Precision Measurement of the Saturation Intensity in Rubidium at 420 nm

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 near $82.02~\pm~ 0.73^\circ$C as the optimal operating temperature, where the signal-to-noise ratio of the Lamb-dip amplitudes with temperature reaches a maximum and Lamb-dip linewidths exhibit 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.