Large-scale linear diffraction gratings with gradually varying pitch were photo-inscribed onto the surface of azobenzene thin films using a 532 nm laser and a modified Lloyd mirror set-up. By placing a cylindrical lens in front of the direct half of the inscribing beam, gratings with a chirping rate as high as 12.9 nm/mm were produced. Subsequently, when these chirped-pitch gratings were coated with silver, over three-fold bandwidth increase was observed in the surface plasmon transmission peaks at FWHM, when compared to constant-pitch gratings. This was made possible due to the simultaneous excitation of surface plasmon resonance in a band of light wavelengths.
© 2017 Optical Society of America
Surface plasmon (SP) grating structures have been gaining increasing attention across a variety of scientific disciplines. Their precise excitation conditions and ability to enhance the light’s electromagnetic field has found applications ranging from bio-sensing , organic light emitting diodes (OLEDs) , and on-chip spectrometers . However, one of their most promising uses may be in increasing the efficiency of thin film solar cells . Light-induced SPs allow for more efficient photon absorption without increasing the thickness of the photoactive layer.
SP waves are generated at a metal-dielectric interface, when incident light interacts with the free electrons in the metal causing them to collectively oscillate in resonance with the light’s electromagnetic field. The light, in turn becomes bound to the surface by these electrons, with the electric field decaying exponentially away from the metal-dielectric interface. These SP waves propagate along the interface for lengths on the order of micrometers, before being dissipated as heat .
Simply shining a light beam onto a metal-dielectric interface cannot generate SP waves because their excitation can only be achieved by matching the incident light wave vector to that of the plasmon along the direction of propagation. The most common methods for achieving this include prism coupling , scattering from sub-wavelength defects  and diffraction from periodic corrugations (gratings) placed at the metal-dielectric boundary . The latter two provide a compact coupling mechanism, with gratings providing the easiest control over the surface plasmon propagation wavelength.
The only draw-back from generating SPs with diffracting elements is that, theoretically, each grating can only excite one surface plasmon at a discrete wavelength, which depends on the light incidence angle, the effective permittivity of the metal, the refractive index of the dielectric and the light polarization, which must be parallel to the grating vector. This drastically limits plasmon enhanced absorption in solar cells to a narrow portion of the full blackbody spectrum produced by the sun. To overcome this, research has begun on a variety of compound and non-periodic gratings to combine surface plasmon generation to multiple wavelengths or to a band of wavelengths using a single grating structure.
For example, Atalla [9,10] used rigorous coupled wave-analysis to model solar cells with a back contact grating, comparing regular gratings with compound and Fourier harmonic gratings. Bi et al  used holographic lithography to create a two-dimensional, crossed metallic grating from two distinct grating periods rotated 90 degrees with respect to each other. Similarly, Khan et al  sequentially exposed a photoresist to three interference patterns yielding three superimposed grating periods (525 nm, 612 nm and 690 nm), then they repeated the procedure with the sample rotated 90 degrees to the original grating. This yielded a two-dimensional crossed multi-diffractive grating. All these superimposed gratings achieved enhanced plasmonic absorption in their respective photoactive materials when compared to single gratings.
In this paper, surface plasmon resonance from chirped-pitch sinusoidal metallic gratings is compared to that of constant-pitch sinusoidal gratings and a significant SP wavelength bandwidth increase is observed. Even though chirped-pitch gratings have been previously constructed and analyzed [13–16], their production methods are often time consuming, expensive and lack control over the grating profile. Furthermore, no literature can be found on the plasmonic activity of chirped-pitch sinusoidal gratings compared to constant-pitch gratings. Herein, a Direct Laser Interference Patterning (DLIP) technique is used to create chirped-pitch diffraction gratings in a short and single-step process, allowing for easy control over the grating parameters, such as the chirp rate and depth, as detailed elsewhere .
The gratings were created upon exposing thin films of azobenzene molecular glass to a laser interference pattern [18–20]. Azobenzene containing compounds are known to undergo a cis-trans photo-isomerization upon exposure to light at an absorbing wavelength. This causes localized mass transport of the material, found to be proportional to the second derivative of the light’s intensity profile . Consequently, when an azobenzene thin film is exposed to a chirped laser interference pattern, the pattern is photo-mechanically imprinted as a chirped-pitch surface relief grating. This process requires only a few minutes of laser exposure time and requires no further post-exposure processing. Then, by coating the resulting grating with a thin layer of a metal, SPs can be excited. This technique, demonstrated here for visible wavelengths, is also applicable to UV and IR wavelengths.
2. Experimental procedure
Corning 0215 glass microscope slides, with dimensions of 38 × 38 mm2 and 1 mm thickness, were used as substrates. They were thoroughly washed with dish soap and the excess water was removed using lint-free wipes. The slides were then dried in an oven at 100 degrees Celsius for 30 minutes and finally, they were blown dry one last time using clean compressed air. After cleaning, approximately 3 ml of Disperse Red 1 azobenzene molecular glass solution, diluted in dichloromethane at a 3 wt% mix ratio, was deposited on each slide and spun at 1100 rpm for 20 seconds, using a Headway Research spin-coater. Samples were then dried in the oven at 100 degrees Celsius for 30 minutes to evaporate any remaining solvent. The resulting films had a thickness of 240 ± 30 nm, as measured using a Dektak IID surface profiler.
To create constant-pitch gratings, a Lloyd mirror set-up was used in conjunction with a Coherent Verdi V5 diode-pumped laser emitting at 532 nm. Light from the laser passed through a spatial filter and was collimated using a lens. Then, the linearly polarized beam passed through a quarter-wave plate to generate circularly polarized light. Finally, it was incident on a Lloyd mirror set-up, consisting of a mirror at 90 degrees with the azobenzene sample. Half of the collimated laser beam was directly incident on the sample, while the other half was reflected off the mirror, creating a sinusoidal interference pattern on the sample’s surface. This pattern was imprinted on the azobenzene film as a diffraction grating. The pitch of each grating was controlled by the incident laser beam angle, while the depth was controlled by the exposure time. An adjustable circular iris was used to control the collimated laser beam diameter, and thus the physical size of each grating. The resulting gratings had the size and shape of the semi-circle beam-half. For constant-pitch gratings, the laser exposure time was set between 10 and 20 seconds, with an iris aperture diameter of 9.4 mm. The irradiance of the laser beam was set to 312 mW/cm2.
To create chirped-pitch diffraction gratings, the same set-up previously described was used, but with a cylindrical lens having a diameter of 4 mm, placed along the beam path at a fixed 69 mm in front of the sample, as illustrated in Fig. 1(a). This lens was imbedded in a custom cardboard rectangular aperture of 4 × 7 mm2, designed to block the rest of the direct half of the laser beam. Careful placement of the lens and lens-aperture was required to insure that the lens was not tilted with respect to the vertical and that the laser beam-half hitting the mirror was not blocked. This cylindrical lens created a curved wave front that interfered with the flat wave front reflected off the mirror. This resulted in a varying period length along the horizontal between interference maxima, and therefore created a chirped-pitch grating on the azobenzene film, as detailed elsewhere . To prevent the deflected light from the cylindrical lens from hitting the mirror and reflecting back on the sample, a piece of paper was placed roughly at 80 degrees to the lens in order to block the stray light, as depicted in Fig. 1(a). For chirped-pitch gratings, the azobenzene films were exposed to the laser beam for 160 seconds and an iris aperture diameter of 18.9 mm was chosen in order to fully irradiate the lens-aperture. The laser irradiance, measured before the cylindrical lens, was kept constant at 312 mW/cm2. This procedure resulted in the creation of an approximately rectangular-shaped chirped-pitch grating with dimensions of 9.5 × 7.2 mm2, as illustrated in Fig. 1(b). An actual picture of six different chirped-pitch gratings, coated with silver, is seen in Fig. 1(c).
After fabrication, each grating was sputter-coated with 60 nm of silver using a Bal-Tec SCD-050 sputter coater. The silver layer was previously shown to take the same shape of the underlying grating, creating a metallic sinusoidal grating on top of the azobenzene grating .
A low-power 632.8 nm Helium-Neon laser with a 1-mm diameter was used to measure the pitch of the gratings. The laser light would pass through the sample, positioned on a motorized turntable, and the angle between the 0th and 1st diffraction orders was measured. Therefore, the grating pitch was calculated using the grating equation. For chirped-pitch diffraction gratings, the He-Ne laser beam cross-section didn’t cover the entire grating surface; however, the diffracted orders appeared as elongated ellipses due to the varying grating pitches being illuminated at once. It was the center of the ellipse that was chosen as the localized grating pitch for the chirped-pitch gratings. Furthermore, as predicted from previous analysis of chirped-pitch gratings [17,21], the positive and negative diffracted orders were asymmetrical in size, with one side having a more focused diffraction order. This occurs because the chirped-pitch grating is simply a replicated holographic representation of the light phase during the inscription process of the grating.
To analyze the transmission spectra of the various diffraction gratings, white light from an Oriel Corp. halogen lamp passed through an adjustable neutral density filter and a linear polarizer to produce either vertically (TE) or horizontally (TM) polarized light. The white light was either focused to a 1-mm diameter spot size on the sample using a lens to probe specific areas on the grating, as depicted in Fig. 2(a), or passed through a rectangular aperture unfocussed in order to study the transmitted light from the entire grating at once, as illustrated in Fig. 2(b). After passing through the sample, light was focused into an Edmund Optics CCD fiber optic spectrometer using another focalizing lens. For all spectra shown in this paper, the angle of incidence on the sample was set to 90 degrees and the spectra were normalized to isolate the surface plasmon signal by dividing the horizontal TM-polarization spectrum with the vertical TE-polarization spectrum. The grating vector was oriented horizontally in all the experiments presented in this work; therefore, only TM polarized light would excite a SP.
3. SP theory for constant-pitch gratings
When a periodic grating with spacing is used as a plasmon coupler, the wavelength of a surface plasmon, along a flat metal-dielectric interface is given by the following equation :Eq. (1), is plotted in Fig. 3(a) and it can be seen that it is roughly linear over visible wavelengths. The plot in this figure takes into account the changes of and with wavelength. The normalized surface plasmon transmission spectra, obtained experimentally from normally incident light that is focused on the central region of constant-pitch gratings, is shown is Fig. 3(b) for several different gratings. The narrow positive peaks, seen in Fig. 3(b), are associated with the SP excitation at the silver-air interface and indicate an enhanced transmission of the metallic layer due to the plasmon resonance. According to Eq. (1), only one surface plasmon peak is expected for each spectrum/grating pitch at normal incidence. The appearance of double peaks near the top of each SP signal is due to a photonic energy gap that is created when a grating becomes deep enough that the metallic surface is no longer effectively flat to an incoming photon . However, the center of this plasmon photonic energy gap in Fig. 3(b) follows well the theory presented by Eq. (1).
The measured and theoretical edge-to-edge grating pitch range for chirped-pitch gratings as a function of the constant-pitch grating that would have been produced at that particular laser incidence angle with respect to the Lloyd mirror, with the cylindrical lens removed, are shown in Fig. 4. For the fabrication of every chirped-pitch gratings studied here, the cylindrical lens was placed at exactly 69 mm in front of the Lloyd mirror/sample corner, along the half laser beam directly incident on the azobenzene film. The placement of the cylindrical lens during the laser inscription process along with a geometrical analysis of the light beam reflection off the Lloyd mirror, propagation and interference on the sample’s surface  permitted the calculation of the theoretical range in the pitch variation for each chirped-pitch grating. Subsequently, the results from this calculation were compared to the actual edge-to-edge pitch range for each chirped-pitch grating, measured using the diffraction angle set-up previously described.
In Fig. 4, the measured chirped gratings’ pitch range matches very well to the theoretical pitch range, obtained using a similar development to a previously-published theoretical model from our research group . This information, combined with the SP wavelength data for constant-pitch gratings from Fig. 3, can be used to develop an expected wavelength bandwidth for the surface plasmon enhanced transmission, when comparing constant-pitch and chirped-pitch gratings.
Also from Fig. 4, it is apparent that the rate of chirping increases as the theoretical pitch, with the cylindrical lens removed, increases. For example, a grating with a theoretical pitch of 550 nm, with the cylindrical lens removed, had a pitch rate of 7.5 nm/mm, while a grating with a 750 nm theoretical pitch had a chirp rate of 12.9 nm/mm. This is a consequence of the increasing angle between the azobenzene sample and the inscribing laser beam.
In regards to the cylindrical lens placement in front of the sample to produce chirped-pitch gratings, every effort was made to place the lens in the center of the direct half of the laser beam, but the theoretical model assumes that the lens is off by 0.3 mm, at a distance of 5.0 mm from the center of the full 18.9 mm diameter beam. It was found that the placement of the lens with respect to the center of the beam greatly affected the rate of chirping for the gratings, especially those with larger pitches . It can be noted that the rate of chirping would increase if the cylindrical lens is positioned closer to the sample. However, for consistency, all chirped-pitch gratings in this work were fabricated with the cylindrical lens fixed in the position described.
The TM/TE normalized transmission spectra of fully illuminated chirped-pitch gratings, compared to constant-pitch gratings, according to the set-up seen in Fig. 2(b), are shown in Figs. 5(a)-5(d). Four different chirped-pitch gratings were tested in this figure, with theoretical center pitches of 600 nm, 650 nm, 700 nm and 750 nm, respectively seen in Figs. 5(a)-5(d). Each of these chirped-pitch gratings was compared to the SP spectra from constant-pitch gratings, obtained from Fig. 3(b).
At first, it was noted that the surface plasmon peaks associated with chirped-pitch gratings were significantly less intense than those generated by constant-pitch gratings. This is because not every groove from a chirped-pitch grating was exciting a SP at the same wavelength, unlike a constant-pitch grating. In order to provide a fair comparison between the two, the intensity of the probing white light was varied until the peaks from the chirped-pitch and constant-pitch gratings were approximately the same height. This procedure did not change the shape or location of the plasmon signals.
It can clearly be seen in Figs. 5(a)-5(d) that chirped-pitch gratings exhibit a broadband excitation of surface plasmons. For chirped-pitch gratings with numerically higher pitch ranges, the measured SP signal does not fully reach across the expected theoretical range. This is potentially due to the fact that the fabricated chirped-pitch gratings were slightly curved towards one edge, inherently producing a lesser surface plasmon signal. It is also possible that as the inscribing laser incidence angle increases, the irradiance becomes lesser toward one edge, hence producing a shallower-depth area on that side of the chirped-pitch grating. This could also significantly decrease the height of the surface plasmon transmission peak .
An estimation of the SP wavelength bandwidth increase for chirped-pitch gratings, compared to constant-pitch gratings, can be obtained from Fig. 5. The full width half maxima (FWHM) of each surface plasmon transmission spectrum, obtained from Fig. 3(b) and Figs. 5(a)-5(d), are presented in Table 1. Constant-pitch gratings have an average SP peak FWHM of 10.2 nm, while chirped-pitch gratings have an average SP peak FWHM of 43.6 nm. From this data, an average increase of 327% is calculated for the SP excitation peaks at FWHM, when a fully illuminated chirped-pitch grating, having a width of 9.5 mm is compared to a fully illuminated constant-pitch grating.
Further transmission plots were obtained for each chirped-pitch grating, initially analyzed in Figs. 5(a)-5(d), upon focalizing the probe white light on five equally spaced locations along an imaginary horizontal line drawn along the center of each chirped-pitch grating. As illustrated in Fig. 2(a), incident white light was focused to a spot size of 1.0 mm in diameter at positions labeled as 0 mm, 2.4 mm, 4.8 mm, 7.1 mm and 9.5 mm from the left edge of each chirped-pitch grating, which corresponds to the flat edge where the azobenzene film was closest to the Lloyd mirror during the inscription process.
The transmission spectra at these different locations along the chirped-pitch gratings are shown in Figs. 6(a)-6(d). It can be seen that indeed the varying pitch of the grating does produce varying plasmon wavelengths across the expected range. Also from these plots, it is apparent that the plasmon peak decreases in height as the focused white light beam is moved from left to right on the grating. Furthermore, the decrease in the SP peaks height from edge-to-edge seems to be more accentuated for gratings with a larger central pitch. This observation points again to the possibility that in addition to a chirping in the grating’s pitch, the depth of the grating grooves might also be changing from left to right. This could be due to the diminishing laser irradiance towards the edge or due to the dispersion created by the lens during the writing process. To test the hypothesis that the chirped-pitch grating depth changed from edge to edge, a Bruker Dimension Edge Atomic Force Microscope (AFM) was used to take images in tapping mode of a chirped-pitch grating, the one whose SP transmission scans are shown in Fig. 6(d), at various positions along the grating. Each scan consisted of a 5-microns square area and the average depth for each position/scan was measured using the Bruker NanoScope Analysis software. The results are located in Table 2. The same AFM scans were done for a 750-nm constant-pitch grating and the grating depth was measured to be almost 60 nm throughout the grating area.
In Fig. 6(a), it is evident that, for the chirped-pitch grating at 600 nm center, the grating’s depth was large enough to produce a photonic energy gap for plasmon-producing photons for the lower range of pitches on the left side of the grating. This could also account for the appearance of raised left edge, near a wavelength of 600 nm, in the transmission spectra of the same chirped-pitch grating in Fig. 5(a). It should also be noted that a second weaker surface plasmon signal is visible in Fig. 6(a) at higher wavelengths above 850 nm. This signal is associated with the SP excitation at the silver-azobenzene interface. This particular SP is also seen in Fig. 5(a) as a sharp increase, near a wavelength of 850 nm, in the SP spectrum from chirped-pitch gratings spectra.
Broadband surface plasmon enhanced transmission was observed for chirped-pitch diffraction gratings fabricated using Direct Laser Interference Patterning on azobenzene thin films. The films were fabricated using a modified Lloyd mirror set-up, in a single-step process. Chirping rates between 8.5 nm/mm and 12.9 nm/mm were obtained experimentally for different gratings. When coated with a layer of silver, these gratings generated surface plasmons, with varying efficiency, across a broader band of wavelengths when compared to their constant-pitch counterparts. The fabricated chirped-pitch gratings in this work offer a quick and inexpensive way to generate a band of surface plasmons with a single and compact diffraction grating. In the future, these diffraction gratings could be used for broadband plasmon enhanced absorption in thin film solar cells to increase their efficiency.
Natural Sciences and Engineering Research Council of Canada (NSERC) 501100000038 2015-05743.
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