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Abstract
Two-dimensional Resonant Waveguide Crossed Gratings (RWCG) were fabricated on azobenzene molecular glass thin films and their resonance behavior was studied once placed in between orthogonally aligned polarizers. Normally-incident polychromatic light was transmitted and/or reflected from these RWCGs only in narrow positive peaks. In addition, the central wavelength and transmitted intensity of these positive peaks were actively modulated by an external light source. Furthermore, a dynamic volume birefringence behavior related to the photomechanical effect of the azobenzene chromophores was observed. A mechanism to explain the polarization conversion of the resonant light using RWCGs at normal incidence was also proposed.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Light incident on a thin-film with a specific polarization, wavelength and angle can be coupled into discrete s- and p-polarized waveguide modes upon diffraction by a grating located at the film’s surface [1]. This resonant coupling of light is normally observed experimentally as narrowband minima in the spectrum of light that is transmitted or reflected by the waveguides [2,3]. Optical waveguides are essential in integrated optics and they have been used in many applications including resonant light filters [4,5], sensors, polarizers and augmented reality [3–6]. One approach to fabricate optical waveguides is to engrave periodic grating nanostructures on top of thin dielectric films [7]. This was done for instance, by inscribing linear surface relief gratings (SRG) on azobenzene thin films and the resulting device was used as a narrowband waveguide filter [8,9]. Azobenzene chromophores are known for their photomechanical mass-transport properties and a laser interference pattern can be used to inscribe surface relief grating structures on azobenzene-functionalized thin films [10–14]. Furthermore, the laser irradiation of azobenzene films with complex interference patterns can lead to the fabrication of quasi-crystals [15] and metasurfaces [16–18].
Active control and manipulation of the polarization of light in transmission and in reflection has been extensively studied and is of great importance in developing modern optical communication systems, light filters and tunable photonic devices [19–28]. In addition, intensity-based readouts (i.e. positive peaks) rather than spectroscopic analysis (i.e. negative peaks) are desirable and advantageous for many applications such as sensing and imaging [29–32]. The behavior of linear 1D resonant waveguide gratings, when placed in between crossed polarizers oriented at 45° with respect to the grating vector, have been studied extensively and it was shown that positive peaks can be obtained [32–34]. Also, some attempts have been done to fabricate guided mode transmission or reflection filters with few film layers and low angles of incident [35,36]. However, there are no reports in the literature of positive-peaked guided resonance modes which occur when a single layer 2D waveguide grating is placed in between crossed polarizers at normal incident. In addition, the suggested mechanisms for 1D structures do not explain the behavior of 2D crossed gratings. Therefore, further research needs to be done to understand the mechanism for the formation of the positive-peaked guided modes in grating structures, specifically in 2D resonant waveguide gratings.
In this study, two-dimensional Crossed Surface Relief Gratings (CSRG) [29,31,37] are inscribed on an azobenzene molecular glass (gDR1) [38] thin film to experimentally create for the first time an optically-tunable dynamic Resonant Waveguide Crossed Grating (RWCG). These RWCGs can couple incident light in and out of resonant waveguide modes [8]. When a RWCG is placed in between orthogonal polarizers parallel to its grating vectors, only positive narrow peaks in the out-coupled and polarization-converted spectra of normally-incident light are detected and all other wavelengths are cancelled out. Wavelength-selective and positive-peaked guided modes from a two-dimensional RWCG is demonstrated for the first time for normally incident light in both transmission and reflection, and a mechanism for the polarization conversion of out-coupled light is proposed. Furthermore, these RWCGs exhibit a dynamic coupling behavior where the intensity and the wavelength position of the polarization-converted resonant light is modulated using an external light beam because of the azobenzene photodynamics [39–41]. These RWCGs having positive resonant peaks can be useful in many intensity-based applications such as sensing and imaging.
2. Sample preparation
Filtered solutions of 3-wt% Disperse Red-1 azobenzene molecular glass (gDR1) [38] in Dichloromethane (CH2Cl2) were prepared and spin-coated on clean Corning 0215 glass microscope slides (38×38×1 mm3). The average thickness of the films was measured using a Dektak XT surface profiler to be 197 nm. The gDR1 coated slides were dried at 75 °C for 15 minutes before the laser inscription. Resonant waveguide crossed gratings (RWCG) were fabricated by inscribing constant-pitch crossed surface relief gratings (CSRG) on gDR1 thin-films using a diode-pumped solid-state laser (Coherent, Verdi V6, λ = 532 nm) as described in our previous work [31], by interfering left- and right-handed circularly polarized collimated laser beams using a Lloyd mirror. The grating pitch (Λ) was controlled precisely by fine-tuning the angle of incidence of the laser (θ) on the gDR1 film via a rotating sample holder assembly (Λ= λ/2sinθ). The Verdi laser irradiance was kept constant at 365 mW/cm2 for all exposures.
3. Results and discussion
The surface topography, modulation depths and pitch size of the resulting RWCGs were measured using a Bruker Dimension Edge Atomic force microscope (AFM) on a random 5×5-µm2 grating area that was analyzed using the Bruker Nanoscope analysis software, corresponding to the two perpendicular grating vectors (Kx, Ky). In Figs. 1(a) and 1(b), typical 2D and 3D AFM images of a RWCG with identical orthogonal pitches of 400 nm and a modulation depth of 125 nm are presented. A 10-mW He-Ne laser with a wavelength of 632.8 nm was also used to confirm the resulting grating pitch by measuring the angles between the 0 and ±1 diffraction orders (m) of the grating (Λ= mλ/sinθ). The two orthogonal grating vectors Kx and Ky are identified in Fig. 1(b). With the gDR1 film absorbing light below 600 nm and having an effective refractive index of about 1.57, a random grating pitch and depth were chosen in this work to achieve resonant light coupling in the red region of the visible spectrum (neff = λres/Λ). Any other grating pitch would have resulted in light coupling at different wavelengths according to the waveguiding theory.
Fig. 1. Atomic Force Microscopy images of a resonant waveguide crossed grating (RWCG) with orthogonal pitches of 400 nm on a gDR1 thin-film of 197-nm thickness. (a) 2D image of a 400-µm2 area (Inset is the Fast Fourier Transform), and (b) its corresponding zoomed-in 3D topography of undulations in the directions of grating vectors Kx and Ky (25-µm2 area).
Transmission through and reflection off the RWCGs were measured by exposing them to normally-incident and polarized white light spectrum from a halogen lamp with an irradiance of 15 mW/cm2. As depicted in Fig. 2, the vertically polarized light (denoted as V) was focused onto a 3 mm-diameter area on a RWCG and subsequently the transmitted or reflected light went through a second polarizer in the horizontal direction (denoted as H) before being collected by a photodiode. Note that V is oriented along Ky and H is parallel to Kx of the RWCG. The polarizer orientation depicted in Fig. 2 is hereby identified as VH. The polarizers were also turned in tandem by 90 degrees and the resulting configuration is identified as HV to explore the effect of the incident light polarization. To enable the capture of the reflection spectra at normal incidence, a beam splitter was used to direct the reflected light towards the second polarizer and the photodiode. In all measurements, a filter was used in front of the white light to block wavelengths below 575 nm and reduce the absorption and the cis-trans photoinduced molecular transformation in the gDR1 film [42].
Fig. 2. Schematic of the experimental setup to measure transmission through and reflection off of the resonant waveguide crossed grating (RWCG).
The resonant coupled light into the azobenzene RWCG was measured in transmission and in reflection, as depicted in Fig. 3. When a RWCG with a 400-nm pitch is placed in between crossed polarizers in either the VH or HV configurations, parallel to its grating vectors, two sharp positive resonance peaks (FWHM < 10 nm) centered at 626 nm and 636 nm (measured in transmission in Fig. 3(a)) and at 627 nm and 638 nm (measured in reflection in Fig. 3(b)) are observed.
Fig. 3. (a) Resonant transmission of RWCG and RWLG, and (b) resonant reflection peaks of RWCG, obtained at normal incidence. (c) a schematic representing RWLG and RWCG. All gratings have 400-nm pitches and are placed in between crossed polarizers in either the VH or HV configurations.
Each peak corresponds to light being in-coupled and out-coupled by the crossed gratings into either the s- or p-polarized waveguide modes in the gDR1 film. The resonance peaks seem to be independent of the VH or HV configuration of the polarizers, confirming that the transmitted and reflected light that was coupled into the film is emitted in a polarization state that is perpendicular to that of the incident light. To confirm this effect, the transmission was also measured through the linear grating area depicted as resonant waveguide linear grating (RWLG) in Fig. 3(c). No resonance peak is observed when RWLG is placed in between crossed polarizers (VH or HV), even when the polarization is perpendicular to the grating vector (Fig. 3(a)). This is in agreement with previous reports on 1D photonic crystal slabs [32–34]. Therefore, the fact that positive peaks are being observed just by lateral shift of incident light from RWLG area to the RWCG area (Fig. 3(c)), proves that the resonance response is not due to any misalignment of the sample and therefore another mechanism is governing the resonant response which was not reported previously in the literature. The positive peaks observed in this study for 2D RWCGs, are a result of a novel phenomenon happening via a three-step process: (1) in-coupling of incident light into the waveguide modes of the film by 2D crossed gratings, (2) light transmission and scattering as a mode inside the gDR1 film, and (3) out-coupling of the light with a polarization that is orthogonal to the incident light. Note that the plots shown in Figs. 3(a) and 3(b) are raw unmanipulated data, collected during 24-ms integration time. This shows that light wavelengths that do not couple to a waveguide mode are cancelled out by the crossed polarizers and therefore the background signal is effectively zero.
According to the waveguiding theory [1,3,9], the central wavelength of the s- and p-polarized resonant modes is dependent on the refractive index of the film, the substrate, the superstrate, the thickness of the film, the grating pitch and the angle of incident light. Therefore, the wavelength of the polarization-converted positive peaks, seen in Figs. 3(a) and 3(b) is sensitive to changes to any of the parameters listed above. To illustrate this, the effective refractive index of the azobenzene RWCG can be modulated with an external light beam due to the photochromic nature of the azobenzene chromophores. The effective refractive index of the film can be remotely varied using an external light beam and in turn, it changes the wavelength and intensity of the resonance peak [41].
To modulate the resonant waveguide modes of RWCGs, a vertically polarized (TE) 488 nm Argon ion laser beam with a varying irradiance was used as an external light source and was made incident on the RWCGs at an arbitrary angle of θ∼30°, simultaneously with the normally-incident white light (Fig. 2). The spectra of the transmitted light were measured initially using filtered white light as previously described, in either the VH or HV polarizers configurations as a reference. Then, the transmission signal through the RWCG was recorded once exposed to the blue laser with an irradiance and exposure time just low enough to not cause the destruction of the gratings.
The laser beam, induced local volume birefringence within the azobenzene film thus altering its effective refractive index (neff). In turn, a shift in both the wavelength and the intensity of the out-coupled resonant modes was detected. A RWCG with a 400-nm pitch was placed in between crossed polarizers in the VH or HV configurations. The transmission resonant light from the RWCG was measured first using filtered white light as a reference and then RWCG was exposed to the argon laser with an irradiance of 425 mW/cm2 for 60 s. The laser beam spot size was about 2 mm. A significant change in the light spectrum after 60 s is clear in Fig. 4, since the two initial peaks start to move to lower wavelengths and merge into a single more intense positive peak centered at 630 nm. This is due to the dynamic photoresponse of the azobenzene molecules [43], which depends on the blue laser irradiance, its polarization and exposure time. For instance, once the polarization of the out-coupled resonant light is the same as the polarization of the external pumping laser, the enhancement in the transmission intensity is amplified. Figure 4 shows that the enhancement in the intensity of the resonance peaks after a 60-s exposure to the vertically polarized laser beam, is about 2 times for VH polarizer configuration and 6 times for HV because the out-coupled light in the HV configuration is along the same TE polarization of the laser beam.
Fig. 4. Transmission resonance peaks of a RWCG with 400-nm pitch placed in between crossed polarizers in (a) horizontal-vertical (HV) configuration and (b) vertical-horizontal (VH) configuration, before and after 60 s exposure to a vertically-polarized (TE) Argon ion 488 nm laser light with irradiance of 425 mW/cm2.
The laser-induced birefringence of the RWCG occurs rapidly and it remains as long as the external blue laser is turned on. As soon as it is turned off, the azobenzene molecules slowly relax back and the out-coupled light intensity decreases. To show this effect, the transmission resonant light intensity from the RWCG between VH polarizers was measured first using white light as a reference and then the grating area was exposed to the argon laser with an irradiance of 212 mW/cm2. The relative transmission intensity jumps rapidly from approximately 3 to approximately 10. After 30 s, the laser was turned off and the intensity of the resonance peak was measured every 5 minutes. As it can be seen in Fig. 5(a), even after 25 minutes since the laser was turned off, the intensity of the resonant transmission peak does not return to its initial value and it reaches a plateau around 5. This means that the azobenzene molecules do not return exactly to their original state and a remnant volume birefringence remains. This residual intensity was still present even after a few days. Furthermore, the remnant volume birefringence of azobenzene molecules is a reversible process which leads to a memory effect. The intensity of the resonance peak was recorded at on-off cycles with 1-min intervals once RWCG is exposed to the argon laser with an irradiance of 70 mW/cm2. As Fig. 5(b) shows, the relative transmission intensity starts from a reference point of approximately 2 and at each on-off period the intensity of the resonant peak increases incrementally until it reaches a plateau. Then, the intensity modulates between two discrete levels of 14 (laser off) and 21 (laser on) as if there is a memory effect.
Fig. 5. Transmission through RWCG (a) when placed between a sequence of vertical-horizontal (VH) polarizers and exposed to a vertically polarized Argon ion 488 nm laser light with the irradiance of 212 mW/cm2 for 30 s, then the laser was turned off and the transmission intensity was recorded, (b) when placed between a sequence of horizontal-vertical (HV) polarizers and exposed to on-off cycles of a vertically-polarized Argon-ion 488-nm laser light with the irradiance of 70 mW/cm2 with 1-min intervals.
Therefore, it was experimentally shown in this study that linearly polarized light can be coupled into the optical waveguide modes of an azobenzene thin-film using crossed gratings that are inscribed on its surface. The out-coupled resonant light has a polarization orthogonal to that of the incident light whether it is in transmission or reflection. Also, the out-coupled resonant light from these RWCGs is sensitive to manipulation by an external light source due to the birefringence of azobenzene molecules.
To understand the polarization conversion mechanism caused by RWCGs, consider a model analogous to a system of multiple non-coplanar mirrors that is used to rotate the linear polarization of incident light by 90 degrees [44]. As Fig. 6(a) illustrates, the vertically polarized incident light is traveling towards the first mirror in + x direction and gets reflected towards + y direction. Later, it gets reflected by the second mirror towards + z direction and its polarization is now converted to horizontal. This polarization-converted light can be reflected towards either + x or -x directions by the third mirror. In the case of RWCGs, when vertically polarized light is incident on the grating area, as depicted in Fig. 6(b), it couples into the waveguide modes inside the gDR1 thin-film, and then out-couples from it with a horizontal polarization. Therefore, the crossed gratings act as meta-mirrors [19,45] for converting the incoming linear polarization of light, in addition to being wavelength sensitive [3]. More specifically, Fig. 6(b) is a typical 3D AFM representation of a RWCG with identical 500-nm pitches. The gDR1 film in the x-y plane has a specific thickness in the z direction that supports one guided mode. The incident vertically polarized light travelling in the -z direction (along k1) is coupled into a s-polarized waveguide mode in the x-z plane of the film by grating area A and it travels in the -y direction (along k2) towards a different grating area having a grating vector at 45 degrees from that of grating area A. The light is then scattered by grating area B into the -x direction (along k3) and is now s-polarized in the y-z plane, as if it is reflected off a slanted meta-mirror [19,44,45]. The light that is still travelling in a guided mode is then scattered out of the waveguide by the third grating area C and is now travelling in either the + z (reflected along k4) or -z (transmitted along k5) directions and is now horizontally polarized.
Fig. 6. (a) Schematic for polarization conversion process happening using three non-coplanar mirrors, (b) Schematic for polarization conversion process happening both in transmission and reflection by a resonant waveguide crossed grating (RWCG).
The above hypothetical scenario illustrates one of many scattering paths that light can couple into the waveguide modes using a crossed grating that is inscribed on a thin-film and converts its polarization in both transmission and reflection. The light that undergoes such conversion must be coupled into the waveguide modes of the thin film. This phenomenon occurs at a specific resonant wavelength which depends on several factors such as the film thickness, the grating depth and pitch, as well as the index of refraction of the film as well as its surroundings due to evanescent components of the light in those regions. Note that if the incident vertically polarized light couples into the film by grating area C, the polarization conversion would occur at a different wavelength since the light couples into the p-polarized mode of the waveguide which has different resonant conditions. Therefore, as the incident light couples simultaneously into s- and p- polarized waveguide modes of the RWCG using both gratings A and B, two resonance peaks are observed in transmission and reflection as what is seen in Fig. 3. In addition, the same scenario can be applied to incident horizontally polarized light and polarization conversion to vertical occurs by coupling the light into the same s- and p-polarization waveguide modes. Finally, for a linear grating (for instance if there is only grating A, and B, C do not exist), the incident vertically polarized light couples into s-polarized waveguide mode of the film as explained before. However, this s-polarized light couples out with the same linear grating A having the same vertical polarization. Therefore, once such RWLG is placed in between orthogonal crossed polarizers, no light is being detected (Fig. 3(a)), which is in agreement with Jones vector calculations in a 1D grating [33].
4. Conclusion
It was shown in this study that 2D Resonant Waveguide Crossed Grating (RWCG) fabricated on a single layer of azobenzene molecular glass thin film, transmit and reflect positive resonance peaks once placed in between orthogonally aligned polarizers. This phenomenon was concluded to be due to the polarization conversion occurring as polarized light interacts with the crossed grating nanostructures inscribed on the gDR1 thin film and couples into the film. In addition, it was shown that the central wavelength and transmitted intensity of these positive peaks can be actively modulated by an external light source due to the dynamic photoresponsive behavior of the azobenzene chromophores.
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.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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