Infra-red rubidium atomic resonant filters for low wavenumber scattering

This dissertation presents new approaches for low wavenumber scattering (LWS) based on infra-red rubidium filters, including blocking filters, dispersion filters, and passband filters. LWS is scattering of light with a small frequency change, such as rotational Raman scattering and Thomson scattering, which enables the measurement of species specific properties. The rubidium filters are used in conjunction with a tunable, pulsed Ti:sapphire laser to suppress background scattering and to achieve high resolution at low wavenumbers. The blocking filter is demonstrated to capture backward Thomson scattering as a single-ended diagnostic tool. The density gradient dispersion filter represents a new approach which is very promising for high dispersion, high collection efficiency Raman measurements. The passband filter extends recent work for high resolution Raman spectroscopy in the ultra-violet into the infra-red.; The Ti:sapphire laser employs a grazing incidence cavity and a quadruple amplifier. It produces 10 nsec, 40 mJ/ pulse, 5∼10 GHz linewidth, tunable output around 780 nm with less than 10−5 of amplified spontaneous emission. Wavelength calibration is achieved by newly obtained one and two photon optogalvanic spectroscopy of argon and neon.; The first diagnostic approach uses a rubidium notch blocking filter to suppress elastic scattering from the laser. The filter application is demonstrated by capturing LWS from oxygen gas and solid sulfur, and backward Thomson scattering from an argon plasma in the infra-red region. Electron temperature and electron density in the plasma have been obtained by fitting to a theoretical model.; The second approach introduces a new density gradient dispersion filter, designed for simultaneously rejecting the stray light and capturing multiple rotational Raman lines. This filter, which is based on the variation of refractive index near resonance, has a higher dispersion power than gratings at low wavenumbers. Pure rotational Raman scattering from CO₂ gas has been obtained by this filter.; The third approach uses an infrared passband filter based on resonant fluorescence of Rb. It features an ultra-narrow passband (less than 0.5 cm−1) and high suppression for the out-of-band emission. This filter is applied in measurement of high resolution Raman spectroscopy of sulfur.