| Summary: | This thesis examines a series of effects associated with condensation in excitonic systems, and in particular in systems of microcavity exciton-polaritons and indirect excitons. A proposal is presented for a terahertz laser based on a microcavity system, in which a polariton condensate is formed, stimulating the terahertz transition from the 2p exciton state. An associated fundamental effect is predicted, in which the threshold to lasing is dependent on the statistics of the pump photons. The potential of hybrid Bose-Fermi systems for the study of coherent many body phenomena is demonstrated, by modelling the effects of interaction between an excitonic condensate and a two-dimensional electron gas. The fermionic subsystem of electrons is first considered, and it is shown that a phase transition to superconductivity may exist due to pairing mediated by virtual excitations of the condensate, analogous to the phonon mechanism in conventional superconductors. The system is modelled within BCS theory, and the gap equation is solved numerically to yield the critical temperature. The complementary effects of the electron gas on the bosonic condensate are also studied; the effective interaction between the constituents of the condensate may be strongly modified, affecting the superfluid properties of the condensate, and leading to the appearance of a roton minimum in the dispersion of elementary excitations. In fact, the dispersion may be modified to the extent that the roton gap closes, creating an instability in the system - it is shown that this instability may be manifested as a transition to a supersolid phase.
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