Ph.D. University of Texas, Dallas, 1984
M.S. Miami University, 1979
B.S. Miami University, 1977
Atomic, Molecular & Optical Physics
My research is focused on the study of coherent control of a particular type of cold collision known as photo-association (PA). In this process, a laser is used to place pairs of neighboring cold atoms having temperatures of ~120 microkelvin, into an electronically excited, bound, molecular state. If allowed to decay, these molecules, having typical internuclear separations of ~50 nanometers, would normally decay back into separate ground state atoms, though they can occasionally decay to bound electronic ground states of the molecule. In our case, some fraction of the excited molecules are further excited into autoionizing molecular states. The resulting molecular ions are readily extracted and used as a measure of how efficiently PA + excitation (PAE) takes place.
We first use cw lasers to cool and trap Rb atoms in a magneto-optical trap (MOT). Then using the JRML's broad bandwidth, ultra-short pulsed laser we photo-associate and then coherently excite the cold molecules to the doubly excited Rb(5p,5p) manifold of molecular states. These states are then probed with a narrow linewidth cw laser which selectively excites to autoionizing molecular states. We measure the number of molecular ions produced as a function of the "shaping" we apply to the ultrafast laser pulse. The ultimate goal is to efficiently create molecules in a specific desired state. The underlying idea is that the better we understand the limitations of controlling this very simple chemical process, the better chance we will have in controlling more complicated systems.
In order to explore coherent excitation by the ultra-short pulse laser, we modify the spectral phase and amplitude of the pulses. That is, we control f(w) and E(w) using acousto-optical devices. Thus far, the emphasis has been on controlling PAE through shaping the pulse in the spectral phase. We have seen dramatic effects with contrast ratios (between the "best" and "worst" phases) of nearly 200 to 1. To date, we have generated "landscapes" in which we scanned phase patterns over a few parameters. In the coming year we will apply learning algorithms which should create optimal phases for exciting to specified molecular states. The challenge will then be to understand why certain phases, as determined by the learning algorithm, work better than others.
This research lies in the cross-over region between the physics of ultrafast, ultra narrow line width, and ultra cold.
- Department of Energy
Recent Selected Publications
D. I. Herman, F. R. Giorgetta, K. C. Cossel, N. A. Malarich, L. C. Hutcherson, C. Weerasekara, J. M. Cutshall, N. R. Newbury, S. M. Welch, B. D. DePaola, I. Coddington, E. A. Santos, and B. R. Washburn, "Broadband dual-comb spectroscopy for open-path field measurement of H216O and H218O," in OSA Optical Sensors and Sensing Congress 2021 (AIS, FTS, HISE, SENSORS, ES), S. Buckley, F. Vanier, S. Shi, K. Walker, I. Coddington, S. Paine, K. Lok Chan, W. Moses, S. Qian, P. Pellegrino, F. Vollmer, G. , J. Jágerská, R. Menzies, L. Emmenegger, and J. Westberg, eds., OSA Technical Digest (Optical Society of America, 2021), paper JTu6E.5. [link]
D. I. Herman, L. C. Hutcherson, C. Weerasekara, F. R. Giorgetta, K. C. Cossel, G. M. Colacion, N. R. Newbury, S. M. Welch, B. D. DePaola, I. Coddington, E. A. Santos, and B. R. Washburn, "Feedlot-produced ammonia emissions quantified using dual-comb spectroscopy," in Conference on Lasers and Electro-Optics, J. Kang, S. Tomasulo, I. Ilev, D. Müller, N. Litchinitser, S. Polyakov, V. Podolskiy, J. Nunn, C. Dorrer, T. Fortier, Q. Gan, and C. Saraceno, eds., OSA Technical Digest (Optical Society of America, 2021), paper STh2E.5. [link]
Daniel I Herman, Chinthaka Weerasekara, Lindsay C Hutcherson, Fabrizio R Giorgetta, Kevin C Cossel, Eleanor M Waxman, Gabriel M Colacion, Nathan R Newbury, Stephen M Welch, Brett D DePaola, Ian Coddington, Eduardo A Santos, Brian R Washburn, "Precise multispecies agricultural gas flux determined using broadband open-path dual-comb spectroscopy," in Science Advances 7, 14 eabe9765. [link]
L. C. Hutcherson, D. I. Herman, C. Weerasekara, F. R. Giorgetta, K. C. Cossel, E. M. Waxman, G. M. Colacion, N. R. Newbury, S. M. Welch, B. D. DePaola, I. Coddington, E. A. Santos, and B. R. Washburn, "Beef cattle feedlot emissions measured using open-path dual-comb spectroscopy," in Frontiers in Optics / Laser Science, B. Lee, C. Mazzali, K. Corwin, and R. Jason Jones, eds., OSA Technical Digest (Optical Society of America, 2020), paper LTu2F.2. [link]
Xinya Wang, Xiaoming Yu, Matthew Berg, Brett DePaola, Hongyu Shi, Pingping Chen, Lianjie Xue, Xuefeng Chang, Shuting Lei, "Nanosecond laser writing of straight and curved waveguides in silicon with shaped beams" in Journal of Laser Applications 32, 022002 (2020). [link]