Spectral properties of an electrically tunable one-dimensional photonic crystal infiltrated with a twisted-nematic liquid crystal (PC/TN) are investigated. Two mesogenic materials with dissimilar optical anisotropies are examined for constituting the central defect layer. With the TN alignment of the defect layer embedded in the dielectric multilayers, the defect modes not only shift with the applied voltage but also switch between two major modes when the linear polarization angle of the incident light is altered. The superposition of the mixed-mode TN (MTN) and the photonic bandgap brings out a tremendous undulation in all range of the transmission spectrum. The defect modes falling at the centers of the MTN spectral humps are allowed to intensely transmit while the others are suppressed. As a result, we propose a monochromatic selector constructed by such a PC/MTN device with electrical tunability.
© 2010 OSA
Photonic crystals (PCs) as a fascinating research field in optics has attracted a lot of attention since 1987 [1, 2]. The attraction comes from the unique characteristics, including the photonic bandgap and negative refraction, within a periodic structure in refractive index, and the scale of the period under the wavelength of light . Owing to the uniqueness, liquid crystals (LCs) as a type of intriguing optical material have been introduced into various PC structures. The combination of PCs and LCs lead to new device notions, such as LCs within PC fibers  and color-tunable lasers . One-dimensional (1D) PCs, also known as optical multilayers, have long become a momentous topic in applied optics even decades earlier than the moment when the concept of PC was proposed. To tune the defect modes, Sang and Li proposed a graded defect layer in a 1D PC  while some other groups utilize LCs as defect layers in 1D PCs. The optical property of a 1D PC infiltrated with a homogeneously-aligned LC defect layer was first studied by Ozaki et al. . With a similar structure, Zyryanov et al. discovered interesting transmittance properties of a 1D PC/LC placed between two crossed polarizers [8, 9]. Spectral characteristics of a 1D PC/LC driven by an in-plane field have also been explored . While most previous works concentrate on the tunability of the effective refractive index of the defect layer, the present study aims to investigate the spectral properties of a 1D PC infiltrated with a 90° twisted-nematic (TN) LC as a central defect layer. The blueshift behavior of the defect modes is disclosed in the transmission spectrum with the single-polarizer (SP) setup and the phenomena including the appearance of a peculiar narrow transmission band obtained in the crossed-polarizers (CP) scheme are presented as well. From the experimental and simulated results, we establish a new design concept of the optical device as a monochromatic selector configured by the 1D PC/TN.
In this study, the multilayer on each conductive glass substrate is composed of nine dielectric layers, including five layers of the high-refractive-index material, tantalum pentoxide (Ta2O5) with the refractive index n H = 2.18 and thickness d H = 68.09 nm, as well as four layers of the low-refractive-index material, silicon dioxide (SiO2) with the refractive index n L = 1.47 and thickness d L = 102.37 nm, sputtered alternatively. The alignment layer, SE-8793 (Nissan Chemical), is spin-coated on the top of each multilayer to provide a planar alignment and then rubbed by fabric in perpendicular directions on two substrates. The spacers of a size about 7.3 μm are utilized to determine the defect thickness. Here, we use two different LC materials with distinctive refractive indices and birefringence for the central defect layer. One is the well-known nematic E7 (Merck), with extraordinary refractive index n e = 1.748 and ordinary refractive index n o = 1.523, and the other is a high-resistivity LC material, designated CYLC43, with optical anisotropy Δn = 0.12 at wavelength λ = 550 nm and temperature T =20 °C. All data reported in this work are obtained from a PC/TN cell containing E7 unless specified.
To obtain the transmittance spectrum, a UV-visible spectrophotometer (Shimadzu UV-1601PC) is used which has a resolution of 0.2 nm. The applied voltage is a 1 kHz square-wave signal supplied from an arbitrary function generator (Tektronix AFG-3021B). The nematic director in the light-entrance side is set to be parallel to the x-axis and the incident optical beam is propagated alone the z-axis (Fig. 1 ). The E-mode and O-mode are known in the TN LC display technology where the input polarization angle β are 0° and 90°, respectively. The third experimental geometry corresponds to the condition of β = 45°, which yields the so-called mixed-mode or M-mode . These terms are used for convenience irrespective of whether an analyzer is employed.
3. Results and discussion
Figure 2 illustrates a typical voltage dependence of a defect mode wavelength for two similar PC/TN cells containing different nematic materials in both E- and O-modes. Unlike the voltage independence of a specific defect peak for the ordinary-ray component found in 1D PCs embedding a homogeneously-aligned LC , both the E-mode and O-mode exhibit blueshift with distinct extents. Note that E-mode has a dramatic shift in wavelength, whose extent is strongly dependent of the birefringence, while the O-mode data are similar in terms of the wavelength shift between the two cells. The explanation of these dissimilar behaviors can be traced back to the adiabatic following of the TN cell, for which the transmission properties are first described by Gooch and Tarry with the Mauguin parameter . Owing to the difference in birefringence between the two LCs we use, the Mauguin parameters u, given byFigure 3 shows the u values of these two nematic bulks with specific d and λ in our experimental conditions with a normally-white transmissive TN configuration. Note that the transmittance is derived with a CP condition. Even so, one can realize from Fig. 3 that the PC/TN cell with E7 has greater waveguiding ability than the cell with CYLC43 does. The F2O-based material CYLC43 has lower birefringence and, thus, poorer waveguiding ability as expected. A good waveguide leads the linearly-polarized beam to traverse the LC layer with the rotation of the molecular twist, which makes the effective refractive index for the incident beam nearly equal to the extraordinary one in E-mode and ordinary one in O-mode. As a result, when an external voltage is applied, the defect modes exhibit greater shifts in the E-mode. The smaller shifts observed in the O-mode are attributable to the fact that the elliptic polarization can hardly be avoided in a TN cell.
Note that no spectral shifts are observed for the ordinary ray in a typical PC/LC cell involving electrically-controlled birefringence ; here we deal with the polarization-rotation effect and the term “O-mode,” referring to the initial polarization state as a linearly-polarized beam enters a TN structure, is not exactly equivalent to the ordinary-ray component. A better adiabatic following leads to the larger difference in defect mode wavelength shift between E-mode and O-mode at null voltage as shown by the data of PC/TN composed of E7 (Fig. 2). In order to better manifest the observed phenomena, the larger-birefringence material is chosen in the following experiments.
When the linear polarizer is rotated to the M-mode in the SP scheme, a distinct spectrum can be observed as shown in Fig. 4 . By overlapping the defect modes for the three incident-polarization-angle conditions in the SP scheme, one can see that the peaks of the M-mode spectrum are located at the exactly same positions of the E-mode and O-mode in wavelength and that the intensity of the transmitted light in either E- or O-mode spreads to the other, making the integrated intensity of the peaks in the M-mode almost the same as that in either E- or M-mode. This implies that the M-mode spectrum is a superposition of those of both E-mode and O-mode; it also explains the fine structure characterized by the little peaks in the E- and O-mode spectra for the nonideal experimental data associated with the output nonlinear polarization state.
To further investigate the polarization-rotation effect contributed by the TN layer, a second polarizer with the transmission axis perpendicular to that of the first one is placed at the light-exit side. Based on the previous analysis from Fig. 3, it is easily understood that almost all spectral range of the emerging light can transmit through the analyzer as depicted in Fig. 5 . One might notice the higher transmittance near the band edges for the scheme with crossed polarizers. This is attributed to the experimental artifact and is not shown in the simulation.
A tremendous reshaping of the spectrum is discovered when the polarizers are set to M-mode. Figure 6 reveals the transmission spectra of the PC/TN cell with the M-mode configuration at various applied voltages. The blueshift of spectral features with increasing voltage is unsurprisingly observed; moreover, a few substantial rises and falls are displayed out of the bandgap. Apart from those humps, an anomalous peak appearing at the wavelength of 694.6 nm at null voltage drops to less than 20% transmittance when the voltage is only 0.8 Vrms. The out-of-bandgap breakup cannot be attributed to the effect of the dielectric multilayers; it actually originates from the mixed-mode twisted-nematic (MTN) when the polarization angle β does not equal an integer multiple of π/2 .
To further inspect the interplay between the photonic bandgap and the MTN, simulations are performed and the results are depicted in Fig. 7 . In order to demonstrate some particular conditions, a hump and a valley of the MTN spectrum are adjusted to the central forbidden wavelength of the bandgap as shown in Figs. 7(a) and (b), where the Mauguin parameters are 8.74 and 9.86, respectively. Here the profile of the bandgap in the simulations is calculated for two dielectric multilayers connected by a thin SiO2 layer. The sinusoidal-like curves of the MTN are computed by the 2 × 2 Jones matrix. The results of the PC/MTN are determined by the 4 × 4 Berreman matrix. Figure 7(a) reveals an outstanding peak with more than 80% transmittance appearing at the middle of the MTN hump inside the bandgap. One can even have two such sharp peaks as shown in Fig. 7(b) with the full width at half maximum (FWHM) of only ~0.5 nm. Recall the experimental result in Fig. 6; the similar spike at λ = 694.6 nm is believed to have the same origin. Such enhancement in transmittance of a defect mode by a MTN hump has never been predicted or observed in PC/LC systems until now. Clearly, the incorporation of a MTN into a 1D PC structure allows one to select a few specific peaks with extraordinary high transmittance compared with the rest of the defect modes. And no matter where the MTN valleys of the humps are, the transmittance of the PC/MTN would always be suppressed to nearly vanishment.
In conclusion, the optical transmission properties of electrically tunable defect modes in a 1D PC containing TN LC as a central defect layer are investigated in this paper. The adiabatic following of the polarized light in the TN region dominates the shifting level of the E-mode as a voltage is applied, and the voltage dependence in the O-mode is due to the existence of the elliptic polarization state beyond the Gooch–Tarry minima. The adiabatic following in the PC/TN is further proved in the CP scheme. The most interesting part in this study is the PC/MTN configuration. The MTN enhances the peaks located at the centers of the spectral humps inside the bandgap and yet suppresses the other defect modes. With noticeably high optical transparency and less than 1 nm FWHM of the spectacularly narrow bandpass, this effect practically makes the device not only of use as a light valve, but also a monochromatic selector.
The funding for this research comes from the National Science Council of the Republic of China (Taiwan) under Grant No. NSC 98-2923-M-033-001-MY3 and Russian Federal Grant No. 02.740.11.0220 and SB RAS Grant No. 144. We wish to thank Professor Jy-Shan Hsu for helpful discussion and Wei-Hsin Chen for initiating the simulation.
References and links
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