| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 95 === Nematic liquid crystals (NLC) are optically uniaxial materials. Their alignments are easily disturbed by applied external fields such as electric, magnetic, acoustic fields. A change in the refractive index of NLC results from the realignment of liquid crystal molecules. The influence of acoustic waves on the optical transmission through the liquid crystal cell is called the acousto-optical effect of NLC. The liquid crystal molecules do not be directly perturbed by obliquely incident acoustic beam. Instead, they are influenced by the acoustic guided waves which are induced by the refracted ultrasound into the sandwiched liquid crystal cell. This thesis investigates the acousto-optical effect on vertically aligned NLC due to acoustic guided waves in liquid crystal cells based on experiments and theories of ultrasound.
In experiments, the liquid crystal cells are placed in a holder within a water tank and are obliquely insonified by an immersion ultrasonic transducer in a broad frequency range from 0.2 to 7 MHz. The acousto-optical effect was observed by a variation of optical transmission through the cell. The optical transmission was measured as a function of the gap of liquid crystal cell, acoustic intensity, incident angle, and so on. Experimental evidence indicates that optical transmission becomes significant at the occurrence of acoustic guided waves in the cell. The optical transmission not only reaches saturation with the increase of acoustic intensity and duration but also collapses and followed by becoming saturated again. It seems that realignment occurs once the molecules rotate over an extreme tilt angle.
The comparison of experimental results and the simulated dispersion curves of acoustic guided waves in liquid crystal cells done by Shih shows that S0, S1 modes have strong effect on the realignment of liquid crystal molecules. The A0 mode has only sight influence. In addition, liquid crystal apparently has faster response in high frequency range than at low frequencies. Even though the response time does not reach the required commercial standard, it is worth further investigation on acoustic realignment of liquid crystals using guided waves.
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