Radio Frequency Magneto-Optical Trapping of CaF with High Density

We demonstrate significantly improved magneto-optical trapping of molecules using a very slow cryogenic beam source and either rf modulated or dc magnetic fields. The rf magneto-optical trap (MOT) confines $1.0(3)\times {10}^5$ CaF molecules at a density of $7(3)\times {10}^6{\mathrm{cm}}^{-3}$, which is an order of magnitude greater than previous molecular MOTs. Near Doppler-limited temperatures of $340(20)\mu \mathrm{K}$ are attained. The achieved density enables future work to directly load optical tweezers and create optical arrays for quantum simulation.

Figure 1

(a) Level diagram of CaF. Solid lines indicate laser excitation frequencies. The 531 nm $X\to B$ transition is used for slowing, while the 606 nm $X\to A$ transition is used for the MOT. Wavy lines indicate potential decay paths from the main photon cycling transitions to higher vibrational levels, with their corresponding Franck-Condon factors indicated. (b) Polarization schemes used to drive the $X\to A$ MOT transitions. The hyperfine structure of the ground state is addressed with 3 AOMs with the proper frequency shifts. The hyperfine structure of the $A$ state is unresolved. (c) The experimental layout.

Figure 2

Radio-frequency and dc MOT oscillations following a 1 ms push with the slowing laser, fit to the form $\sim e^{-\beta t}\sin (\omega t+\varphi )$. The equilibrium positions of the rf and dc MOTs are different since the dc remixing field in the slowing region shifts the center position of the dc MOT.

Figure 3

Time-of-flight expansion of the molecular cloud to measure temperature, shown here for the rf MOT after ramping down to $I_0/32$. Images are captured on an EMCCD with 1 ms exposure and 15 averages. Image field of view is $10\times 10\mathrm{mm}$. The dashed lines are fits to the TOF model (see text). We find nearly equal temperatures for the two dimensions, with the difference in the initial cloud size expected from the gradient produced in anti-Helmholtz coils.

Figure 4

(a) MOT temperature vs MOT beam intensity following 15 ms ramp and 10 ms holding. The temperature is defined as $T_{\text{axial}}^{1/3}\times T_{\text{radial}}^{2/3}$. (b) MOT size (Gaussian rms width) following the ramp and hold.