Two-photon cooling of calcium atoms

We demonstrate cooling of calcium atoms using a two-photon transition from the 1S0 ground state to the upper 4s5s 1S0 state via the 1P1 intermediate state. We achieve temperatures as low as 260 mu K in a magnetooptical trap (MOT), well below the Doppler limit (TD = 0.8 mK) of the 1P1 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 1P1 state, a higher transfer efficiency (close to 100%), and a more straightforward experimental implementation.