Maximising the population inversion, by optimizing the 
depopulation rate, in far-infrared quantum cascade lasers.

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

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

    Superlattices and Microstructures 25, 373 (1999).

Electron-LO phonon and electron-electron transition rates are
calculated for a three level triple quantum well system to be
employed as a laser operating in the far-infrared (30-300 $\mu$m)
or terahertz (1-10 THz) region.  The population ratio is
determined from the ratio of the carrier lifetimes in levels |3>
and |2>.  The most effective way of depopulating the lower laser
level is found to be by LO phonon scattering to a strongly
coupled state, as occurs at an anticrossing.   Back scattering of
carriers from level |1> to level |2> is significant at room
temperature, but a population ratio of approximately 5 is
nonetheless possible.  





Email: Dr.Paul.Kinsler@physics.org