Vibrational cooling of cesium molecules using noncoherent broadband light

We demonstrate selective vibrational population transfer in cold cesium dimers using a simple approach based on the use of a shaped incoherent broadband diode laser near threshold. Optical pumping into a single vibrational level is accomplished with an incoherent light source by eliminating transitions from the targeted vibrational level. The broadband spectrum of the laser is wide enough to electronically excite several vibrational states of the molecule simultaneously. This method is relatively inexpensive, simple, and flexible to allow for development of new applications, in particular, the preparation of optically closed molecular system, opening the way to direct laser cooling of molecules.

Figure 1

(Color online) (a) Cs dimer potential curves relevant to the experiment. The PA connects the $6s+6s$ limit of the $X{{}^1\Sigma }_g^{+}$ state to the $6s+6p$ limit. Part of the created molecules decays spontaneously to the $X{{}^1\Sigma }_g^{+}$ state. The ionization takes place through the $C{{}^1\Pi }_u$ intermediate state. The BDL performs optical pumping between the $X{{}^1\Sigma }_g^{+}$ and the $B{{}^1\Pi }_u$ states. (b) Franck-Condon factors plotted in grayscale. The hatched squares indicate transitions that are excited from the optical pumping radiation shown in the upper part, the arrows describe the possible evolution of population excited from the $v_X=6$ level.

Figure 2

(Color online) (a) Schematic of the BDL shaping realized with three interference filters (IFs) in order to transfer the population to the $v_X=0$ level. The filters are placed at an angle of $\sim 30°$ in order to finely adjust the cut-off frequency to the desired value, resulting in a less sharp “cut” than for $0°$. (b) Spectrum of the diode laser before (upper line) and after passing through the interference filters (lower line). All the BDL frequencies are suppressed above the $v_X=0\to v_B=0$ transition frequency. (c) REMPI spectra of the $X{{}^1\Sigma }_g^{+}\to C{{}^1\Pi }_u$ transitions [13, 17]. The spectrum (black lower trace) without the BDL radiation contains labels of some detected vibrational levels $v_X$. The spectrum (red upper trace, offset of 20 ions for clarity) with the BDL radiation shows lines corresponding to the $v_X=0\to v_C=0,1$ transitions.

Figure 3

(Color online) (a) The BDL shaping realized with the folded 4-f line setup, consisted of three mirrors $\left(m\right)$ a grating $\left(g\right)$ a CL and a mask. (b) Spectrum of the diode source with a bias current of 660 mA before (upper line) and after passing the $4f$ line (lower line). At this bias current the diode is just above the lasing threshold, as the dominant frequency at $\sim 12930{\text{cm}}^{-1}$ becomes more apparent. The frequencies of the $v_X=1\to v_B=0,1$ transitions are removed. (c) The REMPI spectrum of the $X{{}^1\Sigma }_g^{+}$ to $C{{}^1\Pi }_u$ transitions with (upper trace, offset of 10 ions for clarity) and (lower trace) without optical pumping with label to $v_X$ levels and to $v_X\to v_C$ transitions.