When atoms in a crystal are struck by laser light,
their electrons, excited by the light, typically
begin moving back and forth together in a regular pattern,
resembling nanoscale soldiers marching in a
lockstep formation. But according to a new theory developed
by Johns Hopkins researchers, under the
right conditions these atoms will rebel against uniformity.
Their electrons will begin moving apart and
joining together again repeatedly like lively swing
partners on a dance floor.
Moreover, the researchers say, this atomic freestyle
dancing can be controlled, paving the way
for tiny computer components that emit less heat and new
sensors to detect biohazards and medical
"By choosing particular atoms in the proper
configuration and directing the right laser light at
them, we could control the behavior of these
'nanodancers,'" said Alexander E.
Kaplan, a professor in
the Department of
Electrical and Computer Engineering in Johns Hopkins'
Whiting School of
Engineering. "The essential thing is, these are completely
designable atomic structures."
Kaplan and Sergei N. Volkov, a postdoctoral fellow in
Kaplan's lab, described this phenomenon in
a paper published recently in the journal Physical Review
Letters. The next step is for other
researchers to conduct lab experiments in an effort to
validate the theory and predictions advanced
by Kaplan and Volkov.
Kaplan, an internationally renowned nonlinear optics
expert who studies how matter interacts
with strong light, said his and Volkov's "nanoriot" idea
runs counter to a widely accepted concept. For
decades, Kaplan said, scientists have adhered to the
Lorentz-Lorenz theory, which asserts that the
atomic electrons in a crystal, exposed to a laser beam,
will move back and forth in tandem in a uniform
way under any conditions.
"But we've concluded that under certain circumstances,
the nearest atoms will behave much
differently," he said. "Their electrons will move violently
apart and come back together again, staging
a sort of 'nanoriot.'"
For this to happen, Kaplan said, several critical
conditions must exist. First, the system must be
very small, typically involving no more than a few hundred
atoms, and the atoms must be arranged in a
one- or two-dimensional configuration. The atoms also must
be grouped in a sufficiently close
concentration; interestingly, though, this arrangement may
allow more space between atoms than
exists in a typical crystal. In addition, the frequency of
the laser driving the atoms must be closely
tuned to one of the specific frequencies of the atomic
electrons--the so-called atomic resonance--in
the way that a radio receiver might be tuned to a
When these critical conditions are met, the
interacting excited atomic electrons get strongly
"coupled," and their motion is affected by one another. The
atomic dance partners begin to match or
counter-match the motion of each other, while still being
driven by the laser's "music."
When this occurs, the dancing atomic electrons form
waves of collective motion. Kaplan calls
these waves "locsitons," based on the words "local" and
"exciton," the latter referring to a physics
concept. Within the atomic systems envisioned by Kaplan and
Volkov, these locsiton waves are strongly
affected by the boundaries of these structures or any
irregularities, such as holes. The presence of
these boundaries results in size-related resonances, or
highly excited motion at certain frequencies,
resembling those of a violin string fixed at two end
points. In this case, the string's end points would
be the boundaries of the group of atoms. A smooth violin
string produces mostly a main tone, and
nearby points of the string move in unison. But an atomic
array more closely resembles a chain of
connected beads, and with the right laser tuning, the
neighboring beads, or atomic electrons, can
oscillate counter to each other.
"Fortunately, once this atomic structure is built, the
'dancing style' of the atoms can be
controlled by the laser tuning," Kaplan said. "Furthermore,
if the laser intensity is sufficient, we
believe the atoms in this lattice will engage in so-called
nonlinear behavior. That means they can be
made to behave like switches in a computer. They will act
like a computer's memory or logic
components, assuming the positions of either 1 or 0,
depending on the initial conditions."
Computer makers, trying to produce ever-smaller
metallic or semiconductor components, have
run into problems related to the excessive release of heat.
However, the nanoscale switch envisioned
by Kaplan would be a dielectric, meaning it would involve
no exchange of free electrons in the
structure. Because of this, the proposed components would
generate far less heat.
If their theory is confirmed, the Johns Hopkins
researchers foresee other applications for
these nanoscale atomic systems. The tiny lattices, they
say, could be designed so that when a specific
foreign biomolecule enters a system, the atomic electron
dancing would stop. Because of this
characteristic, they said, the system could be designed to
trigger an alarm signal whenever a
biohazard or perhaps a cancer cell was detected.
The research by Kaplan and Volkov was supported by a
grant from the Air Force Office of