Sub-Doppler cooling of bosonic strontium in a two-color MOT

We report a two-color magneto-optical trap that continuously operates on the $(5{\mathrm{s}}^2){}^1{\mathrm{S}}_0-(5\mathrm{s}5\mathrm{p}){}^1{\mathrm{P}}_1$ and $(5\mathrm{s}5\mathrm{p}){}^3{\mathrm{P}}_2-(5\mathrm{s}5\mathrm{d}){}^3{\mathrm{D}}_3$ transitions in ${}^{88}\mathrm{S}\mathrm{r}$. Owing to the sub-Doppler cooling from the metastable $(5\mathrm{s}5\mathrm{p}){}^3{\mathrm{P}}_2$ state, a bimodal momentum distribution is observed, with the cold part of the ensemble reaching a temperature of 105 $\mu$K. A detailed simulation is established to explain the data. The absolute frequency of the $(5\mathrm{s}5\mathrm{p}){}^3{\mathrm{P}}_2-(5\mathrm{s}5\mathrm{d}){}^3{\mathrm{D}}_3$ transition is measured to be 603 976 473 $\pm$ 2 MHz using a frequency comb, improving the literature value. The accompanying magnetic trap is characterized, and is shown to contribute to the sequential production of ensembles with temperatures down to 45 $\mu$K.