Self-phase modulation in azo-dye-doped liquid crystals

We investigate the photo-induced reorientation in azo-dye doped liquid crystal (ADDLC) films by observing the diffraction patterns due to self-phase modulation. The experimental results show that the change of refraction index ($\Delta $n) of the ADDLC sample increases with the intensity of green light (I$_{G})$ to a maximum $\Delta $n = 0.09 at I$_{G}$ = 0.7 W/cm$^{2}$ , then decreases to zero at I$_{G}$ = 1.1 W/cm$^{2}$, and increases again with I$_{G}$ to a saturated$\Delta $n = 0.0376 at I$_{G}$ = 1.8 W/cm$^{2}$. The mechanism is mainly due to the competition between the negative and positive torques resulted from trans$\to $cis and cis$\to $trans isomerizations of azo dyes, respectively. The reorientation effect in the ADDLC sample illuminated by the biphotonic lasers (green and red lasers) is also studied by observing the self-phase modulation diffraction pattern. The experimental results indicate that the biphotonic lasers can used to modulate the change of refractive index of the sample. The reason is due to the fact that the red light can enhance the positive torque resulted from cis$\to $trans isomerization, and then change the direction of reorientation of LCs.