A new route for laser cooling and trapping of cold molecules: Intensity-gradient cooling of MgF molecules using localized hollow beams

Abstract

We propose a promising scheme to prepare ultracold MgF molecules from a slowed cold molecular beam by using three-dimensional pure intensity-gradient induced Sisyphus cooling. The cooling is based on a blue-detuned localized hollow beam and a weak repumping beam. We investigate the dynamic process in the trap with the method of the Monte-Carlo simulations, verifying the feasibility. Our results show that achieving sub-Doppler temperature should be possible in such a well-designed trap; the trapped MgF molecules with initial temperature of 1 mK can be directly cooled to the final equilibrium temperature of about m 46.5 K, well below the Doppler limit (about m 527.9 K for MgF); meanwhile, molecules with initial temperature of 14 mK can be even cooled to about m 68.7 K. + -) cooling [22,23], is totally different in type-II systems, because of the existence of the dark states. There are two main cooling mechanisms in such transitions, the gray molasses and the magnetically-assisted Sisyphus cooling, both of which are able to cool particles to below the Doppler limit [24,25]. On this basis, Truppe et al realized for the first time the cooling of molecules, well below the Doppler limit, using a three-dimensional blue-detuned molasses [26]. temperature was achieved by Λ-enhanced gray molasses and deep cooling with a single laser, both of which can cool CaF to several mK [27,28]. Besides, laser cooling and deceleration of molecules with stimulated force has also progressed a lot both theoretically and experimentally [29][30][31].As the common cooling method before molecules loaded into a type-II MOT, laser sweep slowing or whitelight slowing, both of which are used to compensate Doppler shift, are not a very efficient way for larger number of molecules. This causes the problem of the small number and the low density of trapped molecules in the type-II MOT. In order to produce a diverse set of dense, ultracold diatomic molecular species, it would be interesting and worthwhile to explore how to produce much larger numbers of trapped molecules. Here we propose a new scheme for the preparation of ultracold molecules, which consists of the Stark-decelerated molecules and a localized hollow optical trap, where sub-Doppler cooling works well. We bypass the Doppler cooling, and achieve the process from supersonic molecular beam to sub-Doppler cooling of molecules. The localized hollow optical trap has a strong cooling capacity, and it can derive ultracold trapped molecules with higher density.The cooling mechanism of a hollow optical trap is pure intensity-gradient cooling [32][33][34][35][36][37], another sub-Doppler cooling, which can also efficiently cool the molecules to the temperature of several photon recoils. The magnetic-field-assisted intensity-gradient cooling of molecules has first been observed in a one-dimensional standing wave, which have substantially reduced the transverse temperature of a SrF molecular beam [4]. Magnesium monofluoride (MgF) [38,39], due to its highly diagonal Franck-Condon factors (FCFs) and st...

Cold and ultracold molecules in the twenties

“…Very recently, the magneto-optical trapping of a triatomic molecule CaOH has been demonstrated [44]—see below. Other molecules under consideration, in some cases with in-principle demonstrations, for laser cooling (or Zeeman Sisyphus cooling—see below) include BaH [75], MgF [76], BaF [77], AlF [78], YbF [79], CH [80], SrOH [81], YbOH [82], CaOCH 3 [83] and SrCl [84].…”

Acousto-optic k-space filtering for multifrequency laser beam shaping

“…Among the emerging applications for annular optical traps in biomedical engineering there are cell sorting [39] and vessel bioprinting [40]. Besides that, hollow beams are used in dark magnetooptic traps for cold atoms [41,42]. Tunability of the AOTF makes possible dynamic reconfiguration of the trap in the applications mentioned above, which is a certain advantage over static optical traps since the parameters can be precisely varied during the experiment.…”

“…Such a distribution follows the concept of two-color dipole optical trapping [3,4]: a red-detuned attracting potential in the core is combined with a blue-detuned repulsing potential at the outer ring. Unlike existing large-scale dipole traps based on axicons or diffractive optics [41,42,[45][46][47], AOTF-based traps allow fast dynamical reconfiguring of the trap potentials and different wavelengths can be processed independently if the frequency ranges corresponding to different wavelengths do not overlap.…”

A crossed focused vortex beam with application to cold molecules*

“…When the MgF molecule, upon electronic excitation of X 2 Σ + to A 2 Π states, moves in an inhomogeneous light field, it experiences an optical dipole force, which can be described by the AC Stark shifts of the hyperfine energy levels in the 2 Σ state for the Hunds case (b) to them in the A 2 Π state for the Hunds case (a). [11,31] The effect of the multilevel structure of molecules on their response to an electromagnetic field may be conveniently described with the help of the electric dipole polarizability.…”

“…We can also prepare the ultracold MgF molecules from a slowed molecular beam by using the intensity-gradient induced Sisyphus cooling. [31,33] The cooling is based on a bluedetuned CFVB and a weak repumping laser beam. Because of the strong intensity-gradient force and the low-rate spontaneous emission, the molecules in the CFVB trap can be cooled to the lower temperature of several photon recoils.…”