Analyzing a single-laser repumping scheme for efficient loading of a strontium magneto-optical trap

We demonstrate enhanced loading of strontium atoms into a magneto-optical trap using a repumping scheme from the metastable state via the doubly excited state $5s5p{}^3{P}_2\to 5p^2{}^3{P}_2$ at $481\mathrm{nm}$. The number of trapped atoms is increased by an order of magnitude. The frequency and intensity dependence of the atom number enhancement, with respect to the nonrepumping case, is well reproduced by a simple rate equation model, which also describes single-laser repumping schemes reported previously. The repumping scheme is limited by a weak additional loss channel into the long-lived $5s5p{}^3{P}_0$ state. For low repumping intensities, the signature of a halo formed by magnetically trapped atoms in the metastable state is found.

Figure 1

(a) Relevant levels, transition, and loss channels of strontium for laser cooling and repumping. (b) ${}^{88}\mathrm{Sr}$ MOT fluorescence signal with and without repumping. With repumping, a burst signal appears at $t=0$ when the repumper is turned on. The steady-state MOT fluorescence signals in the yellow regions are for computing atom number enhancement. The loading curves after $1.6\mathrm{s}$ fit very well to simple exponentials (red lines).

Figure 2

Enhancement of the MOT atom number due to the repumping light. (a) Dependence of the enhancement on the repumping laser frequency for different repumping beam intensities. The red lines are Lorentzians fitted to the data. (b) Enhancement on resonance (blue circles) and the FWHM (yellow squares) of the fitted Lorentzians vs intensity of the repumping laser. The red lines are fits to the solutions of a rate equation model as explained in the text.