Inter-subband terahertz lasers using four-level asymmetric 
quantum wells.

P. Kinsler, P. Harrison and R. W. Kelsall

    Institute of Microwaves and Photonics, 
    School of Electronic and Electrical Engineering, 
    University of Leeds, 
    Leeds LS2 9JT,
    United Kingdom.

     J. Appl. Phys. 85, 23-28 (1999).

We demonstrate the potential for laser operation at far infrared
wavelengths (30-300 {$\mu$}m, 1-10THz) by using inter-subband
emission in four-level GaAs/AlGaAs asymmetric (stepped) quantum
wells.  Achieving population inversion in these devices depends
critically on the lifetimes of the non-radiative inter-subband
transitions, and so we have performed detailed calculations of
electron--electron and electron--phonon scattering rates.  Our
four-subband structures show potential for the realization of
room temperature lasing, unlike previously considered
three-subband structures which did not give population inversions
except at impractically low electron densities and temperatures. 
Auger-type electron--electron interactions involving the highly
populated ground subband effectively destroyed the population
inversion in three level systems, but in these four subband
structures the inversion is maintained by strong phonon-mediated
depopulation of the lower laser level.  The largest population
inversions are calculated at low temperatures ($< 30$K), but for
the structures with higher emission energies, room temperature
(300K) operation is also predicted.  





Email: Dr.Paul.Kinsler@physics.org