| Summary: | 碩士 === 國立成功大學 === 物理學系碩博士班 === 97 === Abstract
In this thesis, we study the photo-induced reorientation of liquid crystals doped with azo dyes (D2) by observing the diffraction patterns resulting from self-phase modulation. Self-phase modulation results from that the pump laser induces the change of refractive index of the sample, which influences the phase of the pump laser itself, and then causes a diffraction pattern in the far field. The number of diffraction ring is proportional to the change of refractive index in the central part of illuminated region. The more change of refractive index is, the larger phase shift is resulted in the central part of illuminated region.
Experiments consist of two parts; the first part is performed using a green laser, and the second part is performed with pumping a red and a green laser simultaneously. The experimental results of the first part show that the change of refractive index of the sample caused by a pump green laser is a function of the intensity of the green laser (IG). At intensities of green laser IG =0~ 0.7 W/cm2, IG=0.7~1.1 W/cm2 and IG>1.1W/cm2, we observe different phenomena in these three intensity intervals. Initially, the maximum phase shift increases (IG =0~ 0.7 W/cm2), and then decreases almost to zero at IG=1.1 W/cm2, and finally, increases again with IG being less than 3.1 W/cm2. The azo dye transits from trans to cis by absorbing green light. The transition causes the liquid crystals tend to reorient to the direction perpendicular to the polarization of pump light. Such a transition contributes to a negative torque . The torque is proportional to the number of dyes transiting from trans to cis. The azo dye transits from cis to trans by absorbing red light and heat. The transition causes the liquid crystals tend to reorient to the direction parallel to the polarization of pump light. This transition contributes to a positive torque . The torque is proportional to the number of dyes transiting from cis to trans. The total torque is summation of these two torques. The greater the total torque is, the larger the reoriented angle of liquid crystals is. At IG=0~0.7 W/cm2, the number of transition of trans to cis is larger than that of cis to trans which represents . The total torque is negative and increases with increasing intensity of pump light, so the number of diffraction ring increases with increasing intensity of the pump light. At IG=0.7~1.1 W/cm2, because more dyes transit from cis to trans, is larger than that at IG=0~0.7 W/cm2. The total torque is negative and decreases with increasing intensity of pump light. The number of diffraction ring decreases with increasing intensity of pump light. At IG>1.1W/cm2, more cis-state isomers transit to trans isomer by absorbing heat, and lead to be larger than . The total torque is positive and increases with increasing intensity of pump light. The number of diffraction ring becomes more. Illuminating only green light on the liquid crystals doped with azo dye with IG>3.1 W/cm2, the transition from trans to cis is so fast which causes the temperature in the center of illumination is higher than the nematic-isotropic temperature. The liquid crystals is isotropic in the center of illumination, and a Fraunhofer diffraction in the far field is observed.
Illuminating a red laser and a green laser simultaneously on liquid crystals doped with azo dye, the red light speeds up the transition of dyes from cis to trans and causes the larger positive torque . If the positive torque is larger than negative torque , the total torque is positive and then the liquid crystals and azo-dye molecules tend to reorient to the direction parallel to the polarization of pump light.
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