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AIAA AVIATION Forum

Slow Light Prism Enhanced Spectroscopy (SLOPES)

Amirhossein Abbasszadehrad, Jason M. Meyers, Arthur Dogariu, Richard B. Miles

Published Jul 27, 2024• 0 citations
Publisher

Abstract

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.

DOI
10.2514/6.2024-3723

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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.
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In this study, we investigate the effect of the optical thickness of an atomic vapor cell and the delay of the gated camera on the instrument function in Slow Light Imaging Spectroscopy. In an atomic vapor medium, the slow light effect happens at certain frequency windows where there is a large slope of the index of refraction versus frequency. The extent of these windows depends on the important parameters of the vapor cell (i.e., the optical thickness). Furthermore, the gate delay determines the minimum amount of delay that a signal needs to experience before it is detected. These two parameters determine the shape of the instrument function of the spectrometer in slow light imaging spectroscopy. Simulation and experimental results show that when absorption lines of cesium vapor at 852.1 nm are used as a delay medium, the instrument function at low temperature consists of four peaks corresponding to the steep refractive index gradients at either side of the two hyperfine absorption lines. By increasing the temperature, these peaks move away from the absorption lines. When the two peaks between the two absorption lines move away from their corresponding absorption lines, they move closer to each other until they blend, forming a strong peak. By increasing the gate delay, the four peaks of the instrument function reappear.
This thesis presents the applications of some of the coherent processes in a three-level atomic system, to control spatial and temporal properties of a signal pulse. We use two Raman absorption resonances in rubidium vapor separated by a few MHz to achieve a rapidly tunable slow-light system. We control the slow-light characteristics all-optically by tuning the frequency and power of a coupling beam. A dual absorption slow-light system is known to cause less pulse broadening than a single transmission resonance system, and thus, a tunable double absorption system is advantageous. We use a four-wave mixing process to demonstrate pulse storage in rubidium vapor for times much greater than the pulse width. We demonstrate storage of both the temporal and spatial profile of the pulse. We overcome the diffusion of spatial information during the storage in warm atomic vapor by storing the Fourier transform of the image instead of an image with a flat phase. The Raman absorption resonance is also used to control the transverse refractive index profile of the signal beam. The refractive index of the signal interacting with a coupling beam in a Raman process is dependent on the coupling beam intensity. We use a first order Laguerre-Gaussian (LG01) coupling beam to create a waveguide like transverse refractive index profile. We demonstrate propagation of a focused signal beam for lengths much greater than the Rayleigh length. Finally, we demonstrate a dual absorption atomic prism, which is capable
View Video Presentation: https://doi.org/10.2514/6.2022-3365.vid In this work, we demonstrate a new high throughput, high resolution spectroscopic technique based on dispersion in cesium atomic vapor and use it to image the Rayleigh-Brillouin spectrum from air. This is accomplished by utilizing a prism shaped cesium vapor cell and taking advantage of the steep refractive index in the vicinity of atomic resonances of the cesium D2 hyperfine lines at 852.113 nm. A laser beam from a tunable, continuous, and narrow linewidth Ti:sapphire laser was focused in ambient air and the spectrally dispersed scattering was imaged onto a camera after passing through a 80 mm long cesium prism vapor cell. The strong dispersion enables the measurement of the characteristic thermal and acoustic broadening of the Rayleigh-Brillouin scattering. Mie scattering from particles in the air is also captured and can be separated from molecular Rayleigh-Brillouin scattering by tuning close to the cesium resonance. The trade-off between spectral resolution by dispersion and loss of signal by cesium absorption is analyzed by tuning the laser. This technique shows the feasibility to measure the thermodynamic properties of a gas by reconstruction of the Rayleigh-Brillouin spectrum and could be applicable for advanced LIDAR measurements of atmospheric properties.
There is an interest in consideration of the propagation of the Gaussian monochromatic light beams through a cell filled with the medium of two-level atoms (in our case, the atomic potassium vapor), which has (at small detuning from resonant transition to low or high frequencies) the refractive index other than unity (n > 1 for v < v01 and n < 1 for v > v01, where v is the light frequency and v01 is the resonant-transition frequency). In this case, the properties of the radiation passed through the cell are changed. In the very close proximity of the resonance, the nonlinear processes (the three-photon electronic stimulated Raman scattering (SRS) and the six-photon parametric scattering (SPS)) play a crucial role. It is precisely these processes in the two-level medium that determine the irreversibly transformed spectral and angular structure of the light beams at the output of the cell.