NONLINEAR-OPTICAL PROPERTIES OF A SOLUBLE FORM OF POLYISOTHIONAPHTHENE

The third-order nonlinearity of a low-bandgap conjugated organic polymer has been assessed. Degenerate four-wave mixing experiments (at 1.064 mum using picosecond pulses from a passively mode-locked neodymium-doped yttrium aluminium garnet laser) on solutions of the polymer have permitted the determination of the microscopic third-order nonlinearity gamma. The polymer exhibits unusual behaviour which is solution concentration dependent. This behaviour is thought to be due to aggregation effects and strongly suggests that the bulk response of the material is large, the third-order nonlinearity being of the order of ten times greater than that of polythiophene. Following preliminary studies, a sample of the same polymer with a greater average molecular weight was also measured and was found, within an order of magnitude, to possess the same third-order nonlinearity.