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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.
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In a two-dimensional system, namely, excitons in GaAs or InGaAs quantum wells. When an electric field is applied perpendicular to the plane of the wells, the excitons become polarized and move in response to a gradient in the electric field. With the use of electron beam lithography, we can make complicated gate structures on the surfaces of a coupled quantum well wafer to control the movement of excitons. Ultimately, the goal is to test the viability of these excitonic circuits for light harvesting and other electro-optical applications.
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.
In a "coupled quantum well" system, two
very thin layers of semiconductor (about 6 nm, that is, 6 billionths of a
meter) lie side by side. When electric field is applied to the sample,
negative charges accumulate in one layer (or "well") and positive charges
accumulate in the other one. There is a thin barrier in between the two
layers, and quantum mechanics allows charge to "tunnel" from one layer to the
other without hopping over the barrier; as if passing magically through a
wall.
The optical properties of this system depend sensitively on all kinds of
things. We see large shifts of the luminescence wavelength with laser
intensity, with electric field, and with magnetic field, in addition to shifts
due to stress and temperature. We can control these shifts and use them to
modulate the light signals.