Excited state effects in the dielectric function of a 2D boson system

We present calculations for the dielectric function and collective modes of a two-dimensional interacting gas of bosons (excitons) in the xy plane whi ch can be polarized by the application of an electric field in the z direct ion. previous work has explored the response of the density fluctuations wh ile excitons lie in their intrinsic ground state. We extend the Formalism h ere by taking into account the internal degrees of freedom of the excitons, and their possible interplay with the density fluctuation modes of the sys tem. In the limit of low temperature, we find collective normal modes relat ed to the Various degrees of freedom. Our calculations of the corresponding intrinsic oscillator strength show that optical-like modes associated to t he excited states become increasingly damped when temperature is raised, as the occupation factors change the overall response.