Laser cooling on a narrow atomic transition and measurement of the two-body cold collision loss rate in a strontium magneto-optical trap

A strontium vapor cell magneto-optical trap (MOT) has been constructed for the dual purpose of studying cold collision processes in trapped atoms and developing narrow-line cooling/trapping techniques using the 689nm intercombination line. The trap is a standard six beam MOT configuration that uses the strong 461 nm cycling transition to collect about 108 neutral atoms to a temperature of 5 mK.;Collision studies in strontium are unhindered by the complication of hyperfine interactions (I = 0) and should clarify the processes involved in cold collision physics. Unfortunately, the cooling transition is not completely closed and the atoms eventually become shelved into the 5p 3P2 state via the 4d 1D2 state; this loss mechanism is much more dominant than that due to cold collisions. To eliminate this loss, we repump the 5p3P2,0 states with 679nm and 707nm light from diode lasers, achieving a 10-fold improvement in trap density and lifetime. The observed density is now clearly limited by cold collision losses, and these loss rates have been measured for various trap detunings. A large collision cross-section has been observed and can be attributed to excitation of the long-lived 1pi g molecular state. The two-body cold collision loss coefficient beta is measured to be 5 x 10--10 cm3/s for typical trap detunings.;The absence of ground state structure inhibits sub-Doppler cooling in the even-isotope alkaline earth atoms, and their large cooling transition linewidths restrict trap temperatures to the Doppler limit of a few mK. In Sr, however, the narrow 7 kHz intercombination transition at 689 nm can be used to apply a second-stage of Doppler cooling to the trapped atom sample, with a potential temperature reduction of more than 1000-fold. A high-resolution diode laser spectrometer was constructed at 689nm to perform narrow-line cooling as well as provide a direct readout of the residual Doppler-broadened velocity distribution of our trapped atoms. In one dimension, second-stage cooling to 1 muK has been observed for 5% of the velocity distribution. Extending this narrow-line cooling to three-dimensions and employing spectrally-broadened light to increase the capture efficiency to >30%, second-stage collection into a MOT has resulted in 10 muK temperatures and confinement times of 100 ms. To demonstrate the low temperatures of these atoms, optical Ramsey spectroscopy is performed with only two excitation pulses, obtaining a 50% fringe contrast for a 120 kHz fringe linewidth. The 500-fold temperature decrease from second-stage narrow-line cooling should benefit future optical clock development in Sr by increasing the interaction time and reducing systematic errors. Just as compelling, these studies enter a relatively unexplored regime of laser cooling physics where the recoil frequency shift (10 kHz) is greater than the cooling transition linewidth (7 kHz).