PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 634 April 23, 2003   by Phillip F. Schewe, Ben Stein, and James
Riordon

SHOCKING COLOR EFFECTS.  A photonic crystal is a lattice of structures
(sometimes an arrangement of rods or a solid filled with a pattern of holes)
with a periodic alteration in the index of refraction.  In such a material
waves with only a select band of frequencies may propagate successfully.
Other frequencies are forbidden.  What happens, though, when a shock wave
moves through the lattice, momentarily compressing or expanding the
characteristic spacings?  A new "computational experiment" (detailed
computer simulation) provides an intriguing answer.  Evan J. Reed, Marin
Soljacic, and John Joannopoulos at MIT determine that a light beam moving in
a shock-modified photonic crystal will undergo two unexpected changes: a
Doppler shifting hundreds or even 10,000 times bigger than usual and a
bandwidth  narrowing.  There are plenty of phenomena that can broaden a
signal's bandwidth but none yet known that would narrow the bandwidth of an
arbitrary signal in this way (and by factors of 4 or more).  As for the
Doppler shift (a change in the frequency of the light owing to its
reflection from a  moving target), the light reflecting from the shock wave
can be "up  converted" (e.g., turned from red light into green light) with
an efficiency that should match or exceed the up conversions achieved with
nonlinear optical materials.  Furthermore, the shock conversion process is
tunable and independent of light intensity.
According to Evan Reed (evan{at}mit.edu, 617-253-5482) the MIT research should
generate great surprise and interest among those who work with photonic
crystals.   The next step will be to implement the computational results in
the laboratory with samples and actual shock waves, although for the sake of
eventual commercial applications (frequency conversion and signal
modulation) future modifications in photonic crystals will not have to be
initiated with guns or laser pulses but with less destructive acousto-optic
effects.  The photonic-crystal modulations might even be actuated with some
kind of MEMS (microelectromechanical systems) device.  (Reed et al.,
Physical Review Letters, upcoming article; website http://ab-initio.mit.edu
)

FEMTOGRAM MASS DETECTION has been achieved with cantilever oscillators at
Oak Ridge National Lab.  Once set to vibrating at MHz frequencies with a
diode laser, the tiny cantilevers (tiny slivers of silicon as small as 2
microns long and 50 nm thick) are exposed to an atmosphere of small
particles or molecules.  Depending on how the cantilever is coated, some of
the particles will be absorbed onto the surface of the cantilever, altering
its resonance frequency in a measurable way.  In a recent test the vapor
used was an acidic substance, which was absorbed with a mass change that was
noticeable at the 5 fg mass scale.  Other subject particles, such as DNA,
proteins, cells, or trace amounts of various chemical contaminants, should
be detectable by this process.  The experiment was carried out at ambient
conditions, with no vacuum or cryogenic temperatures.  According to Panos
Datskos of Oak Ridge (pgd{at}ornl.gov, 865-574-6205) the mass sensitivity of
the device can be sharpened to the molecular level if the resonance
frequency can be raised from about 2 MHz at present up to 50 MHz.  (Lavrik
and Datskos, Applied Physics Letters, 21 April; figure at
http://www.aip.org/mgr/png/2003/184.htm; website at www.ornl.mnl.gov )

BECs UNDERGO BRAGG EXPLOSION.  Bose Einstein condensates (BEC) provide a
versatile testbed for looking at quantum phenomena.  And maybe cosmology
too.  In their calculations, physicists at the University of Nottingham
first load an alkali BEC into an optical lattice, a honeycomb of laser light
which holds atoms in a 3D gridwork. (For another recent BEC-in-a-lattice
story, see www.aip.org/enews/physnews/2003/split/626-1.html ) Then they jar
the cloud of atoms, setting the BEC into motion, and have it scatter from
the same "crystal" of light beams.  Instead of x rays undergoing Bragg
scattering from crystallized protein, the BEC waves scatter from a crystal
of light.  But as it threads through the optical lattice, the pattern of
Bragg reflections can create traveling zones (essentially self-perpetuating
solitons and local whirlpools, or vortices) where atoms in the condensate
are actually excluded (see figure at http://www.aip.org/mgr/png/2003/185.htm
).  These solitons can in turn destabilize the BEC, causing it to explode
outward. The Nottingham researchers have been trying to model this explosion
using a nonlinear Schrodinger equation, a modified version of the equation
that governs electron waves inside atoms.  According to Mark Fromhold
(mark.fromhold{at}nottingham.ac.uk, 44-0115-9515192), similar equations are
being used in the statistical study of galaxy distribution.  (See for
example, Scott et al., Physical Review Letters, 21 Mar 2003)

***********
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