Laser cooling and slowing of CaF molecules

We demonstrate slowing and longitudinal cooling of a supersonic beam of CaF molecules using counterpropagating laser light resonant with a closed rotational and almost-closed vibrational transition. A group of molecules are decelerated by about 20 m/s by applying light of a fixed frequency for 1.8 ms. Their velocity spread is reduced, corresponding to a final temperature of about 300 mK. The velocity is further reduced by chirping the frequency of the light to keep it in resonance as the molecules slow down.

Figure 1

Laser cooling transitions. The main cooling cycle takes place on the $A(v^{\prime }=0)\leftrightarrow X(v^{\prime \prime }=0)$ transition, while a vibrational repump laser acts on the $A(v^{\prime }=0)\leftrightarrow X(v^{\prime \prime }=1)$ transition.

Figure 2

Schematic of experimental setup.

Figure 3

Spectrum showing the hyperfine structure of the $A\text{−}X\left(0\text{−}0\right)$ transition both with (solid blue line) and without (dashed red line) the cooling lasers applied. Dots are experimental data and the lines are fits to four Gaussians. The scheme for generating the frequency sidebands using acousto- and electro-optic modulators is shown.

Figure 4

Effect of laser cooling, without frequency chirp, for various durations, $\tau =0.1$, 0.5, 1.0, 1.4, and 1.8 ms. (a) Experimental ToF profiles with cooling off (red) and on (blue). Dashed purple lines show the best-fit ToF profiles predicted from the known initial velocity distribution and the simple model for the velocity-dependent force discussed in the text. (b) Velocity profiles inferred from the measured ToF profiles in panel (a) using this same simple model, with cooling off (solid red) and on (dashed purple). (c) Simulated ToF profiles and (d) velocity profiles, with cooling off (solid red) and on (dashed purple). All profiles are summed over $v^{\prime \prime }=0$ and $v^{\prime \prime }=2$ populations.

Figure 5

ToF profiles for molecules in $v^{\prime \prime }=0$ (solid purple line) and $v^{\prime \prime }=2$ (dashed orange line) after 1.8 ms of cooling time without frequency chirp. Dotted line shows the central arrival time without cooling. (a) Experiment. (b) Simulation.

Figure 6

Effect of laser cooling with various frequency chirps $\beta =0$, 10, 20, and 30 MHz/ms, when $\tau =1.8$ ms. Dashed red curves: cooling off. Solid blue curves: cooling on. (a) Experimental ToF profiles. (b) Simulated ToF profiles. All profiles are summed over $v^{\prime \prime }=0$, 1, and 2 populations.