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The
accomplishments in research and education during the academic year
2000–01 were communicated in 54 refereed papers and 54 presentations
at NASA installations, national laboratories, universities,
and national and international conferences.
Nanophase
Materials Group
The
Nanophase Materials Group has developed laser ablation and optical
trapping techniques for fabricating and spatially manipulating
nanoparticles. The nanoparticles of particular interest are quantum dots
(semiconductor reduced to a size such that the particle size is smaller
than that of the exciton) and metal nanocrystals. Confinement of the
exciton leads to a shift in the band gap, which in turn scales with 1/r2
where r is the particle radius. This property has been used for
fabricating materials with a size-graded band gap, which have potential
applications in solar energy cells. Theoretical calculations indicate
that solar energy cells based on size-graded quantum dot structures
could have a threefold increase in conversion efficiency. Driven by the
expected increase in conversion efficiency, the Nanophase Materials
Group has fabricated size-graded quantum dot structures using pulsed
laser ablation and is currently developing them into devices. On another
front, the Nanophase Materials Group has recognized the need for
developing optical trapping (also known as laser tweezers) as a
technology, which is amenable to the manipulation of these small
particles. This approach has led to spinoff discoveries, demonstrating
that optical trapping can be used to separate nanoparticles of different
sizes based on the threshold power for affecting trapping, spatially
enhance photoluminescence whereby a particle is held by the optical
tweezers and subsequently irradiated with another laser to excite
photoluminescence, show multiple particle trapping in an optical field,
and extend the matrix isolation technique to optical trapping for
isolating a nanoparticle in a matrix of inert polystyrene beads.
Overall, the marriage between the technique of optical trapping and
nanoparticle science is expected to open new doorways of research that
will both advance the fundamental understanding of nanoparticles and
serve as a tool for nanomaterials fabrication.
Semiconductor
Crystals and Films Group
The
Semiconductor Crystals and Films Group aims to increase the knowledge of
the properties/structure/processing relationship in wide bandgap
semiconductors and the way they affect the performance of x-ray and
gamma-ray detectors. The aim is to evaluate Earth- and microgravity-grown
crystals and determine their relative contribution to crystalline
defects and to study room temperature semiconductor detector physics by
focusing on their optimization for space applications. NASA has
identified the use of wide bandgap semiconductor detectors technology as
a promising technology for x-ray and gamma-ray astronomy.
The
group developed the crystal growth of new materials, such as
chalcopyrites, for infrared photonic applications such as optical
parametric oscillators. The growth of laser materials also has
continued, and the scope of work has been enlarged to include ternary
II-VI compounds doped with transition metal ions. Besides basic research
applications, such tunable lasers are of particular importance for
environmental monitoring, military countermeasures, medical
applications, and remote sensing.
Research
on high-resistivity semiconductors HgI2 and CdZnTe (CZT) has continued.
HgI2 is being investigated in cooperation with MSFC as a benchmark
material for a model describing physical vapor transport under
microgravity conditions. A new working relationship with Los Alamos
National Lab (LANL) was established in the area of CZT. These efforts
include collaborations of both Fisk and LANL with GSFC to explore future
uses of CZT detectors for space applications. Additional funds have been
received in the above areas from the U.S. Air Force, DOE, NSF, and BMDO/U.S.
Army. Also, through the efforts of two new relationships, with
Vanderbilt University and University of California at Davis,
student-training avenues have been established.
Optical
Materials Group
The
Optical Materials Group is working with glasses and other optical
materials that can be used to make new fiber laser sources. Fiber laser
systems offer significant advantages for aerospace systems. They are
simpler and more compact than many solid-state and gas laser systems.
They are spectrally cleaner than diode lasers and can be effectively
pumped by semiconductor diodes. Applications of fiber laser systems
include simple ranging and altimetry, windshear-detection, and avoidance
systems for aircraft, and satellite-based, global wind-monitoring
systems. This group has produced and is continuing to develop a new
glass (rare-earth-doped lead-tellurium-germanate) that shows great
promise as a fiber laser material. Recent efforts have concentrated on
thulium doping of this glass and development of a laser to operate near
1.9 µm. Such lasers would be extremely useful as pump sources for the
chromium doped II-VI laser materials currently being developed by the
Semiconductor Crystals and Films Group. The Optical Materials Group is
also working in collaboration with the Semiconductor Crystals and Films
Group on developing new nonlinear optical materials for infrared
wavelengths.
Surface
Physics Group
The
Surface Science Group is working on the relationships between the
surface and interface structures and physical properties on novel
materials. Silicon carbide (SiC) is a promising semiconducting material
with superior characteristics in electrical, mechanical, and thermal
properties. Importantly, SiC-based devices have the desired properties
to yield high-frequency and microwave electronic devices far superior to
present-day devices and have a wide range of applications in civil and
military uses. One project under study is the mechanism of ohmic contact
formation on SiC. A new technique has been developed for high-quality
ohmic contact formation on SiC at significant lower annealing
temperatures and on moderately doped SiC substrates. The annealing
temperature for ohmic contact formation is about 300 °C lower than the
conventional technique. Excellent ohmic contact can be formed on the SiC
with two orders’ lower doping concentration than it is in the
conventional technique. The technique will improve the performance of
high-power and high-frequency devices because the contact resistance is
greatly reduced on SiC, and it will provide more flexibility in device
fabrications. A mechanism of ohmic contact formation on SiC has been
proposed, and a patent application is being prepared. Another project in
SiC research is to enhance the sensitivity and selectivity of SiC-based
high-temperature chemical sensors. Adding a nanosize interfacial layer
on SiC can increase the electron transfer properties. Investigation of
the nanocomposed materials for optical materials continues. The goal of
this project is to design and fabricate nanostructural composites by
sol-gel processes, characterize the structures at the nanometer scale,
and examine the optical limiting properties. Through collaborations with
Vanderbilt and DuPont, we have applied atomic force microscopy (AFM) to
investigate the crystalline structures and crystallization formation
processes of commercial polymeric materials. Theoretical simulations of
dust plasmas also continue, with emphasis on industrial processes in
semiconductors and other photonic devices. Additional funds were
obtained from the U.S. Air Force and the Ballistic Missile Defense
Organization. The research collaborations include the Air Force Research
Lab, NASA GRC, NASA MSFC, and Vanderbilt University.
Professor
H. John Caulfield, who is in a new developing group on optics
informatics, has done work in many areas that has resulted in numerous
papers, several book chapters, and two books this year. The books are Holography
for the New Millennium (Springer Verlag, with Jacques Ludman) and Fundamental
Papers in Applied Holography (SPIE Press, with Hans Bjelkhagen). His
work has concentrated on what methods can be borrowed from human
perception and consciousness to aid in the operation of complex systems.
He has also made a very fundamental breakthrough in pattern recognition.
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