  We employ polarization-shaped ultrafast optical pulses to generate
photocurrents which only arise if the optically induced coherent polarization
is frequency modulated. This frequency modulation is obtained via detuned
excitation of light-hole excitons in (110)-oriented GaAs quantum wells. The
observed photocurrents vanish for resonant excitation of excitons and reverse
their direction with a change of the sign of detuning. Moreover, the currents
do not exist for continuous-wave excitation. Our work reveals the existence of
a new class of photocurrents and visualizes the complexity of current response
tensors. This is helpful for the better understanding of optically induced
microscopic transport in semiconductors.
