Fermi edge polaritons in a highly degenerate 2D electron gas: a diagrammatic theory

We present a theoretical study on polaritons in highly doped semiconductor microcavities. In particular, we focus on a cavity mode that is resonant with the absorption threshold (`Fermi edge'). In agreement with experimental results, the strong light-matter coupling is maintained under very high doping within our ladder diagram approximation. While the lower polariton is qualitatively unaltered, it acquires a finite lifetime due to relaxation of the valence band hole if the electron density exceeds a certain critical value. On the other hand the upper polariton has a finite lifetime for all densities, because it lies in the electron-hole continuum where no bound state exists. Our calculations show that a narrow upper polariton quasiparticle still exists as a result from the interplay between light-matter coupling and final state Coulomb interaction.