Limitation of single-repumping schemes for laser cooling of Sr atoms

Abstract: We explore the efficacy of two single-repumping schemes, $5s5p \,{}^3P_2 - 5p^2 \,{}^3P_2$ ($481\,\mathrm{nm}$) and $5s5p \,{}^3P_2 - 5s5d \,{}^3D_2$ ($497\,\mathrm{nm}$), for a magneto-optical trap (MOT) of Sr atoms. We reveal that the enhancement in the MOT lifetime is limited to $26.9(2)$ for any single-repumping scheme. Our investigation indicates that the primary decay path from the $5s5p \,{}^1P_1$ state to the $5s5p \,{}^3P_0$ state proceeds via the $5s4d \,{}^3D_1$ state, rather than through the upper states accessed by the single-repumping lasers. We estimate that the branching ratio for the $5s5p \,{}^1P_1 \to 5s4d \,{}^3D_1 \to 5s5p \,{}^3P_0$ decay path is $1:3.9 \times 10^6$ and the decay rate for the transition from the $5s5p \,{}^1P_1$ state to the $5s4d \,{}^3D_1$ state is $83(32)\,\mathrm{s^{-1}}$. This outcome underscores the limitation on atom number in the MOT for long loading times ($\gtrsim 1 \,\mathrm{s}$) when employing a single-repumping scheme. These findings will contribute to the construction of field-deployable optical lattice clocks.