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arXiv:1706.03237 · Physical Review A

Sub-Doppler two-photon absorption induced by the combination of a single-mode laser and a frequency comb

Marco Polo Moreno de Souza, Giovana Trevisan Nogueira, D. Felinto, S. S. Vianna

Published Aug 31, 2017• 3 citations
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Abstract

The two-photon transition $5S\ensuremath{-}5P\ensuremath{-}5D$ in rubidium vapor is investigated by detecting the fluorescence from the $6{P}_{3/2}$ state when the atomic system is excited by the combined action of a cw diode laser and a frequency comb. The cw laser plays a role as a velocity-selective filter and allows for sub-Doppler spectroscopy over a large spectral range including the $5{D}_{3/2}$ and $5{D}_{5/2}$ states. For a counterpropagating beam configuration, the response of each atomic velocity group is well characterized within the Doppler profile and the excited hyperfine levels are clearly resolved. The contribution of the optical pumping to the direct two-photon process is also revealed. The results are well described in a frequency domain picture by considering the interaction of each velocity group with the cw laser and the modes of the frequency comb.

DOI
10.1103/physreva.96.023430

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The two-photon transition 5S\ensuremath−5P\ensuremath−5D5S\ensuremath{-}5P\ensuremath{-}5D5S\ensuremath−5P\ensuremath−5D in rubidium vapor is investigated by detecting the fluorescence from the 6P3/26{P}_{3/2}6P3/2​ state when the atomic system is excited by the combined action of a cw diode laser and a frequency comb. The cw laser plays a role as a velocity-selective filter and allows for sub-Doppler spectroscopy over a large spectral range including the 5D3/25{D}_{3/2}5D3/2​ and 5D5/25{D}_{5/2}5D5/2​ states. For a counterpropagating beam configuration, the response of each atomic velocity group is well characterized within the Doppler profile and the excited hyperfine levels are clearly resolved. The contribution of the optical pumping to the direct two-photon process is also revealed. The results are well described in a frequency domain picture by considering the interaction of each velocity group with the cw laser and the modes of the frequency comb.
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Abstract We report on the direct observation of the frequency comb printed in the blue fluorescence excitation spectrum of rubidium vapor induced by the combined action of an ultrashort pulse train and a cw diode laser. Each laser drives one step of the 5S—5P—5D two-photon transition in a copropagating configuration and the excitation spectrum is obtained while the cw-laser frequency is scanned over the rubidium D 2 lines. Measurements of the fluorescence as a function of the diode intensity and atomic density allow us to investigate how the effect of power broadening and absorption of the diode laser blur the excitation process. The experimental results and the printing process of the comb on the Doppler profile are described well by numerical integration of the Bloch equations for a cascade three-level system.
We propose and demonstrate pump-probe spectroscopy of rubidium absorption that reveals the sub-Doppler hyperfine structure of the ${}^{5}{\mathrm{S}}_{1/2}\ensuremath{\leftrightarrow}{}^{5}{\mathrm{P}}_{3/2}$ (D2{\mathrm{D}}_{2}D2​) transitions. The counterpropagating pump and probe lasers are independently tunable in frequency, with the probe operating at the single-photon level. The two-dimensional spectrum measured as the laser frequencies are scanned shows fluorescence, Doppler-broadened absorption dips, and sub-Doppler features. The detuning between the pump and probe lasers allows compensation of the Doppler shift for all atomic velocities in the room-temperature vapor, meaning we observe sub-Doppler features for all atoms in the beam. We detail a theoretical model of the system that incorporates fluorescence, saturation effects, and optical pumping and compare this with the measured spectrum, finding a mean absolute percentage error of 3.11%. This high level of agreement allows us to extract the number density and temperature of the Rb\mathrm{Rb}Rb vapor from the two-dimensional spectrum. In the future this technique could be used as an atomic frequency standard for calibrating and characterising a single-photon or low-light-level source.
We present direct observation of the velocity-selective optical pumping of the Rb ground state hyperfine levels induced by 5S(1/2) --> 5P(1/2) femtosecond pulse-train excitation. A modified direct frequency comb spectroscopy based on the fixed frequency comb and a weak cw scanning probe laser was developed. The femtosecond pulse-train excitation of a Doppler-broadened Rb four-level atomic vapor is investigated theoretically in the context of the density matrix formalism and the results are compared with the experiment.
We study resonantly enhanced cascaded two-photon transitions in Doppler-broadened rubidium vapor, induced by an optical frequency comb. Cascaded two-step excitation of atoms from the ground state 5SSS1/2{}_{1/2}1/2​ to the final 5DDD5/2{}_{5/2}5/2​ state via an intermediate 5PPP3/2{}_{3/2}3/2​ state as well as the influence of the frequency comb repetition rate on the two-photon fluorescence signal have been investigated. It is shown that the careful choice of fR{f}_{R}fR​ leads to enhancement of the two-photon signal with considerable increase of the final 5D5/25{D}_{5/2}5D5/2​-state population due to velocity-selective double resonance. Since the double-resonance condition for two different rubidium isotopes is fulfilled for different fR{f}_{R}fR​, the possibility of isotope-selective two-photon excitation is explored. The dependence of the two-photon signal on external magnetic field values and different repetition rates of the optical frequency comb is also investigated.
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.