Photoionization cross sections of ultracold $^{88}$Sr in $^1$P$_1$ and $^3$S$_1$ states at 390 nm and the resulting blue-detuned magic wavelength optical lattice clock constraints

Abstract: We present the measurements of the photoionisation cross sections of the excited $^1$P$_1$ and $^3$S$_1$ states of ultracold $^{88}$Sr atoms at 389.889 nm wavelength, which is the magic wavelength of the $^{1}$S${0}$-${}^{3}$P${}{0}$ clock transition. The photoionisation cross section of the $^1$P$_1$ state is determined from the measured ionisation rates of $^{88}$Sr in the magneto-optical trap in the $^1$P$_1$ state to be 2.20(50)$\times$10$^{-20}$ m$^2$, while the photoionisation cross section of $^{88}$Sr in the $^3$S$_1$ state is inferred from the photoionisation-induced reduction in the number of atoms transferred through the $^3\text{S}_1$ state in an operating optical lattice clock to be $1.38(66)\times$10$^{-18}$ m$^2$. Furthermore, the resulting limitations of employing a blue-detuned magic wavelength optical lattice in strontium optical lattice clocks are evaluated. We estimated photoionisation induced loss rates of atoms at 389.889 nm wavelength under typical experimental conditions and made several suggestions on how to mitigate these losses. In particular, the large photoionisation induced losses for the $^3$S$_1$ state would make the use of the $^3$S$_1$ state in the optical cycle in a blue-detuned optical lattice unfeasible and would instead require the less commonly used $^3$D$_{1,2}$ states during the detection part of the optical clock cycle.