Self-Organized Nanogratings for Large-Area Surface Plasmon Polariton Excitation and Surface-Enhanced Raman Spectroscopy Sensing

Surface Plasmon Polaritons (SPP) are exploited due to their intriguing properties for photonic circuits fabrication and miniaturization, for surface enhanced spectroscopies and imaging beyond the diffraction limit. However, the excitation of these plasmonic modes by direct illumination is forbidden by energy/momentum conservation rules. One strategy to overcome this limitation relies on diffraction gratings to match the wavevector of the incoming photons with that of propagating SPP excitations. The main limit of the approaches so far reported in literature is that they rely on highly ordered diffraction gratings fabricated by means of demanding nano-lithographic processes. In this work we demonstrate that an innovative, fully self-organized method based on wrinkling-assisted Ion Beam Sputtering can be exploited to fabricate large area (cm^2 scale) nano-rippled soda-lime templates which conformally support ultrathin Au films deposited by physical deposition. The self-organized patterns act as quasi-1D gratings characterized by a remarkably high spatial order which matches properly the transverse photon coherence length. The gratings can thus enable the excitation of hybrid SPP modes confined at the Au/dielectric interfaces, with a resonant wavelength which can be tuned either by modifying the grating period, photon incidence angle or, potentially, the choice of the thin film conductive material. Surface Enhanced Raman Scattering experiments show promising gains in the range of 10^3 which are competitive, even before a systematic optimization of the sample fabrication parameters, with state-of-the art lithographic systems, demonstrating the potential of such templates for a broad range of optoelectronic applications aiming at plasmon-enhanced photon harvesting for molecular or bio-sensing.

In the last decades the field of plasmonics witnessed an outstanding development because of the growing interest in the possibility to manipulate optical fields at the nanoscale, well below the diffraction limit, thus boosting a very broad range of optoelectronic applications [1][2][3][4]. Different so-called plasmonic modes, with diverse properties and resonant conditions are found in nature [ 5 ]. In particular, Surface Plasmons Polaritons (SPP) are electromagnetic modes propagating at the interface between a positive (e.g. a dielectric) and a negative permittivity medium due to resonant oscillations of free carriers [5][6][7][8]. Their propagating nature makes them suitable for waveguiding applications, crucial for the development of photonic circuits and components, which are not enabled by Localized Surface Plasmons (LSP) supported by sub-wavelength disconnected nanoparticles [ 5,6,[9][10][11] ]. SPP modes can be excited by electro-magnetic radiation under selected wavevector coupling conditions, thus confining the photons energy into subwavelength volumes at the conductive/dielectric interface where the electromagnetic evanescent field is strongly enhanced [ 5,6 ]. This peculiar property has recently allowed the highly sensitive near-field detection and imaging of surface plasmon modes with strongly subwavelength spatial resolution at the surface of atomic 2D materials in Scanning Near-field Optical Microscopy (SNOM) [12][13][14][15] ] and molecular nanoimaging in Tip Enhanced Raman Scattering (TERS) [16][17][18][19]. In parallel highly sensitive bio-sensing capabilities has been achieved in surface enhanced spectroscopies such as Surface Enhanced Raman Spectroscopy (SERS), Surface Enhanced Coherent Anti-Stokes Raman Spectroscopy (SECARS) and Metal Enhanced Fluorescence (MEF), which provide several orders of magnitude signal gain over their conventional counterparts [ 4,[20][21][22][23][24] ]. Moreover, travelling SPPs have been exploited to perform Remotely Excited SERS (RE-SERS), a technique where excitation and SERS signal collection are spatially displaced, remarkably reducing the fluorescence background signal originating from diffraction limited excitation volumes [ 25,26 ]. Several applications have been recently developed such as optical refractive index sensor devices, already available into the market, which exploit the sensitivity of the SPP resonance to the refractive index of the surrounding dielectric medium [ 27,28 ]. Alternatively, the non-radiative plasmon decay generating hot electrons into the conductive material can be exploited in order to enable direct plasmon enhanced photocatalysis [ 29,30 ], and/or hot carrier injection into semiconductors, effectively extending the operating range of photonic devices beyond their band-gap limited absorption [ 31,32 ]. However, photon-SPP coupling is not allowed by direct illumination because of energy and momentum conservation selection rules, thus particular strategies have to be adopted such as EM coupling via subwavelength probes or diffraction gratings which are able to modulate the wavevector of the incoming photons [ 5,6 ]. In particular, diffraction gratings characterized by a very high spatial coherence are typically fabricated via top-down lithographic fabrication methods that, however, can limit the perspective of large-scale, low-cost applications [ [21][22][23][33][34][35][36].
In this work we demonstrate that a cheap nano-rippled soda-lime glass template, prepared by an innovative, fully self-organized (SO) fabrication method based on wrinkling-enhanced Ion Beam Sputtering [ 37 ], can be used as large area support for a thin Au film conformally grown on top of it by thermal vacuum deposition, in a IMI (insulator-metal-insulator) configuration. The resulting sub-wavelength, quasi-1D grating endowed with linearly graded periodicity enables the excitation of a tunable hybrid Surface Plasmon Polariton mode. Based on our knowledge this is the first example of fully self-organized gratings promoting the coherent excitation of SPP modes over large area.
The potential of these large area plasmonic meta-surfaces for highly sensitive bio-sensing have been demonstrated in SERS, showing strong enhancement of the Raman signal promoted by SPP modes. Remarkable SERS gain values in the range of 10 3 , competitive with the figures of lithographic-made systems [ 38,39 ] as well as with alternative self-organized approaches [40][41][42][43], are found for a pump laser value of 785 nm. Indeed, this makes the sample appealing for molecular sensing, also considering that a systematic optimization of the sample fabrication parameters, which is discussed but it's beyond the scope of this work, could easily further improve these SERS gain values. Finally, we demonstrate how the SERS gain is strongly correlated with the electric field enhancement produced by the excitation of SPP modes by comparing SERS and co-localized micro-extinction measurements. These self-organized gratings represent a very versatile platform, potentially allowing the broadband tuning of the SPP mode from the Near-UV to the Far-IR by simply selecting the conductive layer grown on top of the template, making them interesting for a broad range optoelectronic applications.

Sample fabrication
A soda-lime glass (2.52.50.2 cm) is heated up to 680 K and irradiated with a defocused ion beam (Ar + ) with a low energy of 800 eV at the incidence angle θ=30° (Fig. 1b) with respect to the sample surface normal direction. Positive charge build-up on the glass surface is prevented during the ion irradiation process by stimulating thermionic electron emission from a tungsten filament negatively biased at Vbias=-13 V with respect to the glass sample. For ion irradiation prolonged for 1800 s at the pressure of 4  10 -4 mbar (i.e. ion fluence of 1.4x10 19 ions/cm 2 ), highly ordered faceted nanopatterns evolve on the glass surface showing a steep, asymmetric faceted sawtooth profile. After the nano-rippled pattern is formed on the sample surface, thermal Au deposition is performed on the rippled template at the pressure of about 10 -5 mbar in a two step process, described in the "Results and discussion" section. The process is designed to grow an Au thin film of approximately homogenous thickness on the slope-modulated glass rippled surface.

Morphology characterization
The self-organized rippled glass morphology is characterized by means of an atomic force microscope (Nanosurf S Mobile). The average periodicity, line profiles and slope of the nanoripples are extracted by means of WSxM and Gwyddion software from the statistical analysis of AFM measurements [ 44,45 ]. Top view back scattered electrons SEM images are acquired by means of a thermionic Hitachi VP-SEM SU3500.

Optical characterization
Polarized transmission measurements are performed by fiber-coupling the radiation emitted by a compensated deuterium-halogen lamp (DH-2000-BAL, Mikropak) to a linear polarizer, illuminating the sample from the bare glass side. The beam spot diameter in about 1 mm. The signal is then coaxially collected by a high-resolution solid-state spectrometer (HR4000, Ocean Optics). Non-normal incidence transmission measurements are performed by placing the sample on a stage which provides a tilting movement. All the transmission spectra are normalized to the optical transmittance of a bare glass reference substrate tilted at the corresponding sample angle.

SERS and co-localized extinction measurements
Samples (rippled Au/soda-lime glass and reference flat Au thin film on bare soda-lime glass) are immersed in a 10 -4 M MB solution for an hour and then rinsed in de-ionized water for 10 minutes.
Polarized SERS and optical micro-extinction measurements are performed by means of a Horiba XploRA Nano. For SERS, pump lasers at 638 and 785 nm wavelengths are employed, using a 100x objective for the excitation of a diffraction-limited area. Different sample positions, a few mm away from each other, are probed by averaging multiple SERS measurements distanced by some μm. Detection is carried out in backscattering. Micro-extinction measurements have been co-locally performed at corresponding sample coordinates by switching the source to a tungsten lamp illuminating the sample from below and to a 10x objective for signal collection.

RESULTS AND DISCUSSION
A defocused Ar + ion beam at low energy (800 eV) irradiates a soda-lime glass sample at an incident angle θ=30° with respect to the surface normal as sketched in Figure 1a The ripple sides opposed to the incoming ion beam direction develop wider facets with a narrow slope distribution sharply peaked at about +35° (Fig. 1c). Conversely, the sides directly exposed to the Ar + beam develop narrower facets with a broad slope distribution weakly peaked at about -50° (Fig. 1c). Moreover, we engineered the IBS process in order to produce a sample with a graded periodicity along its length. We achieved this by exploiting an ion dose gradient along the projection of the Ar+ beam, originating from the increasing sample-source distance at the tilted incidence angles here employed, and from the gaussian intensity distribution of the ion beam.
Considering a periodicity =200 nm in the central area of the glass template (Fig. 1a), we achieve in this way a modulation of the grating periodicity of about ±15% moving from the top to the bottom of the sample along the projection of the ion beam (see Fig. S2). It is worth to underline, as described in a recent study by some of the authors [ 37 ], that the nanopattern formation kinetics is strongly enhanced by a solid-state wrinkling instability which is activated when the substrate temperature is raised near the glass transition threshold and is concurrently exposed to irradiation with an Ar+ ion beam. Under such conditions a compressive stress builds up in a thin surface skin layer partially depleted of alkali atoms; stress release takes place by surface buckling provided enhanced mass transport in the near-surface region is made possible by the reduction in glass viscosity [ 37 ]. We highlight that when ion irradiation is performed at reduced temperatures for which mass transport is strongly reduced, one does not observe the formation of the high aspect ratio faceted wrinkles but rather the low aspect ratio ripples conventionally found after Ion Beam Sputtering of insulating and semiconductor substrates at room temperature [ [46][47][48].
As demonstrated by the authors in recent studies [49][50][51] such faceted, high aspect ratio, ordered rippled patterns are the ideal template for maskless confinement of large area, self-organized arrays of disconnected uniaxial plasmonic nanostructures. The possibility to control tilt and morphology of the nanostructures is an interesting feature for a wide range of applications which exploit tunable dipolar and hybrid localized plasmonic modes. Here instead we propose a novel self-organized fabrication protocol to produce periodically corrugated continuous Au thin films of uniform thickness, conformally supported on the linearly graded glass diffraction grating which sustain propagating Surface Plasmon Polariton (SPP) modes with tunable response. Given the asymmetric faceted profile of the glass template, a conventional thermal deposition process at normal incidence would result in an Au film with non-uniform thickness, evaluated along the local surface normal. To overcome this limitation, we developed a dual-step off-normal thermal deposition procedure which is capable of achieving growth of an Au film with uniform thickness on the two opposite facets. The first deposition step is performed at the angle θ1= -15° with respect to the sample surface normal (negative angles corresponding to an anti clock-wise rotation), over the side of the ripple profile opposing the ion beam during the nano-structuring step (Fig. 1d). In this condition the Au beam forms a 20° angle with respect to the local surface normal of the "wide" facets. At the same time an angle of 65° is formed between the Au beam and the local surface normal to the "steep" facets. The Au thickness deposited during the first deposition step on the left side of the ripple can be estimated as hleft-1=h 0  cos20°, while on the right side the thickness is hright-1=h 0  cos20°, where h0 is the equivalent Au thickness on a flat substrate. During the experiment h0 is the parameter which is measured, by means of a quartz microbalance. The second Au deposition step is performed at the angle θ2=+30° with respect to the sample normal (positive angles correspond to a clockwise rotation), over the side of the ripple profile directly facing the ion beam during the nanopatterning step (Fig. 1d). Now the Au beam forms a 20° angle with respect to the "steep" facets and a 65° angle with respect to the "wide" facets local surface normal; the situation is now opposite compared to θ1 conditions and hleft-2=h 0  cos65° while hright-2=h 0  cos20°. To grow an approximately uniform Au layer over the asymmetric slope modulate ripple profile it's sufficient to impose a desired thickness h and solve one of the following equations for h 0 : As shown in the Scanning Electron Microscopy (SEM) cross sectional image of the sample after metal deposition (Fig. 1e) one can clearly identify the presence of the Au film with thickness h=25 nm (Fig. 1d) which conformally follows the profile of the glass template. Such a rippled air/Authin-film/glass template can be considered as an IMI (insulator-metal-insulator) structure with slanted facets. We stress that such asymmetric, so-called slanted (or blazed), quasi-1D gratings cannot be fabricated by top-down techniques unless complex gray-scale lithography processes are adopted.
For the excitation of the propagating plasmon it is necessary that proper coupling of light momentum takes place at the metal/dielectric interface, mediated by transfer of the grating wavevector k G = n Normal incidence linearly polarized optical transmission measurements are shown in Fig. 2a. For TE polarization (Fig. 2a, dotted black curve) the electric field oscillates parallel to the diffraction grating grooves (ripples long axis); in the considered spectral range, the sample transmission curve resembles the one of a Au flat thin film of equivalent thickness (Fig. 2b). For TM polarization (Fig. 2a -continuous red curve) the electric field oscillates orthogonally to the diffraction grating grooves (ripples short axis) and a strong dichroism is observed as transmission shows a broad minimum centered at the wavelength λ of about 660 nm. This transmission dip (corresponding to a maximum in the extinction spectrum) is attributed to the excitation of a propagating SPP mode along the continuous rippled Au film, which is enabled by the momentum exchange mediated by photon-grating interaction [ 21,23,33 ]. In Fig. 2c we demonstrate that the SPP resonant wavelength, measured in the transmission spectra under TM polarization at normal incidence, can be tuned by varying the grating periodicity at different samples coordinates along our linearly graded nanogratings. By analyzing the 2D self-correlation function of the AFM topographical images [ 44 ] we can associate the grating periodicity co-localized with the optical transmission spectra (see Fig. S2). In Fig. 2d the wavelength of the transmission dip minimum of each spectrum is plotted against the corresponding grating periodicity. The SPP resonant wavelength redshifts linearly and monotonically starting from the smaller considered periodicity (=183 nmblack curve in Fig. 2c) to the bigger one (=230 nmcyan curve in Fig. 2c). Such a monotonic redshift of the SPP wavelength with increasing grating periodicity Λ is to be expected since a smaller momentum exchange k G = n 2π Λ , mediated by photon-grating interaction, depends reciprocally from the period (n is an integer number either positive or negative) [ 5,6 ].
To further investigate and strengthen the attribution of the observed optical extinction loss to the excitation of an SPP mode, transmission measurements as a function of the sample tilt relative to the light beam were performed. The sample is illuminated from the glass side and the signal is coaxially collected in extinction configuration in TM polarization. The sample is tilted forming an angle θ between the incident light beam and the surface normal (Fig. 3a). The sample rotation is performed counter-clockwise, that is reducing the angle formed by the incident light beam and the local surface normal of the wide facets of the rippled Au/glass template (Fig. 3a). In Fig. 3b transmission spectra are plotted for different sample tilts, increasing in 10° steps from 0° to 60°. We stress that the comparison of Figure 4, though simplified and based on an effective medium approximation, allows to describe the general trend of SPP dispersion when the Au film is so thin (t=25 nm) that coupling between plasmon polaritons propagating at the two interfaces of the slab cannot be neglected. This leads to a hybridization of the SPP modes of a thick metal slab into an antisymmetric mode (out-of-phase matching of electric fields at the two sides of the metal film also called short-range hybrid mode), which is red-shifted compared to the single-interface metal/glass SPP, and into a symmetric mode (in-phase matching of electric fields at the two sides of the metal film also called long range hybrid mode), which is blue-shifted compared to the singleinterface air/metal SPP [ [53][54][55][56]. The best match with the experimental data (Figure 3b,c) corresponding to the low energy antisymmetric mode is found for an effective refractive index of the corrugated glass/Au slab n2=2.3, a considerably higher value compared to the typical n=1.5 of glass which would be expected in the thick Au film limit. In our experimental configuration, the high energy symmetric hybrid SPP mode is not observable since it is strongly damped by the Au s-d interband transitions which are active below 520 nm.

Figure 3 -a) Sketch of the tilted transmission (T) measurements configuration. The red arrow kphoton represents the light beam incoming from the glass side. Θ is the angle formed between
In Figure 4b, instead, we highlight the possibility to modify the experimental SPP plasmon energy ℏω SPP by just varying the grating periodicity Λ (see Fig. 2d), when excitation takes place at constant incidence angle (normal incidence, =0°). The theoretical value of the SPP energy  . This allows us to estimate the SERS gain of our rippled substrate, defined as the ratio between the intensity of the peak at 1625 cm -1 , measured on the rippled sample (e.g. black curves in Fig 5 a,b), and on the flat Au film (red curves in Fig. 5 a,b), normalized to the laser power and integration time [ 40 ]. The so defined SERS gain, plotted as a function of the grating periodicity in each considered position for the 638 and 785 nm pump lasers, shows a monotonic increasing trend as reported in Fig. 5c and Fig. 5d respectively.
As previously discussed, the SPP resonance spectrally shifts when moving at different sample coordinates, due to a spatial dependence of the SO grating periodicity (Fig. 2c). It's thus possible to nicely correlate the intensity of the plasmonic extinction strength at the pump lasers frequency, which is responsible for the SERS near field enhancement, with the co-localized measured SERS gains. After background subtraction the extinction values at the pump lasers wavelengths of 638 nm and 785 nm are then plotted as a function of different sample position in Fig. 5e and Fig. 5f, respectively (see Fig. S4 for details). Remarkably, the intensities of TM extinction and co-localized  Raman co-localized transmission measurements (see Fig. S4).
We stress that the discussed SERS gains could indeed be further increased by optimizing easily accessible experimental parameters. We can mention the increase of the grating period resulting in a redshift, beyond the onset of interband transitions, of the high energy long-range hybrid SPP, which is characterized by a longer lifetime and a transverse electric field penetrating more deeply into the dielectric environment where probe molecules are found [ 54 ]. Morevoer an optimization of the Au thin film thickness to find the ideal hybridization and photon-SPPs coupling conditions could further maximize the SERS gain. Alternatively, the plasmonic response of our linearly graded nanogratings could be further tuned in view of plasmon enhanced spectroscopies by e.g.
matching the pump laser frequency across the UV to NIR spectral range by growing conductive layers with different free-carrier density. The SPP dispersion relation is in fact strongly affected by the metal plasma frequency [ 5,6 ]: a blueshift towards the UV could be achieved by growing on top of the rippled dielectric template metal films with high density of free carriers such as Ag and Al, while following a similar approach, one could imagine shifting the SPP excitation in the NIR-IR range by growing a thin film with a reduced free carrier density like e.g. semiconductors with variable doping and 2D materials [ 59,60 ]. The wide spectral tunability of the plasmonic selforganized nanogratings, combined with the possibility to easily scale-up the low-cost and large area fabrication method here described represent crucial features in view of a broad range of real word applications aiming at photon harvesting and at advanced molecular sensing.

CONCLUSIONS
In this work we demonstrate how a self-organized rippled soda-lime glass template, prepared by

Supporting Information
Fig. S1a shows the 2D self-correlation function of the self-organized glass nanograting AFM topography presented in Fig. 1a of the main manuscript, computed by means of Gwyddion software. The grating average periodicity λ is calculated by measuring the real space distance between the maximum and the secondary neighboring peaks in the 2D self-correlation line profile of Fig. S1b.   The corresponding measured grating periodicity λ is reported in the table (right panel).
In Fig. SI 3 we demonstrate the long-range morphological properties of the self-organized Au/glass nano-rippled grating, thought the analysis of a large area 2D auto-correlation extracted from AFM topography measurements (Fig. S3a). Remarkably, in the self-correlation line profile of Fig. S3c (corresponding to the green line in Fig. S3b), which has been measured by moving more than 10 μm away from the origin of the auto-correlation map, the modulations at the characteristic period around 200 nm persist well visible. This ensures that the grating morphological coherence can spatially match the incoming photons transverse coherence length in our experimental conditions, as required for SPP coupling to occur. In Fig. S4a we plot the SERS co-localized transmission measurements for the different grating