Temperature Modelling of Laser-Irradiated Azo Polymer Films


Yager, K.G.; Barrett, C.J. "Temperature Modelling of Laser-Irradiated Azo Polymer Films" Journal of Chemical Physics 2004, 120 1089–1096.
doi: 10.1063/1.1631438


A cellular automata computer simulation was created to model heat flow in azobenzene films during optical irradiation to form surface patterns. It was found that the temperature gradient within the film (due to the spatially varying incident light field) was extremely small. This allows us to rule out photo-patterning mechanisms based upon spatial variation of material properties due to thermal gradients. Moreover, for typical irradiation conditions, the absolute temperature rise is very small. This allows us to further exclude photo-heating effects from the mechanism of patterning. Simulations agree well with analytical solutions (available in limiting cases), and demonstrate that laser-pulsed patterning is photo-ablative in origin.


Azobenzene polymer thin films exhibit reversible surface mass transport when irradiated with a light intensity and/or polarization gradient, although the exact mechanism remains unknown. In order to address the role of thermal effects in the surface relief grating formation process peculiar to azo polymers, a cellular automaton simulation was developed to model heat flow in thin films undergoing laser irradiation. Typical irradiation intensities of 50 mW/cm2 resulted in film temperature rises on the order of 5 K, confirmed experimentally. The temperature gradient between the light maxima and minima was found, however, to stabilize at only 10–4 K within 2 µs. These results indicate that thermal effects play a negligible role during inscription, for films of any thickness. Experiments monitoring surface relief grating formation on substrates of different thermal conductivity confirm that inscription is insensitive to film temperature. Further simulations suggest that high-intensity pulsed irradiation leads to destructive temperatures and sample ablation, not to reversible optical mass transport.