  We report on the generation of few-ps long spontaneous emission pulses by
quantum dots (QDs) in a switched optical microcavity. We use a pulsed optical
injection of free charge carriers to induce a large frequency shift of the
fundamental mode of a GaAs/AlAs micropillar. We track in real time by
time-resolved photoluminescence its fundamental mode during its relaxation,
using the emission of the QD ensemble as a broadband internal light source.
Sub-ensembles of QDs emitting at a given frequency, interact transiently with
the mode and emit an ultrashort spontaneous emission pulse into it. By playing
with switching parameters and with the emission frequency of the QDs, selected
by spectral filtering, pulse durations ranging from 300 ps down to 6 ps have
been obtained. These pulses display a very small coherence length, which opens
potential applications in the field of ultrafast imaging. The control of
QD-mode coupling on ps-time scales establishes also cavity switching as a key
resource for quantum photonics.
