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 (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, 23 May 2003; website
http://ab-initio.mit.edu )
Femtogram Mass Detection
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 (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; see
figure; website at www.ornl.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 Physics
News Update #626)
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). 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 (,
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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