Open Access
Add to BibSonomy
Abstract
Plasmonic crossed surface relief gratings were fabricated using interference lithography. Their topographies were studied by AFM as a function of laser exposure time and their surface plasmon resonance at a gold-air interface was measured between crossed polarizers in transmission and in reflection modes. Both modes resulted in emitted plasmonic light at specific wavelengths related to the grating pitch, with the reflectance SPR having a much higher intensity than the transmittance SPR. The use of these gratings as plasmonic sensors was examined and their sensitivities were measured in the reflectance and transmittance modes to be 601 nm/RIU and 589 nm/RIU, respectively.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Surface relief gratings (SRGs) are linear periodic nano/microstructures that can be inscribed in photosensitive azobenzene thin films through the interference of polarized laser beams. The cyclic trans-cis photoisomerization of the azobenzene chromophores, which occurs at temperatures well below the glass transition temperature of the material, is key to the microscopic mass transportation and molecular motion of the azobenzene molecules which allow the formation of controllable surface features. The pitch of the SRG is dependent on the laser beam incidence angle and its wavelength, and it can either be constant or chirped throughout the grating surface. The grating modulation depth is a function of the laser irradiance, the azo-film thickness and the laser exposure time [1–11].
SRGs can be coated with a thin metal layer that takes the shape of the grating, and, due to their nanoscale nature, they can be used to excite surface plasmon resonance (SPR) when incident light is polarized parallel to the grating vector. Using white light, a single-pitch linear metallic grating can excite a narrowband SPR that is observed as a positive peak in transmission in addition to the transmitted white light spectrum. This narrowband enhanced transmission occurs because the resonant free electrons in the metal increase the transparency of the metallic layer around the SPR wavelength. Since energy is conserved, a corresponding negative peak is observed in reflection from the same single-pitch linear grating [8,12–23].
Several different geometries of SRGs have been reported thus far such as linear, superimposed, crossed and circular grating patterns [14,16–18,24–28]. One of those SRG geometries is the crossed surface relief gratings (CSRG) which is made by orthogonally superimposing linear SRGs during the fabrication process through sequential laser beam inscription on the azo-film. Thus, a CSRG has two orthogonal grating vectors (Kx, Ky) and can excite SPR using either vertically or horizontally polarized light. CSRGs have attracted attention in various applications including SPR-based sensors because they allow for a unique plasmonic energy transfer in-between the orthogonal gratings [14,18,24,25]. When a metallic CSRG is placed in between crossed polarizers, a SPR will be excited by one of the orthogonal in-plane gratings and then, the SPR light will be re-emitted by the other orthogonal grating in a perpendicular polarization state to that of the incident light. This phenomenon allows for the transmission of only the SPR-exchanged light, while completely eliminating all the remaining background light [24]. However, the light polarization and intensity was never analyzed in reflection off CSRGs since SPR signals in reflection have always been observed in the past as negative peaks in the overall reflected light spectrum, and thus deemed non-excitant for CSRGs that are placed in between crossed polarizers. Furthermore, optimization studies on the effect of the relative inscription times of the two orthogonal gratings in a CSRG with respect to their modulation depths were never conducted in the past.
The plasmonic excitation due to polariton enhancement at the metal-dielectric interface of SRGs has been widely utilized in the past as an effective tool in precise, tunable, real-time and label-free biosensing applications [12,13,22]. For instance, Nair et al. [25], reported the fabrication of a CSRG biosensing platform with a bulk sensitivity of 613 pixel intensity unit (PIU)/refractive index unit (RIU) that can detect E. coli bacteria in ranges of approximately 100 CFU/mL.
So far, the occurrence of SPR in metallic CSRGs that use collinear optics were only studied in transmission mode (T-SPR) [14,18,24,25]. However, in comparison with the Kretschmann and Otto configurations, which use total internal reflection to excite SPR, a collinear reflection-SPR (R-SPR), such as the one presented in this work, could have many potential applications in optics, optoelectronics, imaging and sensing. Here, we investigate the formation of CSRGs and their R-SPR and T-SPR behavior, as well as test their bulk sensitivities in both modes.
2. Experimental
Solutions of Disperse Red 1 azobenzene molecular glass (gDR1) were prepared with concentrations of 3 wt% in Dichloromethane (CH2Cl2), as described elsewhere [29]. They were mechanically shaken for 1 hour and then filtered through a 0.45-µm syringe filter. Approximately 70 µm of the gDR1 solution was spin-coated on clean Corning 0215 glass microscope slides (38×38 mm2) at 1050 rpm for 30 s. This yielded solid thin films with a 280-nm thickness, as measured using a Dektak XT surface profiler. Some samples were spin-coated twice using the above parameters, yielding 480 nm gDR1 films. The films were then dried in an oven at 75 °C for 15 min.
A diode-pumped solid-state laser (Coherent, Verdi V6, λ = 532 nm) was used to inscribe constant-pitch linear SRGs. The gDR1-coated glass slides were irradiated by two interfering beams from the laser, one directly incident, one reflected upon a Lloyd’s mirror placed orthogonally to the sample. The circularly polarized incident beam was collimated, expanded and passed through a variable iris as described elsewhere [18]. The grating pitch was controlled via a rotating sample holder assembly, by fine-tuning the angle of incidence of the laser on the sample. Grating pitches of 550 nm and 650 nm were chosen for the fabricated gratings in this work. This ensured that the SPR would be confined to the visible spectrum at an air/gold interface and that the transmission of the SPR signal through the sample is unaffected by the absorption of the azobenzene film (λmax,Abs=476 nm).
Once a linear grating was inscribed on a gDR1 film for a specific time (t1=250 s), the sample was rotated 90 degrees for a second exposure (t2) in order to fabricate a CSRG having an identical pitch to that of the first exposure. In this study, the first exposure time was held constant at 250 s while the second exposure time was changed to vary the relative modulation depths of the orthogonal gratings. The Verdi laser irradiance was kept constant at 365 mW/cm2 for all exposures.
The modulation depths of the resulting CSRGs, which had a total surface area of approximately 2 cm2, were measured using a Bruker Dimension Edge Atomic force microscope (AFM). The 3-dimensional grating profiles from a random 5 × 5 µm2 area were scanned by the AFM and analyzed using the Bruker Nanoscope analysis software. Two distinct modulation depths were determined for each CSRG, corresponding to the two perpendicular grating vectors. The first depth (d1) was the depth along the grating vector (Kx) obtained during the first laser inscription time (t1). The second depth (d2) was the depth along the grating vector (Ky) obtained during the second laser inscription time (t2). The modulation depths d1 and d2 were taken as the average peak-to-valley distances obtained from at least nine unique cross-sections parallel to the grating vector of interest. The grating pitch of the CSRGs was also measured using the AFM in a similar manner to the modulation depth, but by averaging the peak-to-peak distance.
A second method was used to measure and confirm the grating pitches. A 10-mW He-Ne laser with a wavelength of 632.8 nm was made incident on a CSRG secured on a computer-controlled motorized rotating stage. The angles between the 0 and ±1 diffraction orders were measured by rotating the sample stage. The average diffraction angle from the ±1 orders was used to calculated the grating pitch (Λ) according to the grating equation:
where m is the order of diffraction, λ is the laser wavelength and θ is the average angle between the 0 and ±1 diffraction orders.A 65-nm layer of gold was sputtered over the CSRGs using a Quorum sputter coater. The gold layer takes the same shape as the CSRGs and the gold thickness value used for this work has been previously shown to provide a strong SPR signal while still permitting light measurement in transmission [30].
In this work, the SPR in CSRGs were measured both in transmission (T-SPR) and in reflection (R-SPR) modes when the samples were exposed to the white light spectrum from a halogen lamp. The light from the lamp was first made horizontally polarized (TM; along Kx) and then focused onto the tested CSRG. Subsequently, the transmitted or reflected light from the CSRG went through a second vertically-aligned light polarizer (TE; along Ky) before being collected by a spectrometer. The two polarizers were also rotated in the reverse order of polarization (TE-TM). A dark spectrum was collected and subtracted from the SPR spectrum of each grating.
The SPR spectra of all CSRGs were obtained for angles of incidence between ±8 degrees. To enable the capture of the reflected SPR spectra at normal incidence, the sample was tilted at an angle of 1° around the horizontal axis and a mirror was used to re-direct the reflected beam towards the second polarizer and the photodiode.
For the sensing experiments, a sucrose aqueous solution of 20 wt% was made and used to prepare dilutions of 1, 3, 5, 10 and 15 wt%. Deionized water was used as the reference (0 wt%). A PDMS mixture was made using Sylgard 184 elastomer and curing agent at a 10:1 ratio and baked at 100 °C for 30 min to make a smooth slab with a 2-mm thickness. An opening of approximately 3×3 mm2 was cut in the PDMS slab and was placed over the CSRGs. This opening allowed for the retention of the analyte solution over the CSRG during the sensing experiments. Finally, a cover glass was placed over the solution and stuck to the PDMS layer to eliminate lensing effects. The refractive index of the sucrose solutions was measured using an Abbe refractometer. T-SPR and R-SPR spectra were measured at each analyte concentration by pipetting the solution into the PDMS slab void, placing a cover glass on top, recording the SPR signal, then cleaning the CSRG surface with DI water and repeating these steps for another analyte concentration.
3. Results and discussion
CSRGs were made through a two-step process described in our previous work by rotating the sample 90 degrees along a plane perpendicular to the incident light [14,18,24,25]. The inscribing laser irradiance (365 mW/cm2) was kept identical for both exposures, while each sample was exposed to the collimated laser beam for inscription times t1 and t2, as mentioned in the experimental section.
gDR1 molecules go through a cyclic trans-cis transformation during the light exposure which results in a photo-induced molecular transportation [31–36]. The molecules reposition to form a linear grating structure with a specified pitch during the first holographic patterning with interfering laser beams. The second step carves a second linear grating perpendicular to the previously formed one, thus resulting in 2D nanostructures. The effect of the exposure time ratio (t2/t1) on the dynamics of the CSRG formation are shown in Fig. 1 for samples with 280-nm and 480 nm of gDR1 film thicknesses. The modulation depths of crossed gratings were measured using AFM. These measurements were performed over a 5×5 µm2 randomly selected area of crossed grating samples with identical pitches of 550 nm as described in the experimental section. The first exposure time was kept constant for all samples (t1=250s) while the second exposure time (t2) was varied. Figure 1 illustrates that when the time ratio (t2/t1) increases (i.e. the second exposure time increases), the modulation depth ratio (d2/d1) also increases rapidly until it reaches a plateau at around d2/d1 = 5 for exposure ratio times (t2/t1) above ∼1. The error bars in Fig. 1 are standard deviations calculated over the measured data points. The modulation depth of crossed gratings is almost the same across both orthogonal directions (d2/d1=1) at around t2/t1 ∼ 0.3-0.4 and it falls almost on a similar fitting curve for both 280 nm and 480 nm azo thickness. Therefore, the formation process of CSRGs seems to be independent of the original azobenzene film thickness (Fig. 1). The rapid increase and decrease of the modulation ratio at a time ratio between 0.2 and 0.3 is not entirely understood, but it could be related to the photo-mechanical dynamics of the azobenzene chromophores.
Fig. 1. Ratio of the vertical-to-horizontal modulation depths of CSRGs as a function of the inscription time ratio.
Figure 2 shows the AFM images of gDR1 films during the formation of crossed gratings at specific time ratios. As t2/t1 increase from 0.152 to 2 (Fig. 2(a) to Fig. 2(f)), one can clearly observe the evolution of the CSRG nanostructures from an initial linear SRG accompanied by further conversion to another linear SRG orthogonal to the initial one. At t2/t1 = 0.152 (Fig. 2(a)), there exists a linear SRG with a grating vector along the x axis (Kx), in which only slight modulations are formed due to the small exposure time of the second grating. As the second exposure time increases, the modulations deepen and new orthogonal lines are inscribed over the previously formed linear SRG until the initial linear SRG is completely reformed to a new linear grating at 90 degrees with respect to the first grating with a vector along the y axis (Ky).
Fig. 2. AFM images of SRGs inscribed on gDR1 films with increasing time ratio t2/t1 from (a) 0.152, (b) 0.28, (c) 0.56, (d) 0.8, (e) 1.04 and (f) 2.
The trans azobenzene isomer has a planar configuration and is a larger molecule than the cis isomer. Therefore, the trans to cis transformation by light exposure, creates microscopic mass movements which are accompanied by increases in the molecular dipole moments [32,37]. Such transformations continue over time and the azo molecules start to orient at a direction perpendicular to the light polarization direction [34,38–41]. However, when the polarization changes, the azo molecules reorient in the new direction orthogonal to the new polarization [41,42].
Although a circularly polarized light has no preferential direction, when a left and a right circularly polarized beam (LC, RC) interfere (as in this work), a periodically rotating linear polarization pattern of constant intensity occurs and this results in linear surface relief gratings during the first exposure [33,39]. Then, when the film is being rotated 90 degrees for the second exposure, the same periodic rotating linear polarization pattern inscribes the new orthogonal SRG across the previously formed one. Although the photoisomerization occurs in almost nanoseconds [10] and the electromagnetic-field-azo-dipole interactions are also a fast phenomenon [33], azo molecular motion occurs in a longer time frame as indicated in Figs. 1 and 2. This suggests that the physical reorientation of azo units is probably governed by a slower, non-linear phenomenon such as anomalous diffusion of molecules in the film. This is consistent with a complex diffusive model recently proposed by Pawlik et al. [4].
The orthogonal grating vectors of a CSRG, such as those seen in Fig. 2, are denoted by Kx and Ky and represent the two grating vectors perpendicular to each linear SRG peaks and troughs. These vectors are important for the measurement of the SPR signal intensity of the CSRG when placed in between crossed polarizers. Incident light on the sample was either horizontally polarized (TM; along Kx) or vertically polarized (TE; along Ky). The transmitted or reflected light was collected in the orthogonal light polarization to account for the polarization conversion in between the crossed gratings at the SPR wavelength [43]. Figure 3(a) schematically shows an unpolarized incident light that is being polarized vertically along Ky (TE), which then excites a T-SPR after passing through the gold-coated CSRG at the air-gold interface. The second horizontal polarizer (TM) along Kx eliminates the background non-SPR light and allows the transmission of only the emitted T-SPR light to be collected by a silicon photodetector. The setup for collecting R-SPR is similar to Fig. 3(a) except that the second orthogonal polarizer is placed along the reflected beam from the CSRG surface.
Fig. 3. Surface plasmon resonance peaks of a CSRG with two identical pitches of 550 nm (Λx= Λy=550 nm). (a) schematic of the SPR generation between crossed polarizers, (b) R-SPR, and (c) T-SPR.
The SPR wavelength excited by a SRG along a metal-dielectric interface is given by the following equation [43,44]:
As it can be seen in Figs. 3(b) and 3(c), positive SPR peaks are recorded in both R-SPR and T-SPR. This is a stark contrast in the plasmonic reflection signal between CSRGs and SRGs since linear 1D gratings normally exhibit negative plasmonic peaks in reflection [23]. In the case of CSRGs placed in between crossed polarizers, the sharp intensity increase in reflection at the SPR wavelength indicates SPR light emission from the sample due to the plasmonic energy exchange between the crossed gratings. If light energy can be coupled into a SPR mode by one of the CSRG gratings, the SPR energy can be reversibly re-emitted as light by the orthogonal grating. The intensity of the R-SPR signal is more than 3 times larger than that of the T-SPR signal. The polarization conversion of the CSRG is clearly key to the positive SPR peaks observed here. For instance, in our plasmonic CSRG, TE polarization along Ky serves to couple the light into the 2D resonant nanostructure of CSRG through SPR coupling. This light can then be out-coupled by the orthogonal grating in the TM direction along Kx and the polarization-converted light is emitted as a positive signal in both transmission and reflection. However, most of the light is still reflected off of the sample corresponding to a higher relative intensity in reflection (IR-SPR > IT-SPR), since there is no attenuation due to absorption in the gold layer. Another advantage of R-SPR over T-SPR is that the effects on the signal as it passes through the azo-film and glass substrate, such as absorption, are completely avoided. As it was mentioned previously, a layer of 65 nm gold which was used in this work, has shown to provide the strongest SPR signal while still permitting light measurement in transmission [30]. Therefore, using R-SPR over T-SPR improves the detection capabilities and could potentially be much more beneficial in SPR sensing and imaging techniques. However, this plasmonic light emission process is not entirely understood yet in the literature and it should be studied in more details both experimentally and theoretically.
To confirm the tunability of these plasmonic 2D nanostructures, other CSRGs with identical orthogonal pitches of 650 nm (Λx = Λy = 650 nm) and with a modulation depth ratio of ∼ 1 were also fabricated and coated with a 65 nm gold layer. Figure 4 compares 3D AFM images of two different CSRGs having 550 nm and 650 nm pitches (Fig. 4(a) and (b)) and their corresponding modulations’ topographies along Kx axis of the grating (Fig. 4(c) and (d)). The pitch can be controlled precisely in order to obtain certain SPR results.
Fig. 4. Three-dimensional AFM images of CSRGs having pitch sizes of (a) 550 nm, (b) 650 nm and (c, d) the modulation depth variations along the Kx vector of the gratings, respectively.
For the sample with 650 nm pitch, T-SPR and R-SPR measurements were also obtained using incident white light when it was placed in between crossed polarizers. Figure 5 presents a compilation of the experimental SPR signals from this CSRG upon changing the angle of incidence between −8 to +8 degrees. The theoretical SPR wavelength for this grating pitch is predicted by Eq. (2) to be 715 nm. While λR-SPR in both the TE-TM and TM-TE configurations is approximately 702 nm, λT-SPR is higher than λR-SPR and have values of 720 nm and 710 nm respectively, which is similar to what was observed for the 550 nm pitch CSRG, which could also be due to the sample 1° tilt in reflection mode.
Fig. 5. The SPR signal intensity map plot of a CSRG (Λx= Λy=650 nm) as a function of the incident light angle and wavelength. (a) R-SPR through a TE-TM polarization sequence, (b) R-SPR through a TM-TE polarization sequence, (c) T-SPR through a TE-TM sequence and (d) T-SPR through a TM-TE sequence.
Also, similar to the CSRG with the 550 nm pitch, the measured R-SPR intensity of the CSRG with the 650 nm pitch is approximately 12 times higher than the T-SPR which again proves the advantage of using R-SPR over T-SPR in sensing and imaging applications. In addition, the orientation of the first and second polarizers (TE-TM vs TM-TE) plays a role in the intensity of the SPR signal as it defines how the SPR is being excited and spread depending on the diffraction efficiencies along the grating vectors Kx and Ky with respect to the incident light polarization. The slight change in the SPR signal intensities of the TE-TM configuration in comparison with the TM-TE one, is most likely due to the minor difference in the relative modulation depths between the gratings in the CSRG, thus affecting the diffraction efficiency and in turn the coupling efficiency of light into the SPR mode.
The other distinct phenomenon happening in Fig. 5 is the splitting of the SPR signals by changing the incidence angle of light, which is a characteristic of SPR excitations and is described by Eq. (2). The wavelength of the SPR peaks sweep between 680 nm to approximately 800 nm upon 16 degrees of angular rotation (θi= −8° to +8°) of the sample. The SPR at normal incidence is the strongest because it results from a standing wave SPR condition when both the forward and backward diffraction orders couple to the SPR at the same time, while at off-normal incidence, the SPR waves are propagating and typically result from either the forward or backward diffraction orders coupling to the SPR at different wavelengths [21].
CSRGs have been used as sensors previously by our research group and bulk sensitivities of up to 778.6 nm/RIU have been obtained using T-SPR signals [18]. To investigate the performance and sensitivity of the R-SPR signal emitted by CSRGs fabricated in this research, different aqueous sucrose solutions of 0 wt%, 1 wt%, 3 wt%, 5 wt%, 10 wt%, 15 wt% and 20 wt% were prepared and deposited over the CSRG’s surface. Each solution concentration was tested separately after flushing the PDMS well over the CSRG (Λ=550 nm) with deionized water. Figure 6(a) shows that increasing the concentration of the solution results in a red-shift of the R-SPR peak wavelength. Based on Eq. (2), this red-shift occurs due to the change in the refractive index of the dielectric, which is water in this case. As a result, the slope of the R-SPR shift versus the refractive index is defined as the bulk sensitivity of the R-SPR sensor (S=ΔλR-SPR/Δnd). Figure 6(b) indicates that both R-SPR and T-SPR have potential sensor applications with sensitivities of approximately SR-SPR = 601 nm/RIU and ST-SPR = 589 nm/RIU respectively. The intent of these results is to demonstrate that both the R-SPR and T-SPR signals have almost equal bulk sensitivities, while the R-SPR signal is much stronger than the T-SPR signal, meaning that the R-SPR signal can be easily captured and measured even if the intensity of the incident light is low. This sensing experiment was done without optimization of the physical characteristics of the gratings, meaning that the values obtained for the bulk sensitivities could be even higher [18,45]. For instance, S. Long et al. [45] have recently shown that the sensitivity of grating coupled SPR sensors depends on their grating pitch and angle of incident light; increasing the former increases the sensitivity up to a certain value while increasing the latter shows a maximum sensitivity at θi around 90 degrees.
Fig. 6. (a) R-SPR wavelength shift by increasing the concentration of sucrose solution in water. (b) Comparison between the sensitivities of R-SPR and T-SPR of CSRG (Λx= Λy=550 nm) versus concentration and refractive index change of the sucrose solution.
4. Conclusion
Tunable crossed surface relief gratings (CSRG) were made and their formation was studied by Atomic Force Microscopy. It was shown that CSRGs emit SPR light when coated with a metallic layer both in transmission and in reflection modes. The SPR light emission occurs at the gold-air interface when a CSRG is placed between crossed polarizers and it is independent of the direction of polarization. Reflection SPR (R-SPR) has a much higher emission intensity than transmission SPR (T-SPR) and both methods can be used to sense analytes in solution. To that end, aqueous sucrose solution was used and bulk sensitivities of approximately 601 nm/RIU and 589 nm/RIU were measured in reflection and transmission respectively.
Funding
Defence Research and Development Canada; Natural Sciences and Engineering Research Council of Canada (RGPIN-2020-03881).
Disclosures
The authors declare no conflicts of interest.
References
1. O. Henneberg, T. Geue, M. Saphiannikova, U. Pietsch, P. Rochon, and A. Natansohn, “Formation and Dynamics of Polymer Surface Relief Gratings,” Appl. Surf. Sci. 182(3-4), 272–279 (2001). [CrossRef]
2. Y. Gritsai, L. M. Goldenberg, O. Kulikovska, and J. Stumpe, “3D structures using surface relief gratings of azobenzene materials,” J. Opt. A: Pure Appl. Opt. 10(12), 125304 (2008). [CrossRef]
3. T. Sabel, G. V. Lucas, and M. C. Lensen, “Simultaneous formation of holographic surface relief gratings and volume phase gratings in photosensitive polymer,” Mater. Res. Lett. 7(10), 405–411 (2019). [CrossRef]
4. G. Pawlik, T. Wysoczanski, and A. C. Mitus, “Complex dynamics of photoinduced mass transport and surface relief gratings formation,” Nanomat. 9(3), 352 (2019). [CrossRef]
5. O. R. Bennani, T. A. Al-Hujran, J. M. Nunzi, R. G. Sabat, and O. Lebel, “Surface relief grating growth in thin films of mexylaminotriazine-functionalized glass-forming azobenzene derivatives,” New J. Chem. 39(12), 9162–9170 (2015). [CrossRef]
6. K.-H. Kim and Y.-C. Jeong, “One-step fabrication of hierarchical multiscale surface relief gratings by holographic lithography of azobenzene polymer,” Opt. Express 26(5), 5711–5723 (2018). [CrossRef]
7. O. Sakhno, L. M. Goldenberg, M. Wegener, and J. Stumpe, “Deep surface relief grating in azobenzene-containing materials using a low-intensity 532 nm laser,” Opt. Mater.: X 1, 100006 (2019). [CrossRef]
8. R. J. Moerland, J. E. Koskela, A. Kravchenko, M. Simberg, S. V. D. Vegte, M. Kaivola, A. Priimagi, and R. H. A. Ras, “Large-area arrays of three-dimensional plasmonic subwavelength-sized structures from azopolymer surface-relief gratings,” Mater. Horiz. 1(1), 74–80 (2014). [CrossRef]
9. X. Zhao, J. Wang, J. Huang, L. Li, E. Liu, J. Zhao, Q. Li, X. Zhang, and C. Lu, “Path-Guided Hierarchical Surface Relief Gratings on Azo-Films Induced by Polarized Light Illumination through Surface-Wrinkling Phase Mask,” Langmuir 36(11), 2837–2846 (2020). [CrossRef]
10. E. Sava, B. Simionescu, N. Hurduc, and I. Sava, “Considerations on the surface relief grating formation mechanism in case of azo-polymers, using pulse laser irradiation method,” Opt. Mater. 53, 174–180 (2016). [CrossRef]
11. J. Leibold and R. G. Sabat, “Fabrication of micrometer-scale surface relief gratings in azobenzene molecular glass films using a modified Lloyd’s mirror interferometer,” Opt. Mater. 96, 109315 (2019). [CrossRef]
12. B. A. Prabowo, A. Purwidyantri, and K. C. Liu, “Surface plasmon resonance optical sensor: A review on light source technology,” Biosensors 8(3), 80 (2018). [CrossRef]
13. H. N. Daghestani and B. W. Day, “Theory and applications of surface plasmon resonance, resonant mirror, resonant waveguide grating, and dual polarization interferometry biosensors,” Sensors 10(11), 9630–9646 (2010). [CrossRef]
14. S. Nair, J. G. Cruz, Á. M. Hernandez, G. Ascanio, R. G. Sabat, and C. Escobedo, “Rapid label-free detection of intact pathogenic bacteria: In situ via surface plasmon resonance imaging enabled by crossed surface relief gratings,” Analyst 145(6), 2133–2142 (2020). [CrossRef]
15. R. Mahmood, M. B. Johnson, and A. C. Hillier, “Massive Enhancement of Optical Transmission across a Thin Metal Film via Wave Vector Matching in Grating-Coupled Surface Plasmon Resonance,” Anal. Chem. 91(13), 8350–8357 (2019). [CrossRef]
16. E. Bailey and R. G. Sabat, “Surface plasmon bandwidth increase using chirped-pitch linear diffraction gratings,” Opt. Express 25(6), 6904–6913 (2017). [CrossRef]
17. L. Lévesque and P. Rochon, “Surface Plasmon Photonic Bandgap in Azopolymer Gratings Sputtered with Gold,” J. Opt. Soc. Am. A 22(11), 2564–2568 (2005). [CrossRef]
18. Y. Bdour, C. Escobedo, and R. G. Sabat, “Wavelength-selective plasmonic sensor based on chirped-pitch crossed surface relief gratings,” Opt. Express 27(6), 8429–8439 (2019). [CrossRef]
19. S. Kasani, K. Curtin, and N. Wu, “A review of 2D and 3D plasmonic nanostructure array patterns: Fabrication, light management and sensing applications,” Nanophotonics 8(12), 2065–2089 (2019). [CrossRef]
20. P. Törmä and W. L. Barnes, “Strong coupling between surface plasmon polaritons and emitters,” Rep. Prog. Phys. 78(1), 013901 (2015). [CrossRef]
21. P. L. Rochon and L. Lévesque, “Standing Wave Surface Plasmon Mediated Forward and Backward Scattering,” Opt. Express 14(26), 13050–13055 (2006). [CrossRef]
22. H. H. Nguyen, J. Park, S. Kang, and M. Kim, “Surface plasmon resonance: A versatile technique for biosensor applications,” Sensors 15(5), 10481–10510 (2015). [CrossRef]
23. S. Long, J. Cao, S. Geng, N. Xu, W. Qian, and S. Gao, “Optimization of plasmonic sensors based on sinusoidal and rectangular gratings,” Opt. Commun. 476, 126310 (2020). [CrossRef]
24. S. Nair, C. Escobedo, and R. G. Sabat, “Crossed Surface Relief Gratings as Nanoplasmonic Biosensors,” ACS Sens. 2(3), 379–385 (2017). [CrossRef]
25. S. Nair, J. G. Cruz, Á. M. Hernandez, G. Ascanio, R. G. Sabat, and C. Escobedo, “Selective uropathogenic E. Coli detection using crossed surface-relief gratings,” Sensors 18(11), 3634 (2018). [CrossRef]
26. J. Leibold and R. G. Sabat, “Laser-induced controllable chirped-pitch circular surface-relief diffraction gratings on AZO glass,” Photonics Res. 3(4), 158–163 (2015). [CrossRef]
27. J. Leibold, P. Snell, O. Lebel, and R. G. Sabat, “Design and fabrication of constant-pitch circular surface-relief diffraction gratings on disperse red 1 glass,” Opt. Lett. 39(12), 3445–3448 (2014). [CrossRef]
28. G. Xue, H. Lu, X. Li, Q. Zhou, G. Wu, X. Wang, Q. Zhai, and K. Ni, “Patterning nanoscale crossed grating with high uniformity by using two-axis Lloyd’s mirrors based interference lithography,” Opt. Express 28(2), 2179–2191 (2020). [CrossRef]
29. R. Kirby, R. G. Sabat, J. M. Nunzi, and O. Lebel, “Disperse and disordered: A mexylaminotriazine-substituted azobenzene derivative with superior glass and surface relief grating formation,” J. Mater. Chem. C 2(5), 841–847 (2014). [CrossRef]
30. H. Suzuki, M. Sugimoto, Y. Matsui, and J. Kondoh, “Effects of gold film thickness on spectrum profile and sensitivity of a multimode-optical-fiber SPR sensor,” Sens. Actuators, B 132(1), 26–33 (2008). [CrossRef]
31. M. Saphiannikova, V. Toshchevikov, and J. Ilnytskyi, “Photoinduced Deformations in Azobenzene Polymer Films,” Nonlinear Opt,” Quantum Opt. 41(1), 27–38 (2009). [CrossRef]
32. Ľ. Vetráková, V. Ladányi, J. A. Anshori, P. Dvořák, J. Wirz, and D. Heger, “The absorption spectrum of: Cis -azobenzene,” Photochem. Photobiol. Sci. 16(12), 1749–1756 (2017). [CrossRef]
33. S. L. Oscurato, M. Salvatore, P. Maddalena, and A. Ambrosio, “From nanoscopic to macroscopic photo-driven motion in azobenzene-containing materials,” Nanophotonics 7(8), 1387–1422 (2018). [CrossRef]
34. N. S. Yadavalli, M. Saphiannikova, and S. Santer, “Photosensitive response of azobenzene containing films towards pure intensity or polarization interference patterns,” Appl. Phys. Lett. 105(5), 051601 (2014). [CrossRef]
35. J. Kumar, L. Li, X. L. Jiang, D. Y. Kim, T. S. Lee, and S. Tripathy, “Gradient force: The mechanism for surface relief grating formation in azobenzene functionalized polymers,” Appl. Phys. Lett. 72(17), 2096–2098 (1998). [CrossRef]
36. T. Fukuda, K. Sumaru, T. Yamanaka, and H. Matsuda, “Photo-induced formation of the surface relief grating on azobenzene polymers: analysis based on the fluid mechanics,” Mol. Cryst. Liq. Cryst. 345(1), 263–268 (2000). [CrossRef]
37. G. Hartley, “The Cis form of azobenzene,” Nature 140(3537), 281 (1937). [CrossRef]
38. N. S. Yadavalli and S. Santer, “In-situ atomic force microscopy study of the mechanism of surface relief grating formation in photosensitive polymer films,” J. Appl. Phys. 113(22), 224304 (2013). [CrossRef]
39. A. Priimagi and A. Shevchenko, “Azopolymer-based micro- and nanopatterning for photonic applications,” J. Polym. Sci., Part B: Polym. Phys. 52(3), 163–182 (2014). [CrossRef]
40. M. Ivanov, A. Priimagi, and P. Rochon, “Effect of saturation on the diffraction efficiency of holographically recorded gratings in azopolymer films,” Opt. Express 17(2), 844–849 (2009). [CrossRef]
41. M. Hendrikx, A. P. H. J. Schenning, M. G. Debije, and D. J. Broer, “Light-triggered formation of surface topographies in azo polymers,” Crystals 7(8), 231 (2017). [CrossRef]
42. J. H. Lin, J. H. Huang, H.-C. Kan, and C. C. Hsu, “Optical tuning of guided mode resonance in an azo-copolymer waveguide grating structure inscribed with a surface relief grating,” Adv. Device Mater. 1(3), 74–79 (2015). [CrossRef]
43. Y. Mikhyeyev and R. G. Sabat, “Polarization-contrast surface plasmon imaging,” Opt. Express 28(15), 21481–21488 (2020). [CrossRef]
44. J. Zhang and L. Zhang, “Nanostructures for surface plasmons,” Adv. Opt. Photonics 4(2), 157–321 (2012). [CrossRef]
45. S. Long, J. Cao, Y. Wang, S. Gao, N. Xu, J. Gao, and W. Wan, “Grating coupled SPR sensors using off the shelf compact discs and sensitivity dependence on grating period,” Sens. Actuat. Reports 2(1), 100016 (2020). [CrossRef]
