Temperature effect on photoinduced third harmonic generation variation in azo polymer thin films

In this work we study the temperature effect on photo-induced THG change in azo-polyurethane homopolymer and azo guest-host polymer thin films. The study is important as it can shed light on the molecular motions associated with angular hole burning (AHB) and angular redistribution (AR) at different temperatures. In the experiment a Q-switched Nd:YAG laser (1064 nm, 7 ns pulse at 10 Hz) was used as the light source. The second harmonic of the fundamental was used as the pump beam. A small amount of the fundamental was used as a probe to generate THG from the samples. The temperature effect on the decay of the photoinduced THG of the disperse-real-PMMA (DRl-PMMA) thin film, and of the polyurethane homopolymer thin film, for the pump and probe co-polarized configuration are presented. The best fit to the data using the bi-exponential function: f(t) = A exp(-t/τ_A ) + B exp(-t/τ_B ) + y₀ are also shown. The first and second terms of this equation are associated with fast AHB and slow AR decays, respectively. The fitting results show both A and τ_A values of both samples decrease when temperature increases. This arises from the decreases of the thermal relaxation time and the population of cis state as temperature increases. For the AR induced slow THG decay, both samples have longer decay time constant TB at higher temperatures. This indicates that the AR motion is slower at higher temperatures. Recently Sekkat et al. examined photoinduced linear absorption variations of a series of azo-polyurethane polymers, and pointed out that the decrease of cis thermal relaxation time in azo-polymer would reduce the AHB effect and even generated a pure AR motion in one particular azo-polyurethane polymer. Here we only find the decrease of the AHB effect but no increase of the AR motion when the samples are heated up. This is attributed to larger orientational diffusion coefficient at higher temperatures. When polymers get soft, it is easier for the strong pump field to torque the molecular dipole back while the dipole is trying to reorient away from the pump field polarization. It is also noticed that the homopolymer sample has much longer TB time constant and smaller B/A coefficient-ratio compared with the DRl-PMMA sample. Thus,- the AR effect in the homopolymer sample is much smaller than that in the DRl-PMMA sample, due likely to the larger molecular weight of the homopolymer sample.