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Chinese Physics B

Enhancement of the Doppler-free saturated absorption signal based on 87 Rb second excited state transition at 420 nm

Yuechen 悦晨 Zhang 张, Weihao Zhao, Yuan Tian, Yi Zhang, Sihong Gu, Ang Zhang, Jie‐Hua Chen

Published Oct 30, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/ae194c

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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.
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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.
The 5S1/2→6P3/25S_{1/2} \rightarrow 6P_{3/2}5S1/2​→6P3/2​ transition of rubidium at 420420420 nm is a promising candidate for a portable warm-vapor all-optical atomic clock. Despite recent precision spectroscopy studies at 420420420 nm in Rb, an experimental determination of the saturation intensity of this transition has not yet been reported. The saturation intensity is a fundamental parameter that influences the identification of a potential clock transition frequency in terms of optimizing various intensity-dependent parameters and connected systematics. In this work, we report the first experimental measurement of the saturation intensity of the 420420420 nm transition in Rb, obtaining (23.18±0.28)(23.18 \pm 0.28)(23.18±0.28) mW/cm2^22 for the 87^{87}87Rb F=2→F′=3F = 2 \rightarrow F' = 3F=2→F′=3 transition and (25.56±0.37)(25.56 \pm 0.37)(25.56±0.37) mW/cm2^22 for the 85^{85}85Rb F=3→F′=4F = 3 \rightarrow F' = 4F=3→F′=4 transition, in excellent agreement with theoretical predictions. We further investigate the temperature dependence of the Doppler-free Lamb-dip amplitude and linewidth over 59.03 ± 0.3759.03~\pm~0.3759.03 ± 0.37 - 91.20 ± 0.90∘91.20~\pm~0.90^\circ91.20 ± 0.90∘C in a 100100100 mm commercial vapor cell, identifying around 82.02 ± 0.73∘82.02~\pm~ 0.73^\circ82.02 ± 0.73∘C as the optimal operating temperature, where the signal-to-noise ratio of the Lamb-dip amplitude with temperature reaches a maximum and the observed Lamb-dip linewidth exhibits a minimum. We also present precise measurements of the magnetic-dipole (AAA) and electric-quadrupole (BBB) hyperfine constants of the 6P3/26P_{3/2}6P3/2​ state for both isotopes, with the measured values being consistent with previously reported values for the hyperfine constants.
Excited state population distribution created by 5S1/2→5P3/2 laser excitation in room temperature rubidium vapor was quantitatively analyzed applying absorption and saturation spectroscopy.When the laser linewidth was much smaller than the Doppler linewidth,the absorption coefficients were measured for a series of P780 powers between 40 μW and 5mW.Population in the 5P3/2 level obtained from the saturation measurements was also determined by the absorption measurement of narrow spectral line from a Rb hollow cathode lamp.The agreement between the results obtained in these two ways is very good.It was shown that 2% of the ground state population could be transferred to the first excited state by pumping the Doppler broadened line with a single-mode narrow-band laser.
We implement a compact optical frequency standard scheme with laser frequency locked to the 5 S 1/2 ( F = 2) − 6 P 3/2 ( F ′ = 3) transition of the second excited state of 87 Rb atoms in a 3 mm cubic glass cell, using a Doppler-free saturated absorption spectroscopy. The experimental results show the frequency stability at the level of 2.2 × 10 −12 at 1 s. Furthermore, we conduct an experimental study on the effect of a repump laser on the frequency performance of the saturated absorption spectroscopy optical frequency standard, providing valuable experimental results with reference values for implementing this type of optical atomic clock.
The utilization of atomic or molecular spectroscopy for frequency locking of single-frequency laser to improve laser frequency stability plays an important role in the experimental investigation of optically pumped atomic magnetometers, atomic clocks, laser cooling and trapping of atoms, etc. We have experimentally demonstrated a technique for frequency stabilization of a single-frequency laser employing the bright state spectroscopy (BSS) with a rubidium atomic vapor cell. By utilizing the counter-propagating dual-frequency 795 nm laser beams with mutually orthogonal linear polarization and a frequency difference of 6.834 GHz, which is equal to the hyperfine splitting of rubidium-87 ground state 5S1/2, an absorption-enhanced signal with narrow linewidth at the center of Doppler-broadened transmission spectroscopy is observed when continuous scanning the laser frequency over rubidium-87 D1 transition. This is the so-called BSS. Amplitude of the absorption-enhanced signal in the BSS is much larger compared with the conventional saturation absorption spectroscopy (SAS). The relationship between linewidth and amplitude of the BSS signal and laser beam intensity has been investigated. This high-contrast absorption-enhanced BSS signal has been employed for the laser frequency stabilization. The experimental results show that the frequency stability is 4.4×10−11 with an integration time of 40 s, near one order of magnitude better than that for using the SAS.