Molecular Laser Cooling in a Dynamically Tunable Repulsive Optical Trap

Recent work with laser-cooled molecules in attractive optical traps has shown that the differential ac Stark shifts arising from the trap light itself can become problematic, limiting collisional shielding efficiencies, rotational coherence times, and laser-cooling temperatures. In this Letter, we explore trapping and laser cooling of CaF molecules in a ring-shaped repulsive optical trap. The observed dependences of loss rates on temperature and barrier height show characteristic behavior of repulsive traps and indicate strongly suppressed average ac Stark shifts. Within the trap, we find that $\mathrm{\Lambda }$-enhanced gray molasses cooling is effective, producing similar minimum temperatures as those obtained in free space. By combining in-trap laser cooling with dynamical reshaping of the trap, we also present a method that allows highly efficient and rapid transfer from molecular magneto-optical traps into conventional attractive optical traps, which has been an outstanding challenge for experiments to date. Notably, our method could allow nearly lossless transfer over millisecond timescales.

Figure 1

(a) Experimental setup. A ring-shaped repulsive trap (green) runs concentrically with an attractive optical dipole trap (red) formed from a focused Gaussian beam. Molecules are detected by imaging along the same axis. (b) Intensity distribution of the repulsive trap [$r_0=160(10)\mu \mathrm{m}$]. (c) Model potential for the ring trap. (d) Energy level diagram of CaF with relevant transitions.

Figure 2

(a) Normalized molecular number $N$ versus hold time $t$. An exponential fit (solid) yields a $1/e$ time of 46(4) ms. (b) Temperature $T$ versus hold time $t$. (c) Loss rate $\gamma$ versus temperature $T$. (d) Loss rate versus barrier height $U_0$. For (b),(c),(d), solid lines show linear fits.

Figure 3

(a) Temperature versus cooling light intensity $I/I_0$ at various $\mathrm{\Lambda }$-cooling light detunings $\mathrm{\Delta }$, in free space and in trap. Blue circles, green diamonds, and red squares show free space temperatures; Blue triangles, green pentagons, and red hexagons show in-trap temperatures. (b) Ratio of the in-trap to free-space temperature $\kappa$ versus cooling light intensity $I/I_0$. For both plots, $I$ is the single-beam single-axis intensity, $I_0=5.0(5){\mathrm{mW}/\mathrm{cm}}^2$; $\mathrm{\Delta }=6\mathrm{MHz}$ (blue circles, triangles), 13 MHz (green diamonds, pentagons), and 38 MHz (red squares, hexagons).

Figure 4

(a) Experimental sequence for enhanced transfer into an attractive ODT: (i) free-space cooling (10 ms), (ii) compression with cooling (21 ms), (iii) release of untrapped molecules (30 ms), and (iv) transfer into the attractive ODT with cooling. Shown are the ring radius $r_0$, barrier height $U_0$, cooling light intensity $I$, and attractive ODT depth $V$ versus time. (b),(c),(d),(e) In situ images during compression, taken 0, 6, 15, 21 ms, into the ramp, respectively. (f) Transfer fraction $f$ versus loading time $t$. Solid lines are fits to an exponential saturation curve. (g) Loading rate enhancement $\eta$ versus compression ratio $\alpha =r_i^2/r_f^2$. Solid line shows the expected geometric enhancement $\eta =\alpha$. Dashed line shows a linear fit to data with $\eta =\alpha =1$ fixed. The shaded region indicates $\pm 1\sigma$ bands for the fit.