All-optical pulsed writing in azobenzene copolymer films in the sub-millisecond regime

Block copolymers P(MA4-block -MMA)10 and P(MA4- block -MMA)20 and a random copolymer P(MA4-ran-MMA)10 were prepared from azobenzene methacrylate (MA4) and methyl methacrylate (MMA). The former monomer was introduced as both a photoresponsive and a mesogenic component, and the latter as a transparent non-mesogenic component. Thin films of the copolymers were used to investigate all-optical writing by linear-polarized illumination in the pulsed regime to modify the local birefringence at the microscopic scale. The optical properties of P(MA4- block -MMA)10 were compared with those of P(MA4- ran-MMA)10 having the same chemical composition (10 mol% MA4) but different distributions of the two components. It was found that in the block copolymers relatively intense pulses as short as 100 ms produced a remarkable local increase in the birefringence, stable in the time-scale (up to several days) explored in the experiment. In contrast, efficient and stable optical modifications could not be achieved in the random copolymer.