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Optical modulation of guided mode resonance (GMR) is demonstrated in a waveguide grating structure (WGS) which contains a disperse-red1 (DR1)-doped poly(methylmethacrylate) (PMMA) cladding layer. The resonance wavelength of a GMR mode can be tuned by pumping the cladding layer with a 442 nm wavelength laser beam, because of photoinduced refractive index change in the layer. The resonance wavelength shifts to shorter wavelength side, and the shift increases with pumping power, up to a maximum shift of 5 nm. A detector was used to monitor the intensity of the light that was reflected from the WGS at the wavelengths of the GMR peak positions, and the WGS was found to exhibit optical modulation with a shortest switching time of less than 0.3s.
©2011 Optical Society of America
1. Introduction
Waveguide grating structures (WGS), which typically comprise a substrate, a waveguide layer, and a grating layer, can generate sharp reflection and transmission anomalies that are associated with the guided mode resonance (GMR) effect [1–5]. GMR occurs when an incident light beam is reflected or transmitted by coupling with the resonance modes in the waveguide layer. The wavelength at which GMR occurs depends strongly on the orientation and polarization of the incident light as well as the refractive indices of the constituent materials in the structure and that of the environment. The band width of GMR can be designed to be as small as a tenth of a nanometer [2,6–8]. Therefore, GMR has attracted much interest because of its potential applications, including in polarization-dependent filters [1], narrow-band filters [2], color filters [9], bi-sensors [10,11], and the manipulation of optical signals [12–17]. Recently, photoaddressable polymers, such as azobenzene, have been adopted in the optical manipulation of coupling between incident light and optical devices. Nau et al. [16] demonstrated optical switching of a quasi-guided mode in metallic photonic crystal slabs with photoaddressable polymers. Yang et al. demonstrated the optical tuning of a GMR filter in which was incorporated azobenzene liquid crystal [17]. Both works demonstrated that polymer that contains azobenzene is a promising material for use in an optically tunable GMR filter. However, in these works, the tuning of GMR filters was achieved either on a time scale of minutes or simply irreversibly, mainly owing to the long thermal relaxation time of the azobenzene polymer. For practical purposes, the speed of the optical tuning of GMR filters must be improved. Such an improvement can be achieved by reducing the relaxation time of azobenzene polymers.
This work demonstrates the optical modulation of GMR on the time-scale of a few tenths of a second using a WGS design that exploits a disperse-red 1 (DR1)-doped poly(methylmethacrylate) (PMMA) cladding layer. Based on previous investigations [18–22], the DR1 molecules can freely rotate in a DR1/PMMA guest-host system, causing them to respond quickly to the switching of excitation light. A theoretical method that was based on a 2D grating [23,24] and waveguide mode [4] equations were used herein to calculate the GMR peak positions of the WGS. The calculations agree with the experimental results, verifying that the temporal change of the refractive index of the cladding layer caused by the excitation light is responsible for the modulation of GMR. For WGS fabrication, a simple and low-cost double-exposure two-beam interference technique [25,26] was employed to record a two-dimensional sinusoidal square-lattice grating structure with a large area on the top of a waveguide structure.
2. Sample preparation and measurement setup
2.1 Design and fabrication of WGS
Figure 1(a) schematically depicts a two-dimensional (2D) WGS, which contains a 2D sinusoidal square-lattice grating layer (SU8), a guiding layer (SU8) and a cladding layer (DR1/PMMA) on the top of a glass substrate. To form the guest-host DR1/PMMA cladding layer, a chloroform solution of DR1/PMMA, in which DR1 molecules were doped into PMMA with a concentration of 6 wt%, was firstly prepared. This solution was spun-cast on the top of a glass substrate. The DR1/PMMA cladding layer had a refractive index (nc) of 1.534 at 633 nm wavelength and a thickness (TC) of 3.2 µm. The absorption band of the DR1/PMMA layer was from 400 nm to 600 nm with a peak position at 488 nm and an absorbance at the pumping wavelength, 442 nm, of 1.3, decreasing to 0.57 during exposure to the pump beam at 50 mW. The absorption of the cladding layer was expected to be inhomogeneous in the direction normal to the film. However, based on the results below, this effect was not crucial to the optical modulation of the GMR in the WGS. From numerical analysis, the electric filed of the GMR mode tunnels into the bottom cladding layer with a range estimated to be 1.5 µm. The cladding layer of 3.2 µm thickness (>1.5 µm) was chosen to ensure that the GMR mode can experience a sufficient refractive index change. To form the guiding layer, a layer of SU8 photoresist, designed with thickness (TW) of 1.24 µm and refractive index (ns) of 1.58, was first spun-cast on the top of the DR1/PMMA layer and was then photopolymerized. After depositing the SU8 guiding layer, its thickness and refractive index were measured to be 1.1 µm and 1.578 at 633 nm wavelength, respectively. The thickness varied less than 100 nm using the same fabrication parameters. They were close to the designed values and the small difference between designed and fabricated structures did not affect the GMR property; for example, both designed and fabricated structures were multi-mode. The refractive indices and thicknesses of the cladding and guiding layers were determined using a prism coupler and by making alpha-step surface profile measurements, respectively. To produce a GMR mode around 650 nm, a 2D square-lattice grating with a lattice constant (Λ) of 585 nm was designed on the top of the SU8 guiding layer. After spin-coating a second SU8 film on the top of the guiding layer, the double-exposure two-beam interference technique [25,26] was used to pattern the grating structure. The two laser beams was arranged with a mutual angle of 32.3° to get the desired lattice constant of the grating. After examining by a scanning electron microscope (SEM) and an atomic force microscope (AFM), the lattice constant and maximum modulation depth (TG) of the grating structure were determined to be 586 nm and 220 nm, respectively. Figure 1(b) presents a SEM image of the grating structure.
Fig. 1 (a) Schematics of the 2D WGS. From top to bottom are: 2D sinusoidal square-lattice grating layer (SU8), guiding layer (SU8), cladding layer (DR1/PMMA) and glass substrate. (b) SEM image and its zoom-in view (inset) of the 2D grating structure.
2.2 Reflection spectra measurement and pump-probe experimental setup
Figure 2 displays the setup for measuring reflection spectra. A halogen lamp was the white light source, from which the light was passed through an iris to yield a beam with a diameter of 1 mm; its polarization was set using a polarizer. The reflection spectra were collected and analyzed using a grating spectrometer (Ocean 2000) with a wavelength resolution of 0.3 nm. Figure 2 also presents the setup of the pump-probe experiment. A He-Cd laser provided the pump beam with a wavelength of 442 nm, and its power was controlled by the combination of a half-wave plate and a polarizer. The pump beam was incident from the glass substrate side and focused on the sample by a lens with a focal length of 10 cm. The diameter of the focal-spot of the pump beam was about 2 mm which covered the whole area of the probe beam spot. The sample orientation is described with coordinate system at the bottom. The z-axis coincides with the surface normal direction and the incident plane of the reflection measurement coincides with the y-z plane. The symmetry directions of the 2D grating were assumed to align with the x- and y-axes.
Fig. 2 Experimental setup for reflection spectra measurement and inset is a He-Cd laser with 442 nm as the pump beam for the pump-probe experiment. θ: incident angle, λ/2: half-wave plate, L: lens and P: polarizer.
3. Experimental results and discussions
3.1 Angle-resolved reflection spectra of GMR
Figure 3 shows the specular reflection spectra of the WGS for both transverse-electric (TE) and transverse-magnetic (TM) modes at the different incident angle (θ). The dips between 400 nm and 600nm in all of the reflection spectra resulted from the absorption of the DR1/PMMA layer. Figure 3 displays three reflection peaks in the wavelength range from 600 nm to 950 nm under TE polarization, and two under TM polarization. As the incident angle increases, all reflection peaks are blue-shifted. The open squares indicate the GMR peak positions obtained by 2D grating [23,24] and waveguide mode [4] equations for the WGS. Note that the calculation only provided GMR peak positions and could not give reflectivity. The calculation parameters, shown in the caption of Fig. 3, are chosen based on refractive indices and thicknesses of the designed structure. Note in the experimental results that the GMR modes (modes 1 and 2) at long-wavelength region were non-degenerate. This indicates that the sample was rotated azimuthally about 7° around the surface normal direction. As shown in Fig. 3, the calculated and measured reflection peak positions agree with each other qualitatively, which reveals that these peaks are signatures of the GMR modes of the WGS device. There is a small difference between the measured and calculated GMR peak positions at long-wavelength region, which is probably due to the minor difference between the real and designed structures. Compared to the reflectivity of the GMR in a 1D grating WGS [17], the GMR modes have relatively low reflectivity (~10%), which is probably associated with the multiple planes of diffraction that are generated by a 2D grating [24]. Multiple resonances are obtained by matching the guided modes in the different planes of diffraction to different wavelengths [24]. The low diffraction coupling efficiency of the 2D grating may be another reason for the low reflectivity of the GMR modes. The first-order diffraction coupling efficiency of the WGS was found to be 4.1% using a He-Ne laser at 633nm and an incident angle of 25°.
Fig. 3 Angle-resolved reflection spectra of the WGS at the different incident angle (θ) for (a) transverse-electric (TE) and (b) transverse-magnetic (TM) modes. The reflection spectra were normalized with the reflection spectrum from a microscope slide using the same light source. Open square (□) denotes the calculated GMR peak positions obtained with 2D grating [23,24] and waveguide mode [4] equations. The parameters used in the calculation are Tw = 1.24 µm, ns = 1.58, nc = 1.53, Λ = 585 nm, and azimuthal angle ϕ = 7°. For clarity, each curve was shifted to have 0.5 unit spacing between adjacent plots.
To demonstrate the optical modulation of the GMR modes in the grating structure using the pump-probe approach, the TE GMR mode that was associated with the peak at 650 nm at θ = 25°, whose bandwidth was 2 nm, was chosen. This mode was close to the absorption edge of the DR1/PMMA cladding layer, and so was expected to be more sensitive to a change in the refractive index of the cladding layer that resulted from the photoisomerization of DR1 molecules [18–22].
3.2 Optical tunable GMR
Figure 4 shows the reflection spectra of the GMR mode that was pumped by a He-Cd laser at 442nm with a fixed excitation power (70 mW) for various exposure times: 0.3 s, 5.1 s, 14.7 s and 47.4 s. Figures 4(a) and (b) present the results obtained with the parallel (co-polarized) and orthogonal (cross-polarized) relative polarization, respectively, of the pump beam and probe beam. In both cases, the GMR peak was blue-shifted with exposure duration. The co-polarized configuration yielded a faster and larger GMR shift (5 nm) because of a larger change in the refractive index in the direction parallel to the pump beam polarization than in the direction perpendicular to the polarization of the pump beam [18,19]. Notably, no photoinduced shift of the GMR peak was observed for the WGS that did not contain the DR1/PMMA cladding layer. Figure 4(c) plots the dependences of the GMR peak position for both co-polarized and cross-polarized configurations on exposure time. For both configurations, the GMR peak is initially blue-shifted dramatically, slowly reaching a steady wavelength at saturation. Figure 4(d) plots the calculated GMR peak position as a function of nc, obtained using the 2D grating [23-24] and waveguide mode [4] equations. The calculations indicate that a reduction of nc by 0.045 blue-shifts the GMR peak position by 5 nm.
Fig. 4 Reflection spectra of the GMR mode of the WGS pumped by a He-Cd laser at 442 nm with a constant excitation power (70 mW) under different exposure times: 0.3, 5.1, 14.7 and 47.4 (s), for co-polarized (a) and cross-polarized (b) configurations. (c) The exposure time dependences of the GMR peak position for co-polarized and cross-polarized configurations, respectively. (d) The theoretical calculation results of the GMR peak position versus nc.
Figure 5 illustrates the dependence of the maximum GMR peak shift on the pumping power for both co-polarized and cross-polarized configurations after 50 s of exposure. The shift increases linearly with pumping power in both cases. For the same pumping power, the maximum GMR peak shift of the co-polarized configuration exceeds that of the cross-polarized configuration. For both configurations, the maximum GMR peak shift saturates at a pumping power of higher than 70 mW, presumably because of the saturation of change of the refractive index in the DR1/PMMA layer.
Fig. 5 The maximum GMR shift versus the excitation power. The straight lines are for the guidance of eye.
3.3 Optical modulation of GMR
The optical modulation of the GMR mode is demonstrated by monitoring the reflection signal from the WGS at two wavelengths while the cladding layer is excited using a pump beam. A pump beam with a power of 50 mW, which could shift the GMR mode by 3-4 nm, based on the results in Fig. 5, was used. The reflection signals from the WGS were monitored with two channels - one set at 650.2 nm and the other at 647.0 nm. Figure 6 presents the results. The pump beam intensity was cycled between 0 and 50 mW, as shown in the bottom plot. When the pump beam was off, the GMR reflection signal peaked at 650.2 nm. It was high when the channel was set to this wavelength, and was low when the channel was set to 647.0 nm. When the pump beam was switched on, the GMR peak shifted from 650.2 nm to 647.0 nm, causing the signal at the 650.2 nm channel to drop and the signal at the 647.0 nm channel to rise. The observed switching time constant for the 647.0 nm channel was less than 0.3 s for both switching on and switching off. For the 650.2 nm channel, the time constants for switching on and off were less than 0.3 s and 5 s respectively. The difference between the time constants is explained by the time dependence of the GMR peak position shown in Fig. 4(c). Once the pump beam is being switched on, the peak shifts immediately away from the 650.2nm to a position that is quite close to the saturation wavelength, and continues to change gradually. Therefore, the drop in the signal at the 650.2 nm channel almost equals with the rise in the signal in the 647.0 nm channel. A similar result is obtained when the pump beam is switched off. The GMR peak is immediately red-shifted toward 650.2 nm upon switching off of the pump beam but full recovery takes longer: the time depends on the degree of randomization of the orientation of azo-molecules through thermal activation. The time constant for DR1 molecule orientation randomization is of the order of seconds. Accordingly, the signal at 650.2 nm takes longer time to recover. Notably, that no damage to the sample was observed after cycles of on-off switching.
Fig. 6 Demonstration of optical modulation of the GMR. Black line and red line are reflection signals recorded by two individual channels at 650.2 nm and 647.0 nm, respectively. Dotted line at the bottom represents the optical pumping signal.
4. Conclusions
This work presents a simple and promising method for realizing all-optical modulation with a GMR filter that contains WGS that is fabricated by the two-beam interference technique as the top layer, and a cladding layer that is composed of a guest-host DR1/PMMA film. The sharp peaks, with bandwidth of 2 nm, in the angle-resolved reflection spectra from the WGS for TE and TM polarizations reveal the presence of the GMR modes. The GMR mode is highly sensitive to any change in the refractive index in the cladding layer that is caused by the photo-isomerization of DR1 molecules induced by the pump beam. This is manifest in the modulation of the peak position of the GMR mode in both the co-polarization and the cross-polarization measurement configurations. From the prediction of theoretical calculation using 2D grating and waveguide mode equations, the maximum 5 nm shift of the GMR peak observed in experiment is corresponded to a 0.045 change of refractive index in the cladding layer. A detector to monitor the intensity of the light that was reflected from the filter at the wavelengths of the GMR peak positions demonstrated the effectiveness of the proposed GMR filter in optical modulation, with a shortest switching time of less than 0.3 s.
Acknowledgments
The authors gratefully acknowledge financial support from the National Science Council, Taiwan, under grant Nos. NSC 98-2112-M194-008-MY3, NSC 99-2112-M194-008-MY3 and NSC 98-2811-M194-007. J. H. Lin acknowledges the support of postdoctoral fellowship from National Science Council, Taiwan.
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