Research On The Quantum Phenomenon In Producing Ultra-cold Cesium Molecules Via Photoassoociation Method

To develop one effective method to produce mass of cold molecules, even ground state molecules with high transfer efficiency,is one of the hot points in atomic physics field.Due to its complex energy structure,it is not only interesting to quantum theory,but also to experimentally realize the molecular condensates to investigate the quantum phenomenon in producing ultra-cold molecules.Based on the analysis of experimental realization of the cold Cs₂ molecules through photo-association(PA) and experimental research on the character of the spectrum for Cs₂ molecules,one effective four-level model is proposed to describe this process,wherein the two up-level are consisted of two excited molecular states located in the O_g^-(6s+6p_3/2) double wells and coupled by the tunneling mechanism.Starting from this simple model,we foucs on the relative quantum phenomenon in this PA process and the main results are following:Firstly,in the frame of the standard mean-field approximation and after introducing the two stimulated Raman laser pulses,the optimum conditions for producing Cs₂ molecular condensates from ultra-cold Cs atoms are obtained by the numerical and analytical methods.Our results show that a stable STIRAP method with high conversion efficiency could be realized under the extended "two-photon" resonance condition,including two-photon process,mean-field correction and the tunnelling coupling between two upper excited molecular levels.We proposed two methods to obtain the high conversion efficiency under current experimentally less demanding conditions(relative small effective Rabi frequency for pump laser pulse). One is adjusting the detuning difference between two laser pulses,and simultaneously optimizing the delay coefficientαand pulse duration T for same effective Rabi frequencies with up to 87.2%transfer efficiency. Another one is adopting the unequal effective Rabi frequency to obtain the high conversion efficiency.For given effective Rabi frequency and the detuning of pump laser,adjusting the effective Rabi frequency and the detuning of dump laser with up to 80.7%transfer efficiency.Secondly,we investigated the properties of the probe absorption spectrum under the weak probe field condition by solving the master equations for density opreators.The double dark resonance is predicted in the absorption spectrum when the tunnelling coupling strength is large enough. The double dark resonance not only reveals the formation of the ultra-cold molecules,but also provides further evidence for the tunnelling as one effective coupling mechanism between the two excited molecular states.On the other hand,this novel effect can also be checked by measuring the population of the atom.The effect of the various experimental conditions on this phenomenon has been discussed.Then,we theoretically analyzed the linewidth of the probe absorption spectrum in a cold Cs atom-molecule system.We found that the tunnelling coupling between the two excited molecular states could be a new controllable parameter and plays an important role to obtain the sub-natural linewidth of the probe absorption spectrum.For example,when the tunneling couple strength fulfilsσ₁₂=10γ_ab1,the linewidth is only about 0.66MHz. Moreover,since the linewidth of interest is dominated by the tunnelling coupling,the sub-natural linewidth spectrum is still obtained even in the case of the large laser intensity and the detuning of the pump field.Finally,we analyzed the dynamical and Landau instabilities for CPT state in Cs atom-molecule condenstates.We expect to obtain the range of unstatble parameter.This work is in preparing.These works will be helpful to extend the quantum theories to the complex system,and also be interesting in the field of nonlinear optics and measurement of precision spectroscopy etc.