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Journal of Physics B Atomic and Molecular Physics

Heterodyne Doppler-free two-photon absorption and dispersion in rubidium vapour

Daniel Bloch, M. Ducloy, E. Giacobino

Published Dec 28, 1981• 10 citations
Publisher

Abstract

Heterodyne Doppler-free spectroscopy, previously demonstrated on single-photon resonances (heterodyne saturation spectroscopy), has been extended to two-photon transitions in rubidium (5S-5D and 5S-7S). It provides a simple method of observing the two-photon dispersion lineshape directly. The noticeable sensitivity of this absorption technique, which has been used for an optical measurement of the hyperfine structure of the 5D level, is discussed in comparison with that currently obtained in two-photon-induced fluorescence.

DOI
10.1088/0022-3700/14/24/004

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Heterodyne Doppler-free spectroscopy, previously demonstrated on single-photon resonances (heterodyne saturation spectroscopy), has been extended to two-photon transitions in rubidium (5S-5D and 5S-7S). It provides a simple method of observing the two-photon dispersion lineshape directly. The noticeable sensitivity of this absorption technique, which has been used for an optical measurement of the hyperfine structure of the 5D level, is discussed in comparison with that currently obtained in two-photon-induced fluorescence.
In this paper, we describe two complementary nonlinear spectroscopy methods which both allow one to achieve Doppler-free spectra of atomic gases. First, saturated absorption spectroscopy is used to investigate the structure of the 5S 1/2 → 5P 3/2 transition in rubidium. Using a slightly modified experimental setup, Doppler-free two-photon absorption spectroscopy is then performed on the 5S 1/2 → 5D 5/2 transition in rubidium, leading to accurate measurements of the hyperfine structure of the 5D 5/2 energy level. In addition, electric dipole selection rules of the two-photon transition are investigated, first by modifying the polarization of the excitation laser, and then by measuring two-photon absorption spectra when a magnetic field is applied close to the rubidium vapour. All experiments are performed with the same grating-feedback laser diode, providing an opportunity to compare different high-resolution spectroscopy methods using a single experimental setup. Such experiments may acquaint students with quantum mechanics selection rules, atomic spectra and Zeeman effect.
Frequency-modulation spectroscopy was employed successfully to measure the Doppler-free absorption and dispersion signals originating from several two-photon transitions in (85)Rb and (87)Rb. The two-photon cross section of the hyperfine transition 5S(1/2) (F = 2) ? 5D(5/2) (F = 4) in (87)Rb was determined to be sigma((2)) asymptotically equal to 4 x 10(-20) cm(4)/W. The transition isotope shift (TIS) of the transition 5S(1/2) ? 5D(5/2) between (87)Rb and (85)Rb was found to be Deltanu(TIS) = +166 +/- 2 MHz.
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.
An ongoing spectroscopic investigation of atomic rubidium utilizes a two-photon, single-laser excitation process. Transitions accessible with our tunable laser include 5P1/2 (F ′ ) ← 5S1/2 (F) and 5P3/2 (F ′ ) ← 5S1/2 (F). The laser is split into a pump and probe beam to allow for Doppler-free measurements of transitions between hyperfine levels. The pump and probe beams are overlapped in a counter-propagating geometry and the laser frequency scans over a transition. Absorption, fluorescence and polarization spectroscopy techniques are applied to this basic experimental setup. The temperature of the vapor cell and the power of the pump and probe beams have been varied to explore line broadening effects and signal-to-noise of each technique. This humble setup will hopefully grow into a more robust experimental arrangement in which double resonance, two-laser excitations are used to explore hyperfine state changing collisions between rubidium atoms and noble gas atoms. Rb-noble gas collisions can transfer population between hyperfine levels, such as 5P3/2 (F ′ = 3) Collision ←− 5P3/2 (F ′ = 2), and the probe beam couples 7S1/2 (F ′′ = 2) ← 5P3/2 (F ′ = 3). Polarization spectroscopy signal depends on the rate of population transfer due to the collision as well as maintaining the orientation created by the pump laser. Fluorescence spectroscopy relies only on transfer of population due to the collision. Comparison of these techniques yields information regarding the change of the magnetic sublevels, mF , during hyperfine state changing collisions.
We describe an experiment for investigating the 5S1∕2→5D5∕2 two-photon transition in rubidium using a single grating-feedback diode laser operating at 778.1nm (385THz). Continuous tuning of the laser frequency over 4GHz allows for the clear resolution of the Doppler-free spectral features and allows accurate measurement of the hyperfine ground-state splitting. A direct comparison between Doppler-broadened and Doppler-free spectral features is possible because both are distinctly evident in the two-photon spectra. By independently modifying the polarization state of the two laser fields, the impact of electric dipole selection rules on the two-photon transition spectra is investigated. This experiment is a valuable addition to the advanced undergraduate laboratory because it uses much of the same equipment as the single-photon saturated absorption spectroscopy experiment performed on the 5S1∕2→5P3∕2 transition in rubidium (λ=780.24nm) and provides students with an opportunity to investigate characteristics of atomic spectra not evident in the single-photon experiment.
Optical heterodyne spectroscopy is one of the ultrasensitive and high resolution laser spectroscopies. During past years, it has extended from saturation absorption to two-photon transitions in a three level quantum system of atoms and ions. The particularly interesting case is the modulation transfer spectroscopy, where the optical heterodyne detection is performed via nearly degenerate four-wave mixing (NDFWM) in the nonlinear medium. In this report, the modulation transfer spectroscopy is used to observe the optical heterodyne saturation absorption and two-photon transition spectra simultaneously in Na2 without Doppler background. The line shapes of optical heterodyne signal are discussed in detail.