Far-infrared pump-probe measurements of the intersubband lifetime in an AlGaAs/GaAs coupled-quantum well

We report pump-and-probe measurements of the electron intersubband lifetime (T-1) in an AlGaAs/ GaAs heterostructure using a picosecond pulsed far-infrared laser. The subband spacing (11 meV) is less than the optical-phonon energy. Time-resolved measurements yield intersubband lifetimes ranging from T-1 = 1.1 +/- 0.2 ns to T-1 = 0.4 +/- 0.1 ns depending an measurement conditions. Results are in agreement with previous lifetime measurements on the same sample using continuous excitation at intensities less than or equal to 1 W/cm(2). The steady-state measurements yielded shorter lifetimes at high excitation intensities, possibly due to carrier heating leading to intersubband scattering by optical phonon emission. (C) 1996 American Institute of Physics.