Physicists
at the National Institute of Standards and Technology
(NIST) have used lasers to cool and trap erbium
atoms, a "rare earth" heavy
metal with unusual optical, electronic and magnetic
properties. The element has such a complex energy
structure that it was previously considered too wild
to trap. The demonstration, reported in the April
14 issue of Physical Review Letters,* might lead
to the development of novel nanoscale devices for
telecommunications, quantum computing or fine-tuning
the properties of semiconductors.
Laser
cooling and trapping involves hitting atoms with
laser beams of just the right color and configuration
to cause the atoms to absorb and emit light in a
way that leads to controlled loss of momentum and
heat, ultimately producing a stable, nearly motionless
state. Until now, the process has been possible only
with atoms that switch easily between two energy
levels without any possible stops in between. Erbium
has over 110 energy levels between the two used in
laser cooling, and thus has many ways to get "lost" in
the process. NIST researchers discovered that these
lost atoms actually get recycled, so trapping is
possible after all.
The NIST team heated erbium to over 1300 degrees
C to make a stream of atoms. Magnetic fields and
six counter-propagating purple laser beams were then
used to cool and trap over a million atoms in a space
about 100 micrometers in diameter. As the atoms spend
time in the trap, they fall into one or more of the
110 energy levels, stop responding to the lasers,
and begin to diffuse out of the trap. Recycling occurs,
though, because the atoms are sufficiently magnetic
to be held in the vicinity by the trap's magnetic
field. Eventually, many of the lurking atoms fall
back to the lowest energy level that resonates with
the laser light and are recaptured in the trap.
The
erbium atoms can be trapped at a density that is
high enough to be a good starting point for making
a Bose-Einstein condensate, an unusual, very uniform
state of matter used in NIST research on quantum
computing. Cold trapped erbium also might be useful
for producing single photons, the smallest particles
of light, at wavelengths used in telecommunications.
In addition, trapped erbium atoms might be used for "doping" semiconductors
with small amounts of impurities to tailor their
properties. Erbium--which, like other rare earth
metals, retains its unique optical characteristics
even when mixed with other materials--is already
used in lasers, amplifiers and glazes for glasses
and ceramics. Erbium salts, for example, emit pastel
pink light.
* J.J. McClelland and J.L. Hanssen. 2006. Laser
cooling without repumping: a magneto-optical trap
for erbium atoms. Physical Review Letters. April
14.
Contact: Laura Ost
laura.ost@nist.gov
301-975-4034
National Institute of Standards and Technology
(NIST)
