A fast-switching bistable optical intensity modulator is demonstrated. Using a dual-frequency cholesteric liquid crystal, the direct switching is achieved from the scattering focal conic state to the transparent long-pitch planar state. In comparison with the bistable cholesteric devices proposed previously, our device, characterized by its capability of direct two-way transitions between the two bistable states, possesses a very short transition time from the focal conic state to the planar state as short as 10 ms. No voltage has to be applied to sustain the optical states, making the device low energy consuming. Potential applications of this device are addressed.
© 2011 OSA
Optical intensity modulators such as light shutters based on liquid crystals (LCs) have been intensely studied in the past decades [1–7]. The major scattering types of LC light shutters are classified into polymer-dispersed nematic LCs (PDLCs) and cholesteric LCs (CLCs). Compared with PDLCs, CLC light modulators have many advantages such as possibility of stable states and low driving voltage. The dual-frequency CLC (DFCLC) intensity modulators can operate in the transparent homeotropic (H) texture or reflecting planar (P) state by a frequency-modulated voltage . These modulators have low transmittance in the light-off state, enabling their use as light shutters. By incorporating a photocurable monomer into a DFCLC, the cured composite exhibits a H texture as a new stable state, permitting the bistable switching between the P and H states . Stabilized by polymer networks, this type of CLC is called the polymer-stabilized cholesteric texture (PSCT). Bistable PSCT shutters based on DFCLCs can also be electrically switched between a transparent state and a scattering focal conic (FC) state . Although PSCTs have much potential, they have some drawbacks such as high voltage pulses and slow transition time. To broaden the spectrum of applications with a concern of energy saving, a fast-switching light modulator possessing low-voltage-pulse-induced bistability is highly desired.
Fundamental physics and optical properties of CLCs as well as transitions among cholesteric textures are well documented in the literature . Conventionally, CLC switching from the P state to the FC state is induced by an ac signal in square waves. When a high voltage beyond a critical value is applied, the CLC will go into the H state. Subsequent transition to the P state can be achieved if the field is turned off quickly or to the FC state if the high voltage is turned off slowly. In this driving scheme, the transition from the FC to P state is accomplished through an intermediate state; i.e., the H state, and the transition time is very long due to the slow H-to-P transition . Alternatively, the dynamic driving scheme creates a further transient P state to achieve faster switching from the FC to the P state, with a total transition time of the order of 100 ms . Both the conventional and the dynamic drive schemes employ indirect transition paths to switch from the bistable FC state to the P state. In any case, switching from either the P or the light-scattering FC state to the other has been proposed for a variety of LC applications such as cholesteric displays, reverse-mode light shutters, and other electro-optical devices .
In this paper, we report on a fast-switching bistable transmissive DFCLC device made of a nematic host doped with a chiral material. It switches bidirectionally between the P state and the FC state by frequency-modulated voltage pulses as show in Fig. 1 . In the transparent P state the helical axis is normal to the cell substrates and light in the near infrared is selectively reflected while in the scattering FC state the LC exists in randomly-oriented poly-domains. Both the P and FC states are stabilized at zero voltage.
The LC host material used was the nematic MLC-2048 (Merck) with the clearing point 106.2°C. The chiral dopant was S811 (Merck) and the concentrations used in this study were 0, 5, 9, 10, 11, 12, 13 and 15.5 wt%. The resulting DFCLC materials were injected into 11.6-μm-thick planar cells by capillary action in isotropic phase. For the high doping level of 15.5 wt%, mediocre alignment was exhibited in the P state. Fortunately, it could be improved by thermal annealing  and the obtained uniformity persisted after the shutter was switched from the P to FC state and then switched back to the P state. In order to examine the frequency-dependent dielectric properties, a LCR meter was used. A He–Ne laser operating at the wavelength of 632.8 nm was adopted and no polarizers were employed in the electro-optical measurement. An arbitrary function generator (Tektronix AFG-3022B) was employed to supply various frequency-modulated square-wave voltages to switch the states of DFCLCs. The transmission spectra of the DFCLCs were acquired with a spectrophotometer (Shimadzu UV-1601PC). The experimental temperature was fixed at 27 ± 1 °C except the dielectric measurement.
3. Results and discussion
DFLCs have frequency-revertible dielectric anisotropy [10,11]. When a low-frequency electric field is applied, the dielectric anisotropy is positive and the LC director tends to be parallel to the field direction. On the other hand, when the LC is subject to a high-frequency driving signal, the dielectric anisotropy becomes negative and the director tends to be perpendicular to the field. Apart from the frequency dependence of the dielectric relaxation, DFLCs are sensitive to the temperature . Figure 2 shows the relationship between the crossover frequency f c and the absolute temperature T. The behavior of the increase in f c with increasing T satisfies the Arrhenius equationFig. 2 that the crossover frequency blueshifts as the concentration of the chiral dopant increases or the pitch length decreases. After grasping the DFCLC dielectric relaxation properties in relationships with the chiral-dopant concentration and cell temperature, the device characteristics can be easily tailored. Figure 3 presents the optical polarizing micrographs of a 15.5 wt% DFCLC sample imposed by various voltages at 1 kHz. The original (undisturbed) state is planar at 0 Vrms; the cell exhibits the FC state at ~15–45 Vrms. Finally the texture appears to be homeotropic when the voltage is over ~45 Vrms. Note that the similar sequential variations of the optical appearance can be observed for other samples with different dopant concentrations. For example, a 10 wt% DFCLC cell shows the optical textures alike at lower voltages, giving the dark state at the critical voltage of 35 Vrms.
With transmittance smaller than 10%, our 15.5 wt% sample has a stable scattering (translucent) FC state as shown in Fig. 4 . Furthermore, the stable transparent (bright) state of the P texture has about 90% transmittance in the visible and Bragg’s reflection is centered at 870 nm in the near-infrared region. Obviously, the scattering power (or opacity) of the FC state is not high enough to warrant the use of this device as an ideal bistable light shutter. This problem can easily be solved by inserting an optical attenuator at the expense of the light-on transparency.
To demonstrate the realization of the fast, direct two-way transitions, a switching scheme is designed as depicted in Fig. 5 , where the “high frequency” represents 100, 200, 300 or 400 kHz for cell switching actually used in this study. As suggested by Fig. 3, various extents of scattering are possible by using voltage pulses with different amplitudes. To avoid the dielectric heating and consequent f c change (i.e., blueshift) caused by high voltage, the pulse amplitude is fixed at 20 Vrms in this study. Figure 6 illustrates the transition responses and bistable property between the FC state and the P state of the 10 wt% DFCLC switched with 100 kHz and 1 kHz voltage pulses. The transition time, defined as the time interval between 10% and 90% of the maximum and minimum transmittance difference, is ~10–20 ms from the P state to the FC state regardless of the dopant concentration. However, the FC-to-P transition time varies significantly with the chiral-dopant concentration as revealed in Table 1 . For a given high frequency, the transition time increases in general with increasing dopant concentration due to the more randomly oriented multidomains corresponding to the shorter pitch in the initial FC state. Noticeably, the shortest switching time is obtained from the sample with 10 wt% chiral dopant driven by a voltage pulse at 100 kHz, as also displayed in Fig. 6. In comparison, the bistable PSCT light shutter which is also based on a DFCLC possesses a response time of ~20 ms from the light-on H texture to the light-off FC state and a longer transition time of ~100 ms from the FC state to the H state when a 500 ms voltage pulse of the amplitude of 50 Vrms is applied .
Interesting properties of fast-switching bistable DFCLC intensity modulators have been investigated. Two stable states; i.e., the transparent P state and the scattering FC state, can be rapidly switched from one to the other, without the need of a detour, by employing proper pulse voltages on DFCLCs with appropriate pitch lengths. Because of the bistability, the optical states remain at zero voltages. The feasibility of the fast and direct switching from the FC to P state is demonstrated with a transition time of merely 10 ms in a particular case. Such a device can be used as an electrically tunable light valve, a neutral density filter or attenuator, and in the application of polarizer-free transmissive displays. With the nature of fast-switchable bistability, our results further open up new possible applications for the low-power-consumption DFCLC devices in fast-speed shutter and smart glass technologies.
We are indebted to Oleg V. Yaroshchuk of the Institute of Physics, National Academy of Sciences of Ukraine for suggestions for improvement of the revised manuscript. This research is financially supported by the National Science Council of the Republic of China (Taiwan) under Grant No. NSC 98-2112-M-033-004-MY3.
References and links
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