Optically controlled nonlinear absorption and third harmonic generation in azo-PMMA thin films

The third harmonic generation (THG) and nonlinear transmission (NT) were used to probe the change of /spl chi//sup (3)/ in azo-doped polymethylmethacrylate (PMMA) thin films. The observed phenomena can be interpreted by a microscopic model based on angular hole burning (AHB) and angular redistribution (AR) mechanisms of photoisomerization effect. In this experiment, the pumping effect on the THG and NT at different polarization directions is investigated. The variations of THG here are mainly due to the change of third order susceptibility /spl chi//sup (3)/ (-/spl omega/; /spl omega/, /spl omega/, -/spl omega/). As we switch on the pumping beam, the THG in the co-polarized configuration experiences a fast decay associated with the AHB in few seconds and, thereafter, it reaches the stationary state after a slow decay corresponding to the AR lasting for several minutes. When we turn off the pumping beam, THG will recover its original level by thermal relaxation. However, there is a small G increase in the cross-polarized configuration. Since the nonlinear absorption is related to the image part of /spl chi//sup (3)/ (-/spl omega/; /spl omega/, /spl omega/, -/spl omega/), the decrease in trans molecule number density corresponds to the decrease in Im /spl chi//sup (3)/ (-/spl omega/; /spl omega/, /spl omega/, -/spl omega/). When nonlinear absorption decreases, the NT increases accordingly. The anisotropic molecular distribution, which appears during AHB and AR, will also result in the nonlinear dichroism in the sample. We present time evolutions of the NT in the sample measured in the vicinity of focus point of the probing beam for both co-polarized and cross-polarized configurations in NT measurements. A considerable increase of NT in the co-polarized configuration can be observed when we switch on the pumping beam. On the contrary, as we turn off the pumping beam, the NT decreases to its original level. However the NT remains nearly unchanged in the cross-polarized configuration. The considerable change of NT in the co-polarized configuration fits in with our expectation since, in the existence of pumping beam, the trans population density in the direction parallel to the pumping beam will decrease in both AHB and AR mechanisms. On the other hand, the thin film will restore its isotropic distribution by thermal relaxation after pump removal, so the NT returns its original level. Nevertheless, the nearly unchanged NT in cross-polarized configuration is beyond our expectation. It may be due to that the change is too small to be observed in the NT measurements. Furthermore, the expected fast growth associated with AHB disappears in the growing curve. This may be due to that we used another 1 kHz diode-pumped Nd:YAG laser as pumping source. The heat accumulated on thin film is inevitable, so thermal cis-trans relaxation speeds up and the raising population of trans molecules can reorient to the direction parallel to the pump polarization by fast angular diffusion. Accordingly, nonlinear absorption (NT) will not decrease (increase) as fast as expected in AHB.