Chia Rong Lee

Electrically switchable and thermally erasable biphotonic holographic gratings in dye-doped liquid crystal films

This study investigates high-resolution photoinduced biphotonic holographic gratings in azo-dye-doped liquid crystal films. A biphotonic grating (BG) is formed under the illumination of one linearly polarized green light with the simultaneous irradiation of an interference pattern created by two linearly polarized red lights. This study ascribes the formation of this grating to two mechanisms. One mechanism is the green-light-inducing strong dye absorption followed by adsorption through the trans–cis isomerization; the other mechanism is the inhibition effect of adsorption induced by the red light through the cis–trans inverse isomerization. These produce a twisted nematic structure-modulated pattern, which, in turn, causes the BG. Additional experiments demonstrate that the formed BGs are electrically switchable and thermally erasable.

Polarization holographic grating based on azo-dye-doped polymer-ball-type polymer-dispersed liquid crystals

A polarization grating (PG) written in an azo-dye-doped film of polymer-ball-type polymer-dispersed liquid crystals was investigated. The writing beams were two mutually orthogonal (s- and p-polarized) polarized beams. The PG resulted from molecular reorientation of the liquid crystals as a result of their interaction with the dye molecules adsorbed on the surface of the polymer balls. Polarization characteristics of the diffracted beams and the grating pattern were studied under a polarizing optical microscope with a crossed analyzer. The results indicate that the PG diffracts the linearly polarized incident light into beams with various polarizations. Accordingly, the grating can be used as an unpolarized or a polarized beam splitter, depending on the polarization of the incident light. A model based on the Jones matrix approach was developed, and it closely fits the experimental results.

Electrically controllable liquid crystal random lasers below the Fréedericksz transition threshold

This investigation elucidates for the first time electrically controllable random lasers below the threshold voltage in dye-doped liquid crystal (DDLC) cells with and without adding an azo-dye. Experimental results show that the lasing intensities and the energy thresholds of the random lasers can be decreased and increased, respectively, by increasing the applied voltage below the Fréedericksz transition threshold. The below-threshold-electric-controllability of the random lasers is attributable to the effective decrease of the spatial fluctuation of the orientational order and thus of the dielectric tensor of LCs by increasing the electric-field-aligned order of LCs below the threshold, thereby increasing the diffusion constant and decreasing the scattering strength of the fluorescence photons in their recurrent multiple scattering. This can result in the decrease in the lasing intensity of the random lasers and the increase in their energy thresholds. Furthermore, the addition of an azo-dye in DDLC cell can induce the range of the working voltage below the threshold for the control of the random laser to reduce.

All-optically controllable random laser based on a dye-doped liquid crystal added with a photoisomerizable dye

This study investigates, for the first time, an all-optically controllable random laser based on a dye-doped liquid crystal (DDLC) cell added with a photoisomerizable dye. Experimental results indicate that the lasing intensity of this random laser can be all-optically controlled to decrease and increase sequentially with a two-step exposure of one UV and then one green beam. All-optically reversible controllability of the random lasing emission is attributed to the isothermal nematic(N)-->isotropic(I) and I-->N phase transitions for LCs due to the UV-beam-induced trans-->cis and green-beam-induced cis-->trans back isomerizations of the photoisomerizable dye, respectively. The former and the latter can decrease and increase the spatial fluctuations of the order and thus of the dielectric tensor of LCs, respectively, subsequently increasing and decreasing the diffusion constant (or transport mean free path), respectively, and thus decaying and rising the scattering strength for the fluorescence photons in their recurrent multi-scattering process, respectively. The consequent decrease and increase of the lasing intensity for the random laser and thus the rise and descent of its energy threshold are generated, respectively.

Optically switchable biphotonic photorefractive effect in dye-doped liquid crystal films

This study reports the optically swichable photorefractive (PR) effect induced in a biphotonic grating in the presence of an applied dc voltage, in an azo-dye-doped liquid crystal (LC) film. When one green light is switched on (off), the PR grating can be turned on (off) by irradiating an interference field generated by two linearly polarized red lights. Experimental results demonstrate that such a PR effect follows primarily from a biphotonic process, with two mechanisms—the generation of green light-induced space charges by trans–cis isomerization, and the suppression of the formation of space charges by the red light in cis–trans back isomerization. These mechanisms apply to the dark and bright fringes of the red interference field, respectively. A spatially inhomogeneous space-charge field is then established to modulate the orientation of LCs, forming the PR grating in the presence of a dc voltage. Strong coupling of the two red lights is observable and measured dynamically during the formation of th...

Color cone lasing emission in a dye-doped cholesteric liquid crystal with a single pitch

This work investigates a novel color cone lasing emission (CCLE) based on a one-dimensional photonic crystal-like dye-doped cholesteric liquid crystal (DDCLC) film with a single pitch. The lasing wavelength in the CCLE is distributed continuously at 676.7-595.6 nm, as measured at a continuously increasing oblique angle relative to the helical axis of 0-50 degrees . This work demonstrates that lasing wavelength coincides exactly with the wavelength at the long wavelength edge of the CLC reflection band at oblique angles of 0-50 degrees . Simulation results of dispersion relations at different oblique angles using Berremans 4X4 matrix method agrees closely with experimental results. Some unique and important features of the CCLE are identified and discussed.

Optically tunable/switchable omnidirectionally spherical microlaser based on a dye-doped cholesteric liquid crystal microdroplet with an azo-chiral dopant

This paper presents an optically wavelength-tunable and intensity-switchable dye-doped cholesteric liquid crystal (DDCLC) spherical microlaser with an azo-chiral dopant. Experimental results present that two functions of optical control - tunability of lasing wavelength and switchability of lasing intensity - can be obtained for this spherical microlaser at low and high intensity regimes of non-polarized UV irradiation, respectively. If the DDCLC microdroplet is subjected to weak UV irradiation, azo-chiral molecules may transform to the bent cis state at a low concentration rate. The effect can slightly decrease the local order of LCs and thus the helical twisting power of the CLC in the microdroplet. As a result, the CLC pitch may become slightly elongated, which will cause the gradual red-shift of both omnidirectional PBG and lasing emission of the DDCLC spherical microdroplet. In contrast, when the microdroplet is subjected to strong UV irradiation, numerous azo-chiral molecules may simultaneously change to bent cis-isomers to seriously disarrange the helical texture of the CLC, which will quickly deform the PBG and deactivate the lasing emission of the microdroplet. Prolonged irradiation of a blue beam after strong UV irradiation may cause the cis azo-chiral molecules quickly convert back rod-like trans-isomers, which may then regenerate the CLC Bragg onion and PBG structures and reactivate the lasing emission of the microdroplet. Optical control of the DDCLC spherical microlaser is realized on a scale of seconds and minutes when UV irradiation is strong and weak, respectively. The 3D DDCLC spherical microlaser is a highly promising controllable 3D micro-light source or microlaser (e.g., all-optical 3D single photon microlaser) for applications of 3D all-optical integrated photonics, laser displays, and biomedical imaging and therapy, and as a 3D UV microdosagemeter or microsensor.

Optically switchable biphotonic gratings based on dye-doped cholesteric liquid crystal films

This study elucidates optically switchable gratings (BGs) based on biphotonic effect in dye-doped cholesteric liquid crystal films. When one circularly polarized green beam is switched on (off), the gratings can be turned on (off) by illumination with an interference field generated by two linearly polarized red beams. The biphotonic gratings are formed by two mechanisms—green-beam-induced dye reorientation through trans-cis isomerization and red-beam-induced suppression of dye reorientation by cis-trans back isomerization. These mechanisms result in a spatially periodic distribution with homeotropiclike and planarlike structures, respectively, in dark and bright regions of the interference field, generating the BGs.

All-optically controllable random laser based on a dye-doped polymer-dispersed liquid crystal with nano-sized droplets

This study elucidates for the first time an all-optically controllable random laser in a dye-doped polymer-dispersed liquid crystal (DDPDLC) with nano-sized LC droplets. Experimental results demonstrate that the lasing intensity of the random laser can be controlled to decrease by increasing irradiation time/intensity of one green beam, and increase by increasing the irradiation time of one red beam. The all-optical controllability of the random laser is attributed to the green (red)-beaminduced isothermal nematic-->isotropic (isotropic-->nematic) phase transition in LC droplets by trans-->cis (cis-->trans back) isomerization of azo dyes. This isomerization may decrease (increase) the difference between the refractive indices of the LC droplets and the polymer, thereby increasing (decreasing) the diffusion constant (or transport mean free path), subsequently decreasing the scattering strength and, thus, random lasing intensity.

Electrically controllable and polarization-independent Fresnel zone plate in a circularly symmetric hybrid-aligned liquid crystal film with a photoconductive polymer layer

This work reports a Fresnel zone plate in a circularly symmetric hybrid-aligned liquid crystal (LC) film with a photoconductive polymer layer. An ultraviolet-induced electrodelike pattern of polymer layer under a zone plate photomask results in alternating major and minor portions of external voltage dropping on LC layer in conductive and nonconductive regions, respectively. These effects cause the discrepancy in LC reorientation between adjacent zones, generating a Fresnel zone plate. The focusing of the zone plate is electrically controllable and polarization independent. Additionally, the zone plate has advantages of a zero focusing in the voltage-off state and a very small operating dc field range from 0to0.3V∕μm.