Highly optically excited semiconductors lie at the heart of the semiconductor laser and provide an interesting playground for condensed matter physics. They can be modelled within an "effective mass theory", giving a simple Hamiltonian which, nevertheless, shows rich and interesting behaviour. The systems may be characterised by their geometry (2- or 3-dimensional, electron-hole bilayer geometry, etc.), average density, and the electron/hole mass ratio. Despite their simplicity and fundamental and technological importance, even the basic phase diagrams of these systems are very uncertain. These systems probably exhibit exciton formation, biexciton formation, Wigner crystallisation, Mott insulating behaviour, and Bose condensation of excitons at low densities.

We are currently using quantum Monte Carlo techniques to study the two-component plasma, excitonic insulator, and Wigner crystal phases of electron-hole systems.