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 of 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 thermal heating,
suppresses light at longer wavelengths and re-emits light at a shorter
wavelength band, one that may be more useful for such technological
applications as photovoltaic power generation, or building a better
lightbulb.
Shawn Lin and his Sandia colleagues, in the course of their studies
of photonic crystals, have seemed to challenge the venerable formulation,
made by Max Planck 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.,
Applied Physics Letters, 14 July 2003; Lin et al., Optics
Letters, Sep.15, 2003.)
Picosecond X-ray Crystallography of a Protein
Picosecond x-ray crystallography of a protein has been demonstrated
for the first time, by a multinational collaboration (Philip Anfinrud,
NIH, PhilipA@intra.niddk.nih.gov), enabling atom-scale movies of an
important biomolecule as it performs a speedy function. This accomplishment
will be presented at the upcoming
American Crystallographic Association meeting from July 26-31 in
Cincinnati (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 movies of a mutant
myoglobin molecule getting rid of a toxic carbon monoxide (CO) molecule.
Myoglobin is the protein that stores oxygen in muscle tissue. The researchers
chose to study a mutant version of the protein because the highly strained
atomic structure in part of the protein causes it to get rid of a CO
molecule much more quickly than does ordinary myoglobin.
To capture this process, they first sent a 1-ps pulse of laser light
to the protein to eject the CO. Immediately afterward, they illuminated
the protein with intense, 150-ps x-ray pulses from the ESRF synchrotron.
Crucial to this process was the ability to isolate single 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 proteins, the picosecond time-scale of these
movies is commensurate with the timescale of many molecular dynamics
simulations, allowing 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-time holographic system to acquire a fly-through
movie of living tissue using infrared light and a special, semiconductor
holographic 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 using optical coherence
imaging (OCI). OCI is related to the more widely known optical coherence
tomography (OCT). However, OCT involves scanning a laser beam through
a sample and gathering information point by point, which then must be
assembled into a complete image. OCI, on the other hand, captures complete
images of thin tissue sections that can be recorded directly with a
video camera.
The key to the holographic OCI technique is a dynamic holographic film
that filters out the scattered, incoherent background light but passes
the coherent, full-frame images to a camera. Tissue readily reflects
image-bearing infrared light, but it also strongly scatters the light,
and without coherence filtering the scattered light would overwhelm
the coherent pictures.
By adjusting the relative delay between the image beam and the reference
beam in the OCI system's imaging interferometer, the researchers (Ping
Yu, 765-494-3004, pingyu@physics.purdue.edu, David Nolte, 765-494-3013,
nolte@physics.purdue.edu) could control the depth of the images and
assemble a slice-by-slice tour through a tumor while leaving the tissue
intact.
Application of the OCI technique 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).
Ultimately, the researchers explain, holographic OCI could offer a
nondestructive alternative to x-rays and microsectioning methods for
studying living tissue. (P.
Yu et al., Applied Physics Letters, 21 July 2003.)
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