Two-photon cooling of calcium atoms

We demonstrate cooling of calcium atoms using a two-photon transition from the ${}^{1}{S}_{0}$ ground state to the upper $4s5s\phantom{\rule{3.33333pt}{0ex}}{}^{1}{S}_{0}$ state via the ${}^{1}{P}_{1}$ intermediate state. We achieve temperatures as low as $260\phantom{\rule{0.16em}{0ex}}\textmu{}\mathrm{K}$ in a magnetooptical trap (MOT), well below the Doppler limit (${T}_{\text{D}}=0.8\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{K}$) of the ${}^{1}{P}_{1}$ state. We characterize the temperature, lifetime, and confinement of the MOT over a range of experimental parameters, observing no reduction in lifetime due to coupling to the higher state. We perform theoretical simulations of the cooling scheme and observe good agreement with the experimental results. The two-photon cooling scheme presented in this work provides an alternative to the standard Doppler cooling applied to alkaline-earth atoms, based on a sequence of two magnetooptical traps. The advantages of our scheme are the possibility of varying the effective linewidth of the ${}^{1}{P}_{1}$ state, a higher transfer efficiency (close to $100%$), and a more straightforward experimental implementation.