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We present time and spectral resolved pump and probe studies of the intersubband dynamics in a bound to quasibound GaAs/AlGaAs quantum-well infrared photodetector dependent on the external electric field. Without bias, the excited electrons are scattered back to the ground subband with a time constant of about 2 ps. Subsequent intrasubband cooling is observed in the ground state, which gives rise to a relaxation component on a 50 ps time scale. With an applied electrical field, 35%–45% of the excited electrons escape to continuum states and are accelerated along the electrical field. Within time constants between 5 and 8 ps, the extended electrons are recaptured by wells in the low field bulk domain close to the collector contact. The redistribution of the carriers in the picosecond time regime and the screening of the external field due to the separation between electrons and donors leads to a change of the absorption spectrum for a time period shorter than the intrinsic response time of the electrical circuit.© 2002 American Institute of Physics.