LABORATORY PROJECTS - BERKELEY

Investigation of New Raman-shifting Techniques

       One of the primary technical challenges of infrared cavity ringdown spectroscopy is obtaining sufficient infrared intensity from the Raman shifting process. Traditionally the Saykally group has used a multipass high-pressure gaseous hydrogen cell as the shifting medium. However, this approach requires mirrors that are highly reflective over a wide wavelength range. These mirrors, despite having a fairly high damage threshold, tend to become damaged with time, and are difficult to replace. As a result, I have been investigating alternative strategies for Raman shifting.

       One approach, which we have been pursuing in collaboration with Takamasa Momose (Kyoto University) and Takeshi Oka (University of Chicago), is to use solid or liquid parahydrogen as the Raman gain medium. The intrinsically higher Raman gain of these media may allow us to obtain the necessary third-Stokes intensity with only a single pass, thus eliminating the need for multi-pass mirrors. So far, we have not been able to obtain sufficient third-Stokes intensity, but we are optimistic that upcoming experiments with more highly transparent crystals of parahydrogen may be more successful.

       A second approach is to use gaseous hydrogen enclosed in a hollow glass waveguide. Such a waveguide extends the length over which the pump beam is tightly focused (to as much as one meter) and also prevents the escape of the first- and second-order Stokes fields which are necessary to produce the third-Stokes we need. We have obtained a sample of a hollow glass waveguide from Jim Harrington (Rutgers University), and will be testing it out shortly.

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