Dr. George Welch
Texas A&M University
The vibrational structure of a molecule can be a useful signature or "fingerprint" of a substance. Inelastic scattering of light by molecules is an important tool for probing molecular vibrational structure. Thus, inelastic scattering of light from molecules can be a useful method for detecting particular molecular species. This effect is called Raman scattering after Chandrasekhara Venkata Raman, who received the 1930 Nobel Prize in Physics for its discovery.
The Raman effect is typically very weak, and coherence is a very powerful way to produce larger signals. Multiple photons can be used to excite and scatter from molecular vibrations, resulting in a signal that is sensitive to the same vibrations as in Raman scattering, but where the fields emitted from all molecules can add coherently, producing a very large signal that is quadratic in the number of scattering molecules. This effect is called coherent anti-Stokes Raman scattering, or CARS.
CARS is a third-order nonlinear optical process and may be masked by other nonlinear optical processes, such as four-wave mixing. Also, because of its large quadratic dependence, CARS may be unsuitable for detection of small amounts of a substance in the presence of a large background o other molecules.
There are many approaches for resolving the problems of CARS while preserving its high signal level. One method is to use femtosecond lasers with temporally shaped pulses to exploit the vibrational structure of molecules. We refer to these tricks as "FAST-CARS" for "femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman scattering".
I will introduce all these concepts, and present many results from various experiments we have performed over the last few years. I will also show how optical interference can be used for broad-band heterodyne detection of FAST-CARS signals, giving rise to interesting new effects.
Location: Physics Bldg., Room 401