Optical pumping of 5s4d^{1}D_{2} strontium atoms for laser cooling and imaging

Abstract

We present a faster repumping scheme for strontium magneto-optical traps operating on the broad 5s^{2}^{1}S_{0}−5s5p^{1}P_{1} laser cooling transition. Contrary to existing repumping schemes, we directly address lost atoms that spontaneously decayed to the 5s4d^{1}D_{2} state, sending them back into the laser cooling cycle by optical pumping on the 5s4d^{1}D_{2}−5s8p^{1}P_{1} transition. We thus avoid the ∼100µs-slow decay path from 5s4d^{1}D_{2} to the 5s5p^{3}P_{1,2} states that is part of other repumping schemes. Using one low-cost external-cavity diode laser emitting at 448nm, we show our scheme increases the flux out of a 2D magneto-optical trap by 60% compared to without repumping. Furthermore, we perform spectroscopy on the 5s4d^{1}D_{2}−5s8p^{1}P_{1} transition and measure its frequency ν_{^{88}Sr}=(668917515.3±4.0±25)MHz. We also measure the frequency shifts between the four stable isotopes of strontium and infer the specific mass and field shift factors, δν_{SMS}^{88,86}=−267(45)MHz and δν_{FS}^{88,86}=2(42)MHz. Finally, we measure the hyperfine splitting of the 5s8p^{1}P_{1} state in fermionic strontium and deduce the magnetic dipole and electric quadrupole coupling coefficients A=−4(5)MHz and B=5(35)MHz. Our experimental demonstration shows that this simple and very fast scheme could improve the laser cooling and imaging performance of cold strontium atom devices, such as quantum computers based on strontium atoms in arrays of optical tweezers.