Theoretical And Experimental Study Of The Destabilization Of Dark States In Magnesium Fluoride (MgF) And Optical Trapping Of MgF Molecules

There are great progress in cold molecules over the past years and they have attracted great attention.Cold molecules may have applications in precision measurement,cold chemistry,cold collision,quantum information and quantum computing,owing to their rich internal degrees of freedom,long range and tunable interaction and other chemical characteristics.There are many approaches to produce cold molecules,including Stark deceleration,Zeeman deceleration,laser cooling and photo-association and so on.Laser cooling is one of the direct cooling approaches.It is the most important to destabilizing dark states in molecular laser cooling.This thesis focuses on the theoretical and experimental study of the destabilization of dark states and optical trapping the MgF molecules.Then we propose an effective scheme to decelerate molecules by using a strong standing wave,and the velocity reaches the trapping range of the magneto-optical trap.Finally,we study the crossed hollow beams with blue detuning to trap cold MgF molecules.The volume of the trap can be controlled by changing the quantum number of the vortex phase plate.And the intensity gradient cooling in the blue detuning hollow optical trap can be used to further cool and trap molecules.Firstly,a detailed theoretical and experimental study of destabilizing the dark states by the magnetic field is reported in MgF molecules.By applying an external magnetic field,the Zeeman sublevels are made to be time dependent and nondegenerate,forcing the dark states to Larmor precess into the bright states.In theory,the density matrix equations for the multiple-level system are proposed.It is found that the laser induced fluorescence signal depends on the angle and the magnitude of the magnetic field.When theta is equal to?/3 and mean Larmor frequency is between 2?×14 MHz and 2?×28.3 MHz,the fluorescence reaches the maximum,when the LIF is enhanced by a factor of 2.1.The experimental results are completely consistent with the theoretical results.Then,we studied the stimulated radiation slowing of the molecules by using a red-dutuned strong standing wave.The light field is composed of two propagating beams in opposite directions,with the same frequency,polarization and intensity.The magnetic field is applied to the interacting region to destabilize the dark states,thus the deceleration efficient will be higher.Monte-Carlo kinetic simulated results show that using the field with the saturation parameter 400 and the detuning-10?,the velocity of120 m/s is reduced to 13.6 m/s,and the deceleration efficiency 2.8%.Compared with the Doppler cooling,the stimulated radiation slowing not only does not require a complex laser system,but also does not need to compensate for the Doppler shift.Finally,two tunable optical traps in the experiment are proposed,including the localized hollow trap and the crossed-focused vortex trap,and the intensity distributions are studied in free propagation space.The results show that the size of the localized hollow trap is an ellipsoidal trap.The radial spot size of this trap is proportional to f/D and the minimum is 39?m,while the axial one is proportional to(f/D)~2.The crossed-focused vortex trap is a spherical trap.The spot size of the crossed trap is not only proportional to the focal length and the angular momentum quantum number(l)of the vortex phase plate,but also inversely proportional to the diameter of the incident beam,and the minimum is 16.3?m.When l decreases from 16 to 1,the corresponding trapping volume of the CFVB is compressed by a factor of up to 10~4 times and thus the density and phase-space density of the trapped molecules will be increased.