Lasers based on intersubband transitions in quantum wells and strained layers.

Far infrared (FIR) frequencies, have potential applications in spectroscopy, radio astronomy, and space-based communication systems. The current sources are gas lasers which require high input power for operation in addition to being expensive and bulky compared to semiconductor lasers. Conventional semiconductor devices, namely electronic and optoelectronic devices, have not been employed in this frequency range. These frequencies are higher than microwave frequencies generated by electronic devices where the frequency is limited by the maximum velocity of the carriers in the semiconductor. They are lower than optical frequencies emitted by the radiative relaxation of electrons from the conduction band to the valence band in interband optoelectronic devices where the minimum bandgap of the semiconductor is the limiting factor. The energy separation between the heavy hole and light hole bands in the valence band of strained semiconductor layers is proportional to the amount of strain in the layer and therefore can be designed to be in the FIR frequency range. Another potential alternative medium for realizing semiconductor far infrared lasers, is to invoke the intersubband transition in quantum wells where the energy separation between subbands can be adjusted to this frequency range by changing the well width. Many laser structures based on intersubband transitions in quantum wells have been proposed. These structures are either electrically or optically pumped. In this work, the feasibility of these proposed structures, as well as new schemes, including structures based on strained layers, are investigated. Various parameters which are pertinent to a feasibility study of these lasers are calculated and discussed. These include the optical gain, the confinement factor, the relaxation rates, and the optical losses. The potential problems concerning the realization of the proposed structures which are not practical are described while more promising schemes are also determined. The most promising structure found is an optically pumped step quantum well which essentially operates as a 4-level laser to minimize a bottleneck in the lower laser state.PhDApplied SciencesCondensed matter physicsElectrical engineeringNuclear physics and radiationPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/129416/2/9513283.pd