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.

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). |

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). |