1. Introduction

Guided-mode resonance (GMR) filters have attracted much interest owing to their ease of fabrication and excellent wavelength-selecting ability, enabling them to be used in numerous devices, including optical filters [1], color filters [2], and biochemical sensors [3]. Typically, a GMR filter consists of a waveguide layer in which light propagates through the waveguide medium, and a grating layer, which acts as a phase-matching element that couples the light into or out of the waveguide layer. The operation of such a filter is based on the GMR effect [4–6], which is described as follows. When the light beam is incident on the filter, that part of the light that satisfies the resonance condition of the grating structure (phase-matching condition) couples into the waveguide layer and becomes quasi-guided modes, which are also called leaky modes. They cannot then be sustained within the waveguide layer. These quasi-guided modes eventually leak out of the waveguide layer and interfere with the incident light beam, generating anomalous reflection and transmission responses. The resonance wavelengths of GMR filters depend mostly on the materials from which they are made and the design of the grating and waveguide structures. However, the resonance wavelengths and optical properties of GMR filters are fixed upon fabrication, limiting their range of practical application. Substantial research has focused on the development of tunable GMR filters, whose resonance wavelengths can be shifted, expanding the optical functionality. One of the simplest methods of shifting resonance wavelengths is to control the angle of incidence of the incident light. At various oblique angles, the resonance wavelengths can be tuned over a very large range [7, 8]. Other commonly used methods for making GMR filters tunable are based on optical tuning [9], MEMS [10], and the thermo-optic effect [11, 12].

Several tunable GMR filters that contain LCs have been realized. Organic liquid crystals (LC) have been by far the most widely investigated tunable materials for integration into inorganic photonic devices, owing to their highly variable refractive index, which can be controlled by external forces. The advantages of LC-based GMR filters include their compact size, low power consumption, and relatively large tuning range. The two main characteristics of LC-based GMR filters are the tunability of their resonance wavelengths [13–15] and the changing of their output optical behavior [16, 17]. The former function involves the use of LCs in cladding layers to vary the effective refractive index of the device and shift its resonance wavelengths. The latter function involves the use of twisted nematic liquid crystals (TN-LCs) as an achromic polarization rotator that controls the polarization of incident light and supports the tunability of both bandwidth [16] and transmittance [17].

Although many tunable GMR filters have been developed, to the best of the authors’ knowledge, no work has demonstrated a tunable GMR filter whose resonance wavelengths and reflectance can be controlled simultaneously. This work demonstrates a full-color, reflectance-tunable GMR filters that incorporates 90° TN-LCs. The tunability and electro-optical properties of the proposed filter were measured and simulated. The simulation agrees closely with the experimental results.

2. Fabrication of sample

Figure 1(a) schematically depicts the structure of the proposed filter, which comprises two parts, which are a GMR chip and a 90° TNLC layer. The GMR chip is composed of a glass substrate, an ITO layer, an anti-reflection (AR) coating layer for 365nm (Brewer Science iCON-16, n = 1.81) and a sub-wavelength grating (SWG). The SWG was made of a photoresist (PR3170, n = 1.6) and a dielectric material (Ta₂O₅, n = 2.09). The dielectric material with high refractive index was used in a waveguide layer to confine resonance modes. The steps of the fabrication of the SWG structure were as follows. First, 160 nm-thick iCON-16 was spin-coated at 3000 rpm for 40 s on the glass substrate, and soft-baked at 175 °C for 60 s. Second, the 110 nm-thick PR 3170 was spin-coated at 4000 rpm for 60 s on the icon16 film and baked at 100 °C for 60 s. Next, an SWG structure with its period of 410 nm and a duty cycle of 35% was patterned by interference lithography with two UV laser beams [18]. The UV laser utilized here was a 355 nm diode-pumped solid-state laser (Cobolt Zouk^TM) with an output power of 20 mW and a coherence length exceeding 40 m. After the photoresist was developed using NMD-W (2.38% TMAH) developer, a high-index layer of Ta₂O₅ with a thickness of 110 nm was deposited on the PR 3170 layer by e-beam evaporation process, and then the SWG structure was formed. Figures 1(b) and 1(c) display scanning electron microscope (SEM) images of the SWG structure after the development of the photoresist and the deposition of Ta₂O₅, respectively. The SEM images were taken by thermal field emission scanning electron microscope (FE –SEM, FEI Inspect F50). To fabricate the 90° TNLC, another glass substrate that was coated with a rubbed polyimide was placed on the Ta₂O₅ layer; the rubbing direction was perpendicular to the direction of the grooves of the SWG structure. The two layers were separated by a 13 µm-thick spacer. Accordingly, when the NLC mixture (E7, Merck) was injected into the cell, the 90° TN state. Finally, a linear polarizer whose transmission axis was parallel to the direction of rubbing of the alignment layer was stuck on the top glass substrate.

 

Fig. 1 (a) Cross-section of designed filter; SEM images of SWG structure after (b) photoresist development and (c) Ta₂O₅ deposition

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3. Results and discussion

Figure 2 schematically depicts the principle of operation of the proposed filter, whose resonance wavelengths and reflected intensities can be tuned by adjusting the angle of incidence and applying an electric field, respectively. To achieve a GMR reflective filter, which exhibits a very large spectral shift in resonance wavelength covering the entire visible spectrum, the resonance wavelength for normal incidence was set at infrared region (~700 nm). The physical parameters of one-dimensional SWG structure such as the period, thickness, and duty cycle as well as the thickness of Ta₂O₅ layer and refractive index of NLC were designed and optimized using Rigorous Coupled Wave Analysis (RCWA) software (Rsoft software). As to 90° TNLC, a sufficiently thick cell gap of 13 µm was selected to satisfy Mauguin condition, which makes 90° TNLC serve as an achromatic polarization rotator. The proposed GMR filter with the one-dimensional SWG structure depends on polarization and the filter is designed for TE-polarized light, so no GMR effect on TM-polarized light occurs. With respect to the selection of resonance wavelengths, the resonance wavelengths of the designed GMR filter depend strongly on angle of incidence. Normally incident light is reflected with only a single resonance peak of zero-order diffraction mode. For off-normal incidence, the single resonance peak is split into two resonance peaks, which are the positive and negative first-order peaks, respectively. As the angle of incidence increases, the resonance peak of negative first-order diffraction mode is blue-shifted, while that of the positive first-order diffraction mode is red-shifted. Attention is focused here on the negative first-order diffraction mode because it reflects within the visible spectrum. The tunable intensities of the proposed filter are tuned as follows. In the V_off state, the LC layer is in the 90° TNLC state, and the top and bottom LC directions are parallel to the rubbing direction (y axis) and to the direction of the nano-grooves (x axis), respectively. The TM-polarized light is generated when obliquely incident ambient light passes through the top linear polarizer. Then, TM-polarized light is rotated through the 90° TN region, and it converted to TE-polarized light. Thereafter, the TE polarized light that satisfies resonance condition of the GMR filter is reflected and other TE-polarized light is transmitted through the bottom glass substrate. Finally, the reflected TE polarized light is converted to TM-polarized light, and passes through the top linear polarizer. In the voltage-on state, the liquid crystal directors of the 90 ^o TNLC are aligned perpendicular to the substrate. When the TM-polarized light that is created by the polarizer passes through the cell, its polarization remains unchanged. Accordingly, all of the TM-polarized light passes through GMR filter, and none is reflected.

 

Fig. 2 Operation of proposed filter with 90° TNLC cladding in (a) Voff and (b) Von states.

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To verify the alignment condition of the 90° TNLC, the proposed filter was observed under a polarizing optical microscope in transmission mode (T-POM). The direction of the nano-grooves (x axis) was parallel to the transmission axis of the bottom polarizer. The POM images under parallel and crossed polarizers, presented in Figs. 3(a) and 3(b), show good dark and bright states, respectively, revealing that the 90° TNLC satisfies the Mauguin condition, and the polarization of the incident light is rotated along the rotation of the LC director axis. The electro-optical performance of the 90° TNLC in the proposed filter at oblique incidence was also investigated. An He-Ne laser (633 nm) was used as a probing light source and another linear polarizer was placed on the bottom glass substrate with its transmission axis parallel to that of the original linear polarizer that was located on the top glass substrate. The voltage was applied to the device from function generator (33220A, Agilent). Figure 3(c) plots the voltage-transmittance (V-T) curves of the 90° TNLC for various angles of incidence. For normal incidence, when zero voltage was applied to the 90° TNLC, the 90° TNLC had the lowest output light intensity under parallel polarizers. As the applied voltage increased, the LC directors of the 90° TNLC gradually reoriented toward the direction normal to the substrates, reducing the phase retardation. Hence, more light passed through the bottom linear polarizer and the output light intensity gradually increased. When the applied voltage was 3 V, the bulk LC directors were almost normal to the substrates; therefore, the linearly polarized light that was produced after the incident light passed through the top linear polarizer all passed through bottom linear polarizer. As the angle of incidence increased to 60°, the output light intensity of the 90° TNLC at 0V under parallel polarizers was at its lowest value, indicating that the 90° TNLC obeys the Mauguin condition [19–21]. However, the required voltage increased with the angle of incidence. When the angle of incidence reached 40°, the Fresnel effect reduced the maximum output light intensity of the 90° TNLC under a sufficient driving voltage reduced [22]. Furthermore, the response time of the 90° TNLC was also measured. The rising time and falling time measured are around 40ms and 180ms, respectively. To ensure the 90° TNLC satisfying the Mauguin condition for full visible region, a relatively large cell gap of 13 µm was chosen herein because of the limitation of manufacture. Typically, the 90° TNLC has a fast response time (~5ms) when it is operated at Gooch–Tarry’s first minimum condition [23]. Besides, the response time can be effectively improved by means of polymer stabilization [24] or overshoot driving methods.

 

Fig. 3 POM images of proposed filter in T-mode under (a) parallel and (b) crossed polarizers. (c) V-T curves of 90° TNLC in proposed filter at various oblique angles of incidence.

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Figure 4(a) plots the variation of the resonance wavelength of proposed filter with the angle of incidence from 0 to 50°. A halogen lamp (HL-2000-FHSA-LL, Ocean Optics) with a spectrum between 400 nm and 880nm was used as the light source and the proposed filter characteristics were examined using an optical spectrum analyzer (USB 4000, Ocean Optics). For normal incidence, the corresponding resonance wavelength of zero-order diffraction mode was 710 nm, which is in the near-infrared region. As the angle of incidence increased, the resonance wavelength of the negative (positive) first-diffraction mode shifted toward shorter (longer) wavelengths. As the angle of incidence increased from 0 to 50 ^o, the resonance wavelength was shifted by 280 nm from 710 nm to 430 nm. The variation in resonance wavelength in both of the first-diffraction modes was calculated using RCWA software. The simulated curve agrees closely with the measured results. Figure 4(a) presents the reflection spectra of the negative first-diffraction mode under many oblique angles of incidence. The resonance wavelengths of the negative first-diffraction mode at oblique angles of 3, 9, 16, 26, and 34 ^o are 690, 650, 600, 550, and 500 nm, respectively. The corresponding FWHMs of the reflected peak are 5, 6, 7, 9, and 10 nm. Although the proposed filter can be extensively tuned ability to shift the reflected peak by changing the angle of incidence, Fresnel reflection at larger angles of incidence cause stronger reflection from the glass substrates, worsening the contrast ratio of the reflective filters. When the angle of incidence is larger than 40 ^o, the Fresnel reflection is stronger than reflected peak of the proposed filter. Therefore, when the proposed filter is operated at large angle of incidence, the Fresnel reflection must be considered or be eliminated using an anti-reflection coating [22].

 

Fig. 4 (a) Dependence of experimental and simulated resonance wavelengths of proposed filter on angle of incidence. (b) Reflection spectra of proposed filter at angles of incidence of 3, 9, 16, 26, 34°.

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Given the difficulty of normalizing the reflectance at oblique incidence, the transmittance rather than the reflectance of the proposed filter was measured. Figures 5(a)-5(d) present the transmission spectra of the negative first-diffraction mode for various applied voltages at angles of incidence of 0, 9, 26, and 45°, respectively. At 0V, the corresponding resonance wavelengths were 710, 650, 550, 450 nm. As the applied voltage increased, the transmittance increased because less TM-polarized light was generated after the incident light that passed through the top linear polarizer was converted to TE-polarized light by 90° TNLCs. Some extra peaks appeared because of the GMR effect on TM-polarized light. Figure 5(e) plots the voltage-transmittance (V-T) curves of the proposed filter at various angles of incidence. For normal incidence, the transmittance of the filter ranged from 12 to 85%, and the voltage that maximized transmittance was approximately 3V. As the angle of incidence gradually increased, the required voltage increased owing to the electro-optic properties of the 90° TNLCs. When the angle of incidence reached 45°, the strong Fresnel reflection from the glass substrate reduced the tuning range of transmittance of the filter between 10 and 45%.

 

Fig. 5 Transmission spectra of proposed filter for angles of incidence of (a) 0, (b) 9, (c) 26, and (d) 45 degrees; (e) V-T curves of proposed filter at various angles of incidence.

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4. Conclusion

In conclusion, this work demonstrates a full–color and reflectance-tunable filter that is based on a GMR device with a 90° TNLC film as cladding. The resonance wavelength can be selected by controlling the angle of incidence, and the corresponding reflected color can be shifted from near-infrared (710 nm) to blue (430 nm). The 90° TNLC film functions as an achromic polarization rotator that controls the polarization of the incident light, so the reflected intensity can be controlled by electrically driving the 90° TNLC film. Such a reflective filter enables the reflected wavelength and intensities to be separately selected. It therefore has great potential for practical use in color filters, optical switching, and even optical signal processing.