Kevin C. Wright and Dr. Bret C. Hess, Physics and Astronomy
This research project has been much more time-consuming than I anticipated. Because the required lasers and optical elements are very sensitive and highly susceptible to accidental misalignment and damage, many parts of the multi-stage laser amplification system had to be rebuilt from the ground up several times during the course of this research. This was a great hands-on learning opportunity for me, even though it did impede the progress of my research somewhat. I have still been able to accomplish most of the objectives outlined in my proposal. Specifically, I have completed all but the final stage of the proposed optical parametric amplifier, and have used the portions of the laser system already operational to conduct preliminary studies of photo-induced absorption.
A considerable amount of preliminary work was required to prepare for the experiment. I started by restoring a helium-cooled cryostat to working order so that we could conduct low in a vacuum environment at temperatures down to 15 Kelvin. This cryostat is necessary to determine how thermal effects influence the optical excitation properties of the nanocrystals.
Much of the equipment used required control via computer interface, and I designed and/or modified a series of GPIB and RS232 based software applications for operating a temperature controller, a motor-driven delay stage, a monochrometer, a CCD spectrometer, and the lock-in amplifier used as our primary data acquisition device. Most of this work was done while waiting for replacement parts for the laser.
In designing the parametric amplifier, I did some theoretical work determining how the vector phase matching properties of barium borate make broadband parametric amplification possible. I derived a general solution for vector phase matching in birefringent crystals and developed numerical methods for calculating the crystal acceptance angle and spectral bandwidth. The validity of this solution has been confirmed by a sum-frequency generation experiment.
The parametric amplifier is presently in the final stages of construction and alignment. I was able to successfully frequency double the pump laser in May, converting it from near infrared to a bright blue by focusing the beam into a carefully aligned lithium borate crystal. Last month I finally solved the more difficult problem of generating a highly stable white light supercontinuum by tightly focusing a tiny fraction of the infrared beam into a sapphire crystal where a process called self-phase modulation converts the pulse into an ultrafast smear of colors across the rainbow. The final stage of amplifying the supercontinuum by focusing it into the barium borate crystal with the frequency is currently underway and should be completed in a week or two.
Using the blue frequency-doubled beam and the supercontinuum I have begun investigating photo-induced absorption processes. In this experiment a pulse from the blue beam is focused into the sample material, exciting it and changing its absorption characteristics. The probe pulse from the supercontinuum follows a trillionth of a second later, which is partly absorbed and partly transmitted. The transmitted supercontinuum is routed to a detector, which records the intensity at each wavelength. The probe delay can then be varied to show how the absorption characteristics decay with time. The decaying change in transmission at each wavelength gives detailed information about the carrier dynamics of the material being studied. Nanocrystals promise to exhibit particularly interesting behavior because of their extremely small size. The first successful experiment was conducted August 25 using a thin piece of amorphous silicon as test case. A sample of the data collected appears below in Figure 2. We will begin taking measurements using the nanocrystal films at the beginning of September.