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arXiv:2606.30871

Precision Measurement of the Saturation Intensity in Rubidium at 420 nm

Shivam Sinha, Sumit Achar, Sankar Satheesh, Arijit Sharma

physics.atom-phquant-phSubmitted Jun 29, 2026• Announced Jul 1, 2026• 0 citations• Revised Jul 1, 2026 (v2)
v1 · Jun 29, 2026v2 · Jul 1, 2026v3 · Jul 21, 2026
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Abstract

The $5S_{1/2} \rightarrow 6P_{3/2}$ transition of rubidium at 420 nm is a promising candidate for a portable warm-vapor all-optical atomic clock. Despite recent precision spectroscopy studies at 420 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 420 nm transition in Rb, obtaining $(23.18 \pm 0.28)$ mW/cm$^2$ for the $^{87}$Rb $F=2\rightarrow F'=3$ transition and $(25.56 \pm 0.37)$ mW/cm$^2$ for the $^{85}$Rb $F=3\rightarrow F'=4$ 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$~\pm~0.37$ - 91.20$~\pm~0.90^\circ$C in a 100 mm commercial vapor cell, identifying around 82.02$~\pm~ 0.73^\circ$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 ($A$) and electric-quadrupole ($B$) hyperfine constants of the $6P_{3/2}$ state for both isotopes, with the measured values being consistent with previously reported values for the hyperfine constants.

Comments
Shivam Sinha and Sumit Achar equally contributed to this work

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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.
We experimentally study the character of saturated absorption spectroscopy (SAS) of the 87 Rb 5 S 1/2 → 6 P 3/2 transition at 420 nm in a miniature cubic glass cell (3 × 3 × 3 mm 3 ). The influences of the laser intensity and cell temperature on the saturated absorption resonance are comprehensively investigated to determine the optimal parameters for a compact stabilized frequency standard. By stabilizing the laser frequency to the optimized SAS signal, a beatnote stability of 2.4 × 10 −13 at 1 s is measured between two identical SAS systems, corresponding to a single-laser stability of 1.7 × 10 −13 at 1 s, assuming equal contribution from each laser. A further analysis of the noise budget reveals that the main contributions of short-term stability arise from the intermodulation effect and the laser frequency noise through the frequency modulation to amplitude modulation (FM-AM) conversion process. Our work provides a promising route toward high-performance miniaturized optical frequency standards based on high-lying excited-state transitions.
Abstract We analyze an optical atomic clock using two-photon 5 S 1 / 2 → 4 D J transitions in rubidium. Four one- and two-color excitation schemes to probe the 4 D 3 / 2 and 4 D 5 / 2 fine-structure states are considered in detail. We compare key characteristics of Rb 4 D J and 5 D 5 / 2 two-photon clocks. The 4 D J clock features a high signal-to-noise ratio due to two-photon decay at favorable wavelengths, low dc electric and magnetic susceptibilities, and minimal black-body shifts. Ac Stark shifts from the clock interrogation lasers are compensated by two-color Rabi-frequency matching. We identify a ‘magic’ wavelength near 1060 nm, which allows for in-trap, Doppler-free clock-transition interrogation with lattice-trapped cold atoms. From our analysis of clock statistics and systematics, we project a quantum-noise-limited relative clock stability at the 10 − 13 / τ ( s ) -level, with integration time τ in seconds, and a relative accuracy of ∼ 10 − 13 . We describe a potential architecture for implementing the proposed clock using a single telecom clock laser at 1550 nm, which is conducive to optical communication and long-distance clock comparisons. Our work could be of interest in efforts to realize small and portable Rb clocks and in high-precision measurements of atomic properties of Rb 4 D J -states.
The goal of this experiment was to measure the hyperfine energy splittings of the ground to first excited state transitions in rubidium using saturated absorption spectroscopy. Using this technique, we measured these transition energy spectra by taking the difference of two photodiode outputs due to multiple beams of a single laser scanned over a range of frequencies and shone through a cell of Rb vapor. When the laser frequency was resonant with an atomic transition, photons of those frequencies were absorbed, leaving a dip in intensity of the beam measured at the photodiode. One of the two laser beams had its excitations saturated by a more intense, counter-propagating pump beam from the same laser. The pump beam saturated the absorption of the velocity equal zero atoms that the counter-propagating probe beam could have also interacted with due to no Doppler shift. At these frequencies the probe beam experienced less absorption that was measured by a photodiode. The photodiode output was calibrated to the change in frequency of the scanned laser using a Fabry-Pérot interferometer. The output of this interferometer served as a reference for how the laser frequency changed. Final results for the 87Rb F=1 spectrum accounted for the largest error with a maximum of 12.2% deviation from accepted energy spacing values due in part to systematic error. Other spectra measurements were in better agreement, possessing error ranging from 2.1% to 10.1% and some agreeing with accepted values to within our uncertainty.
The commonly quoted saturation intensity of rubidium (Rb) for D2 transitions is 16.7W.m−2\mathrm{W}.\mathrm{m}^{-2}W.m−2. This value, however, is only accurate for the cycling transition, which renders the Rb atom to resemble a two-level system. In practice, the effective saturation intensity is influenced by various factors, such as the polarization of light, optical pumping, and atom collision which can redistribute the population. As a result, it is crucial to experimentally determine the effective saturation intensity. In this context, we demonstrate a novel yet simple experimental method to measure the effective saturation intensities of Doppler-broadened transitions in atomic vapours. Our method only requires on-resonance absorbance data for various input laser intensities, which can be obtained with a tunable CW laser without the need for precise frequency locking. We applied this method to measure the effective saturation intensities of all four Doppler-broadened D2 transitions in natural rubidium (Rb) vapour (27.8%87Rb and 72.2 %85Rb{}^{\text{8}} {}^{\text{5}}Rb85Rb) atr=35∘Cr={35}^{\circ} \mathrm{C}r=35∘C. The results show that the effective saturation intensities for Rb range from 0.752 to 16.8W.m−2\mathrm{W}.\mathrm{m}^{-2}W.m−2. Notably, for certain transitions (specifically, repump transitions), the effective saturation intensities were smaller than 16.7 W.m−2\mathrm{m}^{-2}m−2(up to an order of magnitude), with potentially important implications for optical pumping in Doppler-broadened transitions.
Abstract The Saturated Absorption Spectroscopy (SAS) was performed to measure the hyperfine constants of rubidium 5P1/2 excited state using an external-cavity diode laser (ECDL) operating at 795 nm. We report our measurements for the magnetic dipole coupling constants 120.79(29) and 407.75(57) for 85 Rb and 87 Rb respectively. Our measurement of the 85 Rb hyperfine constant shows a good agreement with previous results and our measurement of the 87 Rb hyperfine constant resolves the discrepancy of results that exists in the literature.