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Spontaneous Coherence of Excitons and Polaritons

Spontaneous coherence is a general effect in physics which includes Bose-Einstein condensation, superfluidity, lasing, and superconductivity. When an ensemble of bosons is cooled to below a critical temperature, a macroscopic number of them can spontaneously be attracted to occupy a single quantum state. The system will then have coherence, acting like a wave, even on large size scales.

Excitons, which are pair states of excited electrons and holes in a solid, are bosons and can undergo Bose-Einstein condensation under certain conditions. Since excitons are created by photons and can convert into photons, exciton motion essentially corresponds transport of optical energy. But because excitons have an effective mass, they move much more slowly than photons and therefore can undergo a spontaneous phase transition to a superfluid state just like atoms. One way of looking at an exciton condensate is that it corresponds to the spontaneous appearance of optical phase coherence even without lasing, i.e. "coherence without stimulated emission.''

A polariton is a mixed state of an exciton and a photon. Since they are more photon-like than a simple exciton, the distinction between Bose-Einstein condensation of polaritons and lasing is less well defined; one can call spontaneous coherence in this system a "polariton laser." In principle, spontaneous coherence of polaritons can occur even at room temperature.

We create excitons or polaritons in semiconductor samples at liquid helium temperatures via an intense, ultrafast (picosecond or femtosecond) laser pulse and then examine the evolution of their momentum distribution and spatial distribution by detecting the light they emit, either via a time-gated CCD camera with 5 ns resolution, time-correlated photon counting with 40 ps resolution, a streak camera with 5 ps resolution, or pulse-probe methods with subpicosecond resolution. We also collaborate with theorists to answer fundamental questions about exciton and polariton condensates.

We are working on two ways of trapping excitons and polaritons. First, a trap can be made in three dimensions by applying an inhomogeneous stress. Excitons are attracted to the region of a shear stress maximum. When an intense laser pulse illuminates the region, excitons are created which remain trapped. A second method involves a two-dimensional system, namely, excitons in GaAs quantum wells. When an electric field is applied perpendicular to two coupled quantum wells, electrons move to one well and holes move to the other well. The excitons which consist of pairs of these electrons and holes can also be trapped by the application of an inhomogeneous stress.

Nonlinear Optics in Semiconductor Nanostructures

In electronics, the transistor plays an essential role as a switch by which one electrical signal turns another electrical signal on and off. Can we do the same thing with light beams? If we could make an "optical transistor" by which one light beam switches another one, we could make an optical computer in which all signals were carried by light instead of electrical signals. This would revolutionize technology in the way the electronic transistor did 50 years ago.

One way to do this is with nonlinear optics. In "linear" optics, the absorption and reflection coefficients of a medium are not dependent on the light intensity. In nonlinear optics, these coefficients can depend on the intensity, polarization, and wavelength of light. Therefore one can devise many schemes in which the presence of one light beam affects the transmission of another.

Besides optical-optical interactions, two other important effects are electro-optics and magneto-optics, in which an electric field or a magnetic field cause a shift of the optical properties of a material. These effects can be used for optical communications and memory devices.