Theory of the generation and detection of carriers in ultrafast photoexcitation experiments in semiconductors

As recently observed, the process of optically generating carriers in semiconductors has to be treated with a fully quantum mechanical description. We demonstrate that the description of carrier generation on the basis of the Bloch equations leads to much broader energy distributions than can be expected semiclassically. We also point out that the detection of such distributions by the probe pulse is a non-trivial process due to quantum effects, and that the probe spectrum is not always a direct measure of the carrier distribution function. A quantum mechanical calculations of the probe spectrum shows that it is considerably broader than the distribution function, and that sharp features in the time domain are smoothed out in the probe transmission.