Lanthanide nanoparticles (LNPs) are promising sensors of chemical, mechanical, and temperature changes; they combine the narrow-spectral emission and long-lived excited states of individual lanthanide ions with the high spatial resolution and controlled energy transfer of nanocrystalline architectures. Despite considerable progress in optimizing LNP brightness and responsiveness for dynamic sensing, detection of stimuli with a spatial resolution approaching that of individual nanoparticles remains an outstanding challenge. Here, we highlight the existing capabilities and outstanding challenges of LNP sensors, en-route to nanometer-scale, single particle sensor resolution. First, we summarize LNP sensor read-outs, including changes in emission wavelength, lifetime, intensity, and spectral ratiometric values that arise from modified energy transfer networks within nanoparticles. Then, we describe the origins of LNP sensor imprecision, including sensitivity to competing conditions, interparticle heterogeneities, such as the concentration and distribution of dopant ions, and measurement noise. Motivated by these sources of signal variance, we describe synthesis characterization feedback loops to inform and improve sensor precision, and introduce noise-equivalent sensitivity as a figure of merit of LNP sensors. Finally, we project the magnitudes of chemical and pressure stimulus resolution achievable with single LNPs at nanoscale resolution. Our perspective provides a roadmap for translating ensemble LNP sensing capabilities to the single particle level, enabling nanometer-scale sensing in biology, medicine, and sustainability.

Structural colors traditionally refer to colors arising from the interaction of light with structures with periodicities on the order of the wavelength. Recently, the definition has been broadened to include colors arising from individual resonators that can be subwavelength in dimension, for example, plasmonic and dielectric nanoantennas. For instance, diverse metallic and dielectric nanostructure designs have been utilized to generate structural colors based on various physical phenomena, such as localized surface plasmon resonances (LSPRs), Mie resonances, thin-film Fabry–Pérot interference, and Rayleigh–Wood diffraction anomalies from 2D periodic lattices and photonic crystals. Here, we provide our perspective of the key application areas where structural colors really shine and other areas where more work is needed. We review major classes of materials and structures employed to generate structural coloration and highlight the main physical resonances involved. We discuss mechanisms to tune structural colors and review recent advances in dynamic structural colors. In the end, we propose the concept of a universal pixel that could be crucial in realizing next-generation displays based on nanophotonic structural colors.

Realization of integrated quantum photonics is a key step toward scalable quantum applications such as quantum computing, sensing, information processing, and quantum material metrology. To enable practical quantum photonic systems, several challenges should be addressed, including (i) the realization of deterministic, bright, and stable single-photon emission operating at THz rates and at room temperatures, (ii) on-chip integration of efficient single-photon sources, and (iii) the development of deterministic and scalable nanoassembly of quantum circuitry elements. In this Perspective, we focus on the emerging field of physics-informed machine learning (ML) quantum photonics that is envisioned to play a decisive role in addressing the above challenges. Specifically, three directions of ML-assisted quantum research are discussed: (i) rapid preselection of single single-photon sources via ML-assisted quantum measurements, (ii) hybrid ML-optimization approach for developing efficient quantum circuits elements, and (iii) ML-based frameworks for developing novel deterministic assembly of on-chip quantum emitters.

Dielectric metasurfaces have emerged as a powerful platform for novel optical biosensors. Due to their low optical loss and strong light–matter interaction, they demonstrate several exotic optical properties, including sharp resonances, strong near-field enhancements, and the compelling capability to support magnetic modes. They also show advantages such as CMOS-compatible fabrication processes and lower resonance-induced heating compared to their plasmonic counterparts. These unique characteristics are enabling the advancement of cutting-edge sensing techniques for new applications. In this Perspective, we review the recent progress of dielectric metasurface sensors. First, the working mechanisms and properties of dielectric metasurfaces are briefly introduced by highlighting several state-of-the-art examples. Next, we describe the application of dielectric metasurfaces for label-free sensing in three different detection schemes, namely, refractometric sensing, surface-enhanced spectroscopy through Raman scattering and infrared absorption, and chiral sensing. Finally, we provide a perspective for the future directions of this exciting research field.

Photovoltaic systems have reached impressive efficiencies, with records in the range of 20–30% for single-junction cells based on many different materials, yet the fundamental Shockley-Queisser efficiency limit of 34% is still out of reach. Improved photonic design can help approach the efficiency limit by eliminating losses from incomplete absorption or nonradiative recombination. This Perspective reviews nanopatterning methods and metasurfaces for increased light incoupling and light trapping in light absorbers and describes nanophotonics opportunities to reduce carrier recombination and utilize spectral conversion. Beyond the state-of-the-art single junction cells, photonic design plays a crucial role in the next generation of photovoltaics, including tandem and self-adaptive solar cells, and to extend the applicability of solar cells in many different ways. We address the exciting research opportunities and challenges in photonic design principles and fabrication that will accelerate the massive upscaling and (invisible) integration of photovoltaics into every available surface.

Thermal photodetectors rely on thermally sensitive materials to convert the heat generated from incoming light to an electrical signal and are used in, for example, focal plane arrays and for room-temperature, high-photon-flux applications. However, the theory for thermal detectors was initially developed before nanophotonic engineering and, thus, typically assume the integrated absorbers are blackened or have a flat spectral response. Here we discuss recent developments in nanophotonics and metamaterials that have allowed for the creation of spectrally selective absorbers capable of suppressing undesired thermal emission and increasing the potential sensitivity of thermal photodetectors. Furthermore, the subwavelength features of nanophotonic or metamaterial absorbers decrease the amount of material required for absorption, which reduces the detector’s thermal capacitance while increasing its response time and sensitivity. The ideal thermal and noise dynamics are derived for both spectrally selective and unselective thermal detectors, revealing exciting opportunities for future thermal photodetectors with increased sensitivities and response times and decreased noise floors.

The field of two-dimensional (2D) materials-based nanophotonics has been growing at a rapid pace, triggered by the ability to design nanophotonic systems with in situ control, unprecedented number of degrees of freedom, and to build material heterostructures from the bottom up with atomic precision. A wide palette of polaritonic classes have been identified, comprising ultraconfined optical fields, even approaching characteristic length-scales of a single atom. These advances have been a real boost for the emerging field of quantum nanophotonics, where the quantum mechanical nature of the electrons and polaritons and their interactions become relevant. Examples include quantum nonlocal effects, ultrastrong light–matter interactions, Cherenkov radiation, access to forbidden transitions, hydrodynamic effects, single-plasmon nonlinearities, polaritonic quantization, topological effects, and so on. In addition to these intrinsic quantum nanophotonic phenomena, 2D material systems can also be used as sensitive probes for the quantum properties of the material that carries the nanophotonics modes or quantum materials in its vicinity. Here, polaritons act as a probe for otherwise invisible excitations, for example, in superconductors, or as a new tool to monitor the existence of Berry curvature in topological materials and superlattice effects in twisted 2D materials. In this Perspective, we present an overview of the emergent field of 2D-material quantum nanophotonics and provide a future perspective on the prospects of both fundamental emergent phenomena and emergent quantum technologies, such as quantum sensing, single-photon sources, and quantum emitters manipulation. We address four main implications: (i) quantum sensing, featuring polaritons to probe superconductivity and explore new electronic transport hydrodynamic behaviors, (ii) quantum technologies harnessing single-photon generation, manipulation, and detection using 2D materials, (iii) polariton engineering with quantum materials enabled by twist angle and stacking order control in van der Waals heterostructures, and (iv) extreme light−matter interactions enabled by the strong confinement of light at atomic level by 2D materials, which provide new tools to manipulate light fields at the nanoscale (e.g., quantum chemistry, nonlocal effects, high Purcell enhancement).

Recent rapid progress in subwavelength optics and nanophotonics with dielectric structures is underpinned by the physics of Mie resonances excited in nanoparticles with a high refractive index, and it suggests a novel platform for the localization of light in subwavelength photonic structures and opens new horizons for metamaterial-enabled photonics or metaphotonics. In this invited paper, we review the recent advances in Mie-resonant metaphotonics (also termed as “Mie-tronics”) for isolated high-index dielectric nanoparticles (or nanoantennas) and nanoparticle structures such as resonant dielectric metasurfaces, as well as their applications to topological photonics. We also oversee the future developments of this active field and its links to other research areas.

Because of their unique and excellent photophysical properties, lead halide perovskites are widely used in photoelectronic devices such as photodetectors, light-emitting diodes, solar cells, and lasers. Recently it was found that lead halide perovskites also exhibit excellent nonlinear optical properties in nonlinear optics (NLO), including saturated absorption, two- or multiphoton absorption, and nonlinear refraction. It is believed that perovskites will be serious nonlinear optical materials in the future. The nonlinear optical devices prepared from perovskites on the basis of their optical nonlinearity may open up a new viewpoint for the application of information and communication technology. Here the research on lead halide perovskites in NLO is reviewed. The mechanism of nonlinear optical phenomena of lead halide perovskites is analyzed, and some possible methods to improve the optical nonlinearity are summarized. Their applications in passive Q-switched lasers, upconversion lasers, optical limiting, and infrared detection are introduced. Several future research directions and challenges of lead halide perovskites in nonlinear optics are proposed.

Optical materials with vanishing dielectric permittivity, known as epsilon-near-zero (ENZ) materials, have been shown to possess enhanced nonlinear optical responses in their ENZ region. These strong nonlinear optical properties have been firmly established in homogeneous materials; however, it is as of yet unclear whether metamaterials with effective optical parameters can exhibit a similar enhancement. Here, we probe an optical ENZ metamaterial composed of a subwavelength periodic stack of alternating Ag and SiO₂ layers and measure a nonlinear refractive index n₂ = (1.2 ± 0.1) × 10^–12 m²/W and nonlinear absorption coefficient β = (−1.5 ± 0.2) × 10^–5 m/W at its effective zero-permittivity wavelength. The measured n₂ is 10⁷ times larger than n₂ of fused silica and 4 times larger than the n₂ of silver. We observe that the nonlinear enhancement in n₂ scales as 1/(n₀Re[n₀]), where n₀ is the linear effective refractive index. As opposed to homogeneous ENZ materials, whose optical properties are dictated by their intrinsic material properties and hence are not widely tunable, the zero-permittivity wavelength of the demonstrated metamaterials may be chosen to lie anywhere within the visible spectrum by selecting the right thicknesses of the subwavelength layers. Consequently, our results offer the promise of a means to design metamaterials with large nonlinearities for applications in nanophotonics at any specified optical wavelength.

We propose a chiral imaging modality based on optical chirality engineering, fluorescence-detected circular dichroism, and structured illumination microscopy. In this method, the optical chirality of the illumination is structured and the circular dichroism dependent fluorescence is detected. With image reconstruction, the spatial distribution of the chiral domains can be obtained at subdiffraction-limited resolution. We theoretically demonstrate this method and discuss the feasibility of using an optical chirality engineering approach based on far-field optics.

Ultraviolet light is essential for disinfection, fluorescence excitation, curing, and medical treatment. An ultraviolet light source with the small footprint and excellent optical characteristics of vertical-cavity surface-emitting lasers (VCSELs) may enable new applications in all these areas. Until now, there have only been a few demonstrations of ultraviolet-emitting VCSELs, mainly optically pumped, and all with low Al-content AlGaN cavities and emission near the bandgap of GaN (360 nm). Here, we demonstrate an optically pumped VCSEL emitting in the UVB spectrum (280–320 nm) at room temperature, having an Al_0.60Ga_0.40N cavity between two dielectric distributed Bragg reflectors. The double dielectric distributed Bragg reflector design was realized by substrate removal using electrochemical etching. Our method is further extendable to even shorter wavelengths, which would establish a technology that enables VCSEL emission from UVA (320–400 nm) to UVC (<280 nm).

Optical magnetometers based on alkali vapors, such as rubidium, are among the most sensitive technologies for detecting and characterizing magnetic fields. Following the recent effort in miniaturizing atomic-based quantum technologies, the last years were marked by a growing interest in developing integrated quantum nanophotonic circuits for a vast range of applications. Motivated by the attractiveness of such chip-scale integration, we present and experimentally demonstrate an integrated magnetic sensing platform, based on a nanophotonic-chip interfaced to a microfabricated alkali vapor cell. Magnetically induced circular dichroism in rubidium vapor is measured using a planar structure that spatially resolves the handedness of incoming photons depending on their spin. The presented approach paves the way toward further integration of highly sensitive magnetometers, with potential for future applications, such as in high-spatial resolution magnetic vectorial imaging.

Ultrafast polarization switching is being considered for the next generation of ferroelectric-based devices. Recently, the dynamics of the field-induced transitions associated with this switching have been difficult to explore, due to technological limitations. The advent of terahertz (THz) technology has now allowed for the study of these dynamic processes on the picosecond (ps) scale. In this paper, intense THz pulses were used as a high-frequency electric field to investigate ultrafast switching in the relaxor ferroelectric, Bi_0.5Na_0.5TiO₃. Transient atomic-scale responses, which were evident as changes in reflectivity, were captured by THz probing. The high-energy THz pulses induce an increase in reflectivity, associated with an ultrafast field-induced phase transition from a weakly polar phase (Cc) to a strongly polar phase (R3c) within 20 ps at 200 K. This phase transition was confirmed using X-ray powder diffraction and by electrical measurements, which showed a decrease in the frequency dispersion of relative permittivity at low frequencies.

A nonlinear shift in the dispersion relation of a surface phonon-polariton (SPhP) is observed with grating-coupled pump–probe reflection spectroscopy. Upon excitation of an SPhP on a 4H-SiC surface, an instantaneous frequency shift of the SPhP mode at a constant wavevector is observed. This pump-induced frequency shift is equivalent to a nonlinear dispersion shift and to a Kerr-like nonlinear phase shift. The effective nonlinear index is evaluated to be orders of magnitude larger than the typical values of nonresonant dielectric responses. A nonlinear forced oscillator model aided by the first-principles calculations reproduce our observation and, furthermore, indicates that the primary origin is either the Born effective charge or the phonon anharmonicity depending on the frequency within the Reststrahlen band. The instantaneous shift is followed by a picosecond recovery, reflecting the energy relaxation and dissipation of the excited SPhP. This observed nonlinearity forms the basis of the self-phase modulation and four-wave mixing of SPhPs and paves the way toward nonlinear phonon-polaritonics.

We present polarization-sensitive Fourier ptychographic microscopy (PS-FPM) capable of generating high-resolution birefringence images of optically anisotropic specimens over a large field of view (FoV). FPM produces high-resolution images of transparent samples over a large FoV based on multiple intensity measurements acquired at various illumination angles and ptychographic phase retrieval. We combine this attractive feature of FPM with a single-input-state illumination and polarization-diverse imaging system to achieve the imaging of both complex and birefringence information on transparent objects. Compared to conventional polarization imaging techniques, PS-FPM does not involve any mechanical rotation of the polarizer/analyzer and achieves birefringence imaging with a half-pitch resolution of 0.55 μm over 3.78 mm² FoV, which corresponds to the space-bandwidth product of 12.5 megapixels. We demonstrate the high-resolution, large-area birefringence imaging capability of PS-FPM by presenting the birefringence images of various anisotropic objects including monosodium urate, Tilia stem, and hemozoin crystals.

Optical nanocircuits, inspired by electrical nanocircuits, provide a versatile platform for tailoring and manipulating optical fields at the subwavelength scale, which is vital for developing various innovative optical nanodevices and integrated nanosystems. Plasmonic nanoparticles can be employed as promising building blocks for optical nanocircuits with unprecedentedly high integration capacity. Among various plasmonic systems, aggregated metallic nanoparticle, known as oligomers, possess great potential in constructing functional metatronic circuits. Here, the optical nanocircuits comprising special plasmonic oligomers, such as trimers with D_3h symmetry, quadrumers with D_2h symmetry, and their variants with reduced symmetry, are systematically investigated in the metatronic paradigm, both theoretically and experimentally. Our proposed circuit models, based on the displacement current in the oligomers, not only reproduce the resonance spectral details, but also retrieve many hidden physical quantities associated with their optical responses. Guided by the metatronic circuits, the spectral engineering of the oligomers with reduced geometric symmetry is predicted, and subgroup decomposition of several plasmonic quadrumers is examined. Our investigation has revealed a close correlation between the metatronic circuitry and strongly coupled plasmonic oligomers. The observed correlation of spatial arrangement and frequency response in oligomers provides a metatronic guide to modulate plasmonic responses via geometric variation.

Infrared dielectric properties of muscovite mica, one of the first van der Waals crystals, exfoliated on silicon and SiO₂ substrates is studied using near-field nano-FTIR spectroscopy. The spectra of mica show strong thickness and wavelength dependence down to the monolayer-scale, with a prominent broad peak centered around ∼1080 cm^–1 assigned to stretching vibrations of Si–O. We reveal that the infrared dielectric permittivity of mica is anisotropic, that is, has opposite signs along the in-plane and out-of-plane axes, implying a Type I hyperbolic behavior in the range 920–1010 cm^–1 and a Type II hyperbolic behavior in the range 1050–1130 cm^–1. Experimentally measured nano-FTIR spectra agree well with analytical model calculations based on an extended finite dipole model for layered systems of the tip–sample interaction when the out-of-plane dielectric values (instead of the in-plane dielectric values) were used in the calculations.

Semiconducting single-walled carbon nanotubes (SWCNTs) are an interesting material for strong-light matter coupling due to their stable excitons, narrow emission in the near-infrared region, and high charge carrier mobilities. Furthermore, they have emerged as quantum light sources as a result of the controlled introduction of luminescent quantum defects (sp³ defects) with red-shifted transitions that enable single-photon emission. The complex photophysics of SWCNTs and the overall goal of polariton condensation pose the question of how exciton–polaritons are populated and how the process might be optimized. The contributions of possible relaxation processes, i.e., scattering with acoustic phonons, vibrationally assisted scattering, and radiative pumping, are investigated using angle-resolved reflectivity and time-resolved photoluminescence measurements on microcavities with a wide range of detunings. We show that the predominant population mechanism for SWCNT exciton–polaritons in planar microcavities is radiative pumping. Consequently, the limitation of polariton population due to the low photoluminescence quantum yield of nanotubes can be overcome by luminescent sp³ defects. Without changing the polariton branch structure, radiative pumping through these emissive defects leads to an up to 10-fold increase of the polariton population for detunings with a large photon fraction. Thus, the controlled and tunable functionalization of SWCNTs with sp³ defects presents a viable route toward bright and efficient polariton devices.

Super-resolution localization microscopy is based on determining the positions of individual fluorescent markers in a sample. The major challenge in reaching an ever higher localization precision lies in the limited number of collected photons from single emitters. To tackle this issue, it has been shown that one can exploit the increased photostability at low temperatures, reaching localization precisions in the subnanometer range. Another crucial ingredient of single-molecule super-resolution imaging is the ability to activate an individual emitter within a diffraction-limited spot. Here, we report on the photoblinking behavior of organic dyes at low temperature and elaborate on the limitations of this ubiquitous phenomenon for selecting single molecules. We then show that recording the emission polarization not only provides access to the molecular orientation, but it also facilitates the assignment of photons to individual blinking molecules. Furthermore, we employ periodical modulation of the excitation polarization as a robust method to effectively switch fluorophores. We benchmark each approach by resolving two emitters on different DNA origami structures.

A Luneburg lens with a gradient index distribution is an aberration-free and coma-free spherical lens. It has wide applications ranging from invisibility cloaks, illusion optics, and superlensing. However, it is challenging to realize an on-chip Luneburg lens with superior performance. In this paper, an on-chip Luneburg lens is implemented through the integration of gradient metamaterial structures and silicon waveguides. The filling ratio mapping of silicon nanorods is used to achieve the gradient index distribution. As an example of the application, a general and scalable approach for a mode size converter is demonstrated to match two waveguide modes with arbitrary widths. Benefiting from the aberration-free property, a 740 nm bandwidth is achieved in simulation. Limited by our measurement setup, the measured device bandwidth is 220 nm, covering the wavelengths of 1.26–1.36 and 1.507–1.627 μm. To the best of our knowledge, this silicon mode size converter exhibits the largest bandwidth.

In this paper, we demonstrate the infrared photoluminescence emission from Ge(Si) quantum dots coupled with collective Mie modes of silicon nanopillars. We show that the excitation of band edge dipolar modes of a linear nanopillar array results in strong reshaping of the photoluminescence spectra. Among other collective modes, the magnetic dipolar mode with the polarization along the array axis contributes the most to the emission spectrum, exhibiting an experimentally measured Q-factor of around 500 for an array of 11 pillars. The results belong to the first experimental evidence of light emission enhancement of quantum emitters applying collective Mie resonances in finite nanoresonators and therefore represent an important contribution to the new field of active all-dielectric meta-optics.

The coupling of two-dimensional materials with optical metasurfaces is a promising avenue to enhance the advantageous properties of both platforms. Here we integrate an ultrathin monolayer of the transition metal dichalcogenide (TMD) MoS₂, grown by chemical-vapor deposition, with a silicon metasurface, to obtain a hybrid system with enhanced nonlinear response. To this end, we utilize a metasurface exhibiting resonances with high quality factors, which provides increased optical fields. Using the nonlinearity of the TMD monolayer, these resonantly enhanced fields enable more efficient nonlinear frequency conversion. In particular, we experimentally observe an enhanced efficiency of second-harmonic generation in our hybrid structure. By comparing second-harmonic generation using different photonic resonances, we furthermore identify optimized conditions for the spatial distribution of the local optical fields to maximize the nonlinear response. Our results enable the precise design of hybrid structures consisting from TMDs and metasurfaces for future applications.

Calcium- and voltage-sensitive indicators allow for the optical monitoring of neuronal activity at both cellular and population levels. However, conventional approaches for the optical detection of electrical activity in an intact brain typically involve a trade-off between tissue depth and resolution. Cameras of high temporal and spatial resolution can detect activity with single-cell resolution, but are restricted to more superficial structures such as the neocortex and require elaborate optical setups. In contrast, optical fibers can collect fluorescent neural activity from deeper brain areas, but with low spatial resolution. Here, we present a new class of high-resolution, light-sensing devices that are capable of detecting ultralow changes in fluorescent neuronal activity without the need for an optical setup. We show that organic photodetectors (OPDs) based on rubrene and fullerene feature a photovoltage responsivity of 2 V m² W^–1 and that can directly detect changes in fluorescent neuronal activity as low as 2.3 nW cm^–2. Primary cortical neurons were loaded with the fluorescent calcium indicator Cal-520, and neuronal activity was evoked with brief pulses of electrical field stimulation. During simultaneous sCMOS camera acquisition, the OPD was observed to reliably detect electrically evoked fluorescent activity with high fidelity and signal-to-noise ratio. The device also detected time-locked spontaneous fluorescent transients, demonstrating sufficient sensitivity for the detection of physiological events. Our results pave the way for a new class of subdermally implanted stereotactic sensors, representing a capacity for minimally invasive, high-resolution in vivo recordings, which are especially suited to record neuronal populations in behaving animals.

In this work, we introduce a radically new approach for achieving doubly resonant light-to-sound conversion with radiofrequency waves, namely, electromagnetic waves in the range of 1–100 MHz. By taking the profit from recently published metamaterials exhibiting plasma-like responses in the radio range, we introduce the concept of “radioplasmonics” that deals with localized surface plasmons in the radio regime. In analogy with conventional plasmonics, radioplasmonics can be exploited to design microtransducers that effectively convert radio-waves into heat through resonant electromagnetic absorption. Then, by tuning the Young’s modulus of the transducers, we can achieve resonant acoustic vibrations in the same range of frequencies as the plasmonic resonances. In this way, plasmonic heating is converted into resonant thermo-acoustic expansion and its consequent generation of pressure waves. The latter can then be used for ultrasound imaging. We show that, in this double resonance framework, the intensity of the generated acoustic waves is above the current detection level under realistic conditions. The importance of using the radio range is related to its ability to deeply penetrate water and biological tissues. Hence, the proposed approach paves the way to the first total-body thermo-acoustic imaging able to reach a single-cell resolution.

Optical fields interacting with solids excite single particle quantum transitions and elicit collective screening responses that define their penetration, absorption, and reflection. The interplay of these interactions on the attosecond time scale defines how optical energy transforms to electronic, setting the limits of efficiency for processes such as solar energy harvesting or photocatalysis. Our understanding of light–matter interactions is primarily based on specifying the electronic structure of solids and initial particles or fields occupying well-defined states, and the outcome of their interaction culminating in photoelectron or photon emission and analysis, with scant ability to follow the transitional, ultrafast many-body interactions that define it. The optical properties of metals transubstantiate from metallic to dielectric when the real part of their dielectric response function, Re[ε(ω)], passes through zero: at low frequencies, Re[ε(ω)] < 0, and the collective free electron plasmonic response confers high reflectivity; at high frequencies, Re[ε(ω)] > 0, and the fields penetrate as charge-density or longitudinal plasmon waves. How such collective plasmonic responses decay on the femtosecond time scale into single particle excitations is cardinal to plasmonics, but not sufficiently well described by experiment or theory. We examine the spectroscopic signatures of the nonlinear single particle and collective excitations of the low index crystals of silver by nonlinear two-photon photoemission spectroscopy, at frequencies where the bulk dielectric response passes through zero. We find that the transition through zero dielectric region is reflected in the nonlinear photoemission spectra, and in particular, the bulk plasmons decay by giving rise to a non-Einsteinian plasmonic photoemission component. This response, where the energy of photoelectrons is not defined by the incoming photons, occurs when photons excite the longitudinal plasmons, which then decay by exciting photoelectrons selectively from the Fermi level. Such mode of plasmon decay into hot electrons is contrary to the general agreement, but confirms a theoretical prediction by J. J. Hopfield from 1965. Our experiment illuminates a more energy efficient optical-to-electronic energy flow in metals that so far has escaped scrutiny.

As narrow band gap nanocrystals become a considerable building block for the design of infrared sensors, device design needs to match their actual operating conditions. While in the near and shortwave infrared, room-temperature operation has been demonstrated, longer wavelengths still require low-temperature operations and thus specific design. Here, we discuss how field-effect transistors (FETs) can be compatible with low-temperature detection. To reach this goal, two key developments are proposed. First, we report the gating of nanocrystal films from SrTiO₃ which leads to high gate capacitance with leakage and breakdown free operation in the 4–100 K range. Second, we demonstrate that this FET is compatible with a plasmonic resonator whose role is to achieve strong light absorption from a thin film used as the channel of the FET. Combining three resonances, broadband absorption from 1.5 to 3 μm reaching 30% is demonstrated. Finally, combining gate and enhanced light–matter coupling, we show that detectivity can be as high as 10¹² Jones for a device presenting a 3 μm cutoff wavelength and 30 K operation.

Photovoltaics has played a significant and increasingly important role in renewable energy harvesting. However, it only works during the daytime when the sun is accessible. In this paper, we propose to extend the functionality of solar panels into the nighttime for water harvesting, using nighttime radiative cooling. We first determine the suitable temperature and humidity range for nighttime water harvesting using solar panels and elucidate the water harvesting potential from solar panels. We further show that, through emissivity engineering, both the water generation rate and the suitable temperature and humidity range can be significantly improved. As a case study, we show that the average weekly water generation for solar panels in Dubai can reach 261 mL/m², sufficient for dust cleaning of solar panels. Moreover, it can be significantly enhanced up to 681 mL/m² with further emissivity engineering. The collected water can also be used for other applications including agrophotovoltaic and evaporative cooling of solar panels during the day, and can be extended to other solar energy harvesting systems. Our results point to new avenues to explore the nighttime utilization of a wide range of existing sky-facing solar energy harvesting systems and highlight the opportunities to use both the sun and outer space in existing energy systems.

Layered quasi-two-dimensional perovskites are promising candidates for optoelectronic applications exhibiting excitons with high emission quantum yields, high stability, and ease of bandgap tunability. Here, we demonstrate a long-range (∼100 μm) exciton transfer in a layered perovskite structure (en)₄Pb₂Br₉·3Br, with the ethylene diammonium (en) as a spacer that takes place via the reabsorption of emitted photons. Using the two-objectives setup, we directly map the spatiotemporal dynamics of photoluminescence (PL) from perovskite microwires that reveal a clear spectroscopic signature of photon recycling: the appearance of PL emission rise times and the corresponding elongation of the PL decay as a function of separation distance between the excitation and emission locations. We further show that a kinetic model based on the photon-mediated mechanism of the lateral exciton propagation indeed successfully describes all the salient features of the experimental data and gives an independent assessment of the radiative efficiency of the exciton recombination. Our demonstration points out the possibility of judiciously exploiting light management strategies for future high-performance optoelectronic devices with layered perovskite structures.

Recent breakthroughs in photonics-based quantum, neuromorphic, and analogue processing have pointed out the need for new schemes for fully programmable nanophotonic devices. Universal optical elements based on interferometer meshes are underpinning many of these new technologies, however, this is achieved at the cost of an overall footprint that is very large compared to the limited chip real estate, restricting the scalability of this approach. Here, we consider an ultracompact platform for low-loss programmable elements using the complex transmission matrix of a multiport multimode waveguide. We propose a deep learning inverse network approach to design arbitrary transmission matrices using patterns of weakly scattering perturbations. The demonstrated technique allows control over both the intensity and the phase in a multiport device at a four orders reduced device footprint compared to conventional technologies, thus, opening the door for large-scale integrated universal networks.

We report the experimental realization of periodically perforated plasmonic metasurfaces capable of integrating several key functionalities, such as light-to-surface plasmon coupling, controllable beam-splitting, wavelength filtering and routing, high resolution differential wavelength measurement, and vectorial displacement sensing. The plasmonic metasurfaces operate at telecom wavelengths, at the vicinity of the eigenmode crossing points where zero group velocity is experienced, and their functionality parameters, such as sensitivity to misalignment, prong angular separation, power ratio, polarization, and bandwidth, can be adjusted by designing the boundary shape and by conveniently manipulating their alignment with the illuminating light beam. In the same context, a circular plasmonic metasurface could also serve as a vectorial displacement sensor capable of monitoring simultaneously the magnitude and direction of the displacement between its center and that of the illuminating beam. The compact, easily controllable, and all-in-one nature of our devices can enable on-chip integrated circuits with adaptable functionality for applications in sensing and optical signal processing.

We propose a new interpretation for light confinement in picocavities formed by ultrasmall metallic protuberances inside the gap of metal–insulator–metal nanoresonators. We demonstrate that the protuberances support dark resonances with mode volumes comparable to their geometric volumes and that their brightness can be enhanced by several orders of magnitude when they are strongly coupled with the modes of nanoresonators with nanometric dielectric spacers. With a simple and accurate closed-form expression, we clarify the role of gap plasmons in this coupling. Based on this understanding, we propose a general strategy, exploiting strong coupling to design extremely localized modes with cubic nanometer volumes and so-far unreached brightness.

Studying the microscopic behavior of free carriers in materials at an ultrashort time scale is critical to developing semiconductor, optoelectronic, and other technologies satisfying the ever-increasing requirements for smaller sizes and higher speeds. Understanding the effect of local potential modulations and localized states due to nanoscale microstructures on carrier dynamics is essential to realize these requirements. Here, we used time-resolved scanning tunneling microscopy/spectroscopy (STM/STS) combined with a carrier-envelope phase (CEP)-controlled subcycle THz electric field, THz-STM, to probe the ultrafast motion of electrons photoinjected into C₆₀ multilayer structures grown on Au substrate. We have succeeded in demonstrating the time-resolved measurement of ultrafast electron dynamics with sub-nanoscale spatial resolution and subcycle time resolution for the first time and successfully visualized the electron motion triggered by the spatial variation in the lowest unoccupied molecular orbital (LUMO). The difference in the effects of molecular defects, such as a molecular vacancy and orientational disorder, was also clearly distinguished with single-molecular-level spatial resolution. This method is expected to play an important role in the precise evaluation of local electronic structures and dynamics for the future development of new functional materials and device elements.

Recent research efforts in optical computing have gravitated toward developing optical neural networks that aim to benefit from the processing speed and parallelism of optics/photonics in machine learning applications. Among these endeavors, Diffractive Deep Neural Networks (D²NNs) harness light-matter interaction over a series of trainable surfaces, designed using deep learning, to compute a desired statistical inference task as the light waves propagate from the input plane to the output field-of-view. Although earlier studies have demonstrated the generalization capability of diffractive optical networks to unseen data, achieving, e.g., >98% image classification accuracy for handwritten digits, these previous designs are in general sensitive to the spatial scaling, translation, and rotation of the input objects. Here, we demonstrate a new training strategy for diffractive networks that introduces input object translation, rotation, and/or scaling during the training phase as uniformly distributed random variables to build resilience in their blind inference performance against such object transformations. This training strategy successfully guides the evolution of the diffractive optical network design toward a solution that is scale-, shift-, and rotation-invariant, which is especially important and useful for dynamic machine vision applications in, e.g., autonomous cars, in vivo imaging of biomedical specimen, among others.

The understanding of nonlinear light–matter interactions at the nanoscale has fueled worldwide interest in upconversion emission for imaging, lasing, and sensing. Upconversion lasers with anti-Stokes-type emission with various designs have been reported. However, reducing the volume and lasing threshold of such lasers to the nanoscale level is a fundamental photonics challenge. Here, we demonstrate that the upconversion efficiency can be improved by exploiting single-mode upconversion lasing from a single organo-lead halide perovskite nanocrystal in a resonance-adjustable plasmonic nanocavity. This upconversion plasmonic nanolaser has a very low lasing threshold (10 μJ cm^–2) and a calculated ultrasmall mode volume (∼0.06 λ³) at 6 K. To provide the unique feature for lasing action, a temporal coherence signature of the upconversion plasmonic nanolasing was determined by measuring the second-order correlation function. The localized-electromagnetic-field confinement can be tailored in titanium nitride resonance-adjustable nanocavities, enhancing the pump-photon absorption and upconverted photon emission rate to achieve lasing. The proof-of-concept results significantly expand the performance of upconversion nanolasers, which are useful in applications such as on-chip, coherent, nonlinear optics, information processing, data storage, and sensing.

The ZnO-based heterostructures are predicted to be promising candidates for optoelectronic devices in the infrared and terahertz (THz) spectral domains owing to their intrinsic material properties. Specifically, the large ZnO LO-phonon energy reduces the thermally activated LO-phonon scattering, which is predicted to greatly improve the temperature performance of THz quantum cascade lasers. However, to date, no experimental observation of intersubband emission from ZnO optoelectronic devices has been reported. Here, we report the observation of THz intersubband electroluminescence from ZnO/Mg_xZn_1–xO quantum cascade structures grown on a nonpolar m-plane ZnO substrate up to room temperature. The electroluminescence peak shows a line width of ∼20 meV at a center frequency of ∼8.5 THz at 110 K, which is not accessible for GaAs-based quantum cascade structures because of the reststrahlen band absorption from 8 to 9 THz. This result is an important step toward the realization of ZnO-based THz quantum cascade lasers.

The many fundamental roto-vibrational resonances of chemical compounds result in strong absorption lines in the mid-infrared region (λ ∼ 2–20 μm). For this reason, mid-infrared spectroscopy plays a key role in label-free sensing, in particular, for chemical recognition, but often lacks the required sensitivity to probe small numbers of molecules. In this work, we propose a vibrational sensing scheme based on Bloch surface waves (BSWs) on 1D photonic crystals to increase the sensitivity of mid-infrared sensors. We report on the design and deposition of CaF₂/ZnS 1D photonic crystals. Moreover, we theoretically and experimentally demonstrate the possibility to sustain narrow σ-polarized BSW modes together with broader π-polarized modes in the range of 3–8 μm by means of a customized Fourier transform infrared spectroscopy setup. The multilayer stacks are deposited directly on CaF₂ prisms, reducing the number of unnecessary interfaces when exciting in the Kretschmann–Raether configuration. Finally, we compare the performance of mid-IR sensors based on surface plasmon polaritons with the BSW-based sensor. The figures of merit found for BSWs in terms of confinement of the electromagnetic field and propagation length puts them as forefrontrunners for label-free and polarization-dependent sensing devices.

Periodic arrays of nanoparticles are capable of supporting lattice resonances, collective modes arising from the coherent interaction of the particles in the array. These resonances, whose spectral position is determined by the array periodicity, are spectrally narrow and lead to strong optical responses, making them useful for a wide range of applications, from nanoscale light sources to ultrasensitive biosensors. Here, we report that, by removing particles from an array in a periodic fashion, it is possible to induce lattice resonances at wavelengths commensurate with the periodicity of these vacancies, which would otherwise not be present in the system. Using a coupled dipole approach, we perform a comprehensive analysis of how the properties of these vacancy-induced lattice resonances depend on the array periodicity, the particle size, and the number of vacancies per unit of area. Furthermore, we find that these lattice resonances have a subradiant character and originate from the symmetry breaking introduced in the unit cell by the presence of the vacancies. Finally, we investigate a potential implementation of an array with vacancies made of nanocylinders embedded in a homogeneous dielectric environment. The results of this work serve to advance our understanding of lattice resonances and provide an alternative method for controlling the optical response of periodic arrays of nanostructures.

High-mobility hexagonal boron nitride (hBN)/graphene/hBN heterostructures are able to reach intrinsic limits of transport. Here, we investigate optoelectronic mixing, which is a demanding function combining efficient photodetection and fast carrier dynamics. Using such a heterostructure embedded in a coplanar waveguide, we obtain a record conversion efficiency of about −40 dB for frequencies up to 65 GHz. This performance is obtained at high doping in the photobolometric regime. We provide a microscopic model of the photodetection, which accurately describes the experimental observations, allows the assessment of the intrinsic limits of our device, and paves the way for device optimization by revealing the different mechanisms at play.

In the past decade, complex networks of light emitters are proposed as novel platforms for photonic circuits and lab-on-chip active devices. Lasing networks made by connected multiple gain components and graphs of nanoscale random lasers (RLs) obtained from complex meshes of polymeric nanofibers are successful prototypes. However, in the reported research, mainly collective emission from a whole network of resonators is investigated, and only in a few cases, the emission from single points showing, although homogeneous and broad, spatial emission. In all cases, simultaneous activation of the miniaturized lasers are observed. Here, differently, we realize heterogeneous random lasers made of ribbon-like and highly porous fibers with evident RL action from separated micrometric domains that alternatively switch on and off by tuning the pumping light intensity. We visualize this novel effect by building for the first time replica symmetry breaking (RSB) maps of the emitting fibers with 2 μm spatial resolution. In addition, we calculate the spatial correlations of the laser regions showing clearly an average extension of 50 μm. The observed blinking effect is due to mode interaction along light guiding fibers and opens new avenues in the fabrication of flexible photonic networks with specific and adaptable activity.