It has been recently recognized that in bulk semiconductors the displacement current caused by ultrafast optical generation of ‘‘polarized pairs’’ in the applied dc field is an important mechanism of charge transport in addition to the usual transport current. In quantum-well systems, this polarized pair creation is thought to be the only source of photocurrent at the early stages of photoexcitation since the bulklike transport current is inhibited by the barriers. In this work we perform a full quantum-mechanical analysis of ultrafast optical excitation in a dc-biased quantum well. We take into account the multiple transitions that become allowed in the dc field which breaks the Δn=0 selection rule. As a result, the carriers are created as wave packets formed by coherent superposition of several eigenstates. When the characteristic size of these wave packets (coherence length) is much larger than the well width (for long pulses and/or narrow wells), we recover the polarized pairs behavior of the photocurrent. For shorter pulses, when the coherence length becomes comparable to the well width, the photocurrent exhibits quantum beats. Finally, for very short pulses (around 10 fs) we find that the carriers in a quantum well can behave as an ensemble of classical particles and produce a transportlike photocurrent.

• Received 21 April 1995

DOI:https://doi.org/10.1103/PhysRevB.52.12045