Daniel Thrasher and Dr. Scott Bergeson, Department of Physics and Astronomy
From May through August I had the privilege of traveling to Tsing Hua National University in Taiwan to work in the lab of Professor Andy Kung and associates. The Office of Creative Research awarded me a grant to help cover travel and living expenses. The purpose of our research is to create a laser capable of recording ultra fast molecular phenomena. In order to resolve these ultra fast processes it is necessary to create a tunable light source with which we can illuminate the object on a time scale which is faster than its motion. Many atomic relaxations probed using spectroscopic techniques oscillate on the atto-second time scale. Producing a laser with pulse durations on this time scale composed of wavelengths centered in the visible regime is a long sought goal of laser physicists.
In Professor Kung’s lab I was responsible for the experimentation of a new method for accomplishing this goal. Theory suggests that High Order Stimulated Raman Scattering (HSRS) may be a viable tool for realizing such a laser. Raman scattering is a four wave mixing technique wherein an ultra fast laser pulse or pulses excite the Raman modes of a raman active gain medium. This excitation scatters the fundamental frequency components, creating a duplicate of the incident pulse at another location in frequency space. That difference in frequency space between the incident pulse and its scattered projection is the Raman spacing. If one selects a laser which has broad enough frequency components then the duplicate scattered pulse will overlap with the original pulse frequencies, thereby producing a quazi supercontinuum.
We choose to work with diatomic Hydrogen as our Raman gain because it has the largest Raman vibrational frequency at 8 femtoseconds (fs) or 4155 wavenumbers. Our incident pulse is a state of the art 5 fs pulse with a bandwidth of 700-900 nm.
As the lead researcher of the project it was my responsibility to research what work had been performed previously on the subject. I then designed modifications which we could do to increase our knowledge of HSRS. I was also responsible for data acquisition and analysis. After several weeks of experimental uncertainty we decided to try to duplicate the work of another research team. We were successful in recognizing similar results in our own system. We are the first research team to report the effects of HSRS for hydrogen in the impulsive regime.
Perhaps the greatest obstacle to producing the laser using HSRS is finding an intensity regime where competing processes can be sufficiently mitigated. Competing processes include ionization and self phase modulation. They usurp precious energy which can not be used to drive the HSRS efficiently. Future work will include addressing these parasitic processes as well as aligning the phase of the spectral components of the output light, demonstrating its coherence, and measuring the pulse duration.
I would like to thank the Office of Creative Research, as well as its gracious donors for making this opportunity possible. Along with the opportunity to participate in cutting edge utlra fast physics, I also was able to make new friends, learn more of Taiwanese culture, and share my testimony of the restored Gospel of Jesus Christ.