Two-photon excitation of launched cold atoms in flight

We demonstrate two-photon bi-chromatic excitation of cold rubidium atoms in flight, using the pathway 5S(1/2) -> 5P(3/2) -> 5D(5/2) with two resonant photons. In our experiment, Rb-85 atoms are laser-cooled in a magneto-optical trap and launched upward in discrete clouds with a controllable vertical speed of 7.1 +/- 0.6 m/s and a velocity spread that is less than 10% of the launch speed. Outside the cooling beams, as high as 14 mm above the original center of the trap, the launched cold atoms are illuminated simultaneously by spatially localized horizontal excitation beams at 780 nm (5S(1/2) -> 5P(3/2)) and 776 nm (5P(3/2) -> 5D(5/2)). We monitor transmission of the 780-nm beam over a range of intensities of 780-nm and 776-nm light. As the center of the moving cloud passes the excitation beams, we observe as much as 97.9 +/- 1.2% transmission when the rate of two-photon absorption is high and the 5S(1/2) and 5P(3/2) states are depopulated, compared to 87.6 +/- 0.9% transmission if only the 780-nm beam is present. This demonstrates two-photon excitation of a launched cold-atom source with controllable launch velocity and narrow velocity spread, as a foundation for three-photon excitation to Rydberg states. (C) 2017 Optical Society of America