Infrared Luminescence and Optical Characterization

My research interests focus on exploring the optical properties of materials with an eye toward understanding novel luminescent materials in terms of the radiative and non-radiative interactions that affect luminescent efficiencies. My graduate work focused on understanding the nature of the luminescent center in Chromium doped crystals that exhibited strong broad band emission in the near-infrared. These materails have sinced been commercialized as solid state tunable lasers.

Common near infrared emission centers in solid state hosts include Rare Earth atoms (Erbium, Ytterbium, Thulium, Holmium, and Neodymium) and transitions metal ions (Cr⁴⁺ and Ti³⁺). My research work has explored materials that are relatively easy to make that are doped with one or more of these atoms to explore the efficiency of emission from the optically active atoms.

Endohedral Fullerenes

Endohedral fullerenes are a class of molecules consisting of one or more metal atoms/ions surrounded by a fullerene molecule. The metals are incorporated inside the fullerene molecules during the fabrication process by doping the graphite rods with a rare earth oxide (RE₂O₃).

The fabrication of fullerenes is accomplished using a water cooled reactor in which we can control the He pressure during the high current discharge between graphite rods to produce the fullerenes. A welder provides a current of about 150 Amps between two closely spaced graphite rods. The arc causes the atoms in the rods to be vaporized into an explanding cloud that is moderated by the residual He gas in the chamber. During the expansion of these atoms, some of the carbon atoms from small sheets that eventually wrap into fullerenes primarily. However, when the RE atoms are also present, of the fullerenes form by wrapping around the  RE atom or atoms to create the endohedral fullerenes. My groups first results on these studies were the observation of emission from Er₂@C₈₂.

 More recently we have been exploring the possibility of observing emission from Er atoms wrapped in C₆₀. The challenge here is to mesure the weak emission from the small endohedralfullerenes that are not produced in large quantities in our reactor. In addition, it is not clear how stable these endohedral fullerenes are after production and what happens to them if they oxidize.



Sol-Gel glasses

With the ubiquitous use of glass fibers for high-speed optical communication, the desire for cheaper glass materials with optimized optical properties drives research in alternate ways of making these fibers. Sol-gel glass is produced in a room temperature chemical reaction that may circumvent some of the doping problems associated with solubility of impurity atoms in molten glass. In addition, because high temperatures are not required, temperature sensitive molecular systems can be doped into sol-gel glasses opening up new opportunities to adjust the emission and absorptions properties of the glass.

Much of this work was part of a collaboration with colleagues Dan Boye (Davidson College) and Ann Silversmith (Hamilton College). We first explored the emission efficiency question using Terbium as the primary probe atom. Our work resulted in several publicaitons elucidating the problem of RE ion clustering during the sol-gel reaction and describing the effects of energy transfer resulting from this clustering behavior.

More recent work has focused on ways to insulate the RE ions from water vapor that diffuses into sol-gel glasses through the pores that remain after the formation of the silicate chains forming the glass. Our work has explored the simple chelating agent Dipicolinic Acid (DPA) . Our most recent work has shown that we can obtain emission from Er doped DPA and have measured the lifetime which if sub microsecond.


Combustion Synthesis 

I have explored this method of crystal formation with a couple of students over the years to explore new ways of fabricating materials in which we can modify the optical properties by doping impurities. So far we have made YAG micro crystals using this method. I may push forward with exploring other crystal hosts with some promise of being a novel optical material. The technique involves mixing together the base elements of teh crystal, dopant atoms, a fuel and an oxidizing agent. The liquid mixture is placed in a crucible that is subsequently heated to the combustion temperature of the fuel and the rapid heating provides the thermal energy for nano crystal formation.  My work in this area remains unpublished.