Ming Shian Li

Sensor for monitoring the vibration of a laser beam based on holographic polymer dispersed liquid crystal films

A continuous multiple exposure diffraction grating (CMEDG) is fabricated holographically on polymer dispersed liquid crystal (PDLC) films using two-beam interference with multiple exposures. The grating is fabricated by exposing a PDLC film to 18 repeated exposure/non-exposure cycles with an angular step of ~10°/10° while it revolves a circle on a rotation stage. The structure of the sample thus formed is analyzed using a scanning electron microscope (SEM) and shows arc-ripples around the center. From the diffraction patterns of the formed grating obtained using a normally incident laser beam, some or all of the 18 recorded arc beams can be reconstructed, as determined by the probing location. Thus, it can be applied for use as a beam-vibration sensor for a laser.

Bichromatic optical switch of diffractive light from a BCT photonic crystal based on an azo component-doped HPDLC

This study demonstrates all optical switches between the four diffractive light levels of a body-centered tetragonal photonic crystal. The sample is based on holographic polymer-dispersed liquid crystals that are fabricated using a two-beam interference with multiple exposures. The switching mechanism bases on the effective index modulation of the PC that contains a liquid crystal/azo-dye mixture could be controlled by two pumping laser beams. The switching time between the blue-laser-pumped and the blue-and-green-laser-pumped levels is fast. The study also discusses the switching time of the various levels.

Two-dimensional holographic polarization grating formed on azo-dye-doped polyvinyl alcohol films

This study describes an electrically tunable two-dimensional (2D) liquid crystal holographic polarization grating. It is a twisted nematic grating array, which is obtained by orthogonally overlaying two crossed one-dimensional (1D) polarization holograms. A 1D polarization hologram has a rotating linear polarization pattern, generated by the interference of two orthogonal circularly polarized beams on a substrate that is coated with an azo-dye-doped polyvinyl alcohol layer. The images under a polarized optical microscope and the diffraction patterns from the 2D grating are simulated using the Jones matrix and a Fourier transformation. The experimental results agree with the simulated results. This work represents a substantial advance toward the realization of highly functionalized passive optical devices in which both the beam propagation direction and the polarization state can be controlled in two dimensions.

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.

Electrically Switchable High-Fold-Helix Spiral Phase Plate Based on Polymer Dispersed Liquid Crystals

In this study, high-fold-helix spiral phase plates (SPPs) based on polymer-dispersed liquid crystals (PDLCs) with fold numbers l > 45 are fabricated. The helical light modes modulated by the plates are demonstrated. The beam profiles of the helical modes emerging from the PDLC SPPs are measured using a laser beam profiler. The results show that the beam radius of the principal maximum is linearly proportional to the l of the plate. The images of a laser beam diffracted from SPPs are recorded and found to agree well with calculations based on Fourier transformation.

Transverse wave propagation in photonic crystal based on holographic polymer-dispersed liquid crystal

This study investigates the transversely propagating waves in a body-centered tetragonal photonic crystal based on a holographic polymer-dispersed liquid crystal film. Rotating the film reveals three different transverse propagating waves. Degeneracy of optical Bloch waves from reciprocal lattice vectors explains their symmetrical distribution.

Observation of conical scattering cones from a two-dimensional hexagonal photonic crystal based on a polymer-dispersed liquid crystal

We explore axial scattering from a two-dimensional hexagonal photonic crystal based on a polymer-dispersed liquid crystal at normal incidence. The scattering reveals symmetric cones over a range of frequencies. The observed cones disperse strongly outward, and their radii are determined by the wavelength of the probe beam. Such cones are generated by the coupling of forward-scattered beams of the refracted beams and the refracted beams themselves under momentum conservation.

Optical properties of two-dimensional square lattice photonic crystal based on holographic polymer-dispersed liquid crystal

Two-dimensional (2-D) square lattice (SL) photonic crystals (PCs) are fabricated and their optical/electro-optical properties are studied. The PCs are based on polymer-dispersed liquid crystals (PDLC) that are formed using twobeam interference with double-exposures. The PC structure that is observed using a scanning electron microscope (SEM) matches with the calculated interference pattern. The results of optical/electro-optical studies demonstrate that superprism and negative refraction effects occur at certain incident angles over a range of frequencies, and are consistent with the simulated ones. Moreover, the negative refraction efficiency is electrically controllable.