We investigated the optical properties of a one-dimensional photonic crystal infiltrated with a bistable chiral tilted homeotropic nematic liquid crystal as the central defect layer. By modulating the nematic director orientation with applied voltage, the electrical tunability of the defect modes was observed in the transmission spectrum. The composite not only is a general tunable device but also involves the green concept in that it can operate in two stable states at 0 V. Under the parallel-polarizer scheme, the spectral characteristics suggest a potential application for this device as an energy-efficient multichannel optical switch.
©2011 Optical Society of America
Photonic crystals (PCs) have received immense attention since their debut in the late 1980’s [1,2]. Thanks for the rapid development of nanotechnology, the concept of the PCs is no longer confined on the paper but becomes reality, permitting close observations, measurements, and applications. The most important feature of PCs is the photonic bandgap (PBG) in which the existence of light is forbidden within a certain range of optical wavelengths. A defect in a PC disturbs the structurally periodic arrangement and localizes the light, consequently generating the defect modes in the PBG. Compared with the two-dimensional and three-dimensional counterparts, one-dimensional (1D) PCs manifested as dielectric multilayers are much easier to fabricate. In 2002, Ozaki et al. introduced a liquid crystal (LC) into a periodical multilayer structure as a defect layer . They discovered the electrically tunable defect modes induced by the tunable effective refractive index due to the optical anisotropy of the LC. Based on this notion, various types of LC defect layers have been designed for 1D PCs and many discoveries of interesting spectrum-tunable characteristics have been made [4–9].
Recently, green energy industries develop vigorously; meanwhile, the green concept is concerned with not only how to generate energy, but also how to save energy. Following this trend, photonic devices of low power consumption are highly desired. Nevertheless, all the above-mentioned PC/LC devices require continuous power supply to achieve a specific spectral property. In this study, we propose a novel device: a 1D PC infiltrated with a bistable chiral tilted homeotropic nematic LC (BHN)  to attain both the tunability of defect modes and the green purpose. The aim is to investigate the transmission spectra which reveal the comprehensive optical characteristics of this PC/BHN device without and with polarizers. The BHN bistable switching mechanisms involve the backflow effect and the frequency-revertible dielectric anisotropy of the dual-frequency (DF) LC as well . The BHN itself and the PC/BHN can perform in two stable states, the tilted homeotropic (tH) and tilted twist (tT) states at 0 Vrms, and two voltage-sustained states, the biased homeotropic (bH) and biased twist (bT) states at 1 kHz and 100 kHz, respectively. We report in this paper what is, to the best of our knowledge, the first demonstration of a PC/LC device with optical bistability.
The BHN made of a mixture of a DF LC and a chiral material was sandwiched between two identical glass substrates each coated with a dielectric multilayer and a conducting film. The BHN was ~10 μm in thickness as determined by the spacers and by simulations as well. The refractive indices of the LC host MLC-2048 (Merck) were n e = 1.7192 and n o = 1.4978 at the wavelength of 589.3 nm and temperature of 20 °C. The chiral dopant S-811 (Merck) yielded a 10.23-μm pitch length in the LC bulk. The periodic structure of the alternative layers of Ta2O5 (n H = 2.18) and SiO2 (n L = 1.47) was fabricated with e-beam evaporation technique upon the cleaned substrate covered with indium–tin oxide (ITO). In order to adjust the central wavelength of the stop band at ~600 nm, the thicknesses of the Ta2O5 and SiO2 layers were determined to be 68.09 nm and 102.37 nm, respectively. The film-thickness control for each dielectric layer was achieved with an optical monitoring system for simultaneous reflectance and transmittance measurements during deposition . The numbers of the Ta2O5 and SiO2 layers were 5 and 4 in the multilayer film on each substrate. A mixed solution of a polyimide typically used for planar alignment of LC molecules (SE-6414) and a polyimide for homeotropic alignment (IDL-V101) was spin-coated on the mirrors and then rubbed with a rubbing machine to promote an antiparallel, chiral tilted homeotropic alignment. The rubbing direction was parallel to the y-axis and the light propagated along the z-axis (Fig. 1 ). The transmission spectra of the PC/BHN were acquired with a spectrophotometer (Shimadzu UV-1601PC). An arbitrary function generator (Tektronix AFG-3022B) was used to supply square-wave voltage to switch the LC molecular orientation between the 4 (tH, bH, bT, tT) states.
3. Results and discussion
Figure 2 shows the transmittance spectra of a dielectric mirror and of an empty cell assembly composed of a pair of identical mirrors. The PBG of the cell is located from 470 nm to 750 nm and the spectral peaks sitting in the bandgap, termed the defect modes, arise from the air gap of ~10 μm as a defect layer. Note the obvious optical losses of the defect modes in the central part of the PBG, which are presumably due to interface roughness and imperfect parallelism of the constituent layers.
Figure 3 illustrates the photographs of the tH, bH, bT, and tT states of the PC/BHN cell between two crossed polarizers with the transmission axis of one linear polarizer being parallel to the rubbing direction. One can see from Fig. 3 that the tH state is not as dark as the bH state (Fig. 3(b)). It is because most directors in the bH state are vertically aligned while the LC directors in the tH state are tilted from the ~70° axis in the y-z plane (Fig. 3(a)). If misalignments between the substrates occur, which is usually the case experimentally, the directors in the tH state form a helical structure with a small conic angle tilted in the pretilt angle. The light leakage due to the askew directors from the transmission axis of the polarizer is responsible for the brighter appearance of the tH state . The bT state appears reddish and the tT state is greenish in the BHN cell (without multilayers) between crossed polarizers, whereas the PC/BHN cell appears cornflower blue and sky blue in the bT and tT states, respectively (see Figs. 3(c) and 3(d)). It is worth mentioning that slight change in brightness (or grey level) of the voltage-sustained states can be controlled by an externally applied voltage with amplitude beyond a threshold. In addition, note that the switching between the bistable tH and tT states can be achieved by supplying short frequency-modulated voltage pulses to permit the BHN to promptly pass through the intermediate bH and bT states . In this case, the voltage-sustained states provide necessary pathways for bistable operation and they, thus, serve as the transient states. Undoubtedly, no voltage has to be applied to sustain the optically bistable tH and tT states.
Transmission spectra of the plain PC/BHN without any polarizers between 550 and 650 nm are shown in Fig. 4(a) . To observe the notable difference between the four states, we choose to focus on the spectral range in the red. Obviously, all of the four states have distinct spectral profiles. While the bH state at 10 Vrms exhibits the defect modes attributed to the sole ordinary refractive index n o, all the other states have more peaks or more complex spectra caused by the effective refractive index n eff. Figure 4(b) shows some defect modes of the bH state and tH state in the wavelength range from 615 to 660 nm. The overlapped signals in the two sets of peaks represent the ordinary defect modes. The other peaks in the tH state can be interpreted as the extraordinary defect modes . They originate from the tilted configuration of this state, yielding an effective index of refraction greater than the ordinary one. As such, the intensity of the extraordinary defect modes in the tH state can be tuned by switching between the tH and bH states. The higher the low-frequency applied voltage is, the lower the transmittance of those peaks becomes. In the extreme condition, the extraordinary peaks diminish as the stable tH state transforms to the highly bH state at high voltages. This finding suggests a similar use of this device, as proposed in an earlier study , to provide the light-on as well as light-off states without the need to employ any polarizers.
It is worth noting that the bT state can be achieved from the bH state by switching the frequency from 1 kHz to 100 kHz of the applied voltage. The interplay between the elastic torque and the suddenly changed electric torque induces the subsequent backflow which, in turn, alters the configuration of the LC director field. The final tilt angle in the bT state is factually dictated by the strength of the high-frequency electric field applied across the cell thickness. High voltage reduces the tilt angle and increases the contribution of the n e component to the resulting effective index of refraction. Redshift [8,14,15] of the defect modes can, thus, be observed when an increasing high-frequency voltage is applied (see Fig. 5 ). This phenomenon is distinct from the blueshift behavior of tunable defect modes monitored in many electrically tunable 1D PC/LC systems where the LC used is of positive dielectric anisotropy [3,9]. The stable tT state is obtained by turning off the high-frequency voltage in the bT state. The configurations of the both bT and tT states are similar to the twisted-nematic configuration or super-twisted nematic alignment inasmuch as the thickness-to-pitch ratio is restrained in a certain range to ensure bistability in the BHN .
Figure 6 depicts the transmittance of the PC/BHN cell placed between two parallel polarizers whose transmission axes lie in parallel to the rubbing direction along the y-axis as shown in Fig. 3. One can see from Fig. 6(b) that either (tH or tT) spectral profile resembles a light comb and that the full width at half-maximum (FWHM) of these defect modes is about 3 nm. Figure 6(b) clearly reveals two different sets of defect modes corresponding to the two stable tH and tT states at null voltage. The bistable switching can be easily realized because the cell can transit from the tH state to the tT state through the transient bH and bT states and the tH state can be switched from the tT state via the bH state [10,11]. This suggests a new promising photonic device with bistability for multichannel optical switch applications. In comparison with the defect modes of the two stable states obtained without a polarizer (Fig. 4(a)), the peaks are more discrete or better resolved under the parallel-polarizer condition. Besides, these defect modes in the cross-polarizer scheme become trivial.
To verify the experimental results, Fig. 7 shows the calculated transmission spectra in the stable tH (Fig. 7(a)) and tT (Fig. 7(b)) states. We calculated the orientation of LC layers using the method of minimization of free energy [17,18]. The optical response was found using the transfer-matrix method, generalized for anisotropic medium [19,20]. Both procedures were implemented in MATLAB. Some of the unknown parameters have been acceptably tuned to reach a reasonable match with the experimental results. They are: ITO film (n ITO = 1.5 + 0.006i, d ITO = 140 nm), glass substrate (n Sub = 1.47), alignment layer (n PI = 1.46, d PI = 500 nm), defect layer (n e = 1.7192 + 0.00078i, n o = 1.4978 + 0.00078i, d = 9050 nm, pretilt angle 73°). The agreement is quite satisfactory between the experimental spectra and the simulated ones. Although the material absorption has been taken into account in the simulations, the peak intensity of the defect modes in our theoretically calculated results are still higher than the in experimental. The discrepancy between measured and simulated spectra results from slight light scattering caused by the structural interface roughness and from other experimental uncertainties including the imperfect fabrication of the multilayers. Moreover, the FWHM of simulated defect modes is about 1.5 nm, which is half-fold smaller than that of the experimental data and must be explained by the same experimental uncertainties.
We propose and demonstrate a bi-functional photonic device with bistability and tunability by using BHN as a defect layer infiltrated in a 1D PC. The existence of four different states of the PC/BHN system stems from the inserted bistable BHN bulk situated at the center of the PC structure. Transmission spectra characterized by various defect modes within the PBGs are obtained and the spectral properties of the two stable states in the parallel-polarizer scheme are verified by simulated results. The defect modes in the voltage-sustained bT state are electrically tunable, shifting to the longer wavelengths with increasing voltage at 100 kHz. The PC/BHN possesses two stable (tH and tT) states and can be switched from one to the other. Under the parallel-polarizer condition, this 1D composite exhibits two separate sets of defect modes with similar strengths belonging to the respective stable states. Such unique property paves a new pathway for the application of 1D PC/BHN cells in, e.g., low-power-consumption multichannel optical switches and integrated photonic devices.
This work was financially supported by the National Science Council of Taiwan under Grant No. NSC 98-2923-M-033-001-MY3 and by Russian Federal Contract Nos. 02.740.11.0220 and RNP.188.8.131.5255 and SB RAS Grant No. 144. The authors gratefully acknowledge the assistance of Min-Hsuan Huang in sample fabrication at the initial stage of this study.
References and links
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