Intersubband scattering of cold electrons in a coupled quantum well with subband spacing below hwLO

We report measurements of the intersubband scattering rate between the first and second subband in a quantum-well structure with subband spacing(ll meV) smaller than the optical phonon energy. We measure the electron population in the second subband under CW excitation by a far-infrared laser tuned to the intersubband absorption frequency. This allows us to determine the intersubband relaxation rate using detailed balance. These measurements are novel because they are performed at very low excitation densities (I greater than or equal to 10 mu W/cm(2)). In this regime the heating of the electron gas is negligible, so that the optically excited population in the upper subband greatly exceeds any thermal population induced by laser heating. Therefore, the relaxation rate we measure is controlled by intersubband scattering rather than carrier cooling. At low temperature we obtain an intersubband lifetime of T-1 = (1.2 +/- 0.4)10(-9) s which is power independent below 10(-1)W/cm(2), and approximately temperature independent for lattice temperatures between T=10 and 2.5 K. (C) 1998 Elsevier Science B.V. All rights reserved.