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arXiv:1106.0959 · Journal of Physics B Atomic Molecular and Optical Physics

Doppler-free two-photon resonances for atom detection and sum frequency stabilization

A. M. Akulshin, B. V. Hall, V Ivannikov, A A Orel, Andrei I. Sidorov

Published Oct 18, 2011• 24 citations
arXivHTMLPDF

Abstract

We investigate the excitation of the 5D_{5/2} level in Rb atoms using counter-propagating laser beams, which are nearly resonant to the one-photon 5S_{1/2} - 5P_{3/2} and 5P_{3/2} - 5D_{5/2} transitions, ensuring that a sum of the optical frequencies corresponds to the 5S_{1/2} - 5D_{5/2} transition. The excitation produced by two-photon and step-wise processes is detected via spontaneously emitted fluorescence at 420 nm arising from the 6P_{3/2} - 5S_{1/2} transition. The dependences of blue fluorescence intensity on atomic density and laser detuning from the intermediate 5P_{3/2} level have been investigated. The sensitivity of the frequency detuned bi-chromatic scheme for atom detection has been estimated. A novel method for sum frequency stabilization of two free-running lasers has been suggested and implemented using two-photon Doppler-free fluorescence and polarization resonances.

DOI
10.1088/0953-4075/44/21/215401

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We investigate the excitation of the 5D_{5/2} level in Rb atoms using counter-propagating laser beams, which are nearly resonant to the one-photon 5S_{1/2} - 5P_{3/2} and 5P_{3/2} - 5D_{5/2} transitions, ensuring that a sum of the optical frequencies corresponds to the 5S_{1/2} - 5D_{5/2} transition. The excitation produced by two-photon and step-wise processes is detected via spontaneously emitted fluorescence at 420 nm arising from the 6P_{3/2} - 5S_{1/2} transition. The dependences of blue fluorescence intensity on atomic density and laser detuning from the intermediate 5P_{3/2} level have been investigated. The sensitivity of the frequency detuned bi-chromatic scheme for atom detection has been estimated. A novel method for sum frequency stabilization of two free-running lasers has been suggested and implemented using two-photon Doppler-free fluorescence and polarization resonances.
A high-resolution two-photon spectrum of 5S 1/2 → 5P 3/2 → 5D 5/2 transitions in a thermal 85 Rb vapor cell is presented by using an optical frequency comb and a cw laser. The fluorescence of 6P 3/2 → 5S 1/2 spontaneous emission is detected when the cw laser frequency is scanned from the 5S 1/2 ground state to 5P 3/2 hyperfine levels and the optical frequency comb repetition rate is fixed. The hyperfine splittings (F f = 2–5) of the 5D 5/2 excited state are well resolved. The dependences of fluorescence intensities on the cw laser intensity and temperature of 85 Rb vapor cell are studied, respectively. The experimental results are in good agreement with the theoretical analyses.
The rubidium $5S\text{\ensuremath{-}}5D$ Doppler-free two-photon transition has been observed in a thermal vapor cell by detecting the infrared radiation emitted during the decay from the 5D5D5D excited level to the 5P5P5P intermediate level. Different to the usual detection scheme based on blue light emission this approach does not suffer from reabsorption and still works at high densities. This not only allows for a dramatically enhanced detectable emission rate but also for investigating collisional effects such as collisional broadening and collisional energy transfer. Furthermore, we observe a yet unknown temperature dependence of the resonance linewidth for transitions into the states of the 5D5/25{D}_{\text{5/2}}5D5/2​ level. Consequences for the design of an optical Rb-frequency standard are discussed.
Abstract We report on the characterization of Doppler-free resonances detected by probing the 5S 1/2 → 6P 3/2 transition of the 87 Rb atom at 420 nm. Compared to the commonly studied 5S → 5P transition, the 5S → 6P transition offers advantages of higher transition frequency and narrower natural linewidth. We investigate how the pump laser intensity and cell temperature affect the linewidth and amplitude of the saturated absorption signal for the 420-nm second excited state transition 5S 1/2 ( F = 2) → 6P 3/2 in 87 Rb atoms. Furthermore, we introduce a 795-nm repump laser to improve the quality of the saturated absorption signal. At a relatively low vapor cell temperature of 43 °C, the amplitude-to-linewidth ratio increased by 110%. These results are highly relevant for the development of stable ultraviolet laser sources and their associated applications.
Abstract The coherent blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a continuous wave diode laser is investigated. Each step of the two-photon transition 5S–5P 3 / 2 –5D is excited by one of the lasers, and the induced coherence between the 5S and 6P 3 / 2 states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure and allow us to identify the resonant modes responsible for inducing the nonlinear process. Further, each resonant mode excites a different group of atoms, making the process selective in atomic velocity. The signal dependency on the atomic density is characterized by a sharp growth and a rapid saturation. We also show that for high intensity of the diode laser, the Stark shift at resonance causes the signal suppression observed at low atomic density.
We present an experimental scheme that produces Doppler-free spectra of the $5s \to 6p $ second resonance transition in atomic rubidium. The experiment uses the saturation of the cascade fluorescence that occurs when thermal rubidium atoms interact with two counterpropagating $420 $ nm laser beams of comparable intensity. Part of the excited atomic population goes through the 5p3/25p_{3/2}5p3/2​ level which then decays by emission of 780780780 nm photons. Narrow dips appear in this otherwise broad 780780780 nm fluorescence, which allows resolution of the 6p3/26p_{3/2}6p3/2​ hyperfine structure. A rate equation model is used to interpret the spectra. It is also shown that these narrow peaks can be used to lock the frequency of the $420 $ nm laser. Using a second beam modulated in frequency produces three sets of spectra with known frequency spacings that can be used to perform an all-optical measurement of the hyperfine splittings of the $6p_{3/2} $ manifold in rubidium.