Infrared-laser induced periodic structures in azobenzene polymer films

Azobenzene containing polymers are spin coated onto glass slides to form 200 to 400 nm thick films. The films are then poled by corona discharge to orient the azobenzene molecules in a direction perpendicular to the surface. A single infrared Nd-YAG laser beam at 1064 nm is incident on the film along the normal to the surface. This beam is within the transmission window of the film and is thus not highly absorbed. We have found that second harmonic SHG (at 532 nm) and third harmonic THG (at 354.7 nm) beams are generated and are emitted in the form of arcs behind the sample. These arcs are consistent with emission from periodic structures with a 700 nm spacing. If the sample is not poled then only the arc from the third harmonic is generated. We have also found that if the laser power is increased above a specific threshold then permanent surface ripples are formed on the polymer surface even though the laser power is well below the ablation threshold. We will also look at the effects of including optically written surface relief gratings to enhance the coupling of the IR light to the azopolymer film to lower power required to observe non-linear response.