Rovibrational Cooling of Molecules by Optical Pumping

We demonstrate rotational and vibrational cooling of cesium dimers by optical pumping techniques. We use two laser sources exciting all the populated rovibrational states, except a target state that thus behaves like a dark state where molecules pile up thanks to absorption-spontaneous emission cycles. We are able to accumulate photoassociated cold ${\mathrm{Cs}}_2$ molecules in their absolute ground state ($v=0$, $J=0$) with up to 40% efficiency. Given its simplicity, the method could be extended to other molecules and molecular beams. It also opens up general perspectives in laser cooling the external degrees of freedom of molecules.

Figure 1

Transitions used to cool and detect the vibration and rotation of ${\mathrm{Cs}}_2$ molecules. The (red) filled and (orange) hatched rectangles cover the vibrational levels affected by the broadband laser and the rotational levels excited by the two swept narrow-band lasers used for rotational cooling and detection. The upper and lower numbers on the right-hand side are $J^{\prime }$ and $J$ values, respectively. The (blue) straight arrow represents the RE2PI laser ionizing molecules through the $C$ or $D$ states (not shown). The wavy arrows indicate the spontaneous emission processes.

Figure 2

Three vibrational spectra obtained by photoionization (RE2PI). (a) When only PA $0_g^{-}(6s+6p_{1/2})(v^{\prime }=26,J^{\prime }=2)$ is applied, the spectrum reveals that molecules are mainly stabilized in the $X$ state [12]. (b) The application of the vibrational cooling provokes the emergence of a few intense lines, while the intensity of the other lines is reduced. This proves the accumulation of molecules in $v=0$. (c) Rotational and vibrational cooling to ($v=0$, $J=1$) leads to a spectrum similar to (b), but some vibrational levels $v\ne 0$ are more populated than in (b). The number pairs above indicate the wave number of some $v_C-v$ transitions. The wave number of the 9-7 transition, indicated by an arrow, is used for rotational spectroscopy.

Figure 3

Upper panel: Fortrat diagram showing the transition wave numbers from the ($v=0$, $0\le J\le 9$) rotational levels of the $X$ state. The $P$, $Q$, and $R$ branches, respectively, correspond to the $J_B-J=-1$, 0, and 1 allowed transitions. The parity of the $J$ state is also indicated on the right-hand side. The spectrum covered by the rotational cooling laser is shown by the hatched area. Lower panel: Comparison of rotational spectra obtained with vibrational pumping (upper black dotted line) and rovibrational pumping (lower red solid line). The narrow-band laser used for rotational cooling is swept over the $P(6)$, $P(4)$, and $P(2)$ transitions with repetitive $100\mu \mathrm{s}$ frequency ramps. The rotational cooling efficiency is $\sim 40\%$.

Figure 4

Rotational spectra demonstrating the rovibrational pumping toward $J=1$ (a) and $J=4$ (b) when the narrow-band laser is swept in the frequency range indicated by the hatched areas. For comparison, the spectra resulting from vibrational pumping (dotted line) are displayed above the spectra resulting from rovibrational pumping (solid line).

Figure 5

Increase of the number of molecules in $J=1$ with its typical error bar versus the period of the frequency ramp applied on the narrow-band laser used for rotational pumping. Because molecules escape from the interaction zone of the vibrational pumping laser, the transfer of molecules in $J=1$ decreases for a period longer than $100\mu \mathrm{s}$.