Multipitched Diffraction Gratings for Surface Plasmon Resonance

Article pubs.acs.org/ac

Multipitched Diffraction Gratings for Surface Plasmon ResonanceEnhanced Infrared Reflection Absorption Spectroscopy Joseph W. Petefish and Andrew C. Hillier* Department of Chemical and Biological Engineering, Iowa State University, Ames, Iowa 50011, United States S Supporting Information *

ABSTRACT: We demonstrate the application of metalcoated diffraction gratings possessing multiple simultaneous pitch values for surface enhanced infrared absorption (SEIRA) spectroscopy. SEIRA increases the magnitude of vibrational signals in infrared measurements by one of several mechanisms, most frequently involving the enhanced electric field associated with surface plasmon resonance (SPR). While the majority of SEIRA applications to date have employed nanoparticle-based plasmonic systems, recent advances have shown how various metals and structures lead to similar signal enhancement. Recently, diffraction grating couplers have been demonstrated as a highly tunable platform for SEIRA. Indeed, gratings are an experimentally advantageous platform due to the inherently tunable nature of surface plasmon excitation at these surfaces since both the grating pitch and incident angle can be used to modify the spectral location of the plasmon resonance. In this work, we use laser interference lithography (LIL) to fabricate gratings possessing multiple pitch values by subjecting photoresist-coated glass slides to repetitive exposures at varying orientations. After metal coating, these gratings produced multiple, simultaneous plasmon peaks associated with the multipitched surface, as identified by infrared reflectance measurements. These plasmon peaks could then be coupled to vibrational modes in thin films to provide localized enhancement of infrared signals. We demonstrate the flexibility and tunability of this platform for signal enhancement. It is anticipated that, with further refinement, this approach might be used as a general platform for broadband enhancement of infrared spectroscopy.

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resonances, which allow more sensitive SPR detection and imaging.14,15 SPR and infrared spectroscopy can be combined to provide complementary information. For example, SPR, which is routinely applied for thickness measurements, and infrared reflection−absorption spectroscopy (IRRAS), which is commonly used to deduce the structure and composition of ultrathin films,16−18 can be combined to provide both outputs in a single measurement. For example, we have previously demonstrated how grating-coupled SPR and IRRAS may be used to determine composition and thickness of alkanethiolate monolayers on gold.19 SPR and infrared spectroscopy can also be combined synergistically in order to enhance measured signals. Surface enhanced infrared absorption (SEIRA) is a version of infrared spectroscopy in which vibrational signals are enhanced by coupling thin films to the surface enhanced electric field associated with plasmon excitation.20−22 This technique has largely used localized surface plasmon resonance (LSPR) at discrete metal nanostructures to amplify vibrational signals.23,24 Collections of metallic nanoscale particles,25 shells,26 antennae,20,27 split-ring resonators,28 and metal-island films24 have all

urface plasmon resonance (SPR) is a widely applicable, label-free, analytical technique commonly used to characterize heterogeneous reactions, binding events, and film formation near metal surfaces.1−5 SPR is also a flexible experimental platform as a light source can be coupled to surface plasmons in various ways, including via metal-coated diffraction gratings, waveguides, prisms, and nanoparticles.3,4 While grating-coupled SPR sensors are typically less sensitive6 and less frequently applied3 than prism-based couplers, they do provide several practical advantages. Diffraction gratings can be inexpensively produced by stamping or injection molding of plastics. Indeed, commonplace items such as digital versatile discs (DVDs) and compact discs (CDs) have served as the basis for SPR grating couplers.7−9 Diffraction grating couplers also provide an inherent tunability via modification of the grating period and angle of incidence.3 In addition, multiple analytical signals can often be obtained from one grating coupler by making use of various diffracted orders.10,11 Although the bulk of SPR research has been performed on systems at visible and near-infrared wavelengths, the full range of the infrared spectrum offers many intriguing avenues for sensor development and spectroscopy. Advantages of SPR at infrared wavelengths include that the optical properties of SPRsupporting metals in this spectral region produce long plasmon decay lengths suitable for probing environs as large as living cells.12,13 These same optical properties yield sharper © 2015 American Chemical Society

Received: June 17, 2015 Accepted: October 12, 2015 Published: October 12, 2015 10862

DOI: 10.1021/acs.analchem.5b02864 Anal. Chem. 2015, 87, 10862−10870

Article

Analytical Chemistry

onto clean glass slides at 4,000 rpm for 60 s using a commercial spin coater (model WS-650MZ-23NPP, Laurell Technology Corp., North Wales, PA, USA). Samples were placed on a hot plate at 90 °C for 1 min and then cooled in a stream of compressed air. The photoresist-coated slides were then placed in a custom-built Lloyd’s mirror apparatus, as described in a previous publication.30 The mirror and sample holder were mounted on a computer-controlled rotation stage (model PRM1Z8, ThorLabs), which allowed for control of the interferometer angle to within ∼0.001°. Spatially filtered light from a 40 mW, 405 nm diode laser (Oxxius, Lannion, France) was directed incident to the Lloyd’s mirror in order to create a sinusoidal interference pattern into the photoresist layer. Up to three additional grating periods were transferred to the photoresist by subjecting the samples to subsequent exposures at different interferometer angles. Scheme 1 illustrates the

been demonstrated as SEIRA substrates. More recently, grating-coupled SEIRA has been demonstrated.29 This configuration has been further developed to exploit the angletunable nature of grating couplers to enhance multiple vibrational modes with a set of sinusoidal grating couplers.30 Another of the distinct advantages of grating couplers in SPR techniques is the potential for sensing through multiple, simultaneous signals. This advantage can be realized through analysis of multiple diffracted orders or through peaks of the same order arising from different periodicities or pitch values in the grating topology. For example, a grating having three constituent harmonics was fabricated via laser lithography and used to simultaneously monitor changes in the background refractive index, film thickness, and film refractive index during multilayer protein growth.31 Two plasmons have also been excited simultaneously in the visible spectrum by a molded grating topped with a metal−dielectric−metal layer structure.32 A chirped diffraction grating coupler having varying pitch and amplitude over its length has also been previously reported.33 Complex grating structures have been constructed using various approaches. For example, e-beam lithography has been utilized to fabricate quasiperiodic gratings with multiple resonances in the visible and near-IR regions.34 In this report, we demonstrate the capability of multipitched gratings as couplers in SEIRA measurements. We describe the use of Lloyd’s mirror laser interference lithography to produce multipitched diffraction gratings.35−37 By repetitive dosing of a photoresist layer at varying interferometer angles, a grating height profile corresponding to a superposition of sine waves can be created. Careful control of the Lloyd’s mirror angle using a motorized rotation stage allows for creation of multiple desired pitch values on one grating. This leads to the appearance of multiple SPR resonances in desired locations in the infrared spectrum. Optical diffraction measurements and atomic force microscopy (AFM) are used to quantify the presence of multiple periodicities in the fabricated gratings. IRRAS spectra of silver-coated gratings demonstrate precise control of the plasmon peak position for two possible applications: widely spaced grating periods for simultaneous interrogation of multiple spectral regions and closely overlaid pitches to produce a broader plasmon signal within a given spectral region. Both applications can be used as strategies to provide broadband enhancement in infrared reflection measurements.

Scheme 1. Schematic of Lloyd’s Mirror Interferometer Illustrating Use of Multiple Exposure Angles To Superimpose Wavefronts To Create Multipitched Gratings

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EXPERIMENTAL SECTION Materials and Reagents. Toluene and glass microscope slides were purchased from Fisher (Waltham, MA, USA). Microposit S1813 photoresist and Microposit 352 developer were from Rohm and Haas Electronic Materials LLC (Philadephia, PA, USA). A UV-curable optical adhesive (NOA 81, Norland, Cranbury, NJ, USA) and polydimethylsiloxane silicone elastomer kit (SYLGARD 184, Dow Corning, Midland, MI, USA), were used per manufacturers’ instructions. Silver wire (99.995%, 0.008 in. diameter) and tungsten wire evaporation baskets were obtained from Ted Pella (Redding, CA, USA). Poly(methyl methacrylate) (PMMA, 120,000 MW) was from Aldrich (St. Louis, MO, USA). Deionized water with resistivity greater than 18 MΩ·cm (NANOPure, Barnstead, Dubuque, IA, USA) was used in sample preparation. Fabrication of Multipitch Gratings. Multipitch diffraction gratings were fabricated on glass slide substrates by laser interference lithography. A layer of S1813 photoresist was spun

process for an example with two exposures. Subsequent exposure doses were necessarily larger than the first dose, as the resist becomes partially transparent upon initial exposure. Exposed samples were removed from the Lloyd’s mirror and post baked at 110 °C for 3 min, cooled with laboratory air to room temperature, and developed using Microposit 352 developer. After developing the photoresist, the gratings were 10863


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solution at a spin rate of 1000 rpm and a 715 nm thick PMMA coated from a 8 wt % solution at a spin rate of 1500 rpm. Infrared Reflection Absorption Spectroscopy. IRRAS spectra were acquired with a Fourier transform infrared spectrometer (FTIR; Nicolet MAGNA 750, Thermo Scientific) and attached variable-angle specular reflectance accessory (Pike VeeMax III, Pike Technologies, Madison, WI, USA). This accessory was employed to collect reflectance spectra at incident angles ranging between 50° and 85° with both sand p-polarized light. Light polarization was controlled with a ZnSe polarizer. IRRAS spectra were obtained for both bare silver and for PMMA-coated gratings relative to a flat silver surface at the same incident angle and polarization state. All spectra represent the average of 200 scans. Select grating samples were further characterized by performing IRRAS with the grating direction both perpendicular and parallel to the incident radiation.

rinsed with DI water and blown dry in a stream of compressed air. The interferometer laser power output did not fluctuate more than 3% over the course of sample fabrication. Optical Diffraction Measurements. Optical diffraction measurements were performed using a 5 mW HeNe laser. Transmission of the laser light through the gratings produced diffraction patterns, which were used to confirm the formation of desired multipitched grating surfaces and to estimate the pitch of each periodicity. The basic apparatus consisted of a polarized HeNe beam directed incident to the grating surface and a screen of cardstock, onto which the diffracted spots were projected (Figures S1A,B, Supporting Information). A digital camera (model DCC1545M, Thorlabs) was used to capture and digitize images of the diffraction patterns. The details of the arrangement varied depending on the spacing of the multiple grating periods. For widely varying pitches on the same grating, the screen was placed 0.5 in. behind the grating, and the −1, 0, and +1 diffracted orders were imaged on the back side of the cardstock,. For smaller pitch differences, a longer travel distance was required to resolve the +1 order diffracted peaks corresponding to each period. In this case, a 32 in. separation existed between the grating and the screen, and a camera recorded the diffraction pattern reflected from the front of the screen. Collected images were analyzed using ImageJ. The separation of all first order peaks from the 0 order transmission was measured at a horizontal distance of 32 in., which was then used to calculate the corresponding pitch value (in nanometers) for each spot. Atomic Force Microscopy. Height profiles of the grating structures were obtained via atomic force microscopy using a Dimension 3100 scanning probe microscope in conjunction with a Nanoscope IV controller (Veeco Metrology, LLC, Santa Barbara, CA, USA). Scanning was performed in tapping mode using Si TESP7 AFM tips (Veeco Metrology) with a spring constant of ∼79 N/m and resonance frequency of ∼255 kHz. Grating Replication and Silver Coating. After characterization of the as-fabricated gratings by AFM and optical diffraction, a silicone elastomer kit (Sylgard 184, Dow Corning) was employed to transfer the grating topology to a PDMS master. Replicas of each grating were then fabricated from this PDMS master by placing a drop of optical-grade, UV-curable polymer (NOA 81, Norland) between the PDMS surface and a clean glass slide and irradiating the assembly with a UV lamp for 20 min. The newly formed gratings in UV-curable polymer, which are optically clear, released easily from the PDMS template, which allowed the same grating structure to be reproduced multiple times. A film of silver with a thickness ∼ 160 nm was then evaporated onto the UV-curable polymer gratings at a rate of 1−1.5 Å/s (Denton Benchtop Turbo III, Denton Vacuum, LLC, Moorestown, NJ, USA). The silver film thickness was monitored in situ using a quartz crystal resonator (model XTM/2, Inficon). Spin Coating of Pol(methyl methacrylate) Films. Poly(methyl methacrylate) films were spun out of various weight percent solutions in toluene onto the silver-coated gratings. A trial-and-error approach, with some guidance from literature,38 was employed to elucidate the necessary solution concentration and spin speed, ranging between 500 and 2000 rpm, for producing an appropriate PMMA film thickness. After coating, the grating samples were held in a −10 psig vacuum oven at room temperature for several hours to remove any surplus toluene from the PMMA film. Specific films used in this work included a 262 nm thick PMMA coated from a 4 wt %

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RESULTS AND DISCUSSION Two different grating structures were fabricated to test the utility of this multipitched grating format. Gratings possessing

Figure 1. (A) Optical diffraction patterns for gratings formed from (i) one, (ii) two, and (iii) three sequential exposures. All samples were exposed first for 15 s at 11.67°; gratings ii and iii were further exposed for 45 s at 9.31°, and grating iii was subsequently exposed for 25 s at 6.97°. (B) Intensity profiles (vertically offset for clarity) taken from the diffraction patterns in A. 10864


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Figure 3. p-Polarized IRRAS spectra collected at 80° vs flat silver mirror reference for silver-coated gratings i, ii, and iii showing one, two, and three distinct plasmon resonance peaks. Spectra represent the average of 200 scans.

⎧ 2π 2π 2π = ±Re⎨ nd sin θ + m λ Λ ⎩ λ ⎪

⎪

ϵdϵm ⎫ ⎬ ϵd + ϵm ⎭ ⎪

⎪

(1)

where λ is the light wavelength, nd is the dielectric medium refractive index, θ is the incident angle measured relative to the surface normal, m is the diffracted order, Λ is the grating pitch, and ϵd and ϵm are the permittivities of the dielectric medium and the metal.3 In the infrared spectral region, with a silver−air interface, the permittivity of silver is much larger than that of air (εm ≫ εd). Thus, eq 1 can be simplified to give the following approximation: sin θ + m

λ = ±1 Λ

(2)

Equation 2 can then be solved to estimate the grating pitch (Λ) required to produce a plasmon peak at a specific wavelength (λ) and angle of incidence (θ). Generally, the −1 diffracted order is the strongest and most relevant for reflectance measurements. Subsequently, this pitch value, diffracted order, and the wavelength of the interferometer laser source, λint, can be inserted into the equation describing the Lloyd’s mirror interferometer (eq 3) to determine the required interferometer angle, θint:39

Figure 2. (A) AFM height profiles (vertically offset) of gratings i, ii, and iii. (B) Fast Fourier transform modulus for AFM profiles in A, corresponding to gratings i, ii, and iii.

several, widely spaced pitch values were first created in order to demonstrate the ability to produce targeted, multiband plasmon resonances. The second construction involved the fabrication of multiple, tightly spaced pitch values, where the resulting plasmon resonances were located near enough to one another to overlap in the infrared spectrum. This was done in an attempt to demonstrate broadband enhancement for applications in SEIRA. Gratings with Multiple Simultaneous Pitch Values. Several target pitch values for each grating were determined prior to fabrication, with the goal of placing plasmon resonances at desired spectral locations in the infrared spectrum. This was achieved through consideration of the SPR matching condition for a grating coupler:6

⎛λ ⎞ θint = sin−1⎜ int ⎟ ⎝ 2Λ ⎠

(3)

As an example of a multipitched grating with clearly separated surface plasmon peaks in the infrared spectrum, we chose to target wavenumbers of 5000, 4000, and 3000 cm−1, corresponding to desired grating pitch values of ∼1007, 1260, and 1679 nm at 80° incident angle according to eq 2. With a 405 nm laser wavelength, estimated interferometer angles from eq 3 were 11.59°, 9.25°, and 6.92°, respectively (Table S1, 10865


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Figure 4. (A) Optical diffraction patterns of +1 order for three different gratings formed from four sequential laser exposures with 0.2° (grating 1), 0.1° (grating 2), and 0.05° (grating 3) interferometer angle steps. (B) Intensity profiles taken from the diffraction patterns in A.

Supporting Information). Gratings were then fabricated using one (sample i), two (sample ii), and three (sample iii) overlapping pitch values. In each case, after exposure and developing, the visually apparent grating structures were initially characterized by optical diffraction. Figure 1A shows transmitted diffraction patterns for gratings i, ii, and iii, having been exposed at one (11.59°), two (11.59° and 9.25°), and three (11.59°, 9.25°, and 6.92°) interferometer angles, respectively. For all three gratings, the central spot corresponds to the zero order or directly transmitted light peak. Diffracted signals for −1 and +1 orders are visible to the left and right sides of the central spot. Gratings ii and iii exhibit multiple −1 and +1 order peaks, corresponding to multiple periods in the grating structure. All three gratings were first exposed for 15 s at 11.59°; gratings ii and iii also received a subsequent 45 s dose at 9.25°, with grating iii also subject to a third 25 s exposure at 6.92°. An iterative bracketing process was used to select exposure times for second and subsequent doses in order to achieve similar diffraction efficiencies across the multiple peaks. Areal pixel intensity profiles of the photographs in Figure 1A appear in Figure 1B, which were used in said iterative process. It was apparent that the photoresist absorptivity at 405 nm decreased by at least 50% of the original value after the first exposure. Thus, longer exposure times for subsequent exposures were required to achieve similar diffraction efficiencies compared to peaks from the original dose. In

Figure 5. p-polarized IRRAS spectra at a range of incident angles for silver-coated four-pitch gratings. Interferometer angle steps used in grating fabrication are (A) 0.2°, (B) 0.1°, and (C) 0.05°. Spectra represent the average of 200 scans.

addition to the photographs of the diffraction patterns at ∼0.5 in. from the grating, the lateral displacement of the +1 order peaks was measured at a distance of 32 in. from the grating to calculate the pitch associated with each spot. In summary, these interferometer angles produced pitch values close to those desired for gratings i, ii, and iii (Table S1, Supporting Information). The surfaces of gratings i, ii, and iii were also characterized using AFM, with profiles of each appearing in Figure 2A. 10866


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Figure 7. p-polarized IRRAS spectra at a range of incident angles for silver-coated four-pitch gratings with a 715 nm overlayer of PMMA. Incident light is oriented perpendicular to the grating direction. Interferometer angle steps used in grating fabrication are denoted for each set of spectra. Spectra represent the average of 200 scans.

Figure 6. p-polarized IRRAS spectra at 65° incident angle for silvercoated four-pitch gratings with a 262 nm top layer of PMMA. Spectra were acquired with the infrared beam at orientations perpendicular (perp) and parallel (par) to the grating direction. Interferometer angle steps used in grating fabrication are (A) 0.2°, (B) 0.1°, and (C) 0.05°. Spectra represent the average of 200 scans.

characteristic of the superposition of multiple constituent harmonics. The fast Fourier transform (FFT) modulus was calculated for each of the height profiles in Figure 2A, and the

Grating i displays a clear single period, whereas the surface profiles of gratings ii and iii demonstrate a convoluted pattern 10867


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these three different samples were consistent with this intention (Table S2, Supporting Information). In order to demonstrate the impact of these closely spaced pitch values on the resulting surface plasmon peak locations and characteristics, p-polarized IRRAS spectra were acquired at a variety of incident angles ranging from 60° to 80°. Results for silver-coated gratings having the aforementioned 0.2°, 0.1°, and 0.05° interferometer angle increments are shown in Figure 5A− C. For each sample and incidence angle, different responses are observed that transition from individually identifiable peaks for the 0.2° step grating (Figure 5A) to a single, broad peak for the 0.05° step grating (Figure 5C). For the grating with the 0.2° angle increment, it is possible to distinguish four distinct surface plasmon resonance peaks (Figure 5A) at most incident angles. These four resonance peaks are quite clear at 80°, and become broader at lower angles. The 48 nm difference between the four pitch values for this grating results in plasmon peaks that are separated by ∼80−90 cm−1. Also apparent by inspection in Figure 5A is the shift of the plasmon responses to lower wavelengths (higher wavenumbers) with decreasing incident angle, as predicted by eq 2. In contrast, four individual resonance peaks are not distinguishable in the grating created with the finer 0.05° angle increment (Figure 5C). The 12 nm difference between pitch values for this grating creates plasmon peaks with a separation of ∼25 cm−1, which are so close that they merge into one observable peak in this measurement. The 0.1° angle step grating displays behavior intermediate between the two extremes, with some evidence of distinct peaks at higher incident angles (Figure 5B). In all of these gratings, a broad response is observed, with closely placed plasmon resonances providing a set of peaks with varying degrees of overlap. SEIRA with Multipitched Grating Couplers. In order to test the utility of these closely spaced surface plasmons for enhancing the infrared signal of an adsorbed film, these gratings were coated with PMMA to a thickness of ∼262 nm. At this thickness, the plasmon and C−H stretching modes of the polymer film overlap. Figure 6 depicts p-polarized IRRAS spectra of the silver-coated, multipitched gratings with this PMMA overlayer. For each grating, the spectrum is shown for 65° incidence angle with the grating direction both perpendicular (plasmon excited) and parallel (no plasmon excitation) to the incident light source. For the parallel configuration, the C−H vibrational signal is evident in the spectrum, but with a magnitude of ∼0.05 reflectance units, which is typical for this film thickness. In all three of the multipitched gratings, there is a significant enhancement of the C−H signal when the sample is rotated to the perpendicular orientation. At this orientation the plasmon resonance is excited and appears at a wavenumber range that overlaps with the C−H peak. The magnitude and quality of the enhancement varies in each case. The 0.2° step sample shows the largest magnitude peak, with an increase of peak intensity to nearly 0.5 reflectance units, which is approximately 10-fold larger than the plasmon-free peak. The 0.1° and 0.05° step samples also exhibit enhanced peaks with an increase of approximately 6 and 4 times, respectively. In terms of signal quality, the enhanced C− H peak on 0.1° and 0.05° samples appears as a multiple of the unenhanced peak, while the 0.2° sample exhibits additional fine structure, including additional peaks at 2750 and 3150 cm−1. These additional peaks are likely from individual plasmon peaks in the substrate as seen in Figure 5A for the uncoated sample.

frequency domain translated to distance, as displayed in Figure 2B. Consistent with the surface profiles, the Fourier spectra for gratings i, ii, and iii show one, two, and three major maxima, respectively. These results confirm the formation of single, double, and triple overlapping periodicities in the grating surfaces. Gratings i, ii, and iii were then subject to the replication and silver-coating process as detailed in Experimental Section. After silver coating, p-polarized IRRAS spectra were obtained for the sample gratings at 80° (Figure 3). Sample i exhibits one peak, sample ii exhibits two peaks, and sample iii exhibits three peaks. Notably, the peak positions within the infrared spectra closely match the targeted values of 3000, 4000, and 5000 cm−1. Also of note is the consistency of the placement for peaks common to multiple gratings; the wavenumber position of the targeted 5000 cm−1 peak is 5082 cm−1, 5125, and 5068 cm−1 for gratings i, ii, and iii.. A similar observation holds for the 4000 cm−1 peak on gratings ii and iii, with values of 4059 and 4046 cm−1, respectively. The ability to use eqs 2 and 3 to accurately and consistently predict plasmon peak placement and guide grating fabrication allows for efficient construction of desired, individually focused, multipitched grating structures. Gratings with Multiple, Closely Spaced Pitch Values. Multipitch gratings having several finely separated peaks were also constructed and characterized in an attempt to produce a broad SPR signal in the infrared spectrum comprised of several overlapping individual resonances. This response could be applicable in various applications, including simultaneous probing of wider vibrational modes or clusters of narrow modes in the spectra of analyte thin films, as well as for SEIRA applications. For these experiments, three different gratings were fabricated by quadruple exposure of the photoresist at angles separated by four, equally spaced steps of either 0.2°, 0.1°, or 0.05° with a base pitch Λ in the range of 1600−1700 nm. AFM height profiles for these three gratings were acquired (Figure S2, Supporting Information). In the case of these finely separated pitch values with 15−50 nm differences between adjacent periods, it proved difficult to resolve distinctions between the periods either by visual observation or by examining the FFT spectra of the height profiles. Nevertheless, the presence of a combination of pitch values is indicated by the nonuniformity of the surfaces profiles. To verify the nature of these diffraction surfaces, we relied on optical diffraction measurements as described earlier. Further characterization of these gratings was achieved by optical diffraction. A larger separation between the grating and the detector (Figure S1B, Supporting Information) was used to resolve individual first order diffraction peaks. Figure 4A depicts images of the +1 order diffraction maxima for three different four-pitched gratings fabricated with 0.2°, 0.1°, and 0.05° angle steps. Intensity profiles for these images (Figure 4B) show four distinct pitch values. The spacing between peaks decreases by a factor of approximately 2 with each halving of the interferometer angle step increment when going from 0.2° to 0.1° and then 0.05° angle steps. These interferometer angle increments create pitch values with separations of approximately 48, 24, and 12 nm for the three gratings, respectively. All gratings in this set were exposed so as to produce pitch values centered around 1700 nm, which would yield SPR resonances located near 3000 cm−1 in the infrared spectrum at an incident angle of 80°. These specific exposure angles and measured pitch values as determined by optical diffraction for 10868



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Notably, the 0.1° and 0.05° samples, having more finely spaced plasmon peaks, do not exhibit this additional fine structure. In order to more clearly observe the shape of these broadband or overlapping plasmon signals at several incident angles, a thicker (∼715 nm) PMMA film was deposited on an additional set of silver-coated gratings. In this way, the overlaid resonances were translated to a region beyond the diffracted −1 order reflection and the PMMA vibrational modes around 3000 cm−1, as shown in Figure 7. As expected, the grating made from four 0.2°-separated exposures created the broadest signal, with a fwhm of ∼400 cm−1 at 70° incidence. Values for the 0.1° and 0.05° angle step gratings at corresponding conditions were ∼180 and ∼120 cm−1, respectively. Differences in signal shape were also readily apparent, with the most widely separated pitches in the 0.2° case clearly composed of four plasmon peaks; the 0.1° angle step sample shows a doublet of peaks, while the most closely spaced pitches in the 0.05° angle step grating appear almost as a single resonance. For ease of analysis as a broadband sensor, it is often desirable to have a broad enhanced signal in the shape approximating a rectangular or trapezoidal notch: a relatively flat top with no prominent peaks or troughs would be ideal. The spectra for the 0.1° angle step grating appears to come closest to ideal with regard to this criterion. The diffraction efficiency and plasmon resonance height is strongly dependent on the amplitude corresponding to each pitch in the grating topology; refinement of the sensor response could be performed by careful adjustment of the exposure times for each fabrication sequence.

AUTHOR INFORMATION

Corresponding Author

*Tel.: 515-294-3678. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grants CHE 0809509 and 1213582).

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REFERENCES

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CONCLUSIONS Surface enhanced infrared absorption (SEIRA) is a useful analytical technique for amplifying signals in IR spectroscopy. Previous advances in SEIRA largely utilized nanoparticle-based surface plasmons to generate increased electromagnetic fields and enhanced absorption in analyte vibrational modes. Recent work has reported SEIRA using grating-coupled propagating surface plasmons, with responses that are adjustable via the pitch of the grating and angle of the incident light. In this report, we used a Lloyd’s mirror interferometer and multiple exposure angles in the construction of multiband as well as broadband grating couplers having multiple pitches. Multiband SPR responses were designed in such a way that a silver-coated grating produced plasmon peaks at 3000, 4000, and 5000 cm−1. Four interferometer exposures at small angle steps were employed to fabricate broadband sensors featuring a wide response comprised of four overlaid plasmon resonances. We expect that these developments may prove useful for simultaneous enhancement of vibrational modes in multiple regions of the infrared spectrum and for concurrent enhancement of several peaks within an interval of several hundred wavenumbers, respectively.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02864. Laser diffraction transmission imaging apparatus, AFM height profiles, and interferometer angles and corresponding pitch values (PDF) 10869


Article

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Multipitched Diffraction Gratings for Surface Plasmon Resonance

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