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arXiv:2602.07161 · Physical Review Applied

Two-photon-excited fluorescence spectroscopy of Rb atoms in a magneto-optical trap

Alan McLean, Christian Drago, Daniel Podos, Chengyi Luo, Caleb Brzezinski, Ting-Wei Hsu, John Sipe, Ralph Jimenez

physics.atom-phphysics.app-phquant-phSubmitted Feb 6, 2026• Announced Feb 10, 2026• 0 citations• Revised Oct 3, 2026 (v3)

v1 · Feb 6, 2026 v2 · Aug 21, 2026 v3 · Oct 3, 2026

Abstract

We report the results of two-photon-excited fluorescence (TPEF) measurements of the $5\mathrm{S}_{1/2} \rightarrow 5\mathrm{D}_{5/2}$ transition of $^{85}$Rb and $^{87}$Rb cooled in a magneto-optical trap (MOT). We observe TPEF at excitation powers as low as 1 $μ$W or fluxes as low as $2.98_{-0.94}^{+1.37} \times 10^{18}\ \text{photons}\,\text{cm}^{-2}\,\text{s}^{-1}$ ($^{85}$Rb) and $3.31_{-1.33}^{+2.21} \times 10^{18}\ \text{photons}\,\text{cm}^{-2}\,\text{s}^{-1}$ ($^{87}$Rb). Our results demonstrate that optically cooled Rb is a promising platform for observing sensitive two-photon spectral signatures at low photon fluxes.

Journal reference: Phys. Rev. Appl. 26, 034067 (2026)
DOI: 10.1103/c8gq-zp19

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We report the results of two-photon-excited fluorescence (TPEF) measurements of the

5\mathrm{S}_{1/2} \rightarrow 5\mathrm{D}_{5/2}

transition of

^{85}

Rb and

^{87}

Rb cooled in a magneto-optical trap (MOT). We observe TPEF at excitation powers as low as 1

μ

W or fluxes as low as

2.98_{-0.94}^{+1.37} \times 10^{18}\ \text{photons}\,\text{cm}^{-2}\,\text{s}^{-1}

(

^{85}

Rb) and

3.31_{-1.33}^{+2.21} \times 10^{18}\ \text{photons}\,\text{cm}^{-2}\,\text{s}^{-1}

(

^{87}

Rb). Our results demonstrate that optically cooled Rb is a promising platform for observing sensitive two-photon spectral signatures at low photon fluxes.

A two-color magneto-optical trap (MOT) for trapping cesium (Cs) atoms is experimentally realized. This two-color MOT employs the radiation forces due to photon scattering from the Cs 6P(3/2) F' = 5 - 8S(1/2) F" = 4 excited-state transition, which replaced one pair of the three pairs of cooling/trapping laser beams operating on a single-photon red detuning to the Cs 6S(1/2) F = 4 - 6P(3/2) F' = 5 cycling transition in a standard six-beam Cs MOT, and can cool and trap atoms on both the red-detuning and blue-detuning sides of the two-photon resonance. Employing the two-color MOT, the background-free fluorescence detection of trapped atoms has been demonstrated. This technique will be useful for observation of weak fluorescence signal radiated from single atoms in MOT.

We describe cooling and trapping of both

{}^{85}

Rb and

{}^{87}

Rb in a range of magnetic fields up to 2.6 T. Atoms are injected from a low-field pyramidal magneto-optical trap and recaptured in a high-magnetic-field atom trap. The atoms are cooled and trapped by a six-beam optical molasses via the

5{S}_{1/2}|{m}_{I},{m}_{J}=

1/2$\ensuremath{\rangle}$$\ensuremath{\rightarrow}$

5{P}_{3/2}|{m}_{I},{m}_{J}=

3/2$\ensuremath{\rangle}$ transition (

{m}_{I}=

5/2 for

{}^{85}

Rb and

{m}_{I}=

3/2 for

{}^{87}

Rb). We study the trap fluorescence spectra, atom temperatures, density distributions, and lifetimes as a function of magnetic field and detuning parameters. The trap fluorescence spectra are both narrow and asymmetric, as is characteristic for laser cooling of atoms in an external trapping potential. The trap is modeled using a Monte Carlo trajectory simulation technique.

We report the measurement of the photoionization cross sections of the

5{S}_{1/2}

and

5{P}_{3/2}

states of

^{87}\mathrm{Rb}

in a two-species Hg and Rb magneto-optical trap (MOT) by the cooling laser for Hg. The photoionization cross sections of Rb in the

5{S}_{1/2}

and

5{P}_{3/2}

states at 253.7 nm are determined to be ${1}_{\ensuremath{-}1}^{+4.3}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}20}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{2}$ and $4.63(30)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{2}$, respectively. To measure the

5{S}_{1/2}

and

5{P}_{3/2}

state fractions in the MOT we detected the photoionization rate of the

5{P}_{3/2}

state by an additional 401.5 nm laser. The photoionization cross section of Rb in the

5{P}_{3/2}

state at 401.5 nm is determined to be $\text{1.18(10)}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}\phantom{\rule{0.28em}{0ex}}{\text{cm}}^{2}$.

We report the spectroscopy of the

{8S}_{1/2}

energy level in

{}^{210}\mathrm{Fr}

by two-photon excitation of atoms confined and cooled in a magneto-optical trap. The resonant intermediate level is the upper state of either the

{7P}_{3/2}

trapping transition or the

{7P}_{1/2}

repumping transition. The center-of-gravity energy difference between the

{7S}_{1/2}

ground level and the

{8S}_{1/2}

level is $19732.523\ifmmode\pm\else\textpm\fi{}0.004 {\mathrm{cm}}^{\ensuremath{-}1}.$ The hyperfine separation of the

{8S}_{1/2},F=11/2

and

F=13/2

states is $10256\ifmmode\pm\else\textpm\fi{}7 \mathrm{MHz},$ giving a magnetic dipole hyperfine constant A of $1577.8\ifmmode\pm\else\textpm\fi{}1.1 \mathrm{MHz}.$