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Laser Physics

Sub-Doppler dichroism as a useful tool in alkali atom hyperfine spectroscopy

Mohammad Karami, Zahra Heydarinasab, Farrokh Sarreshtedari

Published Oct 30, 2023• 4 citations
Publisher

Abstract

Abstract Analytical modeling of the sub-Doppler dichroic atomic vapor laser lock error signal shows that the slope of this error signal could be of interest for the identification of transitions in alkali atom hyperfine spectroscopy. Among four D1 line transitions of alkali atoms, the slope sign of the sub-Doppler error signal is different only for the case when the Landé g-factor is negative for both the ground and excited states. The analytical model used, which includes all sublevel transitions, shows very good agreement with the experimental spectroscopy results. The use of this technique allows us to accurately characterize hyperfine transitions in alkali atoms without the need for advanced spectroscopic tools.

DOI
10.1088/1555-6611/ad04c7

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Journal of Experimental and Theoretical Physics Letters•Mar 1, 2015•19 citations
Abstract Analytical modeling of the sub-Doppler dichroic atomic vapor laser lock error signal shows that the slope of this error signal could be of interest for the identification of transitions in alkali atom hyperfine spectroscopy. Among four D1 line transitions of alkali atoms, the slope sign of the sub-Doppler error signal is different only for the case when the Landé g-factor is negative for both the ground and excited states. The analytical model used, which includes all sublevel transitions, shows very good agreement with the experimental spectroscopy results. The use of this technique allows us to accurately characterize hyperfine transitions in alkali atoms without the need for advanced spectroscopic tools.
We present a comprehensive theoretical and experimental study of the lineshape in sub-Doppler dichroic atomic vapor laser lock (DAVLL) spectroscopy of the D1 transition lines of 87 Rb and 85 Rb atoms. By calculating sub-Doppler DAVLL spectra with both density matrix equations and rate equations, we found that the coherence effect is significantly dependent on the branching ratios of the transition lines. In addition, we investigated the dependence of the amplitude and slope of the spectra on the applied magnetic field and pump beam power, and found good agreement with the results calculated from the rate equations.
A theoretical and experimental study of lineshapes in sub-Doppler DAVLL (dichroic atomic vapor laser lock) spectroscopy for the D lines of Rb atoms was presented. From the calculation of sub-Doppler DAVLL spectra using both density matrix equations and rate equations we found that the coherence effect depended significantly on the branching ratios of the transition lines. We also investigated the dependence of the amplitude and the slope of the spectra on the magnetic field and the pump beam intensity, and found good agreement with the results calculated from the rate equation.
In this letter we study magnetic circular dichroism in alkali atoms exhibiting asymmetric behaviour of magnetically induced transitions. The magnetic field B ∥ k induces transitions between Δ F = ± 2 hyperfine levels of alkali atoms and in the range of ∼ 0.1 -- 3 kG magnetic field, the intensities of these transitions experience significant enhancement. We have inferred a general rule applicable for the D 2 lines of all alkali atoms, that is the transition intensity enhancement is around four times larger for the case of σ + than for σ − excitation for Δ F = + 2 , whereas it is several hundreds of thousand times larger in the case of σ − than that for σ + polarization for Δ F = − 2 . This asymmetric behaviour results in circular dichroism. For experimental verification we employed half-wavelength-thick atomic vapor nanocells using a derivative of the selective reflection technique, which provides a sub-Doppler spectroscopic linewidth ( ∼ 50 MHz ). The presented theoretical curves well describe the experimental results. This effect can find applications particularly in parity violation experiments.
Abstract Various efforts have been made to overcome Doppler broadening in hyperfine measurement limitations in the atomic vapors spectroscopy and associated applications. The present study measured and calculated hyperfine resolved ellipsometric parameters through the near-normal reflectance spectra of the rubidium vapor cell in two experimental setups based on continuous and modulated pathway. The results indicated that valuable information could be extracted from the ellipsometric parameters about the atomic medium. Change in the ellipsometric parameters in each transition line confirms the existence of the elliptical polarization of the reflected light when it is exposed to the alkali metal vapor. Our results show that the ellipticity at 5S1/2 (Fg = 1, 2) → 5P1/2 (Fe = 1, 2) hyperfine transitions of 87Rb (D1 line) is small, and accordingly hyperfine transitions between the ground 5S1/2 (Fg = 2, 3) and excited 5P1/2 (Fe = 2, 3) states of the 85Rb isotope are considerable. These ellipsometric parameters, as phase difference, can trace the behavior of the relative orientation of the electric field and atom velocity in the interface based on van der Waals dipole–dipole interaction and is directly proportional to the strength of the light-matter interaction which extremely useful instead complicated atomic spectroscopic methods.
Abstract We experimentally generate polarization spectroscopy (error) signals corresponding to the D2-line hyperfine transitions, F g = 2 → F e = 1 , 2 , 3 of 87Rb, and F g = 3 → F e = 2 , 3 , 4 of 85Rb, and show that the strongest error signals correspond to the closed hyperfine transitions (oscillator strength ∼ 0.7 ), F g = 2 → F e = 3 and F g = 3 → F e = 4 , respectively. We make the generated error signal robust to fluctuations in external parameters by finding optimum values for the vapor cell temperature and pump-probe intensities at two different beam diameters. We further employ these optimized error signals to directly (without the need for frequency/phase modulation) lock the laser frequency—reducing the rms drift/linewidth from ∼ 10 MHz to < 500 kHz, when measured over a ∼ 60 min duration. In addition, by comparing theoretically calculated and experimentally measured error signals for the pump intensities ranging from ∼ 0.1 I sat 0 − 10 I sat 0 (where, I sat 0 ∼ 1.6 mW cm−2 is the two-level saturation intensity for the Rb D2-line transitions), we discuss the applicability and limitations of existing numerical and analytical approaches based on a full multi-level rate equation model for polarization spectroscopy.
It has been shown that the D 1 line of atomic vapors of alkali metals excited by π-polarized radiation in a strong transverse magnetic field includes specific “guiding” (indicating) atomic transitions between the magnetic sublevels of the hyperfine structure. The dependence of the frequency shift of the guiding transitions on the magnetic field, as well as on their dipole moments, is asymptotic for all other transitions. An experiment with a nanocell with Rb vapor with a thickness of half the wavelength (λ/2 method) for ensuring a sub-Doppler spectral resolution has completely confirmed the presence of guiding transitions. Two groups of six transitions for 85 Rb and two groups of four transitions for 87 Rb have been detected in the absorption spectra in magnetic fields above 4 kG. A guiding transition has been identified in each of four groups. Four transitions forbidden at B = 0 have been also detected; with an increase in the magnetic field, their probabilities also approach the probabilities of the guiding transitions.