  Intense terahetz (THz) pulses induce a photoluminescence (PL) flash from
undoped high-quality GaAs/AlGaAs quantum wells under continuous wave laser
excitation. The number of excitons increases 10000-fold from that of the steady
state under only laser excitation. The THz electric field dependence and the
relaxation dynamics of the PL flash intensity suggest that the strong electric
field of the THz pulse ionizes impurity states during the one-picosecond period
of the THz pulse and release carriers from a giant reservoir containing
impurity states in the AlGaAs layers.
