Abstract
We report on an experimental test of the spin selection rule for two-photon transitions in atoms. In particular, we demonstrate that the $5S_{1/2}\to 6S_{1/2}$ transition rate in a rubidium gas follows a quadratic dependency on the helicity parameter linked to the polarization of the excitation light. For excitation via a single Gaussian beam or two counterpropagating beams in a hot vapor cell, the transition rate scales as the squared degree of linear polarization. The rate reaches zero when the light is circularly polarized. In contrast, when the excitation is realized via an evanescent field near an optical nanofiber, the two-photon transition cannot be completely extinguished (theoretically, not lower than 13\% of the maximum rate, under our experimental conditions) by only varying the polarization of the fiber-guided light. Our findings lead to a deeper understanding of the physics of multiphoton processes in atoms in strongly nonparaxial light.
Highlights
Two-photon excitation, explored theoretically in 1931 [1] and first experimentally demonstrated in the early 1960s [2,3], revolutionized the fields of spectroscopy [4,5], fluorescence microscopy [6], and optical metrology [7]
If the simultaneously absorbed photons have the same frequency and are provided by counterpropagating beams, the fluorescence spectrum is free of Doppler broadening [8–11]; hyperfine transition lines can be clearly seen, allowing one to achieve a robust frequency reference [12,13]
This leads to a selection rule for the allowed change of the orbital angular momentum of the electron: L = 0, ±2
Summary
Two-photon excitation, explored theoretically in 1931 [1] and first experimentally demonstrated in the early 1960s [2,3], revolutionized the fields of spectroscopy [4,5], fluorescence microscopy [6], and optical metrology [7]. We focus on the spin selection rule, which further restricts the angular momentum changes in electric dipole allowed, single-frequency two-photon transitions between S levels in atoms where the intermediate level is detuned from the single-photon resonance frequency. In this case, F = 0 and mF = 0 (with mF being the magnetic quantum number) apply, meaning that the total spin of the atom must be preserved. We experimentally study the polarization dependence of an S → S two-photon transition in a 87Rb gas for two conceptually different excitation conditions: (i) warm atoms in a vapor cell with Gaussian beam illumination and (ii) laser-cooled atoms in the evanescent field of a single-mode ONF, where the light is strongly nonparaxial
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