High-throughput micropatterning of plasmonic surfaces by multiplexed femtosecond laser pulses for advanced IR-sensing applications

Accepted Manuscript High-throughput micropatterning of plasmonic surfaces by multiplexed femtosecond laser pulses for advanced IR-sensing applications

S.I. Kudryashov, P.A. Danilov, A.P. Porfirev, I.N. Saraeva, T.H.T. Nguyen, A.A. Rudenko, R.A. Khmelnitskii, D.A. Zayarny, A.A. Ionin, A.A. Kuchmizhak, S.N. Khonina, O.B. Vitrik PII: DOI: Reference:

S0169-4332(19)31040-2 https://doi.org/10.1016/j.apsusc.2019.04.048 APSUSC 42356

To appear in:

Applied Surface Science

Received date: Revised date: Accepted date:

15 October 2018 30 March 2019 4 April 2019

Please cite this article as: S.I. Kudryashov, P.A. Danilov, A.P. Porfirev, et al., Highthroughput micropatterning of plasmonic surfaces by multiplexed femtosecond laser pulses for advanced IR-sensing applications, Applied Surface Science, https://doi.org/ 10.1016/j.apsusc.2019.04.048

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ACCEPTED MANUSCRIPT High-throughput micropatterning of plasmonic surfaces by multiplexed femtosecond laser pulses for advanced IR-sensing applications S.I. Kudryashov,1,2 P.A. Danilov, 1,2 A.P. Porfirev,2,3,4 I.N. Saraeva,1 T.H.T. Nguyen,1,5 A.A. Rudenko,1 R.A. Khmelnitskii,1 D.A. Zayarny,1 A.A. Ionin,1 A.A. Kuchmizhak,2,6 S.N. Khonina,3,4 O.B. Vitrik2,6 1

Institute for Automation and Control Processes FEB RAS, 690041 Vladivostok, Russia 3

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Lebedev Physical Institute, Moscow 119991, Russia

Samara National Research University, Samara 443086, Russia,

Image Processing Systems Institute - Branch of the Federal Scientific Research Centre

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Ha Tinh University, precinct Dai Nai, Ha Tinh, province Ha Tinh, Viet Nam

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“Crystallography and Photonics” of Russian Academy of Sciences, Samara 443001, Russia,

School of Natural Sciences, Far Eastern Federal University, 690041 Vladivostok, Russia

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Corresponding author: Sergey I. Kudryashov [email protected]

Abstract Tightly focused, highly spatially multiplexed femtosecond laser pulses, coming at sub-

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MHz repetition rates, were used to mask-less pattern thin plasmonic films film at ultrafast rates, approaching 25 million of microelements per second. For this purpose, the initial pulses were multiplexed by fused silica diffractive optical elements into linear arrays of 31, 51 and 101

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circular light spot and then scanned over the films by a galvanometric scanner through a long-

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focus objective or high-NA aspherical lens. These optical scheme and 5-µJ pulse energy supported the only 31- and 51-beam micro-patterning, with the corresponding aperture, field-ofview and sub-100 nJ energetic limitations for the laser processing of the film. The resulting large

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(105-106 holes per array) arrays of micro-holes of variable diameters and periods in thin films of different thickness and diverse plasmonic materials  Ag, Cu, Al and Au-Pd alloy (80%/20%)

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 were for the first time systematically characterized in the broad IR-range (1.5-25 µm) in terms of plasmonic effects in extraordinary optical transmission, indicating for the increasing wavenumber a smooth transition from the common Bethe-Bouwkamp transmission to its plasmon-enhanced analogue, ending up with common geometrical (wave-guide-like) transmission. Finally, promising label- and luminescence-free laboratory-scale, robust and highsensitivity sampling of chemicals and biosamples via plasmonic and chemical contributions, uneven and structurally-sensitive regarding different functional groups of the model analyte molecules and band structure of the plasmonic metal, was demonstrated for the large IR-sensing arrays of micro-holes in plasmonic films, with the obvious perspectives for down-scaling of sensing elements for vis-IR surface-enhanced spectroscopies. 1

ACCEPTED MANUSCRIPT Key words: femtosecond laser pulses; tight focusing; spatial multiplexing; plasmonic thin films; high-throughput micropatterning; IR transmission; extraordinary transmission; plasmonic effects; surface-enhanced IR-sensors

1.

Introduction Large-scale arrays of nanophotonic elements are key building blocks of plasmonic or all-

dielectric metasurfaces, enabling nano-scale light control [1-4], as well as ultrasensitive chemo-

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and bio-sensing [5-7]. A key concept underlying the functionalities of many metasurface approaches is their use of constituent elements with spatially varying optical properties. Very

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recently, two-dimensional pixelated all-dielectric metasurfaces with a series of spatial domains, exhibiting ultrasharp variable discrete-frequency IR-spectral responses, were presented to detect

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mid-infrared molecular fingerprints of surface-bound analytes via broad-band IR-range illumination and imaging selective, nondestructive and label-free surface-enhanced infrared

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absorption or reflection (SEIRA/R) at the multiple spectral sensing points [8]. This approach paves the way toward sensitive and versatile miniaturized mid-IR surface-enhanced IR-sensors,

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resolving molecular fingerprints without the need for spectrometry, frequency scanning or moving mechanical parts.

Currently, both plasmonic and all-dielectric metasurfaces are commonly fabricated by

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optical, electron or ion-beam lithography [1-8]. However, multi-scale elements of metasurfaces,

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when fabricated through traditional lithographic procedures, become dramatically more expensive and require significantly longer fabrication cycles. Alternatively, vis/IR-range metasurfaces can be fabricated by versatile laser-based approaches, including single-shot large-

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scale multi-beam laser interference of moderately focused laser pulses [9,10] and shot-per-spot nano- and micro-patterning, using tightly focused multi-kHz laser repetition rates and fast

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scanning/positioning systems [11-13]. With the ultrashort-pulse lasers, providing the ultimate spatial resolution via reduced heat-affected zone [14], multi-beam laser interference capability is considerably diminished by coherence length considerations, limiting single-shot fabrication areas by sub-mm dimensions. In contrast, throughput of laser nano- and micropatterning can be additionally increased via beam multiplexing by means of solid-phase diffractive optical elements [13,15-16] or liquid-crystal spatial light modulators [17,18]. Laser, scanning and multiplexing modalities provide broad flexibility during ultrashort-pulse laser nano- and micropatterning in fabrication of multi-scale plasmonic and all-dielectric metasurfaces. Recently, simple, single-step mask-less micro-patterning of thin plasmonic films by tightly-focused sub-MHz-rate femtosecond laser pulses was demonstrated as the enabling technology for fabrication of laboratory-scale micro-hole array sensors for robust and high2

ACCEPTED MANUSCRIPT sensitivity, label- and luminescence-free surface-enhanced IR-spectroscopy (SEIRS) in chemoand biosensing applications [5,19,20], based on extraordinary transmission (EOT) of electromagnetic radiation. In visible and IR-ranges, EOT through small (diameter D < 0.3, with wave-guide transmission for wider holes [21]) holes was discovered as a plasmonic effect in 90s [22,23], following the diffraction-based theory of Bethe and Bouwkamp for transmission through such single tiny holes [24,25]. This theory predicts for an infinitely thin film with its infinite conductivity (perfect metal without dissipation) the absolute transmittance [24,25] 4

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64  D (1) T ( , D)  2  , 2  27  2 indicating rapid increase of transmittance T versus hole diameter D and spectral wavenumber  =

1/ as a tunneling effect. In comparison with the classical Bethe-Bouwkamp theory, EOT is

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mediated by surface plasmon-polaritonic and localized plasmonic effects on metal surfaces (in thin films  both on front and rear ones [21]) and on hole edges, respectively, effectively

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increasing the actual hole size available for transmission [26] and revealing, cfg., a few resonant transmission bands for different directions {1,0} and {1,1} in square arrays of holes as 2D

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photonic crystals [23]:

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m2  n2 m2  n2  M   D , (2)  SP (m, n)   nSP P P MD where m,n are the integer indexes for different directions {m,n} in the array (grating), nSP is the

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effective refractive index for surface plasmon-polaritons, M and D are the dielectric functions of the metallic and the dielectric media [23]. Different hole shapes, film thickness etc. were investigated to explore opportunities in harnessing EOT in color filtering, single-photon sources,

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etc. Meanwhile, systematic studies of EOT in terms of hole diameters and periods, film thickness and materials were not performed since so far, resulting in some unusual EOT demonstrations (e.g., T  9 [26], rather than T  4 in Eq.1), apparently, because of high cost of lithographic

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fabrication of laboratory-scale hole arrays (typically, <103 holes per array). Obviously, macroscopic hole arrays could provide more accurate and reproducible data without unavoidable edge and positioning effects, but with more distinct spectra transmittance resonances, for common-facility macro-spectral inspection of samples with better statistical parameters, easier deposition of chemical or biological analytes, and sample manipulations. In this work, we report on advanced mask-less high-throughput micro-patterning of thin plasmonic films by highly-multiplexed, tightly-focused sub-MHz-rate femtosecond laser pulses, explored in terms of laser pulse energy and repetition rate, different optical focusing schemes and multiplexing diffractive optical elements, for fabrication of laboratory-scale, robust and high-sensitivity micro-hole array sensors for surface-enhanced IR-spectroscopy. Then, we 3

ACCEPTED MANUSCRIPT comprehensively characterize IR-range transmission in such large (105-106 holes per array) arrays of micro-holes of variable diameters and periods in thin films of different thickness and diverse plasmonic materials  Ag, Cu, Al and Au-Pd alloy (80%/20%)  in terms of a smooth transition from the common Bethe-Bouwkamp transmission to its plasmon-enhanced analogue (EOT), ending up with common geometrical (wave-guide-like) transmission. Finally, their SEIRchemosensing modalities were tested regarding a model analyte Rhodamine 6G.

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2. Experimental details Multi-shot surface ablation of 50-nm thick silver films prepared by magnetron sputtering

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(SC7620, Quorum Technologies) of 99.99%-pure commercial silver target in argon atmosphere (4-5 mbar) onto commercial BK-7 silica glass slides, was performed by fundamental-harmonic

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(central wavelength  1030 nm), 300-fs (FWHM) pulses of Satsuma laser (Amplitude Systemes). The TEM00-mode pulses (M2 ≈ 1.07) were coming at variable energies E  5 J and

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the repetition rates in the range of 0-2 MHz, through an optical scheme (Fig. 1A), consisting of a negative lens (25-mm wide aperture, focal distance  –40 mm), a fused-silica DOE, a

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galvanometric scanner ATEKOTM (maximal scan velocity V ≈ 7 m/s) and its F-Theta objective (anti-reflective coatings, 100-mm long focal length and 70-mm wide field of view), or a 0.5-NA coated aspherical silica-glass lens C240TME-B (aperture ≈ 12 mm, working distance ≈ 8 mm,

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Thorlabs), onto a thin-film sample arranged on a vibration-isolated optical table (Standa).

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Surface topography of the micropatterned film spots was characterized by means of optical (Altami-6) and scanning electron (SEM, JEOL 7001F) microscopes. The DOEs of interest were designed, calculating their phase-only transmission functions

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(Fig. 1B) by the Gerchberg-Saxton algorithm [27]. This well-known algorithm allows the reconstruction of the unknown wavefront from the known intensity distributions on a few planes

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of an optical system, for example in the input plane and focal plane. To manufacture the designed DOEs on fused silica substrates, we used a technological process, including photolithography and plasma etching. The resulting DOE elements provided linear patterns of 31, 51 and 101 light spots (Fig. 1B). Their efficiency was found to range from 72% for the 101beam DOE to 76% for the 31-beam DOE. The uniformity of the generated light patterns calculated as 1 – (∑|Ij-Iavg|)/(NIavg), ranges from 0.78 for the 101-beam DOE to 0.87 for the 31beam DOE. Finally, 4 mm-sized periodical square arrays of through micro-holes with variable diameters D and periods P were produced via the high-throughput micropatterning in plasmonic films of different thickness and diverse plasmonic materials  Ag, Cu, Al and Au-Pd alloy (80%/20%), deposited on 2-mm thick IR-transparent fluorite (CaF2) substrates. The mm-sized square patterned regions were produced, 4

ACCEPTED MANUSCRIPT using a square aperture atop the aspherical lens, and properly adjusted regarding to each other on the film surface to provide their matching within the large arrays. SEM chemical microanalysis (energydispersion x-ray spectroscopy, EDX) profilometry was performed across the microholes to ensure their non-modified elemental composition and the absence of oxidation or carbonization. Finally, SEIRA spectroscopy of the produced large micro-hole arrays was performed by means of a broad-range (UVmid-IR) Bruker spectrometer V-70.

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3. Experimental results 3.1. High-throughput microfabrication regimes

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As a primary test, direct shot-per-spot ultrafast laser micropatterning of the film via the long-focus F-theta objective (without the 0.5-NA lens) at f = 500 kHz and variable pulse

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energies E = 2-3 µJ of the incident non-multiplexed (N = 1) Gaussian beam for the nonsynchronized laser and galvano-scanner results in displaced linear arrays of through microholes

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and the overall patterning throughput limited by the repetition rate f

for

fD( E )  V ,

(3)

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P( D( E ),V , f , N  1)  f

which effectively represents the limiting scanning velocity V = 7 m/s for the microhole diameters D  2-6 µm (Fig. 2A). The decrease of the Gaussian laser pulse energy from 3 J to 2.3 J resulted in the corresponding logarithmic reduction of the squared hole size ( D  w lnF / Fabl  ) almost by ten 2

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times above the ablation threshold fluence Fabl  0.3 J/cm2 (threshold energy 2 µJ), favorable for increased patterning throughput at the lower energy-dependent microhole size D(E). However, it also indicated the very limited pulse energy range, available for beam multiplexing, and the observed

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inappropriate near-threshold microhole shape instability (Fig. 2A,bottom and Fig. 2B,top). Meanwhile, as a result, the micropatterning throughput of 5105, 7-m wide through holes ablatively produced per

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second in the form of 3 mm  3 mm wide arrays of holes with 12-m transverse and 14-m longitudinal periods, was achieved in this experimental arrangement. Its high-throughput modality in fabrication of mm-sized arrays of through micro-holes for potential SEIRA applications was demonstrated, being limited at the given microhole size only by the pulse repetition rate (f  2MHz) and the scanning velocity (V  7m/s). Generally, in advanced fiber laser systems of ultrashort (fs-ps) pulses repetition rates approach 10 MHz or more [28], while modern polygon scanning systems provide scanning velocities up to 1 km/s [29]. Altogether, these advanced technical modalities potentially enable multi-million micropatterning rates per second for fabrication of multi-element photonic metadevices for various applications.

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ACCEPTED MANUSCRIPT In this work, multi-million micropatterning rates per second were achieved via efficient multiplexing of the rapidly scanned 5-µJ pulses accompanied by 0.5-NA focusing (Fig. 1). Initially, series of through microholes were produced at different incident energies E of the non-multiplexed collimated Gaussian-shaped fs-laser pulses in the 50-nm thick silver film by means of the test focusing high-NA objectives with their slightly lower NA = 0.25 and slightly higher NA = 0.65 (Fig. 2B,C). The presented SEM images of the typical microholes indicate the operation window for micropattening of the film in terms of pulse energy per one light spot – 100 nJ (NA= 0.25) and 50 nJ (NA = 0.65), indicating the

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highly excessive maximal incident pulse energy of 5 µJ. As a result, the potential multiplexing factor 100 was figured out for the measured high-NA threshold pulse energies of 45 nJ (NA = 0.25) and 14

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nJ (NA = 0.65), providing the minimal through hole opening in the film.

Then, the derived range of the overall incident pulse energies for through-hole micropatterning of

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the silver film with multiplexed fs-laser pulses was used in the multiplexing scheme (Fig. 1) for the single-pass ultrafast laser patterning with 31, 51 and 101 micro-beams at the 0.5-NA focusing, the

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maximal pulse energy E ≈ 5 J and the maximal scan velocity V ≈ 7 m/s, and variable pulse repetition rates in the range f = 50-500 kHz (Fig. 3). The rapid galvano-scanning of the linear 51-fold pattern of the

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multiplexed laser micro-beams across the 12-mm wide aspherical lens aperture in the single pass provided a linear pattern of through microholes only within its limited field of view (≈ 1.5-2 mm, Fig. 3A-C, left-side low-magnification images), while the common view of the patterned film area was

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rhomb-like. At the given multiplexing factor N, scanning velocity V and energy-dependent microhole

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size D(E) the variable pulse repetition rates f effectively varied the patterning throughput according to the expression

for

fD ( E )  V ,

(4)

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P( D( E ),V , f , N )  f  N

indicating the raising throughput at the increasing f and N (specifically, P  25 millions of through holes

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per second at N = 51 and f = 500 kHz) until the overexposure limit fD(E)  V, corresponding to undesirable micro-hole overlapping. The 101-spot patterns were not completely filled in the utilized optical scheme, while 31-spot patterns resulted in the almost two-fold reduced throughput (the results are not shown in Fig. 3). In this context, in order to increase laser micropatterning rates till industrial level of 109 elements/s, advanced high-repetition rate (f  10 MHz) fiber laser systems of ultrashort (fs-ps) pulses and ultra-rapid polygon scanning systems (V  1 km/s) could be used along with the highefficiency beam multiplexing.

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ACCEPTED MANUSCRIPT 3.2. Surface-enhanced IR spectroscopy of large-scale micro-hole arrays In our study, we systematically explored, for the first time, IR-transmission response of large (105-106 holes per array) arrays of micro-holes of variable diameters and periods in thin films of different thickness and of plasmonic materials  Ag, Cu, Al and Au-Pd alloy (weight 80%/20%) (Fig. 4). As a precaution, chemical composition of the micro-holes and their neighborhood was preliminarily analyzed by EDX profiling and mapping to evaluate undesirable oxidation of the sensing surfaces during their micro-fabrication. The results presented in Fig.5

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indicate that the only Cu films become slightly post-pulse oxidized at the periphery of the laser focal spot around the microholes and inside them, but this is not the fact for the films of the other

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materials. Also, such EDX profiling and mapping enables to evaluate the analyte R6G thickness

surfaces as 4-6 at. % (2-3 molecular layers).

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i) Thickness effect

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on the surface as 10-nm layer, accounting for the natural hydrocarbon contamination on the

Highly opaque Ag films (F, T < 1% in the range of 800-3000 cm-1, Fig. 4A) of variable thickness (h = 30, 50, 100 and 150 nm) were used for laser fabrication of micro-hole arrays

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(gratings G, D  4 m and P  6 m) shown in the inset in Fig. 4A. FT-IR transmission spectra of these gratings versus wavenumber , normalized to the IR-transmittance of their CaF2

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substrate with its cut-off edge below 1000 cm-1 (Fig. 4A), indicate rapid increase of T at lower

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wave-numbers, coming to the multi-peak maximum of the transmittance at  1140 cm-1 with the thickness-dependent maximal values and the following thickness-dependent plateau values at higher wavenumbers > 1500 cm-1 (the characteristic curve appearance previously reported in

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[30]).

Linear fitting of these dependences in the double logarithmic coordinates logT-log in the low-wavenumber range (Fig. 4B) demonstrates their slopes, tending to 4 for the thicker films

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(3.980.01 at h = 150 nm, 3.790.02 at h = 100 nm) and to 2 for the thinner films (2.290.05 at h = 30 nm), with the intermediate slope magnitude of 2.840.01 at h = 50 nm. According to the Bethe-Bouwkamp theory [24,25], all these films with D < 0.3 should exhibit the scaling T  4, which is actually valid only for the thicker films, while the thinner one reveals almost waveguide-like (geometrical) transmission T  (D)2 of bulk or surface (SPP) electromagnetic waves for the fixed hole diameter D  4 m and the film of the intermediate thickness shows the transition character. The observed tendency for this transition from IR-wave tunneling to waveguide propagation can be related to SPP-enhanced transmission of the micro-hole arrays, as indicated by the off-set magnitudes of the fitting curves, representing the decimal logarithmic value of the calibration coefficient in Eq.(1). For the thickest film (h = 150 nm) this offset equals 7

ACCEPTED MANUSCRIPT to –12.610.01 (slightly less, than the theoretical value  –12.2 from Eq.(1) – about 2.5 times lower in absolute transmittance growth rate), while for the 100-nm thick film its value – 11.930.06 exceeds the latter, apparently demonstrating EOT almost by 2 times higher in its absolute magnitude growth rate. For the thinner films (h = 30 and 50 nm) this SPP-enhanced transmission trend becomes even more pronounced and even dominant both in their slopes and offsets (–7 and –9, respectively). We believe to present the first experimental physical demonstration of such gradual transition from light tunneling to its SPP-enhanced waveguiding

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through tiny holes in metallic screens in the IR-range.

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ii) Diameter effect

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Similarly, the opaque 30-nm Ag film (F, T < 1% in the range of 800-3000 cm-1, Fig. 4C) with its abovementioned SPP-enhanced waveguide-like transmission in the micro-hole grating with diameter D  4 m and period P  6 m was explored versus variable D = 2, 3 and 4 m,

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being another key parameter of the Bethe-Bouwkamp theory – see Eq.(1). In our case, such variation of D resulting not only in prominent increase of the normalized maximal IR-

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transmittance and its slightly red-shifted main peak about  1100 cm-1 (Fig. 4C,D), but also in different trends on the red spectral shoulders of these dependences. In particular, linear fitting of these curves in the double logarithmic coordinates logT-log

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in the low-wavenumber range (Fig. 4D) demonstrates their slopes about 4 for the smaller holes

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(3.900.06 at D = 2 m, 3.890.03 at D = 3 m) in the perfect agreement with the BetheBouwkamp theory, and the transition magnitude 2.970.01 for the larger holes with D = 4 m. Again, the off-set magnitudes of the fitting curves, representing the decimal logarithmic value of

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the calibration coefficient in Eq.(1) for the different hole diameters D = 2, 3 and 4 m, indicate their experimental values considerably larger, than the corresponding theoretical ones from

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Eq.(1) (see the table in the insets in Fig. 4B,D). As a result, absolute transmittance growth rate becomes almost 5 times higher for the smallest holes, about 2 times higher for the medium-size holes and almost 3 orders of magnitude higher for the largest holes (in the transition regime), being apparently related to the abovementioned SPP enhancement effect [21-26]. Comparing to some previous studies [26], the theoretically predicted scaling T  4 [24,25] is unambiguously demonstrated in the low-wavenumber range (the long-wavelength limit) in Fig. 4D for the two hole diameters D = 2 and 3 m, strictly fulfilling the requirement D < 0.3, while the larger holes exhibit the transition regime to the waveguide-like transmission.

iii) Period and material effects 8

ACCEPTED MANUSCRIPT Eq.(2) based on the SPP-mediated transmission model for hole arrays in metal films [22,23] enables to assign the main peaks in the transmittance maximum in Figures 4,6 to different directions in the gratings as 2D photonic crystals. In Fig. 6 such normalized transmittance spectra are presented for different materials (Ag, Al, Cu, Au-Pd alloy) and different grating periods P. In agreement with this equation, the corresponding (1,0), (1,1) and (2,0)-peaks blue-shift monotonously versus decreasing P with their particular experimental spectral positions in the quantitative agreement with the predicted ones, considering the metal-

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dielectric (CaF2, IR dielectric constant CaF2  2 [31]), rather than metal-air (IR dielectric constant air  1), interface of the gratings. Previously, such air-metal interface of nanohole

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arrays was demonstrated as predominating in their transmission [23].

Moreover, since both localized surface plasmons on micro-hole edges/walls and SPP

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propagating on the metal-air and metal-CaF2 film interfaces are considered to be involved in the EOT phenomenon [21-26], it is instructive to evaluate their relative contributions from our

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experimental data for the thin (30 and 50 nm) metallic films, supporting waveguide-like transmission. In this study, our comparative analysis qualitatively indicate that the increasing D

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makes the (1,1)- and (2,0)-peaks in Figure 4C,D less contrast and apparently less pronounced, despite the hole perimeters and squares simultaneously increase at the constant hole number (surface) density in favor of the localized surface plasmons. In this lieu, one can suggest that the

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Ag film stronger corrugated for larger D, provides stronger SPP scattering to reduced its

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enhancement factor in the transmission (Fig. 4C,D). Similarly, in Figure 6 the fixed 4-m hole diameter (perimeter, square) is favorable for increasing amplitudes (contrast) of the (1,1)- and (2,0)-peaks at increasing P, as the corresponding surface density of hole decreases versus P,

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decreasing the surface corrugation.

Finally, the abovementioned SPP enhancement of the IR-transmittance appears differently

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for the micro-hole gratings with the fixed 4-m hole diameter and different similar periods on the films of the diverse metals used in this study. Specifically, the micro-hole gratings on the AuPd alloy film demonstrate considerably – by almost 50% – lower (1,0)-peak transmittance, comparing, e.g., to the similar gratings on the Ag-film (Fig. 6A). Likewise, in the whole series of gratings the 4-m hole diameter with similar periods of 6 and 7 m in Figure 6, the ranking of metals according to (1,0)-peak amplitude is 1) Cu (0.75/0.6), 2) Al (0.7/0.5), 3) Ag (0.45/0.3) and 4) Au-Pd (0.3/0.2), indicating the best plasmonic response for copper and aluminum, the medium one for Ag and the worst one for the Au-Pd alloy (80%/20%), which has indeed main transport characteristics strongly – five-fold – diminished due to the high Pd-impurity concentration [39]. In the case of silver films, their grain-like (island) structure and minor 9

ACCEPTED MANUSCRIPT surface oxidation during low-vacuum (10-2 bar) magnetron sputtering can hamper the usual outstanding plasmonic properties of this metal.

iv) chemo-sensing with mm-sized laboratory samples of micro-structured IR-sensors Chemo-sensing of the produced 44 mm2 gratings of micro-holes (approx. half-million of micro-holes) on the 30-nm thick silver film on the IR-transparent CaF2 substrate (Fig. 7A,B) was investigated in terms of their resonant EOT-enhanced IR-transmission as surface-enhanced IR-

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absorption (SEIRA) [32]. FT-IR transmission spectra of the pristine CaF2 substrate, as-deposited Ag-film and the micro-hole grating (D  4 m, P  6 m) with an analyte – Rhodamine 6G

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(R6G, chemical formula – C28H31ClN2O3, molar mass – 479.01 g/mole) molecules deposited in each case from its 100-pM ethanol solution by dropping one 0.05-ml drop of its 100-pM ethanol

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solution with 51012 molecules, equivalent, on average, to one tenth of monolayer of R6G molecules, was measured in vacuum in the near-mid IR range of spectral wave-numbers  = 400-

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5000 cm-1, using a FT-IR spectrometer V-70 (Bruker). These spectra normalized to the CaF2 transmittance near the cut-off wavenumber  1000 cm-1 are presented in Fig. 7C,D, indicating

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the very minute characteristic fingerprint dips in the spectra in the ranges  = 900-1300 cm-1 and 2800-3000 cm-1, as R6G analytical signals. The extra-large micro-hole array with its SPP-

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enhanced EOT provide strong and robust transmission signals, enabling enlightening studies of the R6G distribution (i.e., potential pre-concentration or dis-concentration effects) and spectral

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characteristics on the array surface owing to different ultrafine physi- and chemisorption interaction with the structured Ag surface and the CaF2 substrate. First, the actual, apparently, inhomogeneous R6G surface coverage was considered,

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accounting for the different hydrophilicity of metallic and CaF2 spots on the CaF2 substrate and the grating. Such internal calibration was performed, considering the waveguide-like

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transmission through the array in the mid-IR range of 2800-3000 cm-1 at the level 0.08 (8%) though the clean grating and 0.069 (6.9%) through the grating with the R6G monolayer. The corresponding transmittance dips of R6G bands are 0.11% (2850 cm-1), 0.35% (2920 cm-1) and 0.2% (2960 cm-1) on the CaF2 substrate, representing combination CH-stretching modes of the main xanthene ring (XR), phenyl, ethyl and methyl groups with the lower-wavenumber external group modes (EGM) [33], but with the overall wave-numbers less than those of C-H stretching overtones (Fig. 7D). On the grating, these R6G absorption bands demonstrated, on average, much – almost 7-fold – lower signals of 0.04% (2850 cm-1), 0.03% (2920 cm-1), while the R6G absorbance at 2960 cm-1 unexpectedly appears even two-fold higher (0.4%). Comparing to the 13-fold stronger IR-transmission through the CaF2 substrate – 92% vs 7% (this 10

ACCEPTED MANUSCRIPT grating), such 7-fold lower R6G absorbance on the grating effectively means its 2-fold, on average, enhancement of R6G absorbance, with the corresponding 26-fold enhancement of the absorption band at 2960 cm-1. Generally, such enhancement could occur due to plasmonic (spectrally homogeneous over the narrow range of 2800-3000 cm-1) or chemical (R6G-Agcomlexing [33], spectrally-selective for the different R6G bonds) factors, which will be discussed below during the analysis of the characteristic absorption bands. Meanwhile, taking the minimal enhancement conditions at 2920 cm-1, 13-fold difference in the R6G absorbance on

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the CaF2 and the grating could be expected over the entire IR spectrum at the absence of the relevant plasmonic and chemical enhancement factors.

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In the striking contrast, the several main characteristic absorption bands of R6G in the range of 900-1300 cm-1 (the strongest one at 1261 cm-1) appear strongly – by 20-40 times –

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enhanced, comparing to its non-enhanced IR-absorption bands on the CaF2 substrate (Fig. 7C). Specifically, in the original FT-IR spectrum (Fig. 7C) and in the normalized one (inset in this

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figure) of the R6G monolayer on the CaF2 substrate there are well-known strong broad bands at 1020 cm-1 (EGM, 0.6%) and 1090 cm-1 (phenyl group, 0.4%), while rather narrow one at

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1262 cm-1 (XR C-O-C stretching, 0.3%). These bands of the polar EGM ethyl/methyl-amino and XR C-O-C groups, as well as of the phenyl group emerge at the visible absence of very strong IR-bands of EGM C-H deformation (1305 cm-1), non-identified EGM vibrations at 1500

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cm-1 and 1528 cm-1, and skeletal stretching vibrations of the external phenyl group (1606 cm-1)

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[33]. Obviously, on the polar (ionic) CaF2 substrate should benefit IR-absorption mainly for the corresponding polar (amino-, C-O-C, C-O and C=O) groups and to less extent for the polarizable (conjugated xanthene ring, phenyl) groups, inducing minor “red” spectral shifts due to small

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bond weakening (cfg., in this work the XR C-O-C stretching band at 1262 cm-1 versus its normal appearance at 1269 cm-1 [33], but no spectral shifts for the Raman-inactive EGM-

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absorption band at 1020 cm-1 and the IR/Raman phenyl group band at 1090 cm-1). In comparison, the grating demonstrates much stronger shifts of these strong broad bands, now appearing at 1025 cm-1 (EGM, 12%) and 1100 cm-1 (phenyl group, 13%), but the narrow one appears at the same position of 1262 cm-1 (XR C-O-C stretching, 13.5%). The latter one is known to participate in Ag-R6G complexing via charge transfer [33], damping the usual high quantum-yield luminescence of R6G, apparently, via charge-transfer of photo-excited electrons. The other groups acquire slightly stronger bonding (the “blue”-shifted broad bands at 1025 and 1100 cm-1 in Fig. 7C and its inset), apparently, because of orientation conjugation of the phenyl group, which is normally twisted nearly perpendicular to XR plane [33], and the EGM (possibly, C-O group) to the -electron system of the planar xanthene ring (possibly, via the metallic 11

ACCEPTED MANUSCRIPT surface). This indicates that these absorption bands are enhanced regarding the other main IRactive absorption bands in R6G not only by the 5-fold SPP-mediated electromagnetic EOT (Fig. 7C, compare to the high-wavenumber plateau), but also by quasi-chemical – d-electron pair donor charge-transfer and image-potential – effects on the Ag surface. Such strong chemical interactions during Ag-R6G complexing could explain the appearance of the predominant combination IR-absorption band at 2960 cm-1 in Fig. 7D. Hence, the general high-wavenumber 13-fold reduction in R6G absorbance in the EOT

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region of 900-1300 cm-1 is (4-6)-fold enhanced by the EOT effect and then by 60-100 times enhanced by the chemical effects on the silver surface (“chemical field enhancement” [34]),

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resulting in the actual analytical enhancement of 20-40 times for the specific polar XR and EGM bands at 1025 cm-1, 1100 cm-1 and 1262 cm-1, as compared to the CaF2 substrate in Fig. 7C.

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In the same line, none of other plasmonic materials – Cu, Al and Au-Pd alloy – exhibited similar SEIRA effect in their gratings with the same structural parameters. This indicates not only the

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crucial and not yet completely understood impact of chemical bonding in analyte molecules (even in the case of the common model analyte R6G), but also the important chemical

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contribution of analyte-metal complexing via charge transfer. Moreover, the evaluated total enhancement factors (300-400 times, as the ratio of the R6G absorbance on the grating in the ranges of 900-1300 cm-1 and 2800-3000 cm-1 – similarly to the CaF2 substrate) are comparable to

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the previously reported values 102-103 [35] (cfg., the theoretically evaluated enhancement up to

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104 [36,37]), where no chemical enhancement effects were accounted for.

4. Conclusion

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In conclusion, DOE-facilitated high spatial multiplexing of femtosecond laser pulses coming at sub-MHz repetition rates was demonstrated to provide, as an emerging single-step

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“green” and low-cost, robust, flexible and competitive mask-less lithographic tool, ultra-high micropatterning rates (up to 25M-holes per second) and mm-sized SEIRA sensors with 1 million periodical arrays of microholes (in general sense, metasurfaces) in 50-nm thick plasmonic silver film, paving the way for advanced IR environmental, chemo- and bio-sensing applications. This technological breakthrough enables not only cost-effective fabrication of advanced optical, acoustic and other functional metasurfaces, but also their accurate characterization in terms of topology- and material-dependent physical responses by common laboratory spectroscopic tools. Additionally, label- and luminescence-free laboratory-scale, robust and high-sensitivity sampling of chemicals and biosamples via plasmonic and chemical contributions, uneven and structurally-sensitive regarding different functional groups of the analyte molecules and band structure of the plasmonic metal, becomes possible in large IR12

ACCEPTED MANUSCRIPT sensing arrays of micro-holes in plasmonic films, with the obvious perspectives for down-scaling of sensing elements for vis-IR surface-enhanced spectroscopies.

Acknowledgements This work was supported by the Russian Science Foundation (project no. 16-12-10165).

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Figures and captions

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Fig. 1. A) Layout of the optical scheme used in these experiments: BS – beam-splitter, L – negative lens, DOE – diffractive optical element, GSM – galvano-scanner mirror, FO – focusing objective or AL – aspherical lens, PC – laptop for laser, galvano-scanner and stage control, EM –

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energy meter, AC – autocorrelator. (B) Phase patterns of the designed DOEs, providing generation of linear arrays of 31, 51 and 101 light spots, and their corresponding numerically

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calculated transverse intensity distributions in the focal plane.

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Fig. 2. (A) Optical images of directly laser-micropatterned 3x3 mm2 arrays on the silver film at f = 500 kHz, v = 7 m/s, filling factor  80 lines/mm (interline separation  12 m), at variable pulse

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energies E = 3 (top), 2.6 (middle) and 2.3 (bottom) J, using the standard F-Theta objective with the 100-mm focal length. (B,C) Top-view SEM images of separate through microholes in the silver

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film the on silica-glass substrate at different focusing NA=0.25 and 0.65, respectively, with the

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corresponding pulse energies indicated in the bottom corners of the images.

Fig. 3. Top-view optical images of through microhole arrays in the 50-nm thick silver film at focusing NA = 0.5, E = 5 µJ, scan velocity V = 7 m/s (direction shown by the arrow) and different repetition rates f = 100 (A), 200 (B) and 500 (C) kHz (no synchronization). Low-, medium and highmagnification images are shown from the left to the right, the scale bars are 500 µm, the bilateral arrow 18

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frequency mechanical vibrations.

Fig. 4. Normalized transmittance spectra of bare (F) and perforated (gratings G, hole diameter and period – 4 and 6 m, respectively) Ag films with the thickness of 30, 50, 100 and 150 nm

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(top row), bare and perforated (hole diameters D  2, 3 and 4 m at P  6 m) 30-nm Ag film (bottom row), on the CaF2 substrate in normal (A,C) and log-log (B,D) coordinates (in the latter

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case – with their linear fitting curves at the low-wavenumber shoulder). The reference spectrum of the CaF2 substrate is given for comparison. Insets: (A) top-view SEM images of the gratings on the 30, 50-, 100- and 150-nm thick Ag films and (C) on the 30-nm Ag film, on the CaF2 substrates (top to bottom, scale bar – 10 m).

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Fig. 5. (left)10-keV EDX elemental profiles on Al, Ag and Cu microholes arrays (frame size – 710 m) and their spectra; (right) Content of main elements (C,O) at different positions across

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the arrays according to their 10-keV EDX mapping.

Fig. 6. Normalized transmittance spectra of micro-hole gratings (G, D  4 m) on the 50-nm thick Ag (A), Al (B), Cu (C) and Au-Pd alloy (D) films on the CaF2 substrates with the variable periods (shown by the same colors as the corresponding spectra), the colored numbers showing the spectral positions of their (1,0), (1,1) and (2,0)-peaks and the red dashed lines showing their evolution versus P. Insets: top-view SEM images of the gratings with periods shown in microns in the frames (scale bars can vary). 20

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Fig. 7. (A) Optical image (general view) of the 11-mm wide CaF2-slab with the top 30-nm thick

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Ag film and a number of micro-hole gratings (typical square  44 mm2). B) Top-view SEM image of the grating with D = 4 m, P = 6 m (inset: its magnified view). C) Normalized FT-IR

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transmission spectra of the grating with D = 4 m, P = 6 m (G, (1,0),(1,1) and (2,0) – resonance transmission peaks) and of the grating with the R6G monolayer atop (G+R6G); FT-IR transmission spectrum of the CaF2 substrate with the R6G monolayer is given for comparison. Inset: the magnified view of their normalized low- transmittance with the assignment of the R6G absorption bands on the CaF2 substrate and the grating (see the different top and bottom scales) discussed in details in the text. D) Magnified view of their normalized high- transmittance in the overtone region (CH-OV – C-H-stretching overtones) with the assignment of the overtone vibrations (see the details in the text).

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 Fs-laser beam multiplexing by diffractive optical elements enabled highthroughput printing of plasmonic microholes  Large-scale microhole arrays exhibit a transition from plasmon-enhanced IR transmission to wave-guide-like one  Large-scale microhole arrays demonstrate surface-enhanced IR transmission and chemosensing

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