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

We report the results of two-photon-excited fluorescence (TPEF) measurements of the $5{\mathrm{S}}_{1\mathrm{/}2}\to 5{\mathrm{D}}_{1\mathrm{/}2}$ transition of ${}^{85}$Rb and ${}^{87}$Rb cooled in a magneto-optical trap (MOT). We observe TPEF at excitation powers as low as 1 $\mu$W or fluxes as low as $4.30\pm 0.22\times {10}^{18}\text{ photons}{\text{cm}}^{-2}{\text{s}}^{-1}$ (${}^{85}$Rb) and $5.17\pm 0.26\times {10}^{18}\text{ photons}{\text{cm}}^{-2}{\text{s}}^{-1}$ (${}^{87}$Rb). Our results show that Rb, with additional benefits due to its ability to be optically cooled to the point where Doppler-broadening is negligible, is a promising platform for observing sensitive two-photon spectral signatures at low photon fluxes.