Optimized optical cycling in MgF molecules toward laser cooling and slowing

Efficient laser cooling of molecules requires the realization of a closed optical cycling transition in the presence of vibrational, rotational, and hyperfine structure. In this work, we investigate and optimize optical cycling in magnesium monofluoride (MgF), which is a key prerequisite for laser cooling. The highly diagonal Franck–Condon factors of MgF make it possible to achieve on the order of 10⁴ photon scattering events using the main cooling laser and two vibrational repumpers.

We first optimize optical cycling on the rotationally closed P₁/Q₁₂(1) transition of the A–X(0,0) band at 359 nm. Due to unresolved hyperfine structure in both the excited and ground states of MgF, all relevant transitions can be addressed using three frequency components. These components are generated using acousto-optic modulators, allowing independent control of detunings and power ratios. By systematically scanning these parameters, we identify optimal conditions for optical cycling and observe a laser-induced fluorescence enhancement of approximately a factor of three compared to single-frequency excitation. An additional enhancement is achieved by applying a tilted magnetic field to remix dark Zeeman states, resulting in a total fluorescence increase of nearly a factor of 7.

In addition, we demonstrate transverse excitation of MgF molecules using both the main cooling laser and the first vibrational repumper(B-X(0,1) band) at 274 nm, confirming efficient population cycling through the relevant rovibrational transitions. These measurements further validate the feasibility of closed optical cycling in MgF under realistic experimental conditions.

Finally, we will present recent experimental progress toward laser slowing of MgF molecules using these optimized optical cycling schemes and discuss prospects for future laser slowing and trapping of molecular beams.