A highly efficient
room-temperature nanolaser has been demonstrated by
scientists at Yokohama National University in Japan. Made of the
semiconductor gallium indium arsenide phosphate, the overall device
has a width of several microns, but the active region where laser
light actually gets produced has nanometer-scale dimensions in all
directions. The device is the first nanolaser to emit continuous
coherent near-IR light at room temperature and uses only a microwatt
of power, one of the smallest operating powers ever achieved. The
laser's small size and high efficiency were made possible by its
photonic-crystal design. The researchers etched a repeating pattern
of holes through the semiconductor and deliberately introduced a
defect into the pattern—for example, by slightly shifting the
positions of two holes, as shown here. The imperfect pattern defined
a narrow frequency band of light that could exist in the defect
region. Curiously, the team found that a high quality factor
Q was not necessarily advantageous for optimized device
behavior. A high Q may be desirable for low-threshold
lasing, but a low Q should be more effective in such
applications as a single-photon emitter. (K. Nozaki, S. Kita, T.
Baba, Opt.
Express15, 7506, 2007[SPIN].) —PFS
Turning heat
into electricity through sound has been demonstrated by
a team of researchers at the University of Utah. The group, led by
Orest Symko, has built devices that can create electricity from the
heat generated by computer chips, nuclear power plants, or even the
Sun. At the June meeting of the Acoustical Society of America in
Salt Lake City, five of Symko's students reported the latest
advances in their devices, which first convert heat into sound and
then sound into electricity. Typically, each device is a palm-sized
cylinder containing a stack of material such as plastic, metal, or
fiberglass. Applying heat to one end of the stack creates a movement
of air that travels down the cylindrical tube. The warm, moving air
sets up a sound wave in the tube, similar to the way in which air
blown into a flute creates a tone. The sound wave then strikes a
piezoelectric crystal, which converts the wave's pressure
oscillations into electricity. According to Symko, a ballpark range
of 10–25% of the heat typically gets converted into sound energy.
The piezoelectric crystals then convert 80–90% of the sound energy
into electrical energy. The team expects the devices to be used in
real-world applications within two years. (ASA Meeting talks 5aPA2,
5aPA4, 5aPA5, 5aPA7, 5aPA8.) —BPS
A self-generated
maze can be explained with simple physics. Remarkable
patterns can spontaneously emerge in out-of-equilibrium systems with
competing forces. The labyrinthine structures in this illustration
were created experimentally (gold and black) and in simulations
(black and white) by Bjørnar Sandnes and colleagues at the
University of Oslo, Norway. The lines indicate a residue of glass
beads that remains after the liquid has been slowly drained from a
bead–fluid suspension confined between two glass plates. Shortly
after the draining begins, air deforms the air–liquid interface at
the perimeter of the suspension and produces fingers in the liquid.
The slow draining means that viscous forces are not important
determinants of the fingering. Sandnes suggests that the pattern
results instead from a combination of capillary pressure that acts
at the air–liquid meniscus to advance the finger and a retarding
frictional force. As the illustration shows, the simulations run by
the Oslo team to test that simple model produce patterns that look
much like the experimental results. The group tested further by
exploring how the characteristic finger width of the labyrinth
varies with the concentration of glass beads in suspension. Again,
simulated and experimental results were in good agreement,
particularly when the residue of beads was significantly thicker
than the plate spacing. (B. Sandnes et al., Phys. Rev.
Lett., in press.) —SKB
Polonium's
simple cubic structure. Polonium, with atomic number
84, is the only element with a simple cubic crystal structure, and
new theoretical work by a team of scientists at the Academy of
Sciences of the Czech Republic, Brno University of Technology, and
Masaryk University explains why. In a solid piece of Po, the atoms
sit at the corners of a cubic unit cell and nowhere else. Most other
metallic elements, in contrast, have body-centered cubic,
face-centered cubic, or hexagonally close-packed structure. Using
state-of-the-art ab initio electronic structure calculations, the
researchers have produced a detailed theoretical explanation for
Po's unique crystal structure: It is the result of the complicated
interplay of relativistic effects that become important in heavy
atoms such as Po. In particular, they found that the mass–velocity
effect, which describes the relativistic increase in mass of
electrons traveling at speeds comparable to that of light, increases
faster than the effect of spin–orbit coupling in going down the
periodic table from trigonal selenium and tellurium to Po. Another
Po oddity: Its elastic anisotropy is greater than for any other
solid. It is about 10 times easier to deform a Po crystal along the
unit cell's body diagonal (the [111] direction) than along a
direction perpendicular to any of the cubic faces (the [100]
direction). The team found that this property results directly from
the simple cubic structure of Po. (D. Legut, M. Friák, M. ob, Phys. Rev.
Lett.99, 016402, 2007[SPIN].) —PFS
