SELECTION-RULES OF INTERSUBBAND TRANSITIONS IN CONDUCTION-BAND QUANTUM-WELLS

In this work, optical intersubband transitions in conduction-band quantum wells have been reexamined in the multiband scheme. A generalized theory, with emphasis on the selection rules for the inplane polarization, is developed in Kane's k.P formalism. By taking into account the interband couplings, it is shown that the optical transition between any pair of electron subbands can occur with either the normal-to-plane polarized light or the in-plane polarized light. The characteristics of intersubband transitions depend upon whether the subband index differences Delta n are odd integers (Delta n=1, 3,...) or even integers (Delta n=2,4,...). In the case of Delta n being even integers, intersubband transitions of both polarizations are allowed for electrons with a finite in-plane wave vector. In fact, the transition rates are proportional to the value of the in-plane wave vector, and the in-plane polarized transition is dominant. In the case of Delta n being an odd integer, optical intersubband transitions of both polarizations can occur with a zero in-plane wave vector, but the normal-to-plane polarized transition is dominant. Consequences for device implementation such as a normal-incidence infrared photodetector that makes use of the allowed in-plane polarized optical transitions are discussed.