Dynamics of the Formation of Composite Bosons from Fermions

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The exciton resonance in semiconductors is often used to demonstrate resonant phenomena first observed with atoms, such as photon echoes, quantum beats, vacuum Rabi splitting, and coherent-field Rabi flopping. Nonresonant optical excitation into the free-carrier continuum produces an entirely different initial condition, much closer to that of a gas plasma whose electrons and ions eventually recombine into atoms that emit photons to return to their ground state. In a semiconductor, the initial plasma consists of free carriers, electrons and holes, whose kinetic energies depend upon how much the excitation energy exceeds the band gap. Exciton formation from the free carriers has been studied for more than four decades, yet little is known. With few exceptions data have been analyzed assuming that each photon emitted with the energy of the 1s electron/heavy-hole transition must have come from an exciton recombining. A recent theoretical breakthrough showed that the plasma emission is also peaked at the 1s resonance. This important finding calls for the reevaluation of all previous conclusions. It is proposed to study the dynamics of exciton formation in a quantum well and to learn how to control the population of the 1s ground state. The goal of the project is to understand the fundamental transition from an initially purely fermionic system (free-carrier plasma generated by optical absorption) to a bosonic system (exciton population). If the bosonic nature of low-density excitons dominates over the fermionic nature of its components, as is the case for atoms, then Bose-Einstein condensation could be achieved in semiconductors at much higher temperatures than those of atomic systems.