Hung-Chang Jau

Red, Green and Blue Reflections Enabled in an Optically Tunable Self‐Organized 3D Cubic Nanostructured Thin Film

A new light-driven chiral molecular switch doped in a stable blue phase (BP) liquid crystal allows wide optical tunability of three-dimensional cubic nanostructures with a selective reflection wavelength that is reversibly tuned through the visible region. Moreover, unprecedented reversible light-directed red, green, and blue reflections of the self-organized three-dimensional cubic nanostructure in a single film are demonstrated for the first time. Additionally, unusual isothermal photo-stimulated less ordered BP II to more ordered BP I phase transition was observed in the system.

Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal

We demonstrated a highly efficient, polarization-independent and electrically tunable Fresnel lens based on dye-doped liquid crystal using double-side photoalignment technique. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ~41%. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. In addition, the device is simple to fabricate, and has fast switching responses between focusing and defocusing state.

Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

This work examines a planar cholesteric liquid crystal (CLC) cell with a negative dielectric anisotropy, doped with laser dye, as an electrically tunable one-dimensional photonic crystal laser device. The lasing wavelength is demonstrated to be tunable by applying a voltage. Additionally, lasing can be switched on and off changing the frequency of the applied voltage. Wavelength tuning caused by the shift of the reflection band of CLC is attributed to the electrohydrodynamical effect in the negative dielectric cell.

Random lasing in blue phase liquid crystals

Random lasing actions have been observed in optically isotropic pure blue-phase and polymer-stabilized blue-phase liquid crystals containing laser dyes. Scattering, interferences and recurrent multiple scatterings arising from disordered platelet texture as well as index mismatch between polymer and mesogen in these materials provide the optical feedbacks for lasing action. In polymer stabilized blue-phase liquid crystals, coherent random lasing could occur in the ordered blue phase with an extended temperature interval as well as in the isotropic liquid state. The dependence of lasing wavelength range, mode characteristics, excitation threshold and other pertinent properties on temperature and detailed make-up of the crystals platelets were obtained. Specifically, lasing wavelengths and mode-stability were found to be determined by platelet size, which can be set by controlling the cooling rate; lasing thresholds and emission spectrum are highly dependent on, and therefore can be tuned by temperature.

Influence of Polymerization Temperature on Hysteresis and Residual Birefringence of Polymer Stabilized Blue Phase LCs

This paper investigates the voltage hysteresis effect and residual birefringence in the polymer-stabilized blue phase I and II, under various phase separation conditions. By irradiating samples with ultraviolet (UV) light and curing the samples at a reduced temperature, the polymer in the blue phase formed a compact network along the direction of the UV light irradiation. The dense polymer network pinned the cubic lattice structure of the blue phase enabling it to return to its original optical isotropic state. In this manner, hysteresis and residual birefringence were suppressed from 4.16% to 1.67% and 0.057% to 0.002%, respectively. Although the driving voltage remained high, the reduction in hysteresis and residual birefringence are precisely the advancements required for the accelerating development of blue-phase liquid crystal displays.

Temperature dependence of refractive index in blue phase liquid crystals

The refractive indices of a class of Blue-Phase liquid crystals (BPLCs) and their temperature dependence have been measured and analyzed. In general, the thermal index gradients in blue phases, BPI and BPII, are both larger than in isotropic liquid state; the index gradient of BPII phase is steeper than that of BPI, and is attributed to the difference between the expansion coefficients of simple and body-centered cubic lattices. Besides their obvious importance in photonics and nonlinear optical processes and applications, the investigation of the phase dependence of the index gradient also provides a useful way for phase identification of BPLCs, namely the second-order and weakly first order phase transitions corresponding to the ISO/BPII transition and the BPII/BPI transition.

Photoaddressable bistable reflective liquid crystal display

This work presents a bistable reflective liquid crystal display. The mechanism is based on the laser-induced adsorption of azo dyes on the polymer-coated glass substrate. This reflective liquid crystal display is very simple to fabricate and can be optically written, erased, and rewritten with a high contrast. A simulation is performed to fit the measured electro-optical characteristics of this device.

Polarization-independent rapidly responding phase grating based on hybrid blue phase liquid crystal

This work demonstrates a polymer-stabilized blue phase (PSBP) liquid crystal phase grating, which is made of hybrid PSBPs with two different Kerr constants. The Kerr constant of a PSBP is related to the morphology of the polymer network which can be controlled by the phase separation temperature. Owing to the non-patterned electrode and the optical isotropy of the PSBP, the diffraction effect can be completely switched off when the voltage is absent. The diffraction intensity increases when a uniform applied electrical field induces the phase difference in the hybrid PSBP. The phase grating is completely independent of the polarization of the incident light. Furthermore, the response time to switching is in the sub-millisecond range.

Bistable cholesteric liquid crystal light shutter with multielectrode driving

An electrically activated bistable light shutter that exploits polymer-stabilized cholesteric liquid crystal film was developed. Under double-sided three-terminal electrode driving, the device can be bistable and switched between focal conic and homeotropic textures with a uniform in-plane and vertical electrical field. The transparent state with a transmittance of 80% and the opaque/scattering state with a transmittance of 13% can be realized without any optical compensation film, and each can be simply switched to the other by applying a pulse voltage. Also, gray-scale selection can be performed by varying the applied voltage. The designed energy-saving bistable light shutter can be utilized to preserve privacy and control illumination and the flow of energy.

Full-color reflectance-tunable filter based on liquid crystal cladded guided-mode resonant grating

This work proposes a tunable reflective guided-mode resonant (GMR) filter that incorporates a 90° twisted nematic liquid crystal (TNLC). The GMR grating acts as an optical resonator that reflects strongly at the resonance wavelength and as an alignment layer for LC. The 90° TNLC functions as an achromic polarization rotator that alters the polarization of incident light. The resonance wavelength and reflectance of such a filter can be controlled by setting the angle of incidence and driving the 90° TNLC, respectively. The designed filter exhibits a very large spectral shift in resonance wavelength from 710 to 430 nm, which covers the entire visible spectrum. The transmittance can be tuned to within 10 V at various resonance wavelengths. The hybrid GMR - LC filter is compact, has a simple design, and is easy to fabricated. It can therefore be used in practical applications.