Semiclassical theory of multisubband plasmons: Nonlocal electrodynamics and radiative effects

Coherent multisubband plasmons in doped semiconductor quantum wells have recently attracted large interest as they allow us to strongly enhance light-matter interaction via collective Coulomb coupling among different intersubband transitions. In this work, we develop a semiclassical theory of intersubband plasmons in quantum wells, on the basis of nonlocal electrodynamics. The nonlocal treatment provides a proper description of collective effects in the electromagnetic response of the system and, in the long-wavelength approximation, it predicts the same resonance frequencies as the quantum mechanical description. The nonlocal formalism is applied to the study of the radiative decay rate of multisubband plasmons and plasmon polaritons, both in the case of an isolated quantum well and of a planar microcavity. We show that subpicosecond radiative lifetimes are to be expected for intersubband plasmons in semiconductor quantum wells, similarly to quantum well excitons. The theory is formulated in the context of the transfer-matrix method and it can be applied in a straightforward way to stratified geometries of any degree of complexity.