Sensitive characterization of phase and amplitude semiconductor nonlinearities for broadband 20 fs excitation

We present an experimental technique that allows for the detection of pump- induced transmission and phase changes with high sensitivity and ultrafast temporal resolution over an arbitrarily wide time window. This is achieved combining spectral interferometry with high-frequency-chopping differential transmission measurements. With this setup, exciton and continuum nonlinea rities in a semiconductor are studied for broadband excitation. We find tha t the pump-induced phase changes at the exciton and in the continuum decay on distinctly different time scales, indicating different microscopic origi ns. (C) 2000 American Institute of Physics. [S0021-8979(00)04514-X].