Chun-Ta Wang

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.

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.

A stable and switchable uniform lying helix structure in cholesteric liquid crystals

This investigation demonstrates an electrically switchable uniformly lying helix (ULH) stable state in cholesteric liquid crystal. A stable ULH state can be achieved by applying the low-frequency (30 Hz) pulse electrical field via an electro-hydrodynamatic effect. The ULH state can be stably maintained with a helical pitch in the visible range (450 nm–630 nm) and exhibit a tunable uniaxial crystal wave plate property under 1 kHz electrical field. The study examines the electro-optical property of ULH state and driving scheme for switching among the three stable states. A multi-stable and electrically switchable cholesteric liquid crystal can provide various optical properties and has extensive potential applications.

Bistable reflective polarizer-free optical switch based on dye-doped cholesteric liquid crystal [Invited]

The work demonstrates a polarizer-free bistable reflective electro-optical switch that is based on dichroic dye-doped cholesteric liquid crystal (DDCLC). Bistable opaque and transparent states can be achieved using the planar and ULH textures of CLC, respectively. In the planar texture, the liquid crystal and dye molecules have a periodic helical structure with axes perpendicular to the substrate surface. They therefore absorb arbitrarily polarized light. In the ULH texture, the liquid crystal and dye molecules twist along the helical axes parallel to the substrate, and allow most of the arbitrarily polarized light to pass. Both the planar and ULH textures of CLC are stable states, and each can be switched to the other by applying a low-frequency (30Hz) electrical field, owing to the electro-hydrodynamic effect. A bistable electro-optical switch has the advantages of being polarizer-free and consuming low power. It can therefore potentially be used in portable information systems.

Polarization independent Fabry-Pérot filter based on polymer-stabilized blue phase liquid crystals with fast response time

This work demonstrates a polarization-independent electrically tunable Fabry-Pérot (FP) filter that is based on polymer-stabilized blue phase liquid crystals (PSBPLCs). An external vertical electric field can be applied to modulate the effective refractive index of the PSBPLCs along the optical axis. Therefore, the wavelength-tuning property of the FP filter is completely independent of the polarization state of the incident light. The change in the birefringence in PSBPLCs is governed by Kerr effect-induced isotropic-to-anisotropic transition, and so the PSBPLCs based FP filter has a short response time. The measured tunability and free spectral range of the FP filter are 0.092 nm/ V and 16nm in the visible region, and 0.12nm/ V and 97nm in the NIR region, respectively, and the response time is in sub-millisecond range. The fast-responding polarization-independent electrically tunable FP filter has substantial potential for practical applications.

Pinning effect on the photonic bandgaps of blue-phase liquid crystal

This study demonstrates the surface alignment induced pinning effects on blue phases. The morphologies of blue-phase platelets in a nonaligned cell become less uniform, and the photonic bandgap shifted over 120 nm during the cooling process. Comparing the different boundary conditions, the anchoring forces provide by homogeneous alignment can pin the blue-phase platelets, confine the photonic bandgap variation, and increase uniformity of the blue phase. This study also examines the pinning effect by the patternable photoalignment technique. Boundary anchoring forces have a significant effect on the morphology and photonic characteristics of the blue phase, making them applicable to practical applications.

Multi-wavelength laser emission in dye-doped photonic liquid crystals

Multi-wavelength lasing in a dye-doped cholesteric liquid crystal (CLC) cell is demonstrated. By adding oversaturated chiral dopant, the multi-photonic band CLC structure can be obtained with non-uniform chiral solubility. Under appropriate excitation, multi-wavelength lasing can be achieved with a multi-photonic band edge CLC structure. The number of lasing wavelengths can be controlled under various temperature processes. Nine wavelength CLC lasings were observed simultaneously. The wavelength range covers around 600-675nm. Furthermore, reversible tuning of multi-wavelength lasing was achieved by controlling CLC device temperature.

Bistable effect in the liquid crystal blue phase

This study investigates the bistable effect and transition mechanism between various lattice orientations in the negative liquid crystal blue phase. The blue phase exists over a wide temperature range ∼16 °C, and three lattices (110), (112), and (200) are confirmed with Kossel diagrams. The red platelet (110) lattice and blue platelet (200) lattice can be stabilized and switched to each other by particular pulse voltages. An electric field induced planar state and electrohydrodynamatic effect in the blue phase is also investigated. Additionally, the reflected color of the (200) lattice can be adjusted from 455 to 545 nm by temperature induced lattice distortions and provided with reversibility.

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.

Vertically Integrated Transflective Liquid Crystal Display Using Multi-Stable Cholesteric Liquid Crystal Film

The work demonstrates a switchable transflective liquid crystal display using a cholesteric liquid crystal (CLC) film as a switchable reflector. The planar and uniformly-lying helical (ULH) states of CLCs support the reflective and transmissive modes of the display, respectively. Both planar and ULH textures are stable states, and each can be switched to the other via the electro-hydrodynamic effect by applying a low-frequency (30 Hz) electrical field. A transflective display with a CLC as switchable reflector can be utilized in any currently used display mode, such as TN, MVA, and IPS. Such a display exhibits high display performance in both indoor and outdoor environments and low power consumption. It therefore has great potential to be used in portable information systems.