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

We report [1] the laser cooling and trapping of neutral potassium on an open transition. Fermionic $^{40}$K is captured using a magneto-optical trap (MOT) on the closed $\mathrm{4S_{1/2}} \rightarrow \mathrm{4P_{3/2}} $ transition at 767\,nm and then transferred, with unit efficiency, to a MOT on the open $\mathrm{4S_{1/2}} \rightarrow \mathrm{5P_{3/2} }$ transition at 405\,nm. Because the $\mathrm{5P_{3/2} }$ state has a smaller line width than the $\mathrm{4P_{3/2}}$ state, the Doppler limit is reduced. We observe temperatures as low as 63(6)\,$\mu$K, the coldest potassium MOT reported to date. The density of trapped atoms also increases, due to reduced temperature and reduced expulsive light forces. We measure a two-body loss coefficient of $\beta = 2 \times 10^{-10}$\,cm$^3\,$s$^{-1}$, and estimate an upper bound of $8 \times 10^{-18}$\,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 pre-cooling for quantum gas experiments. A qualitatively similar enhancement is observed in a 405\,nm MOT of bosonic $^{41}$K. \\[4pt] [1] Physical Review A, 84, 063420 (2011)