Chong Hoon Kwak

Orientational photorefractive holograms in porphyrin:Zn-doped nematic liquid crystals

We investigated the diffraction properties of dynamic holograms recorded in porphyrin:Zn doped nematic liquid crystals (NLCs) under the influence of an applied dc electric field for various conditions of the grating period, the writing beam intensity and the applied electric field. We also derived an analytic expression for diffraction efficiency from NLCs material equations and torque balance equations and compared the experimental results with the theory, revealing excellent agreement.

Effects of applied electric field on orientational photorefraction in porphyrin:Zn-doped nematic liquid crystals

We observed an orientational photorefractive effect in porphyrin:Zn-doped nematic liquid crystals by measuring two beam coupling gain and diffraction efficiency under the influence of applied electric field. The gain and diffraction efficiency curves against applied electric field typically reveal resonant type curves, rapidly increasing to its maximum values and then gradually decreasing, but have clearly distinct peak positions. Based on the material equations and the torque balance equation of director axis reorientation of liquid crystals, we theoretically derived the expressions for the orientational photorefractive gain and diffraction efficiency, showing good agreement with the experimental results.

Kinetics of two wave mixing gain for moving grating technique in photorefractive BaTiO 3 crystal

We present analytic expressions for time-dependent space charge field and two wave mixing gain under the external applied field accompanying the grating translation. The variations of complex space charge field are analyzed in a complex plane, and the explicit expressions for the resonance and optimum frequencies (or moving grating velocities), which maximize the magnitude and imaginary part of space charge field, respectively, are also obtained. In addition, two wave mixing experiment was conducted with a grating translation technique without the external applied field in a BaTiO(3) crystal and the transient behaviors of measured gain look like damped harmonic oscillations, showing excellent agreement with the theory for the entire time range.

Fabrication of bifocal holographic lenses using holographic polymer-dispersed liquid crystal film

A holographic polymer-dispersed liquid crystal (HPDLC) thin film is composed of multifunctional acrylate monomer blended with the nematic liquid crystal mixture, and then investigated the real-time diffraction efficiencies for various physical parameters of amount of liquid crystals and applied AC electric fields. It is experimentally shown that the holographic gratings recorded in HPDLC film can be reversibly erased and reconstructed by switching on and off of appropriate applied AC electric field. By using these electro-optic properties we have developed bifocal holographic lenses having two different focal lengths of 300 mm and 400 mm.

Observation of a fast-formed absorption grating and a slowly formed phase grating in undeveloped dichromated gelatin

Real-time diffraction efficiency is measured during hologram formation in undeveloped dichromated gelatin (DCG) film. The main features of real-time diffraction efficiency of undeveloped DCG reveal double peaks with increasing exposure time. Adopting the grating translation technique, we show that the first peak originates from the absorption grating and that the latter is related to the phase grating. To explain such complicated hologram-formation dynamics, we present a phenomenological model for mixed gratings associated with photoinduced refractive-index changes and absorption changes. We have compared the model results with the experimental results, which show good agreement.

Simple method for measuring the rotational viscosity coefficient of nematic liquid crystals

We propose a simple method for determination of rotational viscosity coefficient of nematic liquid crystal under the influence of an external square wave electric field and theoretically analyzed by using Huygens-Fresnel diffraction theory with a small phase shift approximation. The rotational viscosity and moment of inertia are experimentally determined, showing good agreement with previous results.

Z-scan measurement for two photon absorption and Kerr coefficients in amorphous chalcogenide As/sub 3/S/sub 3/ thin film

We performed openand closed-aperture Z-scan experiments in amorphous chalcogenide As2S3 thin film and also present Z-scan theory. Both nonlinear Kerr coefficient and two photon absorption coefficient of As2S3 were determined.

Two beam coupling gain enhancement in porphyrin:Zn-doped nematic liquid crystals with an applied DC field and moving grating technique

We measured two beam coupling gain in porphyrin:Zn-doped nematic liquid crystals with an applied dc field by using the moving grating technique and theoretically derived an expression of the gain coefficient, showing good agreement with the experimental results.

Z-scan analysis of porphyrin:Zn doped nematic liquid crystal

We measured Z-scan transmittance in porphyrin:Zn doped nematic liquid crystal and developed Z-scan theories for plausible physical mechanisms such as Kerr, thermo-optic and saturable Kerr effects. It is shown that the saturable Kerr effect is most relevant physical origin of our sample.

Variational Analysis of Z-scan in a Saturable Kerr-like LiNbO 3 Crystal

By using the variational formulation, we analyzed the beam waist profile and the Z-scan transmittance in a thick saturable Kerr medium. We also measured Z-scan transmittance in a saturable Kerr-like LiNbO3and compared with the variational analysis.