Abstract

The femtosecond dynamics and the coherence properties of resonant secondary emission (RSE) from quantum wells is a subject of considerable current interest.^1–3 We use a novel approach in which the sample is resonantly excited by a pair of phase-locked pulses, and the spontaneously emitted radiation is de tected by femtosecond upconversion. This novel experimental technique allows us, for the first time, to investigate the temporal evolution and the coherence properties of RSE simultaneously. In particular, we demonstrate that the spontaneously emitted light consists of three different contributions which have different coherent relations to the excitation pulses. In addition to the well known incoherent resonant photo-luminescence (RPL) we identify, for the first time, an optical coherent component with a macroscopic electric field which can interfere with a replica of the excitation pulse.^1,2 After the complete decay of this component within 8 ps quantum coherences between the various excitonic states in the disorder potential last on a much longer time scale (up to 20 ps) which show up as speckles in the spatial domain and as quantum beats in the time domain.

© 1999 Optical Society of America