Potential of femtosecond chirp control of ultrabroadband semiconductor continuum nonlinearities

We present an experimental study of the influence of frequency chirp in 20- fs optical pulses on broadband semiconductor continuum nonlinearities. Diff erential-transmission (DT) measurements were performed in which either the exciting pump pulse or the readout probe pulse, or both pump and probe, wer e chirped. We demonstrate that in certain chirp configurations the spectral ly integrated DT is enhanced on an ultrafast time scale compared with measu rements with unchirped pulses. Therefore pulse chirping has the potential t o improve and optimize all-optical ultrafast switching. Spectrally resolved DT measurements explain these findings. Positive and negative DT contribut ions are observed in different spectral ranges. The spectral position and t he magnitude of these contributions change in time. Proper chirping of the pulses optimizes the readout of the positive contributions and maximizes th e spectrally integrated DT. A simple quantitative model. confirms these con siderations. (C) 1999 Optical Society of America [S0740-3224(99)00312-4].