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Journal of Raman Spectroscopy

High signal-to-noise detection of rotational Raman scattering through refluorescent and dispersive atomic filters

Richard B. Miles, Zhen Tang, Sohail Zaidi, Azer P. Yalin, Noah D. Finkelstein

Published Jan 1, 2000• 4 citations
Publisher

Abstract

We introduce new methods for capturing rotational Raman scattering that simultaneously suppress background scattering and have exceedingly high spectral dispersion at low wavenumbers. These methods are based on resonant absorption and refractive index dispersion of atomic vapor, and take advantage of the availability of tunable lasers. Three different configurations are presented. The first is an atomic notch transmission filter which is based on refluorescence from an optically thick atomic vapor cell. This configuration is demonstrated in air using mercury vapor at 254 nm. The second configuration is an atomic resonance prism cell which is demonstrated in CO2 using mercury vapor, again at 254 nm. The third configuration is a density gradient atomic vapor cell which uses rubidium vapor at 780 nm. In the last two configurations, the atomic cell simultaneously blocks the elastic scattering while spatially dispersing the rotational Raman lines. In each case, a tunable Ti : sapphire laser was employed as an illuminating source. Copyright © 2000 John Wiley & Sons, Ltd.

DOI
10.1002/1097-4555(200008/09)31:8/9<843::aid-jrs618>3.0.co;2-t

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We introduce new methods for capturing rotational Raman scattering that simultaneously suppress background scattering and have exceedingly high spectral dispersion at low wavenumbers. These methods are based on resonant absorption and refractive index dispersion of atomic vapor, and take advantage of the availability of tunable lasers. Three different configurations are presented. The first is an atomic notch transmission filter which is based on refluorescence from an optically thick atomic vapor cell. This configuration is demonstrated in air using mercury vapor at 254 nm. The second configuration is an atomic resonance prism cell which is demonstrated in CO2 using mercury vapor, again at 254 nm. The third configuration is a density gradient atomic vapor cell which uses rubidium vapor at 780 nm. In the last two configurations, the atomic cell simultaneously blocks the elastic scattering while spatially dispersing the rotational Raman lines. In each case, a tunable Ti : sapphire laser was employed as an illuminating source. Copyright © 2000 John Wiley & Sons, Ltd.
We report a new spectroscopy method achieved by combined refraction and propagation delay of light pulses passing through a prism cell filled with an atomic vapor. The atomic vapor medium has strong refractive index gradients near the atomic resonant absorption features, and these features enable the separation in time and space of different spectral features of a light pulse propagating through the vapor cell. The gradient of the refractive index leads to a slowing of the light pulse, while simultaneously, the magnitude of the refractive index and the prismatic windows or internal prism elements of the atomic vapor cell spatially separate the frequencies due to dispersion. The combination of these two effects leads to high-resolution spectral selectivity and strong suppression of out-of-band light. Applications are achieved for Rayleigh, Raman, and Thomson spectroscopy for which the wavelength of the scattering spectra tracks the wavelength of the excitation laser source. The laser is tuned such that the spectral line of interest falls near the resonance of the atomic vapor. Applications for rotational Raman spectroscopy are of particular interest since specific Raman lines can be selected while other Raman lines as well as background Rayleigh and other scattering are strongly rejected.
Narrow-bandwidth optical sources are readily available. However, tunable narrow-bandwidth, wide field-of-view optical filters are not available. The laser-pumped atomic vapor optical filters presented here are in principle tunable narrow-bandwidth, wide field-of-view optical filters. These filters can simultaneously provide high resolution (0.001 nm), wide field-of-view (±90°), high quantum efficiency, and electrically tunable optical filtering. These devices are ideally suited for extracting weak narrow-bandwidth signals buried in strong nonresonant optical background radiation. Potential applications of these fitters include laser communications, laser radar, detection of Raman radiation, and atomic spectroscopy.
Abstract The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.
The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.
Raman spectroscopy is a widely used, highly versatile, and non-destructive method for characterizing the molecular makeup of unknown samples. In nearly all applications the vibrational modes of the constituent molecules provide the mechanism by which the laser light is inelastically scattered and consequently identified. However, the rotational modes of these molecules provide much greater relative rates of inelastic scattering than the vibrational modes. Unfortunately, due to the nature of current filters used in Raman spectroscopy when the laser light is filtered out so is the light scattered by the pure rotational modes. In this research, we propose the use of a Rubidium vapor cell filled with a buffer gas of Argon as a viable ultra-narrow notch filter. This filter would provide a robust laser rejection without removing the light scattered by the pure rotational modes of molecules. Measurement of the pure rotational spectrum of gases can have numerous applications such as precision measurement of exhaled gases for medical diagnostics, highly accurate remote measurement of atmospheric temperatures, and extremely sensitive detection of dangerous air contamination in dusty environments.
A Faraday anomalous dispersion optical filter (FADOF) and an atomic resonant absorption filter are used in tandem to demonstrate a low-frequency shift Raman measurement down to few cm−1. The FADOF, with an ultralow bandwidth of 0.08 cm−1 at 780 nm, serves as a bandpass filter, while the rubidium atomic cell acts as a notch filter which has a bandwidth of 0.3 cm−1. A proof-of-concept study to measure a Raman signal generated from a silica optical fiber is performed, demonstrating a low-frequency measurement of both the Stokes and the anti-Stokes shift down to 3 cm−1 at an equivalent signal level. These results indicate the prospect for gigahertz–terahertz low-energy Raman spectroscopy based on atomic filters.