Cheng-Chang Li

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.

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.

All-optical transistor- and diode-action and logic gates based on anisotropic nonlinear responsive liquid crystal

In this paper, we show that anisotropic photosensitive nematic liquid crystals (PNLC) made by incorporating anisotropic absorbing dyes are promising candidates for constructing all-optical elements by virtue of the extraordinarily large optical nonlinearity of the nematic host. In particular, we have demonstrated several room-temperature ‘prototype’ PNLC-based all-optical devices such as optical diode, optical transistor and all primary logic gate operations (OR, AND, NOT) based on such optical transistor. Owing to the anisotropic absorption property and the optical activity of the twist alignment nematic cell, spatially non-reciprocal transmission response can be obtained within a sizeable optical isolation region of ~210 mW. Exploiting the same mechanisms, a tri-terminal configuration as an all-optical analogue of a bipolar junction transistor is fabricated. Its ability to be switched by an optical field enables us to realize an all-optical transistor and demonstrate cascadability, signal fan-out, logic restoration, and various logical gate operations such as OR, AND and NOT. Due to the possibility of synthesizing anisotropic dyes and wide ranging choice of liquid crystals nonlinear optical mechanisms, these all-optical operations can be optimized to have much lower thresholds and faster response speeds. The demonstrated capabilities of these devices have shown great potential in all-optical control system and photonic integrated circuits.

Bistable light-driven π phase switching using a twisted nematic liquid crystal film.

A light-activated optical phase switch was developed, exploiting the conversion between left-handed and right-handed twisted nematic liquid crystals. Theoretical and experimental analyses revealed that the handedness inversion of the twisted nematic film altered the optical phase of the output waves by π. Herein, the competition between the helical twisting powers of the two reverse-handed chiral dopants determines the handedness of the twisted nematic film. The photo-responsibility and the bistability are attributed to the azobenzene chromophores in one of the chiral additives.

Large three-dimensional photonic crystals based on monocrystalline liquid crystal blue phases

Although there have been intense efforts to fabricate large three-dimensional photonic crystals in order to realize their full potential, the technologies developed so far are still beset with various material processing and cost issues. Conventional top-down fabrications are costly and time-consuming, whereas natural self-assembly and bottom-up fabrications often result in high defect density and limited dimensions. Here we report the fabrication of extraordinarily large monocrystalline photonic crystals by controlling the self-assembly processes which occur in unique phases of liquid crystals that exhibit three-dimensional photonic-crystalline properties called liquid-crystal blue phases. In particular, we have developed a gradient-temperature technique that enables three-dimensional photonic crystals to grow to lateral dimensions of ~1 cm (~30,000 of unit cells) and thickness of ~100 μm (~ 300 unit cells). These giant single crystals exhibit extraordinarily sharp photonic bandgaps with high reflectivity, long-range periodicity in all dimensions and well-defined lattice orientation.Conventional fabrication approaches for large-size three-dimensional photonic crystals are problematic. By properly controlling the self-assembly processes, the authors report the fabrication of monocrystalline blue phase liquid crystals that exhibit three-dimensional photonic-crystalline properties.

All-optical control of polarization splitting with a dielectric-clad azobenzene liquid crystal

We report the design, fabrication, and characterization of an optically switchable polarizing beam splitter with a prism/azobenzene liquid crystal/prism hybrid structure. The beam splitter can operate in the polarization-splitting mode and the non-splitting mode. The switching between the modes is realized by the photoisomerization-induced phase transitions in the azobenzene liquid crystal, featuring all-optical control, bistability, and fast response. Such an active polarization-handling element is highly desirable as it not only simplifies and compacts sophisticated optical systems but also increases the degree of freedom in optical circuit design.

Electrically assisted bandedge mode selection of photonic crystal lasing in chiral nematic liquid crystals

Selection of the bandedge lasing mode of a photonic crystal laser has been realized in a fluorescent dye doped chiral nematic liquid crystal by exerting electrical control over the mode competition. The bandedge lasing can be reversibly switched from the short-wavelength edge mode to the long-wavelength edge mode by applying a voltage of only 20 V, without tuning the bandgap. The underlying mechanism is the field-induced change in the order parameter of the fluorescent dye in the liquid crystal. The orientation of the transition dipole moment determines the polarization state of the dye emission, thereby promoting lasing in the bandedge mode that favors the emission polarization. Moreover, the dynamic mode-selection capability is retained upon polymer-stabilizing the chiral nematic liquid crystal laser. In the polymer-stabilized system, greatly improved stability and lasing performance are observed.

Enhanced image quality of OLED transparent display by cholesteric liquid crystal back-panel

The transparent active-matrix organic light-emitting diode (AM-OLED), which is a self-luminous polarizer-free display, is regarded as one of the most potential candidate of high-transparency transparent display. However, the image quality of the transparent AM-OLED depends strongly on the background, resulting in poor contrast and image distortion. In this work, a transparent display with high image quality that combines AM-OLED and dye-doped cholesteric liquid crystal is demonstrated. Its electro-optical properties are optimized, enabling the transparent display to be not only compatible with active-matrix driving but also the contrast and distortion are sufficiently improved without sacrificing the transparency.

Broadband mid-infrared polarization rotator based on optically addressable LCs.

This work proposes a mid-infrared polarization rotator that incorporates a twisted nematic liquid crystal (TNLC) cell with a photo-controllable alignment layer. The TNLC device with a sufficient phase retardation can act as an achromic polarization rotation device over a wide wavelengths range and thus can rotate the polarization of a mid-IR laser beam. The photo-alignment technique enables TNLCs with arbitrary twisting angles to be generated by the use of visible polarized addressing light to control the directors of the photo-alignment layer. Therefore, arbitrary rotation angles of the polarization axis of a linearly polarized mid-IR laser beam can be realized. Moreover, the rewritable property and reliability of this polarization rotator are experimentally verified. The flexibility of polarization control for broadband mid-IR opens up a large range of potential mid-IR applications.