PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 646  July 16, 2003   by Phillip F. Schewe, Ben Stein, and James Riordon

PHOTONIC CRYSTAL SHIFTS ENERGY.  Photonic crystals are artificial
structures, sometimes consisting of stacked rods, or arrays o
f holes bored into a solid, which permit light in some wavelength bands to
pass through while rejecting light at other bands.
New work at Sandia National Lab indicates that a photonic crystal made from
half-micron-diameter tungsten rods, excited by ther
mal heating, suppresses light at longer wavelengths and re-emits light at a
shorter wavelength band, one that may be more usefu
l for such technological applications as photovoltaic power generation, or
building a better lightbulb.  Shawn Lin and his Sand
ia colleagues, in the course of their studies of photonic crystals, have
seemed to challenge the venerable formulation, made by
 Max P
lanck a hundred years ago, of what kind
of emission spectrum a body should have.  The Sandia photonic crystal seems
to emit between 4 and 10 times as much radiation in
 the near infrared than a body at that temperature (the sample had been
heated to 1250 C) should be emitting.  (Lin et al., App
lied Physics Letters, 14 July 2003; Lin et al., Applied Physics Letters, 14
July 2003; Lin et al., Optics Letters, Sep. 15 2003
)

PICOSECOND X-RAY CRYSTALLOGRAPHY of a protein has been demonstrated for the
first time, by a multinational collaboration (Phili
p Anfinrud, NIH, PhilipA{at}intra.niddk.nih.gov), enabling atom-scale movies
of an important biomolecule as it performs a speedy f
unction.  This accomplishment will be presented at the upcoming American
Crystallographic Association meeting from July 26-31 i
n Cincinnati (http://www.che.uc.edu/aca/index.html ; see also Schotte et
al., Science, 20 June 2003).  While crystallographers
have previously obtained frozen snapshots of thousands of proteins, they
have yet to capture the full range of motion in even a
 single protein.  Previous x-ray movies of proteins have been on the
nanosecond time scale, which is too slow for capturing the
 steps
 of many protein processes.
Recently, however, at the European Synchrotron and Radiation Facility
(ESRF) in France, researchers made picosecond-scale movie
s of a mutant myoglobin molecule getting rid of a toxic carbon monoxide
(CO) molecule.  Myoglobin is the protein that stores ox
ygen in muscle tissue.  The researchers chose to study a mutant version of
the protein because the highly strained atomic struc
ture in part of the protein causes it to get rid of a CO molecule much more
quickly than does ordinary myoglobin.  To capture t
his process, they first sent a 1-ps pulse of laser light to the protein to
eject the CO.  Immediately afterward, they illuminat
ed the protein with intense, 150-ps x-ray pulses from the ESRF synchrotron.
 Crucial to this process was the ability to isolate
 singl
e x-ray pulses from the synchrotron. A CCD camera recorded the patterns
from the successive x-ray pulses as they passed through
 the protein.  The resulting movie showed the CO migrating to various sites
in the protein, with the myoglobin rearranging its
shape to accommodate the expulsion of the CO.  In addition to enabling
researchers to study many important transitions in prote
ins, the picosecond time-scale of these movies is commensurate with the
timescale of many molecular dynamics simulations, allow
ing for closer comparison between theory and experiment.

TUMOR FLY-THROUGH MOVIES.  Researchers at Purdue University and the
Imperial College of Science in London have created a real-t
ime holographic system to acquire a fly-through movie of living tissue
using infrared light and a special, semiconductor hologr
aphic film. The acquired images showed structure inside rat tumors that,
with conventional techniques, would only be visible if
 the tumor was sectioned into thin slices or imaged with ionizing
radiation.  The researchers created the fly-through movie usi
ng optical coherence imaging (OCI). OCI is related to the more widely known
optical coherence tomography (OCT). However, OCT in
volves scanning a laser beam through a sample and gathering information
point by point, which then must be assembled into a com
plete
 image. OCI, on the other hand, captures complete images of thin tissue
sections that can be recorded directly with a video cam
era.
The key to the holographic OCI technique is a dynamic holographic film that
filters out the scattered, incoherent background li
ght but passes the coherent, full-frame images to a camera. Tissue readily
reflects image-bearing infrared light, but it also s
trongly scatters the light, and without coherence filtering the scattered
light would overwhelm the coherent pictures. By adjus
ting the relative delay between the image beam and the reference beam in
the OCI system's imaging interferometer, the researche
rs (Ping Yu, 765-494-3004, pingyu{at}physics.purdue.edu, David Nolte,
765-494-3013 nolte{at}physics.purdue.edu) could control the dep
th of the images and assemble a slice-by-slice tour through a tumor while
leaving the tissue intact.  Application of the OCI te
chniqu
e to cultured rat tumors revealed structures that appeared to be necroses
(regions of dead tissue) and calcifications much like
 those found in human cancers (see image at www.aip.org/mgr/png).
Ultimately, the researchers explain, holographic OCI could of
fer a nondestructive alternative to x-rays and microsectioning methods for
studying living tissue. (P. Yu et al., Applied Physi
cs Letters, 21 July 2003.)

***********
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