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

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• Atomic spectra
• Cooling & trapping
• Nonlinear optics
• Time & frequency standards
• Alkali metals
• Atomic gases
• Fluorescence spectroscopy
• Precision measurements