The Theory of Direct Transitions in Semiconductors

Within the past year there has been an increasing interest in semiconductor behavior, resulting from the development of the optical maser. In addition to the fact that the semiconductor can be used as an optically active medium, this interest results, in part, from a variety of nonlinear effects which can occur in semiconductors at infrared and optical frequencies. Optical masers are capable of providing very high-intensity fields at photon energies corresponding to the energy gap between valence and conduction bands. Since nonlinear effects generally vary as some power of field strength, the laser has stimulated keen interest in semiconductor nonlinear phenomena. Examples of such phenomena are the FranzKeldysh 1,2 effect and the multiple-photon process.3 Closely allied with these nonlinear interband effects is the process of photomixing 4 5 (photoconductivity), where two coherent optical signals are beat together to produce a photosignal which is proportional to the instantaneous density of photoexcited electrons and hence contains sum and difference frequency terms. The aforementioned phenomena have been analyzed previously by 805