Theoretical And Experimental Researches On Buffer Gas Cooling Of MgF And Molecular Laser Cooling

The rapid development and remarkable achievements of cold atom research in recent decades have led people to turn their attention to molecules with broader application prospects.The molecules have more complex internal structures,permanent electric dipole moments,and other chemical properties,and have been an ideal research platform and tool in the field of precision measurement and test of fundamental physics,quantum computation and quantum simulation,cold chemistry and cold collision.The research on cold molecules has important scientific value and far-reaching significance.However,due to the complexity of the molecular structure,the traditional methods of atomic laser cooling and trapping are difficult to directly transfer from atoms to molecules.The preparation of cold molecules is a project with both opportunities and challenges.At present,people have mainly developed two ways to prepare cold molecules or ultracold molecules,including direct and indirect methods.Direct laser cooling of molecules is one of the important means to obtain cold molecules.This thesis focuses on theoretical and experimental researches on buffer gas cooling and molecular laser cooling.Based on the selected molecule of our group,that is,magnesium monofluoride?MgF?,we perform the pioneering and in-depth researches on the selection of laser-coolable candidate molecules,molecular energy level structures,cryogenic buffer gas molecular beam source,molecular high-resolution spectroscopy,Ti:sapphire laser's frequency-doubling and frequency-stabilization system,molecular laser cooling and deceleration schemes,applications of cold molecules?beams?,and other key theoretical and experimental techniques related to molecular laser cooling.First,we introduced the selection basis of the molecules used for laser cooling and the energy level transitions and energy level structures of the selected MgF molecules,mainly focusing on how to construct a quasi-closed energy level transition cycle.By analyzing the Frank-Condon?FC?factor for molecular vibrational transitions and the rotational transitions and hyperfine energy levels of MgF,we would employ the X²?⁺?=0,=1,-??²?_1/2?^?=0,^?=1/2,+?transition to construct quasi-closed transitions to achieve laser cooling.Theoretical analysis shows that MgF has a highly diagonalized FC factor,a higher spontaneous emission rate of the upper level,and a smaller mass,which is an advantageous candidate molecule for laser cooling.Then we describe the design and construction of a buffer-gas beam source for the preparation of a pre-cooled MgF molecular beam,and the absorption spectrum of MgF molecules and the laser-induced fluorescence spectrum of molecular beams measured using the cryogenic molecular beam source.We used the chemical reaction method to prepare MgF molecules,and designed a cryogenic cell based on the theory of buffer gas cooling,and built a 6 K,high-vacuum environment.By analyzing the fine structures of the molecular absorption band measured by the experiment,we calibrated the quantum number of the transition line,determined the transition frequency required for molecular laser cooling,and proved that the²?state of MgF is not an inverted,but a normal state.We also verified the hyperfine structure of the MgF ground state by molecular beam fluorescence spectroscopy.Then we introduce the experimental preparations of two Ti:Sapphire lasers,which are respectively the main pumping light and the repumping light in laser cooling of MgF,including laser frequency-doubling,linewidth-narrowing,and the long-term frequency stabilization.We measured the power and efficiency of the two Ti:sapphire lasers and the frequency-doubling resonator,and combined two frequency stabilization methods?transfer-cavity method and side of fringe method?to develop an effective laser stabilization method with arbitrary-tuning and long-term frequency stability.The frequency stabilization scheme achieved a narrow linewidth?6 kHz?UV laser and a long-term frequency stability of±2.8 MHz,which is sufficient for the main transition of MgF laser cooling with a natural linewidth of 2?×22 MHz.Then we introduce two laser deceleration schemes designed for MgF molecules,namely the spontaneous radiation force deceleration scheme and the stimulated radiation force deceleration scheme,and the deceleration effects of the two schemes were theoretically simulated.For the spontaneous radiation force deceleration,in order to compensate for the Doppler shift and hyperfine splitting during molecular deceleration,we designed a laser sweep scheme based on two electro-optic modulators.Theoretical simulations show that for a 200 m/s MgF buffer gas molecular beam,this solution can complete the deceleration of the molecule within 18 cm.For stimulated radiation deceleration,it has advantages in magnitude and velocity capture range,but the constraint of this solution is that it has higher requirements for laser power.In addition,based on the understanding of the advantages and characteristics of the stimulated radiation force,we have also proposed a constant phase deceleration scheme that can be applied to fast,short-distance deceleration of heavy molecules.Finally,we discuss the application prospect of cold molecules,and took a novel molecular deposition scheme using surface plasmon interference field as an example to specifically describe the application of cold molecular beam obtained by buffer gas cooling or laser cooling.We proposed a simple device and a laser to obtain two-dimensional surface plasmon interference fields with different patterns.We also studied the feasibility of obtaining different patterns of nanoscale optical lattices by changing the laser polarization.Then we simulated the molecular deposition based on the surface lattice field and discussed the factors that affect the deposition resolution.We found that a periodic deposition point array with a resolution width of 53.2 nm can be obtained when the used cold molecular beam is optimized for collimation.