Introduction

The aim of the Fisk University Center for Photonic Materials and Devices is to perform research and develop technologies relevant to NASA’s mission, focusing in the field of photonics.  Research in photonics has made possible the development of new technologies that have produced revolutionary changes in communications, computing, robotics, medicine, environmental control, and many industrial processes.

In particular, the Center has focused its research on one of the most promising branches of photonics - one that produces new materials or improves the production of known materials, which are the initial stages of development of the latest advanced technologies.  Additionally, the potential reputation of the Center will attract an increased number of disadvantaged and underrepresented students, both graduate and undergraduate, and will motivate them to pursue careers relevant to the NASA mission.

Center Research

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

Nanophase materials research focused on  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 Fisk scientists have fabricated size-graded quantum dot structures using pulsed laser ablation and is currently developing them into devices. Optical trapping (also known as laser tweezers) is a technology which is amenable to the manipulation of these small particles. This approach has led to several spinoff discoveries-- demonstrating that optical trapping can be used to separate nanoparticles of different sizes based on the threshold power for affecting trapping, spatially enhancing photoluminescence whereby a particle is held by the optical tweezers and subsequently irradiated with another laser to excite photoluminescence, showing multiple particle trapping in an optical field, and extending 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 

Semiconductor crystals and films research 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. 

Fisk faculty and students working in this area have 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 Marshall Space Flight Center (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 Goddard Space Flight Center (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.

Glass and Optical Materials

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 Materials Science and Applications group at Fisk.

Surface Physics

Surface physics research within the Center deals with 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.