RESEARCH INTEREST

Our experimental group uses a wide array of optical methods to study fundamental questions of quantum mechanics in semiconductor systems. Our optical methods include ultrafast spectroscopy on femtosecond and picosecond time scales, single-photon counting and correlation, real-space and momentum space (Fourier) imaging with CCD cameras, and nonlinear optics such as two-photon absorption and the optical Stark effect. We can also apply variable stress to samples to create potential gradients to move particles inside solids, vary temperature down to cryogenic temperatures, and measure transport with electronics. 

 

One of the main efforts in our lab at present in the study of polariton condensates in microcavities. The polaritons are essentially photons dressed with an effective mass and strong interactions due to the special design of the solid-state microcavity structures we use. These interacting photons can undergo Bose-Einstein condensation, which is a state of matter with spontaneous coherence. We can see the superfluid flow of the polariton condensate over millimeter distances; we can also trap the condensate in various potentials, and we can see interference due to the coherence of the condensate. 

 

This work connects to several fundamental questions. One topic is how coherence can occur spontaneously ("enphasing") in systems like lasers and condensates and how coherence is lost ("dephasing") in standard quantum systems.  This, in turn, relates to the deep question of why there is irreversibility in nature, that is, the arrow of time. Another topic is how phase transitions can occur in nonequilibrium systems. We have developed sophisticated numerical methods to compare the solution of a quantum Boltzmann equation (which gives the temporal evolution of a system in nonequilibrium) to our data on the momentum and energy distributions of gases of various particles.

 

A new effort in our group is looking at the effect of a polariton condensate on electronic transport. This may allow a "light-induced superconductor", in which there are dramatic effects on conduction when the polariton condensate appears. We are also looking at new material such as the transition metal dichalcogenides (TMDs) systems so that the polariton condensate effects can be moved to room temperature. [Link]

 

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 2 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.

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