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Ultrafast coherent and incoherent dynamics of intersubband excitations in semiconductor quantum wells

The generation, manipulation and relaxation of optical intersubband excitations in n-type GaAs/AlGaAs and p-type SiGe/Si quantum wells are studied by different techniques of ultrafast spectroscopy in the mid-infrared. For electrons in GaAs/AlGaAs quantum wells, femtosecond time-resolved four-wave-mixing studies demonstrate de-phasing times of coherent intersubband polarizations of several hundreds of femtoseconds which are determined by electron-electron scattering. The measured dephasing times fully account for the width of the stationary intersubband absorption line, giving evidence of a predominant homogeneous broadening. Using phase-locked mid-infrared pulses and phase-resolving detection schemes, coherent optical control of such intersubband polarizations is demonstrated. In a second experiment, we study the intersubband relaxation of heavy holes (HH) in p-type SiGe/Si quantum wells. Intersubband scattering from the HH2 back to the HH1 subband occurs with a time constant of 250 fs determined by scattering with optical phonons through the deformation potential interaction.