Inverse distribution of hot electrons in a two-level quantum well with nonsymmetric scattering

We have studied the nonequilibrium distribution of electrons accelerated by an in-plane electric field in a stepped quantum well with two closely spaced levels. The spontaneous emission of optical phonons provides an effective cutoff of the electron distribution in the low-temperature case. Taking into account the quasielastic scattering with acoustic phonons (both for intra- and intersubband) and an effective elastic scattering in the second subband, we have found the conditions for the intersubband population inversion of electrons. Next, we have considered the resonant response of a multiquantum well structure placed at the center of a waveguide and found the conditions for the stimulated emission of the THz radiation.