Shing-Trong Wu

Optically switchable twist nematic grating based on a dye-doped liquid crystal film

The adsorption rate of methyl red dyes on a polymer surface is studied and determined to be much faster than that on indium-tin-oxide-coated glass. Therefore, an optically switchable twist-nematic grating is fabricated using a dye-doped liquid crystal cell, with a glass substrate coated with a polymer relief grating.

Optical switch of diffractive light from a BCT photonic crystal based on HPDLC doped with azo component

This study demonstrates an optical switch of the diffractive light from a body-centered tetragonal photonic crystal based on holographic polymer-dispersed liquid crystals that are fabricated using two-beam interference with multiple exposures. The liquid crystal-rich regions form the lattice points of the PC, which contains a liquid crystal/azo-dye mixture. The concentration of the cis isomer changes under laser light exposure; this change, in turn, modulates the effective index of the LCs, and then switches diffractive light.

Multimode lasing from the microcavity of an octagonal quasi-crystal based on holographic polymer-dispersed liquid crystals

An eightfold photonic quasi-crystal (PQC) sample is fabricated holographically using two-beam interference with multi-exposure based on polymer-dispersed liquid crystals. The transmission spectra from the finite-difference time-domain (FDTD) simulation prove the photonic stop band of the rotational symmetry structure of the sample. The resonant mode of the circular microcavity formed in the PQC is calculated. Amplified spontaneous emission and multimode lasing action are demonstrated from the pumped laser-dye-doped PQC microcavity using a Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) pulse laser.

Unusual refractions in photonic crystals based on polymer-dispersed liquid crystal films

This work describes unusual refractive phenomena from a hexagonally close-packed (hcp) photonic crystal (PC) based on a holographic polymer dispersed liquid crystal film. The refracted collimated beams comprise positive and negative refractions and can be switched to exhibit the negative refraction part only by changing the incident angle or the wavelength of the incident beam. The equal frequency surfaces are adopted to analyze the refractions of light both inside and outside the hcp PC.

Two-Dimensional Diffraction Grating Based on Polymer-Dispersed Liquid Crystals

An electrically tunable two-dimensional (2D) diffraction grating is demonstrated using nanosized liquid crystal droplets dispersed in a polymer matrix. A pixel-wall photomask is positioned in contact with a polymer-dispersed liquid crystal (PDLC) film during curing to control the rate of polymerization. The cured inhomogeneous composite film exhibits high transmittance and phase retardation between the high- and low-intensity regions. Such a film can be utilized as a diffraction grating that is polarization dependent and electrically tunable. It has a potential in optical beam steering.

Polymer-Dispersed Liquid-Crystal Holographic Gratings Doped with a High-Dielectic-Anisotropy Dopant

The first-order diffraction efficiency of holographic gratings formed in polymer-dispersed liquid-crystal (PDLC) films doped with a high-dielectric-anisotropy compound, HRL-410, were investigated. PDLC films doped with various HRL-410 concentrations were used to write gratings under different laser powers. By conducting a light-scattering study using a single Ar+ laser beam to excite the sample while probing with a He–Ne laser, we found that the dielectric material increases the rate of polymerization, which in turn affects the formation and the resultant morphology of the grating. The voltage required for switching the grating decreases significantly for the doped films. We propose a model to explain these experimental observations.

Fabrication of Twisted Nematic Gratings Using Polarization Hologram Based on Azo-Dye-Doped Liquid Crystals

This study establishes the feasibility of fabricating twist nematic gratings (TN-gratings) using a polarization holographic approach on the basis of the laser-induced adsorption of azo dyes by a polymer-coated glass substrate. This holographic recording is fast, and the formed TN gratings are permanent. Measurements indicate that the first-order diffracted-beam intensity from this TN grating is independent of the polarization of the incident beam. The polarization states of the zeroth- and first-order diffracted beams are perpendicular to each other. The polarization state and intensity distribution of the diffracted beams agree well with their theoretical values derived using the Jones matrix.

Independent Manipulation of Topological Charges and Polarization Patterns of Optical Vortices.

We present a simple and flexible method to generate various vectorial vortex beams (VVBs) with a Pancharatnam phase based on the scheme of double reflections from a single liquid crystal spatial light modulator (SLM). In this configuration, VVBs are constructed by the superposition of two orthogonally polarized orbital angular momentum (OAM) eigenstates. To verify the optical properties of the generated beams, Stokes polarimetry is developed to measure the states of polarization (SOP) over the transverse plane, while a Shack–Hartmann wavefront sensor is used to measure the OAM charge of beams. It is shown that both the simulated and the experimental results are in good qualitative agreement. In addition, polarization patterns and OAM charges of generated beams can be controlled independently using the proposed method.

Optical filter with tunable wavelength and bandwidth based on phototunable cholesteric liquid crystals

An optical filter with tunable wavelength and bandwidth is demonstrated using two phototunable cholesteric liquid crystals (CLCs) configured in a reflection mode. In this mode, incident light is first reflected by one Azo-chiral-doped CLC and then by another one. The tuning mechanism, which is based on the pitch modulation of the CLCs that contain an Azo-chiral dopant, can be controlled by two pumping laser beams. The central wavelength can be tuned from 510 to 628 nm, and the bandwidth can be changed from 13 to 79 nm.

Electrically controllable smart window with greyscale based on polymer-stabilised cholesteric texture films

ABSTRACT The study reports a novel liquid crystal (LC) film that can be applied on the transmissive substrates or stuck on existing casement window to upgrade the shield. The shadow area and transmittance can be adjusted according to the condition of the outer sunshine. The window is energy saving for which maintains the transmittance after a once-off operation of pulse amplitude modulation (PAM) electric driving. Such a film can be applied in smart curtain, display frame edge and other transmissive electro-optical devices. GRAPHICAL ABSTRACT