Andy Ying-Guey Fuh

Cholesteric liquid crystal laser with wide tuning capability

This letter examines a planar cholesteric cell (CLC) doped with two collocated laser dyes as a one-dimensional photonic crystal. Adding phototunable chiral material (AzoB) allows the CLC photonic crystal to be lased at the band edges of the photonic band gap with a tuning range of over 100nm. Tuning is performed by irradiating the chiral AzoB material with UV light, causing the material to undergo trans-cis isomerization in the CLC film. The tuning range is the visible region from 563to667nm. Moreover, the tuning is reversible.

Highly efficient and polarization-independent Fresnel lens based on dye-doped liquid crystal

We demonstrated a highly efficient, polarization-independent and electrically tunable Fresnel lens based on dye-doped liquid crystal using double-side photoalignment technique. The maximum diffraction efficiency reaches 37%, which approaches the theoretical limit ~41%. Such a lens functions as a half-wave plate, and this feature could be well preserved under the applied voltage. In addition, the device is simple to fabricate, and has fast switching responses between focusing and defocusing state.

Electrically controllable laser based on cholesteric liquid crystal with negative dielectric anisotropy

This work examines a planar cholesteric liquid crystal (CLC) cell with a negative dielectric anisotropy, doped with laser dye, as an electrically tunable one-dimensional photonic crystal laser device. The lasing wavelength is demonstrated to be tunable by applying a voltage. Additionally, lasing can be switched on and off changing the frequency of the applied voltage. Wavelength tuning caused by the shift of the reflection band of CLC is attributed to the electrohydrodynamical effect in the negative dielectric cell.

Lasing in chiral photonic liquid crystals and associated frequency tuning

This letter addresses a dye-doped planar cholesteric cell as a one-dimensional photonic crystal, which can be lased at the band edges of the photonic band gap. The effect of the composition of the material and the thickness of a cholesteric cell (CLC) on the lasing action, and the photo-control of the lasing frequency, are experimentally investigated. Adding a tunable chiral monomer (TCM) allows the CLCs reflection band to be tuned by varying the intensity and/or exposure time of the UV curing light, enabling the lasing frequency of the CLC sample to be tuned.

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.

Dynamic studies of holographic gratings in dye-doped liquid-crystal films.

The dynamic behavior of a holographic grating induced in a homeotropically aligned dye-doped liquid-crystal film is investigated. In the presence of an applied dc voltage, photoexcited azo dyes induce a photorefractive grating and then diffuse and are adsorbed onto cell substrates. The reorientation of liquid crystals as a result of adsorbed dyes leads to a phase grating that is phase shifted 90 degrees from the photorefractive grating. Competition of these two gratings induces two-beam coupling of the writing beams, initially transferring energy from beam 1 to beam 2 and then, after a pause, from beam 2 to beam 1.

Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals

This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.

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.

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.

Nonlinear optical property of azo-dye doped liquid crystals determined by biphotonic Z-scan technique

This work investigates the optical Kerr property of azo-dye doped nematic liquid crystal films using the biphotonic Z-scan technique. The results indicate that the nonlinear effect measured using the Z-scan technique with a red light can be modulated or switched with the simultaneous application of a green light, because of photoisomerization and thermal effects, as determined by dynamic measurements. The former dominates in the early stage when the green light is applied, while the latter dominates in the later stage.