We demonstrate in this paper an electrically tunable photonic device based on one-dimensional photonic crystal (PC) infiltrated with polymer-stabilized cholesteric texture (PSCT) as a central defect layer. With the hybrid PC/PSCT structure, not only is the wavelength of each defect mode switchable among three major stable states by various appropriate frequency-modulated voltage pulses, but also the intensity can be electrically tuned in multi-metastable states. As a result, an electrically controllable multichannel photonic device with several alluring features is proposed. It is wavelength-switchable, intensity-tunable, and polarizer-free and possesses optical tristability in the defect modes to reduce power consumption.
©2011 Optical Society of America
Studies of photonic crystals (PCs) have been a fascinating area in the research field of modern optics since 1987 [1, 2]. The PC is a periodic structure characterized by the photonic bandgap (PBG), in which no photons in the corresponding energy range can enter the structure. A high-reflectance optical multilayer film can be the simplest one-dimensional (1D) PC consisting of alternate layers of two dielectric materials with different refractive indices . By inserting a third dielectric substance as a defect layer in the PC structure, narrow spectral windows in the PBG, also known as defect modes, can be generated. The optical properties of a 1D multilayer PC containing an optically anisotropic nematic liquid crystal (LC) layer was first investigated by Ozaki et al., who revealed the wavelength tuning of the extraordinary components of the defect modes by externally applied voltage [4–6]. With a similar hybrid structure placed between two crossed polarizers, Zyryanov et al. demonstrated attractive optical properties of a 1D PC that allows the design of a field-tunable multicolor pixel composed of a single optical cell [7, 8]. Recently, Lin and associates reported a study of spectral properties of a 1D PC incorporating a twisted-nematic (TN) defect layer and discovered an interesting defect-mode selection effect in the mixed-mode TN configuration . Along this line, Wu et al. proposed a tunable, bi-functional, 1D PC device by replacing the TN LC with a bistable chiral tilted homeotropic nematic layer . Moreover, Matsuhisa et al. examined low-threshold single-mode laser action based on the combination of the defect mode in a 1D PC structure comprising the defect layer and the photonic band edge effect in a dye-doped chiral nematic, often termed cholesteric LC (CLC) . Hsiao et al. suggested another bistable 1D PC/LC structure by using a dual-frequency CLC as the defect layer .
With inclusion of a photo-polymerizable monomer in a CLC, the cured composite contains polymer networks to stabilize the LC, yielding the so-called polymer-stabilized cholesteric texture (PSCT). PSCTs can be employed in low-energy-consuming devices due to the generation of a stable state in the homeotropic (H) texture. This permits the bistable switching between the planar (P) and H states in, for example, transflective light shutters . Bistable PSCT shutters can also be electrically switched between the transparent H state and the scattering focal conic (FC) state . As a matter of fact, PSCTs may possess more than two stable modes; it is a pity that neither of previous works takes such advantage presumably due to the difficult tuning of many parameters for cell fabrication.
In this paper, we report on a multichannel PC device with a key layer of PSCT made from a dual-frequency CLC impregnated with a photocurable monomer. Dual-frequency LCs are characterized by the frequency-revertible dielectric anisotropy such that it is positive to impel the tendency for the LC director in parallel to the field direction in the low-frequency range and becomes negative to incline the director to be perpendicular to the field at frequencies beyond the crossover frequency . Our 1D PC/PSCT device can be switched back and forth from one state to another among the P, FC and H textures by frequency-modulated voltage pulses and each state remains stable after the removal of a pulse signal (see Fig. 1 ). The helical axis in the P state is normal to the cell substrates while in the scattering FC state the LC exists in randomly-oriented polydomains. When a high voltage pulse (V2 in Fig. 1) at a low frequency (1 kHz in this work) is applied, dielectric forces cause the helix axis to reorient and the helix unwinds to generate a homeotropically aligned nematic phase. Here the H state persists in the absence of the electric field because of the stabilizing polymer network. To the best of our knowledge, this work is the first to realize such electrical, tristable switching in a CLC and thus the first demonstration of a PC/LC device with optical tristability. Some metastable states between the tristable states; namely, the P–FC mixed (PFM) states and FC–H mixed (FHM) states as displayed in Fig. 1, are also investigated. Those metastable states are valuable for tuning the defect modes .
The mixture to be introduced into an empty PC cell was prepared by mixing a chiral agent (S811, Merck), a photocurable monomer (RM257, Merck), a photoinitiator (DAROCUR-1173, Ciba) and a dual-frequency nematic LC (MLC-2048, Merck) with a clearing point of ~106.2°C. The weight ratios were 4.7:6:0.6: 88.7, respectively, giving rise to a pitch of ~1754 nm to reflect in the infrared. The mixture, possessing a crossover frequency of ~30 kHz at 26 ± 1°C, was sandwiched by capillary action in isotropic phase (above the clearing point) between two electrically conducting glass substrates covered with identical dielectric multilayer films playing the role of 1D PC structures. The multilayer film on each substrate consisted of alternatively five layers of Ta2O5, with the refractive index nH = 2.18 and layer thickness dH = 68.09 nm, and four layers of SiO2 (nL = 1.47 and dL = 102.37 nm) as shown in Fig. 2(a) . The resulting PBG ranged from 470 to 740 nm. Ref. 9 provides detailed information concerning the dielectric mirrors and the structure of the empty PC cell including alignment layers to promote planar alignment. The cell gap was about 11 μm. The transmission spectra of a single dielectric mirror and an empty cell with an air gap can be found in Ref. 10. To polymerize the monomer, the cell was irradiated by ultraviolet (UV) light at wavelength of 370 nm. The curing intensity was 10 mW/cm2 and the curing time was 65 s. During polymerization, a sufficiently high low-frequency voltage (60 Vrms at 1 kHz) was applied across the cell to impose a homeotropic texture where the LC and polymer molecules were aligned along the cell normal , thereby forming the desired PC/PSCT structure (Fig. 2). In order to fabricate a uniform PSCT, a reflecting mirror was arranged beneath the cell when the UV light illuminated the primitive PC/LC hybrid from above. An arbitrary function generator (Tektronix AFG-3022B) was employed to supply various frequency-modulated square-wave voltage pulses to switch the states of the PC/PSCT cell. The transmission spectra were measured with a spectrometer (Ocean Optics HR2000 + ). All experimental data were acquired at 26 ± 1 °C.
3. Results and discussion
Figure 3 shows three photographs and the corresponding micrographs of the PC/PSCT cell placed between two crossed linear polarizers in the tristable P, FC and H states. One can clearly observe that the transmissive colors are distinctive in the three different states at null voltage. The P state appears light purple because the defect-mode transmittance is higher in red wavelengths as shown in Fig. 3(a). The FC state exhibits a light-scattering texture due to the randomly oriented multi-domains in the PSCT; its royal-blue appearance is caused by the spectral characteristic of the sole PC in conjunction with the vanishing defect modes as presented in Fig. 3(b). Owing to the LC carefully made to be optically tristable, this photonicdevice can also be demonstrated in the stable H state with some light leakage under crossed polarizers as depicted in Fig. 3(c). It is worth emphasizing that the P, FC, and H states are preserved after removal of the voltage, and that the cell can be switched directly from one to another by suitable frequency-modulated voltage pulses.
The spectral properties hereafter were obtained at various agitating voltage pulses and frequencies without using any polarizers. Figure 4 depicts the transmission spectra of a pristine (empty) PC cell and the filled (PC/PSCT) counterpart in the three distinctive states at null voltage. The spectral oscillations within the PBG as seen in Fig. 4 are caused by the defect layer, either the air gap or LC bulk in this study, in the periodic PC structure. The number of these oscillations, known as the defect modes or defect peaks, increases with increasing defect-layer thickness or, to be more precise, optical path length. Here we apply a fixed voltage (50 Vrms) at various frequencies to display the tristable states. Figure 4(a) shows the most intense defect modes in the empty PC cell due to the air gap. With a PSCT layer embedded in the PC, it is clear from Fig. 4(b) that the cell, initially in the H state, creates more spectral windows in the PBG because of the higher (ordinary) refractive index ~no of the LC layer compared with that of the air. The FC state is demonstrated when a 30-kHz voltage pulse is applied across the cell thickness, giving a low level of transmission of the defect modes in the PBG as shown in Fig. 4(c). This light-scattering state can be employed to switch off the defect modes. When the frequency increases to 100 kHz, the PC/PSCT is in the P state, giving rise to the redshifted defect modes as well as increase in defect-mode number in comparison with the spectrum in the H state (Fig. 4(d)). This phenomenon is attributable to the increasing effective index of refraction ~(ne + no)/2 in the P state with the contribution of the extraordinary refractive index ne of the LC.
Figure 5 illustrates the transmission spectra of the PC/PSCT cell in the H, FHM, and FC states induced by various pulse signals of varying voltage amplitude at a fixed high frequency of 100 kHz. The H state of the cell is the initial state without bias (0 Vrms). By applying a voltage pulse of, say, 10 Vrms or 25 Vrms, a FHM state can be achieved from the H state. Furthermore, a 40-Vrms voltage pulse brings about a typical light-scattering state, causing the transmittance of the defect modes to drop (< 0.5%) in the spectral range between 550 and 650 nm. The intensity strength of the defect modes can be tuned by varying the voltage. If one continues to increase the voltage to 60 Vrms, the P state will be attained in the PSCT. Similarly, one can employ a 1-kHz pulse of nontrivial voltage, say, 20 or 60 Vrms, to switch the PSCT from the FC state back to a FHM state or to the H state.
Figure 6 delineates the transmission spectra of the defect modes induced by various voltage pulses of 0, 10, 25, 40 Vrms at a low frequency of 1 kHz. The P state is exhibited at null voltage. Moreover, the intermediate (PFM) states are shown at 10 and 25 Vrms in the transmission spectra. When the voltage pulse is raised to 40 Vrms, the strongest scattering FC state is demonstrated in the PC/PSCT structure. On the other hand, the H state results when the pulse amplitude is increased to 60 Vrms. Note that the FC state can be switched back to P or PFM states by applying a nontrivial high-frequency (e.g., 100 kHz) voltage pulse, with 70 Vrms to achieve the P state. Evidently, one can modulate the strength of defect modes within a certain extent in mixed states near the H and P states. This powerful photonic device has potential to expand the application, enabling its use as an electrically controllable and optically tristable multichannel filter without requiring any polarizers. All the defect modes remain stable for months for the P, FC, and H states and are stable for hours in all mixed states after removal of the eliciting voltage pulse.
Optical transmission properties of an electrically switchable multichannel photonic device have been investigated in this study. The tunability is achieved by incorporation of an electrically controllable PSCT as a defect layer in a 1D multilayer PC. This hybrid structure possesses three stable states; i.e., the P, FC and H states, for defect modes and it can be directly switched from one to another state by employing a proper frequency-modulated voltage pulse on the PC/PSCT cell. Because of the tristability, the optical defect modes remain at zero voltages. This 1D PC/PSCT composite exhibits different defect-mode transmission spectra in different optical states. On the other hand, the intensity of the defect modes can be regulated by the amplitude of voltage in the mixed states and the wavelengths can be switched by the frequency of voltage in the H and P states. Owing to the capabilities for tristable switching as well as wavelength and intensity tunability in the defect modes, the PC/PSCT device investigated in this study can be used as a low-power-consumption multichannel filter, light shutter or an electrically controllable intensity modulator without any polarizers, making the PC/PSCT device promising for photonic applications.
This research is financially supported by the National Science Council of Taiwan under Grant No. NSC 98-2923-M-033-001-MY3 and by SB RAS Grants Nos. 5, 21.1 and 144.
References and links
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