Low-temperature high-density magneto-optical trapping of potassium using the open 4S → 5P transition at 405 nm

We report the laser cooling and trapping of neutral potassium on an open transition. Fermionic K-40 is captured using a magneto-optical trap (MOT) on the closed 4S(1/2) -> 4P(3/2) transition at 767 nm and then transferred, with high efficiency, to a MOT on the open 4S(1/2) -> 5P(3/2) transition at 405 nm. Because the 5P(3/2) state has a smaller linewidth than the 4P(3/2) state, the Doppler limit is reduced from 145 mu K to 24 mu K, and we observe temperatures as low as 63(6) mu K. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of beta = 1.4(1) x 10(-10) cm(3)/s near saturation intensity, and estimate an upper bound of 8 x 10(-18) cm(2) for the ionization cross section of the 5P state at 405 nm. The combined temperature and density improvement in the 405 nm MOT is a twenty-fold increase in phase-space density over our 767 nm MOT, showing enhanced precooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405 nm MOT of bosonic K-41.