We present an experimental study of the influence of frequency chirp in 20-fs optical pulses on broadband semiconductor continuum nonlinearities. Differential-transmission (DT) measurements were performed in which either the exciting pump pulse or the readout probe pulse, or both pump and probe, were chirped. We demonstrate that in certain chirp configurations the spectrally integrated DT is enhanced on an ultrafast time scale compared with measurements with unchirped pulses. Therefore pulse chirping has the potential to improve and optimize all-optical ultrafast switching. Spectrally resolved DT measurements explain these findings. Positive and negative DT contributions are observed in different spectral ranges. The spectral position and the magnitude of these contributions change in time. Proper chirping of the pulses optimizes the readout of the positive contributions and maximizes the spectrally integrated DT. A simple quantitative model confirms these considerations.
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Eiji Tokunaga, Akira Terasaki, and Takayoshi Kobayashi
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