Browsing by Author "Hsu, HK"

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Crucial effects of the anisotropy on optical field induced pattern formation in nematic liquid crystal films
(2004-04-05)
Hsu, HK
;
Chen, SH
;
Lai, Y
;
光電工程學系
;
Department of Photonics
We present here the crucial effects of material anisotropy on optical field induced pattern formation in the one-feedback-mirror arrangement which utilizes the nematic liquid crystal film as the nonlinear medium. By using the quasi-static electric-field-biased planar-aligned homogeneous nematic liquid crystal (NLC) films, we observe both the hexagon and the roll patterns which can be switched optically due to the intrinsic anisotropic distribution of the threshold intensity. The anisotropy comes from the anisotropic nonlinear response of the NLC film and is the crucial factor for such a one-feedback-mirror system to form both the roll and hexagon patterns. The observed phenomena can be explained from the linear stability analysis of the governing diffusion-like equation. The experimental results indicate that the stable roll patterns are formed at low input light power and the stable hexagon patterns formed at high input power. (C) 2004 Optical Society of America.
Enhanced motion estimation for interframe wavelet video coding
(2004)
Tsai, CY
;
Hsu, HK
;
Huang, HC
;
Hang, HM
;
Wu, GZ
An enhanced motion estimation scheme is incorporated into the interframe wavelet coding architecture in this paper. Interframe wavelet coding has the advantage of SNR, temporal, and spatial scalability, and is a potential candidate for the on-going MPEG-21 scalable video coding standard. Motion-compensated temporal Filtering (MCTF) is one of its essential components. Therefore, motion estimation plays an important role in deciding the coding performance. In this paper, we modified the motion estimation syntax/scheme originally specified in the MPEG Advanced Video Coding (AVC) and use it in the interframe wavelet structure. Besides, the techniques of I-block, bi-directional motion estimation, lambda-value adjustment and motion information partitioning are employed. Simulation results show very promising performance particularly on subjective quality.
Influence of Frank elastic constant anisotropy on optical pattern formation using nematic liquid crystal films
(2004-09-01)
Hsu, HK
;
Lai, YC
;
Chen, SH
;
光電工程學系
;
Department of Photonics
We investigate the influence of the Frank elastic constant anisotropy on optical pattern formation using nematic liquid crystal (NLC) films in one-feedback-mirror systems. The governing diffusion-like equation for the parallel planar-aligned and the vertical-aligned NLC films with an externally biasing quasi-static electric field under the hard boundary assumption is derived. The intrinsic Frank elastic constant anisotropy induces the anisotropy of the diffusion length, which results in the anisotropic distribution of the threshold intensity for optical pattern formation. The effects of the elastic constant anisotropy on optical pattern formation are studied for the first time and the obtained numerical results can reasonably explain the optical patterns that are formed.
Phase-conjugate reflection and self-starting optical phase-conjugate oscillation in planar nematic liquid-crystal cells
(2003-01-01)
Shen, YR
;
Hsu, HK
;
Chen, SH
;
光電工程學系
;
Department of Photonics
A theoretical model for phase-conjugate reflection of planar nematic liquid-crystal cells is presented based on the continuum theory of liquid crystals and the optical theory of degenerate four-wave mixing. An analytical solution for the phase-conjugate reflectivity R is derived under the undepleted-pump approximation, and the necessary conditions for self-starting optical phase-conjugate oscillation are obtained. The dependence of these conditions on the related physical parameters is also discussed. The reconstruction phenomenon of phase-conjugate light and the self-starting optical phase-conjugate oscillation have been observed experimentally. The intensities of the phase-conjugate light versus the-biased voltages for various incident optical intensities have been measured, and the results agree very well with the prediction of our theoretical model. (C) 2003 Optical Society of America.
Quasistatic electric-field-modulated optical pattern transition in a thin nematic liquid-crystal film with a single feedback mirror
(2004-10-04)
Hsu, HK
;
Lai, Y
;
Chen, SH
;
光電工程學系
;
Department of Photonics
We present here the pattern formation in the transverse profile of a continuous-wave laser beam passing through a parallel planar-aligned nematic liquid crystal (NLC) film biased by a quasistatic electric field and then reflected back to the sample cell by a single planar mirror. The effect of the biasing voltage is studied through the ability to change the nonlinearity by modulating the orientation of the NLC molecules electrically. By suitably modulating the quasistatic electric field the optical pattern transition from the hexagon to the roll is achieved. The pattern transition can be explained by the linear stability analysis for the nonlinear liquid crystal film with optical feedback. (C) 2004 American Institute of Physics.