Proposal for the formation of ultracold deeply bound RbSr dipolar molecules by all-optical methods
Résumé
We propose a new type of Feshbach resonance occurring when two different ultracold atoms in their ground state undergo an s-wave collision in the presence of a continuous-wave laser light. The atoms collide in the dissociation continuum of the molecular electronic ground state which is coupled by the light to a rovibrational level of the same electronic ground state: we name this a Laser-Assisted Self-Induced Feshbach Resonance (LASIFR). This mechanism, valid for all polar molecules, is analyzed on the example of ultracold 87 Rb and 84 Sr atoms, for which the laser frequency falls in the THz range. The control of the LASIFR with the laser frequency and intensity allows for a strong increase of the pair probability density at short distances, which tremendously increases the number of atoms pairs transfered toward the absolute ground state level by STImulated Rapid Adiabatic Passage (STIRAP). The LASIFR results in the observation of a standard Fano profile in the pump transition of the STIRAP process, and is also promising for the optical control of the interspecies scattering length without atom losses. Initially introduced in the context of nuclear reactions [1, 2], and of atomic autoionizing states [3], a Fano-Feshbach resonance (FR) refers to the interaction of a quasibound state of a so-called "closed channel" embedded in the continuum of a so-called "open channel". Anticipated in the early days of evaporative cooling of atomic hydrogen [4], the concept of Magnetic Feshbach Resonance (MFR) has been predicted as a very powerful tool to control the scattering properties of an ultracold atomic gas by tuning the strength of a magnetic field [5]. MFR revolutionized this research field by enabling the long-sought observation of ultracold quantum degenerate gases [6, 7] and the efficient formation of ultracold molecules [8, 9]. The process of magnetoassociation relying on a MFR is at the heart of the successful formation of ultracold heteronuclear alkali-metal diatomic molecules in their absolute ground state, as demonstrated in several spectacular experiments [10-15]. Ultracold polar paramagnetic molecules composed by an alkali-metal atom and an alkaline-earth atom, are promising for instance for quantum simulation of lattice-spin systems [16]. However they have not been created yet in their absolute ground state. It has long been thought that if one of the atoms has no magnetic moment, MFR could not be implemented. A subtle coupling mechanism has been recently invoked to predict MFRs between paramag-netic Rb(2 S) and non-magnetic Sr(1 S) atoms [17, 18]. The observation of the very narrow resulting MFR remains however challenging [18]. Another kind of FR that could be implemented for any atomic pair is the Optical Feshbach Resonance (OFR) [19-21]: the dissociation continuum of the molecular electronic ground state associated to the colliding atom pair is coupled to a bound level of an excited electronic R Energy R Energy R Energy R Energy R 0 Energy hν (a) hν e〉 g〉 g〉 g,0,0〉 e,1,-1〉 g,0,0〉 g,1,1〉 (b) (c) (d) FIG. 1. (Color online) Scheme for an OFR ((a) and (b)), and for the present LASIFR ((c) and (d)). Panels (a) and (c) (resp. (b) and (d)) hold for the undressed (resp. dressed with nγ photons) picture of the potential energy of the ground (|g) and excited (|e) electronic states including the centrifugal barrier 2 J(J + 1)/(2µR 2). The dressed ro-electronic states (or channels) are labeled as |g/e, J, nγ. The inset of the panel (c) illustrates the long-range behaviour dominated by the isotropic harmonic trapping potential of the particles (see text). state by a laser with a frequency detuned to the red of an atomic transition (Fig.1a, b). An OFR allows for the control of the scattering length with both the laser intensity and frequency. However, the limited radiative lifetime of the excited state induces the formation of vibrationally hot molecules and losses of atoms restraining the efficiency of the scattering length control. In this Letter, we propose a new type of FR: the Laser-Assisted Self-Induced Feshbach Resonance (LASIFR), where the closed and open coupled channels are both associated with the ground electronic state |g of a heteronuclear diatomic molecule