Fundamental Physics in Semiconductor Systems

There are many different goals in semiconductor physics. Some researchers aim at making new devices, and some aim at materials characterization. In our group we look at semiconductor structures primarily as test beds for principles of fundamental physics. The quality of materials has improved over the past decades to the point that we can look at nearly "perfect" structures to understand basic concepts such as two-dimensional physics, the conductor-insulator phase transition, quantum phase transitions such as Bose-Einstein condensation, renormalized mass and energy of particles, nonequilibrium dynamics, dephasing, and control of quantum coherence.

In the past few years we have achieved extraordinary results with spontaneous coherence ("Bose-Einstein condensation") of polaritons in microcavities.

This is a neat video introduction to exciton-polaritons put out by a group at Sheffield University in the U.K.
Video credit goes to LDSD semiconductor research group of Sheffield University. Source

Polaritons Turning in a Gradient
The above image shows polaritons travelling in a potential-energy gradient, starting by moving uphill and then stopping and reversing direction. The colorscale shows the polariton density. The red line is a fit to a parabola for classical ballistic motion: in other words, the polaritons act like an object with mass in a gravity field. Since polaritons are essentially photons renormalized to have mass and to repel each other, we can call this "gravity for photons". From M. Steger et al., Optica 2, 1 (2015).
                  Condensate in a Ring Trap
The above image shows a polariton condensate confined in a two-dimensional ring trap. The interference fringes arise because two copies of the image from the two legs of a Michelson interferometer are overlapped. The clear fringes show that the condensate is coherent across the entire ring. Analysis of the phase shifts in the interference pattern show that the condensate is circulating. From G.-L. Liu et al., Proc. Nat. Acad. Sci. (USA) 112, 2676 (2015).