Engineering an all-optical route to ultracold molecules in their vibronic ground state

We propose an improved photoassociation scheme to produce ultracold molecules in their vibronic ground state for the generic case in which nonadiabatic effects facilitating transfer to deeply bound levels are absent. Formation of molecules is achieved by short laser pulses in a Raman-like pump-dump process where an additional near-infrared laser field couples the excited state to an auxiliary state. The coupling due to the additional field effectively changes the shape of the excited-state potential and allows for efficient population transfer to low-lying vibrational levels of the electronic ground state. Repetition of many pump-dump sequences together with collisional relaxation allows for accumulation of molecules in $v=0$.

Figure 1

(Color online) Pump-dump photoassociation scheme with a near-ir laser field providing resonant coupling between two excited states: potentials (a), timing of the three fields (b), and collisional relaxation to $v=0$ (c).

Figure 2

(Color online) Minimal model for ${\mathrm{Ca}}_2$ including all relevant physics: potential energy curves (a), transition dipole moments (b), and spin-orbit couplings (c) with labels corresponding as 1, $X{}^1\Sigma _g^{+}$; 2, $B{}^1\Sigma _u^{+}$; 3, $\left(1\right){}^1\Pi _g$; 4, $\left(1\right){}^3\Sigma _g^{+}$; and 5, $\left(1\right){}^3\Pi _g$. Spacings of the vibrational levels of the $B{}^1\Sigma _u^{+}$ excited state (d) and of the $B{}^1\Sigma _u^{+}$ and $\left(1\right){}^1\Pi _g$ states (e) dressed by a moderate near-ir laser field ($I=3.2\times {10}^9\mathrm{W}∕{\mathrm{cm}}^2$, ${\omega }_L=881\mathrm{nm}$). The red box in (d) indicates the range relevant for PA.

Figure 3

(Color online) Dynamics of the pump-dump photoassociation process under the influence of a moderately strong near-IR laser field ($I=3.2\times {10}^9\mathrm{W}∕{\mathrm{cm}}^2$, ${\lambda }_c=881\mathrm{nm}$): The squared amplitude of the wave function in the three states is shown just after the pump pulse $(t=t_{\max }^P+24\mathrm{ps})$, at the maximum of the dump pulse ($t=t_{\max }^D=t_{\max }^P+t_{\text{delay}}$ with $t_{\text{delay}}=83\mathrm{ps}$), and at the end of the whole process $(t=t_{\max }^D+24\mathrm{ps})$. The dashed line with light-gray filling in the lowest left panel corresponds to the initial scattering wave function. In the lowest middle and right panel, only population that was transferred by the dump pulse to the electronic ground state is plotted. (b) Transfer probability from the excited states to $v=1$ of the electronic ground state for different intensities of the near-ir coupling laser.