Efficient formation of ground-state ultracold molecules via STIRAP from the continuum at a Feshbach resonance

We develop a theoretical description of photoassociative stimulated Raman adiabatic passage (STIRAP) near a Feshbach resonance in a thermal atomic gas. We show that it is possible to use low-intensity laser pulses to directly excite pairs of atoms in the continuum near a Feshbach resonance and to transfer most of the atomic cloud to the lowest rovibrational level of the molecular ground state. For a broad resonance, commonly found in several mixtures of alkali atoms, our model predicts a transfer efficiency up to 97% for a given atom pair, and up to 70% when averaged over an atomic ensemble. The laser intensities and pulse durations needed for such an optimal transfer, 10²-10 ³ W cm^-2 and several microseconds, are easily achievable experimentally. A single pair of STIRAP pulses converts an estimated fraction ∼ 10^-6-10^-4 of atoms in an atomic ensemble, leading to the production of 10-1000 molecules in a large sample of 10⁷ atoms. A total of ∼10⁴-10⁶ pulse pairs are thus required to transfer most atoms into molecules. Such an efficiency compares with or surpasses currently available techniques for creating stable diatomic molecules, and the versatility of this approach simplifies its potential use for many molecular species.