This work studies photo-induced reorientation in azo-dye-doped liquid crystal (ADDLC) films by observing the
diffraction patterns produced by self-phase modulation (SPM). The experimental results show that not only the
magnitude but also the sign of the refractive index change (Δn) of an ADDLC sample pumped by a green laser can be
modulated by the beam intensity (IG). The reason is that different IG give rise to different trans/cis concentration ratios,
and then induce different overall torques on LC molecules. The trans and cis isomers contribute negative and positive
torques, respectively, that reorient LC molecules perpendicular and parallel to the polarization of the pump laser. The
reorientation effect in the ADDLC sample illuminated by the biphotonic lasers (green and red lasers) is also investigated
by observing the self-phase modulation diffraction pattern. The results indicate that red light can induce |Δn| to decrease
and increase at low and high IG, respectively. Besides, the
dye-induced torque can be changed from negative to positive
by adding red light. This is due to the fact that cis isomers produce an extra positive torque to the LCs after absorbing red
light.
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