Continuous Loading of Ultracold Ground-State ${}^{85}{\mathrm{Rb}}_2$ Molecules in a Dipole Trap Using a Single Light Beam

We have developed an approach to continuously load ultracold ${}^{85}{\mathrm{Rb}}_2$ vibrational ground-state molecules into a crossed optical dipole trap from a magneto-optical trap. The technique relies on a single high-power light beam with a broad spectrum superimposed onto a narrow peak at an energy of about $9400{\mathrm{cm}}^{-1}$. This single laser source performs all the required steps: the short-range photoassociation creating ground-state molecules after radiative emission, the cooling of the molecular vibrational population down to the lowest vibrational level $v_X=0$, and the optical trapping of these molecules. Furthermore, we probe by depletion spectroscopy and determine that 75% of the $v_X=0$ ground-state molecules are in the three lowest rotational levels $J_X=0$, 1, 2. The lifetime of the ultracold molecules in the optical dipole trap is limited to about 70 ms by off-resonant light scattering. The proposed technique opens perspectives for the formation of new molecular species in the ultracold domain, which are not yet accessible by well-established approaches.

Figure 1

Computed ${\mathrm{Rb}}_2$ Hund’s case of potential energy curves [43] relevant for the present Letter. Processes induced by the light source are represented by arrows. (1) Short-range PA from the continuum of the $X^1{\mathrm{\Sigma }}_g^{+}$ ground state toward the rovibrational level $v^{\prime }=138$, $J=1$ of the $0_u^{+}(A^1{\mathrm{\Sigma }}_u^{+},b^3{\mathrm{\Pi }}_u)$-coupled states, achieved by the peak part of the laser spectrum at $9379.5{\mathrm{cm}}^{-1}$. (2) Spontaneous emission back to $X^1{\mathrm{\Sigma }}_g^{+}$ levels, thus creating ultracold molecules in a distribution of levels represented by the purple vertical bar. (3) Optical pumping of the vibrational distribution of the ultracold molecules induced by the broadband part of the light source. (4), (5) RE2PI detection of ${\mathrm{Rb}}_2^{+}$ ions. (6) Rotational-state depletion spectroscopy of $v_X=0,J_X$ molecules.

Figure 2

Laser and SLD combined light spectrum before and after amplification. Note the break in the vertical scale. The vertical lines represent transition energies from the $v_X=0$, 1, 2 levels of $X^1{\mathrm{\Sigma }}_g^{+}$ to $v^{\prime }=0$ of the $0_u^{+}$-coupled states. The 0-0 transition is indeed excluded from the amplified light spectrum.

Figure 3

RE2PI spectra recorded when the PA laser frequency is locked at $9379.5{\mathrm{cm}}^{-1}$. The spectra are associated with excitation of $X^1{\mathrm{\Sigma }}_g^{+}$ levels towards coupled $(2{)}^1{\mathrm{\Sigma }}_u^{+}-(2{)}^3{\mathrm{\Pi }}_u$ levels, which are then ionized. (a) Without and (b) with optical pumping induced by the broadband part of the light spectrum. (b) The peaks are numbered from 1 to 12, and their correspondence with those in (a) is highlighted with dashed lines.

Figure 4

Depletion spectrum of peak 10 (at $20966.9{\mathrm{cm}}^{-1}$) in Fig. 3, when the depletion laser is tuned at the energy of the transition $(v_X=0,J_X\to v_B=0,J_B)$. The origin of the spectrum is set at the energy of the $(v_X=0,J_X=0)\to (v_B=0,J_B=0)Q(0)$ transition at $14660.35{\mathrm{cm}}^{-1}$. The vertical bars show the expected position of the lines, using the spectroscopic data of Ref. [51]. They are labeled with conventional spectroscopic notation, i.e., $Q(J_X)$ for $(v_X=0,J_X)\to (v_B=0,J_X)$ and $R(J_X)$ for $(v_X=0,J_X)\to (v_B=0,J_X+1)$.

Figure 5

Evolution of the vibrational ground-state molecule population as a function of time during the loading (below 1 s, MOT on) stage, and when the MOT is switched off (above 1 s), thus reflecting the loss from the ODT. The red lines are exponential fits.