Artist's conception of
microscopic "quantum droplet" discovered by JILA physicists in a
gallium-arsenide semiconductor excited by an ultrafast red laser pulse.
Each droplet consists of electrons and holes (representing absent
electrons) arranged in a liquid-like pattern of rings. The surrounding
area is plasma. The discovery adds to understanding of how electrons
interact in optoelectronic devices. Credit: Baxley/JILA
JILA physicists used an ultrafast laser and help from German theorists to
discover a new semiconductor quasiparticle -- a handful of smaller
particles that briefly condense into a liquid-like droplet. Quasiparticles are composites of smaller particles that can be created
inside solid materials and act together in a predictable way. A simple
example is the exciton, a pairing, due to electrostatic forces,
of an electron and a so-called "hole," a place in the material's energy
structure where an electron could be, but isn't.The new quasiparticle, described in the Feb. 27, 2014, issue of Nature
and featured on the journal's cover, is a microscopic complex of
electrons and holes in a new, unpaired arrangement. The researchers call
this a "quantum droplet" because it has quantum characteristics such as
well-ordered energy levels, but also has some of the characteristics of
a liquid. It can have ripples, for example. It differs from a familiar
liquid like water because the quantum droplet has a finite size, beyond
which the association between electrons and holes disappears.
Although its lifetime is only a fleeting 25 picoseconds (trillionths
of a second), the quantum droplet is stable enough for research on how
light interacts with specialized forms of matter.
"Electron-hole
droplets are known in semiconductors, but they usually contain thousands
to millions of electrons and holes," says JILA physicist Steven
Cundiff, who studies the properties of cutting-edge lasers and what they
reveal about matter. "Here we are talking about droplets with around
five electrons and five holes.
"Regarding practical benefits,
nobody is going to build a quantum droplet widget. But this does have
indirect benefits in terms of improving our understanding of how
electrons interact in various situations, including in optoelectronic
devices."
The JILA team created the new quasiparticle by exciting
a gallium-arsenide semiconductor with an ultrafast red laser emitting
about 100 million pulses per second. The pulses initially form excitons,
which are known to travel around in semiconductors. As laser pulse
intensity increases, more electron-hole pairs are created, with quantum
droplets developing when the exciton density reaches a certain level. At
that point, the pairing disappears and a few electrons take up
positions relative to a given hole. The negatively charged electrons and
positively charged holes create a neutral droplet. The droplets are
like bubbles held together briefly by pressure from the surrounding
plasma.
JILA's experimental data on energy levels of individual
droplet rings agreed with theoretical calculations by co-authors at the
University of Marburg in Germany. JILA researchers found they could tap
into each energy level by tailoring the quantum properties of the laser
pulses to match the particle correlations within the droplets. The
droplets seem stable enough for future systematic studies on
interactions between light and highly correlated states of matter. In
addition, quasiparticles, in general, can have exotic properties not
found in their constituent parts, and thus, can play a role in
controlling the behavior of larger systems and devices.
JILA is a
joint institute of the National Institute of Standards and Technology
(NIST) and University of Colorado Boulder. Cundiff is a NIST physicist.
The JILA research is supported by the National Science Foundation, NIST
and the Alexander von Humboldt Foundation.
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