Nonlinear optical processes in a polymer waveguide: Grating coupler measurements of electronic and thermal nonlinearities

We report here the first clear demonstration of intensity-dependent phase shift due to electronic nonlinearity in a nonlinear polymer waveguide in which propagation distances over 5 cm have been achieved with total attenuation of ∼1.2 cm^-1. Intensity-dependent coupling angle, intensity-dependent coupling efficiency, and limiter action behavior have been observed in the polyamic acid waveguide using grating excitation with 400 fs, 80 ps, and 10 ns pulses. A nonlinear grating coupler analysis identifies the subpicosecond and picosecond processes with electronic nonlinearity, but the dominant effect in the nanosecond experiment is due to thermal nonlinearity derived from weak absorptions. The magnitude and sign of n₂ of electronic nonlinearity are measured.