego, Javier; Garcia-Vidal, Francisco J.; Feist, Johannes
      Physical Review X (2015), 5 (4), 041022/1-041022/14CODEN: PRXHAE; ISSN:2160-3308. (American Physical Society)
      In most theor. descriptions of collective strong coupling of org. mols. to a cavity mode, the mols. are modeled as simple two-level systems. This picture fails to describe the rich structure provided by their internal rovibrational (nuclear) degrees of freedom. We investigate a first-principles model that fully takes into account both electronic and nuclear degrees of freedom, allowing an exploration of the phenomenon of strong coupling from an entirely new perspective. First, we demonstrate the limitations of applicability of the Born-Oppenheimer approxn. in strongly coupled mol.-cavity structures. For the case of two mols., we also show how dark states, which within the two-level picture are effectively decoupled from the cavity, are indeed affected by the formation of collective strong coupling. Finally, we discuss ground-state modifications in the ultrastrong-coupling regime and show that some mol. observables are affected by the collective coupling strength, while others depend only on the single-mol. coupling const.
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    5. 5
      Törma, P.; Barnes, W. L. Strong Coupling between Surface Plasmon Polaritons and Emitters: A Review. Rep. Prog. Phys. 2015, 78 (1), 013901,  DOI: 10.1088/0034-4885/78/1/013901
      5
      Strong coupling between surface plasmon polaritons and emitters: a review
      Torma P; Barnes W L
      Reports on progress in physics. Physical Society (Great Britain) (2015), 78 (1), 013901 ISSN:.
      In this review we look at the concepts and state-of-the-art concerning the strong coupling of surface plasmon-polariton modes to states associated with quantum emitters such as excitons in J-aggregates, dye molecules and quantum dots. We explore the phenomenon of strong coupling with reference to a number of examples involving electromagnetic fields and matter. We then provide a concise description of the relevant background physics of surface plasmon polaritons. An extensive overview of the historical background and a detailed discussion of more recent relevant experimental advances concerning strong coupling between surface plasmon polaritons and quantum emitters is then presented. Three conceptual frameworks are then discussed and compared in depth: classical, semi-classical and fully quantum mechanical; these theoretical frameworks will have relevance to strong coupling beyond that involving surface plasmon polaritons. We conclude our review with a perspective on the future of this rapidly emerging field, one we are sure will grow to encompass more intriguing physics and will develop in scope to be of relevance to other areas of science.
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    6. 6
      Ebbesen, T. W. Hybrid Light-Matter States in a Molecular and Material Science Perspective. Acc. Chem. Res. 2016, 49 (11), 2403– 2412,  DOI: 10.1021/acs.accounts.6b00295
      6
      Hybrid Light-Matter States in a Molecular and Material Science Perspective
      Ebbesen, Thomas W.
      Accounts of Chemical Research (2016), 49 (11), 2403-2412CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. The notion that light and matter states can be hybridized the way s and p orbitals are mixed is a concept that is not familiar to most chemists and material scientists. Yet it has much potential for mol. and material sciences that is just beginning to be explored. For instance, it has already been demonstrated that the rate and yield of chem. reactions can be modified and that the cond. of org. semiconductors and nonradiative energy transfer can be enhanced through the hybridization of electronic transitions. The hybridization is not limited to electronic transitions; it can be applied for instance to vibrational transitions to selectively perturb a given bond, opening new possibilities to change the chem. reactivity landscape and to use it as a tool in (bio)mol. science and spectroscopy. Such results are not only the consequence of the new eigenstates and energies generated by the hybridization. The hybrid light-matter states also have unusual properties: they can be delocalized over a very large no. of mols. (up to ca. 105), and they become dispersive or momentum-sensitive. Importantly, the hybridization occurs even in the absence of light because it is the zero-point energies of the mol. and optical transitions that generate the new light-matter states. The present work is not a review but rather an Account from the author's point of view that first introduces the reader to the underlying concepts and details of the features of hybrid light-matter states. It is shown that light-matter hybridization is quite easy to achieve: all that is needed is to place mols. or a material in a resonant optical cavity (e.g., between two parallel mirrors) under the right conditions. For vibrational strong coupling, microfluidic IR cells can be used to study the consequences for chem. in the liq. phase. Examples of modified properties are given to demonstrate the full potential for the mol. and material sciences. Finally an outlook of future directions for this emerging subject is given.
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    7. 7
      Sanvitto, D.; Kéna-Cohen, S. The Road towards Polaritonic Devices. Nat. Mater. 2016, 15 (10), 1061– 1073,  DOI: 10.1038/nmat4668
      7
      The road towards polaritonic devices
      Sanvitto, Daniele; Kena-Cohen, Stephane
      Nature Materials (2016), 15 (10), 1061-1073CODEN: NMAACR; ISSN:1476-1122. (Nature Publishing Group)
      A review. Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton-polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient 4-wave mixing to superfluidity in the condensed state. These quasiparticles possess strong intrinsic nonlinearities, while keeping most characteristics of the underlying photons. Here the authors review the most important features of exciton-polaritons in microcavities, with a particular emphasis on the emerging technol. applications, the use of new materials for room-temp. operation, and the possibility of exploiting polaritons for quantum computation and simulation.
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    8. 8
      Weihs, G.; Deng, H.; Snoke, D.; Yamamoto, Y. Polariton Lasing in a Microcavity. Phys. Status Solidi Appl. Res. 2004, 201 (4), 625– 632,  DOI: 10.1002/pssa.200304061
      8
      Polariton lasing in a microcavity
      Weihs, Gregor; Deng, Hui; Snoke, David; Yamamoto, Yoshihisa
      Physica Status Solidi A: Applied Research (2004), 201 (4), 625-632CODEN: PSSABA; ISSN:0031-8965. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review is given of the recent observation of exciton-polariton lasing, and new time-resolved spectra, imaging and angular dispersion data are discussed with regards to a possible Bose-Einstein condensate of polaritons. A brief outlook on the activities towards a clearer observation of polariton lasing is given.
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    9. 9
      Christopoulos, S.; Von Högersthal, G. B. H.; Grundy, A. J. D.; Lagoudakis, P. G.; Kavokin, A. V.; Baumberg, J. J.; Christmann, G.; Butté, R.; Feltin, E.; Carlin Room-Temperature Polariton Lasing in Semiconductor Microcavities. Phys. Rev. Lett. 2007, 98 (12), 1– 4,  DOI: 10.1103/PhysRevLett.98.126405
      There is no corresponding record for this reference.
    10. 10
      Kéna-Cohen, S.; Forrest, S. R. Room-Temperature Polariton Lasing in an Organic Single-Crystal Microcavity. Nat. Photonics 2010, 4 (6), 371– 375,  DOI: 10.1038/nphoton.2010.86
      10
      Room-temperature polariton lasing in an organic single-crystal microcavity
      Kena-Cohen, S.; Forrest, S. R.
      Nature Photonics (2010), 4 (6), 371-375CODEN: NPAHBY; ISSN:1749-4885. (Nature Publishing Group)
      The optical properties of org. semiconductors are almost exclusively described using the Frenkel exciton picture. In this description, the strong Coulombic interaction between an excited electron and the charged vacancy it leaves behind (a hole) is automatically taken into account. If, in an optical microcavity, the exciton-photon interaction is strong compared to the excitonic and photonic decay rates, a second quasiparticle, the microcavity polariton, must be introduced to properly account for this coupling. Coherent, laser-like emission from polaritons has been predicted to occur when the ground-state occupancy of polaritons 〈ngs〉, reaches 1 (ref. ). This process, known as polariton lasing, can occur at thresholds much lower than required for conventional lasing. Polaritons in org. semiconductors are highly stable at room temp., but to our knowledge, there has as yet been no report of nonlinear emission from these structures. Here, we demonstrate polariton lasing at room temp. in an org. microcavity composed of a melt-grown anthracene single crystal sandwiched between two dielec. mirrors.
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    11. 11
      Eizner, E.; Brodeur, J.; Barachati, F.; Sridharan, A.; Kéna-Cohen, S. Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light-Matter Coupling. ACS Photonics 2018, 5 (7), 2921– 2927,  DOI: 10.1021/acsphotonics.8b00254
      11
      Organic Photodiodes with an Extended Responsivity Using Ultrastrong Light-Matter Coupling
      Eizner, Elad; Brodeur, Julien; Barachati, Fabio; Sridharan, Aravindan; Kena-Cohen, Stephane
      ACS Photonics (2018), 5 (7), 2921-2927CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      In org. photodiodes (OPDs), light is absorbed by excitons that dissoc. to generate photocurrent. Here, we demonstrate a novel type of OPD in which light is absorbed by polaritons, hybrid light-matter states. We demonstrate polariton OPDs operating in the ultrastrong coupling regime at visible and IR wavelengths. These devices can be engineered to show narrow responsivity with a very weak angle-dependence. More importantly, they can be tuned to operate in a spectral range outside that of the bare exciton absorption. Remarkably, we show that the responsivity of a polariton OPD can be pushed to near-IR wavelengths, where few org. absorbers are available, with external quantum efficiencies exceeding those of our control OPD.
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    12. 12
      Campos-Gonzalez-Angulo, J. A.; Ribeiro, R. F.; Yuen-Zhou, J. Resonant Catalysis of Thermally Activated Chemical Reactions with Vibrational Polaritons. Nat. Commun. 2019, 10 (1), 1– 8,  DOI: 10.1038/s41467-019-12636-1
      12
      Resonant catalysis of thermally activated chemical reactions with vibrational polaritons
      Campos-Gonzalez-Angulo, Jorge A.; Ribeiro, Raphael F.; Yuen-Zhou, Joel
      Nature Communications (2019), 10 (1), 1-8CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
      Interaction between light and matter results in new quantum states whose energetics can modify chem. kinetics. In the regime of ensemble vibrational strong coupling (VSC), a macroscopic no. N of mol. transitions couple to each resonant cavity mode, yielding two hybrid light-matter (polariton) modes and a reservoir of N - 1 dark states whose chem. dynamics are essentially those of the bare mols. This fact is seemingly in opposition to the recently reported modification of thermally activated ground electronic state reactions under VSC. Here we provide a VSC Marcus-Levich-Jortner electron transfer model that potentially addresses this paradox: although entropy favors the transit through dark-state channels, the chem. kinetics can be dictated by a few polaritonic channels with smaller activation energies. The effects of catalytic VSC are maximal at light-matter resonance, in agreement with exptl. observations.
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    13. 13
      Casey, S. R.; Sparks, J. R. Vibrational Strong Coupling of Organometallic Complexes. J. Phys. Chem. C 2016, 120 (49), 28138– 28143,  DOI: 10.1021/acs.jpcc.6b10493
      13
      Vibrational Strong Coupling of Organometallic Complexes
      Casey, Shaelyn R.; Sparks, Justin R.
      Journal of Physical Chemistry C (2016), 120 (49), 28138-28143CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Strong coupling of optical and mol. vibrational states to form polariton states is a promising route towards the modification of mol. bond properties without changing the constituent atoms or formal bonding structure. Vibrational strong coupling of the ligands of aq. organometallic complex ions to microfluidic Fabry-Perot cavity modes is demonstrated for the first time using the ferrocyanide ion as a model system. The complex can exhibit tunable strong vibrational coupling while dissolved in soln. at moderate concns., with a lower limit of ∼15 mM, due to its large molar absorptivity and narrow resonance bandwidth. Combining the exquisite fluid control of microfluidic devices with the ability to modify ligand bond properties of transition metal complexes via vibrational strong coupling may lead to novel methods for the examn. of catalytic reaction mechanisms and provide a new means for tailoring catalyst mols.
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    14. 14
      Galego, J.; Garcia-Vidal, F. J.; Feist, J. Suppressing Photochemical Reactions with Quantized Light Fields. Nat. Commun. 2016, 7, 1– 6,  DOI: 10.1038/ncomms13841
      There is no corresponding record for this reference.
    15. 15
      Hutchison, J. A.; Schwartz, T.; Genet, C.; Devaux, E.; Ebbesen, T. W. Modifying Chemical Landscapes by Coupling to Vacuum Fields. Angew. Chem., Int. Ed. 2012, 51 (7), 1592– 1596,  DOI: 10.1002/anie.201107033
      15
      Modifying Chemical Landscapes by Coupling to Vacuum Fields
      Hutchison, James A.; Schwartz, Tal; Genet, Cyriaque; Devaux, Eloise; Ebbesen, Thomas W.
      Angewandte Chemie, International Edition (2012), 51 (7), 1592-1596, S1592/1-S1592/3CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Modifying chem. landscapes by coupling to vacuum fields is discussed.
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    16. 16
      Mandal, A.; Huo, P. Investigating New Reactivities Enabled by Polariton Photochemistry. J. Phys. Chem. Lett. 2019, 10 (18), 5519– 5529,  DOI: 10.1021/acs.jpclett.9b01599
      16
      Investigating new reactivities enabled by polariton photochemistry
      Mandal, Arkajit; Huo, Pengfei
      Journal of Physical Chemistry Letters (2019), 10 (18), 5519-5529CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      We perform quantum dynamics simulations to investigate new chem. reactivities enabled by cavity quantum electrodynamics. The quantum light-matter interactions between the mol. and the quantized radiation mode inside an optical cavity create a set of hybridized electronic-photonic states, so-called polaritons. The polaritonic states adapt the curvatures from both the ground and the excited electronic states, opening up new possibilities to control photochem. reactions by exploiting intrinsic quantum behaviors of light-matter interactions. With quantum dynamics simulations, we demonstrate that the selectivity of a model photoisomerization reaction can be controlled by tuning the photon frequency of the cavity mode or the light-matter coupling strength, providing new ways to manipulate chem. reactions via the light-matter interaction. We further investigate collective quantum effects enabled by coupling the quantized radiation mode to multiple mols. Our results suggest that in the resonance case, a photon is recycled among mols. to enable multiple excited state reactions, thus effectively functioning as a catalyst. In the nonresonance case, mols. emit and absorb virtual photons to initiate excited state reactions through fundamental quantum electrodynamics processes. These results from quantum dynamics simulations reveal basic principles of polariton photochem. as well as promising reactivities that take advantage of intrinsic quantum behaviors of photons.
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    17. 17
      Herrera, F.; Spano, F. C. Cavity-Controlled Chemistry in Molecular Ensembles. Phys. Rev. Lett. 2016, 116 (23), 1– 5,  DOI: 10.1103/PhysRevLett.116.238301
      There is no corresponding record for this reference.
    18. 18
      Feist, J.; Galego, J.; Garcia-Vidal, F. J. Polaritonic Chemistry with Organic Molecules. ACS Photonics 2018, 5 (1), 205– 216,  DOI: 10.1021/acsphotonics.7b00680
      18
      Polaritonic Chemistry with Organic Molecules
      Feist, Johannes; Galego, Javier; Garcia-Vidal, Francisco J.
      ACS Photonics (2018), 5 (1), 205-216CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      A review. The authors present an overview of the general concepts of polaritonic chem. with org. mols., i.e., the manipulation of chem. structure that can be achieved through strong coupling between confined light modes and org. mols. Strong coupling and the assocd. formation of polaritons, hybrid light-matter excitations, lead to energy shifts in such systems that can amt. to a large fraction of the uncoupled transition energy. This has recently been shown to significantly alter the chem. structure of the coupled mols., which opens the possibility to manipulate and control reactions. The authors discuss the current state of theory for describing these changes and present several applications, with a particular focus on the collective effects obsd. when many mols. are involved in strong coupling.
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    19. 19
      Thomas, A.; Lethuillier-Karl, L.; Nagarajan, K.; Vergauwe, R. M. A.; George, J.; Chervy, T.; Shalabney, A.; Devaux, E.; Genet, C.; Moran, J.; Ebbesen, T. W. Tilting a Ground-State Reactivity Landscape by Vibrational Strong Coupling. Science 2019, 363 (6427), 615– 619,  DOI: 10.1126/science.aau7742
      19
      Tilting a ground-state reactivity landscape by vibrational strong coupling
      Thomas, A.; Lethuillier-Karl, L.; Nagarajan, K.; Vergauwe, R. M. A.; George, J.; Chervy, T.; Shalabney, A.; Devaux, E.; Genet, C.; Moran, J.; Ebbesen, T. W.
      Science (Washington, DC, United States) (2019), 363 (6427), 615-619CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Many chem. methods have been developed to favor a particular product in transformations of compds. that have two or more reactive sites. We explored a different approach to site selectivity using vibrational strong coupling (VSC) between a reactant and the vacuum field of a microfluidic optical cavity. Specifically, we studied the reactivity of a compd. bearing two possible silyl bond cleavage sites-Si-C and Si-O, resp.-as a function of VSC of three distinct vibrational modes in the dark. The results show that VSC can indeed tilt the reactivity landscape to favor one product over the other. Thermodn. parameters reveal the presence of a large activation barrier and substantial changes to the activation entropy, confirming the modified chem. landscape under strong coupling.
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    20. 20
      Nikolis, V. C.; Mischok, A.; Siegmund, B.; Kublitski, J.; Jia, X.; Benduhn, J.; Hörmann, U.; Neher, D.; Gather, M. C.; Spoltore, D. Strong Light-Matter Coupling for Reduced Photon Energy Losses in Organic Photovoltaics. Nat. Commun. 2019, 10 (1), 3706,  DOI: 10.1038/s41467-019-11717-5
      20
      Strong light-matter coupling for reduced photon energy losses in organic photovoltaics
      Nikolis Vasileios C; Siegmund Bernhard; Kublitski Jonas; Jia Xiangkun; Benduhn Johannes; Spoltore Donato; Vandewal Koen; Nikolis Vasileios C; Siegmund Bernhard; Mischok Andreas; Gather Malte C; Hormann Ulrich; Neher Dieter; Vandewal Koen
      Nature communications (2019), 10 (1), 3706 ISSN:.
      Strong light-matter coupling can re-arrange the exciton energies in organic semiconductors. Here, we exploit strong coupling by embedding a fullerene-free organic solar cell (OSC) photo-active layer into an optical microcavity, leading to the formation of polariton peaks and a red-shift of the optical gap. At the same time, the open-circuit voltage of the device remains unaffected. This leads to reduced photon energy losses for the low-energy polaritons and a steepening of the absorption edge. While strong coupling reduces the optical gap, the energy of the charge-transfer state is not affected for large driving force donor-acceptor systems. Interestingly, this implies that strong coupling can be exploited in OSCs to reduce the driving force for electron transfer, without chemical or microstructural modifications of the photo-active layer. Our work demonstrates that the processes determining voltage losses in OSCs can now be tuned, and reduced to unprecedented values, simply by manipulating the device architecture.
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    21. 21
      Gouterman, M. Spectra of Porphyrins. J. Mol. Spectrosc. 1961, 6 (C), 138– 163,  DOI: 10.1016/0022-2852(61)90236-3
      21
      Spectra of porphyrins
      Gouterman, Martin
      Journal of Molecular Spectroscopy (1961), 6 (), 138-63CODEN: JMOSA3; ISSN:0022-2852.
      Absorption spectra for 3 series of porphyrins derived from the basic skeleton are given: (a) compds. obtained by simple substitution; (b) compds. obtained by redn. of 1 or more pyrrole rings; and (c) compds. obtained from fusion of aromatic rings onto the basic skeleton. The spectra are discussed in terms of a 4-orbital model. Intensity changes and energy shifts are related to the properties of 2 top filled and 2 lowest empty π orbitals. Emission spectra of metal porphyrins are discussed. In closed shell metals, the continuous enhancement of phosphorescence at the expense of fluorescence is attributed to spin-orbit coupling. In paramagnetic metals, observed effects are attributed to the existence of a state at the same energy as the usual triplet, but with multiplicity the same as the ground state; its intensity is ascribed to exchange interactions. In diamagnetic metals with unfilled d shells, peculiar emission properties are attributed to enhanced spin orbit coupling due to low-lying metal triplets.
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    22. 22
      Gouterman, M.; Wagnière, G. H.; Snyder, L. C. Spectra of Porphyrins Part II. Four Orbital Model. J. Mol. Spectrosc. 1963, 11, 108– 127,  DOI: 10.1016/0022-2852(63)90011-0
      22
      Spectra of porphyrins. II. Four-orbital model
      Gouterman, Martin; Wagniere, Georges; Snyder, Lawrence C.
      Journal of Molecular Spectroscopy (1963), 11 (2), 108-27CODEN: JMOSA3; ISSN:0022-2852.
      cf. CA 55, 20615d. A quant. attempt is made to apply a 4-orbital model, described earlier (loc. cit.), to porphyrin mols. The model is a combination of L.C.A.O.-M.O. and a simplified treatment of configuration interaction. The parameters for the latter are detd. on Zn tetraphenylporphine. Orbital and transition energies and oscillator strengths are detd. for the reduced porphyrins: dihydroporphine, tetrahydroporphine, hexahydroporphine, and octahydroporphine. Qual. agreement with the observed spectra is obtained. Similar calcns. are made for phlorin-6-dihydrochloride. Spectra calcns. are given for the metal salts of mono-, di-, and triazaporphine, tetrabenzoporphine, tetrabenzotetraazaporphine (or phthalocyanine), and monobenzo-, 2 dibenzo-, and tribenzotetraazaporphine. The calcns. are in fair agreement with observed spectra. Quant. applications of the model are limited to regions of small perturbations. The application of the model to the calcn. of chem. and magnetic properties is discussed.
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    23. 23
      Esteso, V.; Caliò, L.; Espinós, H.; Lavarda, G.; Torres, T.; Feist, J.; Garcia-Vidal, F. J.; Bottari, G.; Míguez, H. Light Harvesting Properties of a Subphthalocyanine Solar Absorber Coupled to an Optical Cavity. Sol. RRL 2021, 5, 2100308,  DOI: 10.1002/solr.202100308
      23
      Light-Harvesting Properties of a Subphthalocyanine Solar Absorber Coupled to an Optical Cavity
      Esteso, Victoria; Calio, Laura; Espinos, Hilario; Lavarda, Giulia; Torres, Tomas; Feist, Johannes; Garcia-Vidal, Francisco J.; Bottari, Giovanni; Miguez, Hernan
      Solar RRL (2021), 5 (8), 2100308CODEN: SRORAW; ISSN:2367-198X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein, both from the exptl. and theor. point of view, the optical absorption properties of a subphthalocyanine (SubPc), an org. macrocycle commonly used as a sunlight harvester, coupled to metallic optical cavities are analyzed. How different electronic transitions characteristic of this compd. and specifically those that give rise to excitonic (Q band) and charge transfer (CT band) transitions couple to optical cavity modes is investigated. It is obsd. that whereas the CT band couples weakly to the cavity, the Q band transitions show evidence of hybridization with the photon eigenstates of the resonator, a distinctive trait of the strong coupling regime. As a result of the different coupling regimes of the two electronic transitions, very different spectral and directional light-harvesting features are obsd., which for the weakly coupled CT transitions are mainly detd. by the highly dispersive cavity modes and for the strongly coupled Q band by the less angle-dependent exciton-polariton bands. Modeling also allows discriminating parasitic from productive absorption in each case, enabling the estn. of the expected losses in a solar cell acting as an optical resonator.
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    24. 24
      Frisk Kockum, A.; Miranowicz, A.; De Liberato, S.; Savasta, S.; Nori, F. Ultrastrong Coupling between Light and Matter. Nat. Rev. Phys. 2019, 1 (1), 19– 40,  DOI: 10.1038/s42254-018-0006-2
      There is no corresponding record for this reference.
    25. 25
      Kéna-Cohen, S.; Maier, S. A.; Bradley, D. D. C. Ultrastrongly Coupled Exciton-Polaritons in Metal-Clad Organic Semiconductor Microcavities. Adv. Opt. Mater. 2013, 1 (11), 827– 833,  DOI: 10.1002/adom.201300256
      There is no corresponding record for this reference.
    26. 26
      Barachati, F.; Simon, J.; Getmanenko, Y. A.; Barlow, S.; Marder, S. R.; Kéna-Cohen, S. Tunable Third-Harmonic Generation from Polaritons in the Ultrastrong Coupling Regime. ACS Photonics 2018, 5, 119– 125,  DOI: 10.1021/acsphotonics.7b00305
      26
      Tunable Third-Harmonic Generation from Polaritons in the Ultrastrong Coupling Regime
      Barachati, Fabio; Simon, Janos; Getmanenko, Yulia A.; Barlow, Stephen; Marder, Seth R.; Kena-Cohen, Stephane
      ACS Photonics (2018), 5 (1), 119-125CODEN: APCHD5; ISSN:2330-4022. (American Chemical Society)
      Strong interparticle interactions between polaritons have traditionally stemmed from their exciton component. A strong 3rd-order nonlinearity is imparted to polaritonic mode by embedding a nonlinear polymethine dye within a high-Q all-metal microcavity. Nonlinear microcavities are demonstrated operating in the ultrastrong coupling regime with a normalized coupling ratio of 62%, the highest reported to date. When pumping the lower polariton branch, the authors observe tunable 3rd-harmonic generation spanning the entire visible spectrum, with internal conversion enhancements >3 orders of magnitude larger than in bare films. Transfer matrix calcns. indicate that the obsd. enhancements are consistent with the enhanced pump elec. field.
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    27. 27
      Suzuki, M.; Nishiyama, K.; Kani, N.; Yu, X.; Uzumi, K.; Funahashi, M.; Shimokawa, F.; Nakanishi, S.; Tsurumachi, N. Observation of Ultrastrong-Coupling Regime in the Fabry-Pérot Microcavities Made of Metal Mirrors Containing Lemke Dye. Appl. Phys. Lett. 2019, 114 (19), 191108,  DOI: 10.1063/1.5080623
      27
      Observation of ultrastrong-coupling regime in the Fabry-Pe´rot microcavities made of metal mirrors containing Lemke dye
      Suzuki, Makoto; Nishiyama, Kouichi; Kani, Nobutaka; Yu, Xinping; Uzumi, Keiji; Funahashi, Masahiro; Shimokawa, Fusao; Nakanishi, Shunsuke; Tsurumachi, Noriaki
      Applied Physics Letters (2019), 114 (19), 191108/1-191108/5CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We investigate the transmission properties of Fabry-Pe´rot microcavities made of metal mirrors contg. the org. dye mol., generally called Lemke. We synthesized Lemke dye and fabricated the microcavities by using simple vacuum evapn. and spin-coating methods. The vacuum Rabi splitting energy increases in proportion to the square root of the absorption coeff. of the cavity layers and exceeds 1 eV at max. The ratio of the vacuum Rabi splitting energy to the matter transition energy reaches 0.42, so we consider that the ultrastrong-coupling regime was attained. The dispersion relation is reasonably interpreted by using the full Hopfield Hamiltonian. (c) 2019 American Institute of Physics.
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    28. 28
      Winterfeld, K. A.; Lavarda, G.; Guilleme, J.; Sekita, M.; Guldi, D. M.; Torres, T.; Bottari, G. Subphthalocyanines Axially Substituted with a Tetracyanobuta-1,3-Diene-Aniline Moiety: Synthesis, Structure, and Physicochemical Properties. J. Am. Chem. Soc. 2017, 139 (15), 5520– 5529,  DOI: 10.1021/jacs.7b01460
      28
      Subphthalocyanines Axially Substituted with a Tetracyanobuta-1,3-diene-Aniline Moiety: Synthesis, Structure, and Physicochemical Properties
      Winterfeld, Kim A.; Lavarda, Giulia; Guilleme, Julia; Sekita, Michael; Guldi, Dirk M.; Torres, Tomas; Bottari, Giovanni
      Journal of the American Chemical Society (2017), 139 (15), 5520-5529CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A 1,1,4,4-tetracyanobuta-1,3-diene (TCBD)-aniline moiety has been introduced, for the first time, at the axial position of two subphthalocyanines (SubPcs) peripherally substituted with hydrogen (H12SubPc) or fluorine atoms (F12SubPc). Single-crystal x-ray anal. of both SubPc-TCBD-aniline systems showed that each conjugate is a racemic mixt. of two atropisomers resulting from the almost orthogonal geometry adopted by the axial TCBD unit, which were sepd. by chiral HPLC. Remarkably, the single-crystal x-ray structure of one atropisomer of each SubPc-TCBD-aniline conjugate has been solved allowing to unambiguously assign the atropisomers' abs. configuration, something, to the best of our knowledge, unprecedented in TCBD-based conjugates. Moreover, the physicochem. properties of both SubPc-TCBD-aniline racemates have been investigated by using a wide range of electrochem. as well as steady state and time-resolved spectroscopic techniques. Remarkably, each of the two SubPc-TCBD-aniline presents a unique photophys. feature never obsd. before in SubPc chem. As a matter of fact, H12SubPc-TCBD-aniline showed significant ground state charge transfer interactions between the H12SubPc macrocycle and the electron-withdrawing TCBD unit directly attached at its axial position. In contrast, F12SubPc-TCBD-aniline gave rise to an intense, broad emission, which red-shifts upon increasing the solvent polarity and stems from an excited complex (i.e., an exciplex). Such exciplex emission, which has also no precedent in TCBD chem., results from intramol. interactions in the excited state between the electron-rich aniline and the F12SubPc π-surface, two mol. fragments kept in spatial proximity by the "unique" 3-dimensional geometry adopted by the F12SubPc-TCBD-aniline. Complementary transient absorption studies were carried out on both SubPc-TCBD-aniline derivs. showing the occurrence, in both cases, of photoinduced charge sepn. and corroborating the formation of the aforementioned intramol. exciplex in terms of a radical-ion pair stabilized through-space.
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    29. 29
      Anappara, A. A.; De Liberato, S.; Tredicucci, A.; Ciuti, C.; Biasiol, G.; Sorba, L.; Beltram, F. Signatures of the Ultrastrong Light-Matter Coupling Regime. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79 (20), 3– 6,  DOI: 10.1103/PhysRevB.79.201303
      There is no corresponding record for this reference.
    30. 30
      Houdré, R.; Stanley, R. P.; Ilegems, M. Vacuum-Field Rabi Splitting in the Presence of Inhomogeneous Broadening: Resolution of a Homogeneous Linewidth in an Inhomogeneously Broadened System. Phys. Rev. A: At., Mol., Opt. Phys. 1996, 53 (4), 2711– 2715,  DOI: 10.1103/PhysRevA.53.2711
      30
      Vacuum-field Rabi splitting in the presence of inhomogeneous broadening: resolution of a homogeneous linewidth in an inhomogeneously broadened system
      Houdre, R.; Stanley, R. P.; Ilegems, M.
      Physical Review A: Atomic, Molecular, and Optical Physics (1996), 53 (4), 2711-15CODEN: PLRAAN; ISSN:1050-2947. (American Physical Society)
      The effect was studied of inhomogeneous broadening of the electronic state on vacuum-field Rabi splitting. The broadening has no effect on the size of the splitting and, in general, does not lead to an inhomogeneous broadening of the split states. From a spectroscopic point of view, these results have interesting consequences, since they allow the extn. of a homogeneous line in an inhomogeneously broadened system.
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    31. 31
      Forouhi, A. R.; Bloomer, I. Optical Dispersion Relations for Amorphous Semiconductors and Amorphous Dielectrics. Phys. Rev. B: Condens. Matter Mater. Phys. 1986, 34 (10), 7018– 7026,  DOI: 10.1103/PhysRevB.34.7018
      31
      Optical dispersion relations for amorphous semiconductors and amorphous dielectrics
      Forouhi, A. R.; Bloomer, I.
      Physical Review B: Condensed Matter and Materials Physics (1986), 34 (10), 7018-26CODEN: PRBMDO; ISSN:0163-1829.
      An expression for the imaginary part, k, of the complex index of refraction, N = n - ik, for amorphous materials was derived as a function of photon energy E: k(E) = A(E - Eg)2/(E2 - BE + C) where A, B, and C are pos. nonzero consts. characteristics of the medium such that 4C - B2 > 0. Eg Represents the optical energy band gap. The real part, n, of the complex index of refraction is then n(E) = n(∞) + (B0E + C0)/(E2 - BE + C) using Kramers-Kronig anal., where B0 and C0 are consts. that depend on A, B, C, and Eg, and n(∞) is a const. greater than unity. Excellent agreement was found between these formulas and exptl. measured and published values of n and k of amorphous Si, hydrogenated amorphous Si, amorphous Si nitride, and TiO2.
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  • Supporting Information

    Supporting Information

    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpclett.1c02898.

    • Dispersion curves attained from the angular dependence measurements of the absorptance, reflectance and transmittance, and wavelength integrated SSWA versus angle of incidence of sunlight for each one of the layers comprising the optical resonator for the cavity thicknesses considered in the text (PDF)


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