WFU Physics Colloquium

Honors and Pre-Ph.D. Theses presentations

TIME: Thursday Apr. 24, 2008 at 3:45 PM^*

PLACE: Room 101 in Olin Physical Laboratory

^* Note early starting time.

In my experiments, a laser prepares (rather suddenly) a population of conduction electrons and valence holes in zinc oxide. Following this by a few picoseconds, we pass an infrared continuum through the collection of free and trapped carriers to measure optical absorption transitions to higher states. These are the first such observations ever conducted on the ps time scale or at low temperature in ZnO, and the results are unexpected (by us, at least). Meanwhile, another measurement that can be made on these electrons and holes is their recombination to produce luminescence. In collaboration with Oak Ridge National Laboratory (where we will present these and other results next week), we have made such measurements on a variety of ZnO crystals doped, implanted, and treated to try to get the shortest lifetime and highest yield of luminescence. We are working on a common understanding of the absorption and luminescence together.

Optical traps are being applied to an ever-growing list of interesting problems in basics sciences, and engineering. In our lab we are interested not only in optical traps, i.e. optical forces, but also optical torques that will result in a rotating particle on the microscopic and/or nanoscopic scale. With the goal of measuring the polarizability anisotropy of interesting materials, we have optically trapped C₆₀ polymer nanorods with diameters of 800 to 1000 nm and lengths of 1-3 microns in water in a single beam optical trap. While in the trap, the nanorods were optically torqued by rotating the plane of polarization of the trapping light. The torque on the rod can be found by measuring the rod rotation rate as a function of the polarization rotation rate, and then finding a theoretical fit to this curve. This torque is dependent on the polarizability of the particle. However, to back out the polarizability from this dependency, we need find theoretical values of the torque as a function of the polarizability. Toward this end we used several computational methods to calculate forces and torques on trapped C₆₀ rods. We will report on the experimental and theoretical work we have done toward determining the polarizability of C₆₀ rods.