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Intersubband carrier lifetimes and population ratios are calculated for three-and four-level optically pumped terahertz laser structures. Laser operation is based on intersubband transitions between the conduction band states of asymmetric GaAs-Ga 1Ϫx Al x As quantum wells. It is shown that the carrier lifetimes in three-level systems fulfill the necessary conditions for stimulated emission only at temperatures below 200 K. The addition of a fourth level, however, enables fast depopulation of the lower laser level by resonant longitudinal optical phonon emission and thus offers potential for room temperature laser operation.

A theoretical study has been made to compare the quantum efficiencies of interwell and intrawell radiative transitions in quantum cascade lasers based on Al x Ga 1 x As/GaAs technology. Radiative and non-radiative intersubband transition rates were calculated for a range of temperatures between 4 K and 300 K at an applied electric field of 4 kVcm 1 for structures designed to emit in both mid-infrared and far-infrared frequency ranges. It is found that the internal quantum efficiency of mid-infrared devices is a maximum for diagonal, or interwell, transitions. Conversely, for the far-infrared devices, the quantum efficiency is a maximum for vertical, or intrawell, transitions.

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A superlattice of GaAs/Ga 1Ϫx Al x As quantum wells forms a Stark ladder under the influence of a perpendicular electric field. A two level incoherent emitter system, formed by radiative intersubband transitions between adjacent wells, is investigated as a tunable far-infrared radiation source. Intersubband transition rates are calculated at 4, 77, and 300 K for applied fields from 0 to 40 kV cm Ϫ1. It is shown that the quantum efficiency of the radiative emission reaches a maximum at low temperatures for a field of 32 kV cm Ϫ1. Under these conditions the emission wavelength is 38 m with an estimated power output of 1.1 mW.