Selection rules of intersubband transitions in conduction-band quantum wells

Abstract

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 in-plane 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 Δn are odd integers (Δn=1,3,. . .) or even integers (Δn=2,4,. . .). In the case of Δn 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 Δn 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.