Generation Of A Cw Cold Molecular Beam By Using Laser Guiding Of Neutral Molecules

Cold molecules offer new opportunities for molecular spectroscopy, precision measurement, cold collisions, and cold chemistry. Over the decade, there is a great interest in the production of cold molecules and their manipulation. This thesis is dedicated to the theoretical study of laser guiding of neutral molecules and generation of cw cold molecular beams.We propose a promising scheme to guide arbitrary neutral molecules in a hollow-core photonic band gap (HC-PBG) fiber by using a red-detuned Gaussian mode. We theoretically calculate the guiding efficiencies for both the straight guide and bent guide by using a classical model, and study the dynamic behavior of the laser guided molecules by Monte-Carlo simulation. The simulated values for the guiding efficiency, the transverse and longitudinal velocity distributions of the guided molecular beams agree well with those obtained theoretically. When the input laser power is 2kW, the straight guiding efficiency for I2 molecules is about 24.6%. While the input laser power is 200W and the radius of the curvature of the laser fiber is R=0.2cm, the bent guiding efficiency for I2 molecules is about 1.2%, and a cw cold molecular beam will be generated.We propose a novel scheme to bently guide neutral molecules in a hollow optical fiber using a red-detuned HE11 mode. We theoretically calculate the optical potential for I2 molecules inside the hollow fiber and numerically investigate the dependences of the guiding efficiency, the transverse and longitudinal temperatures of guided I2 molecules on both the laser power and the radius of the curvature of the bent guide. Our study shows that and when an input laser is 6kW, and an input I2 molecular beam with a transverse temperature of 0.5K and a longitudinal temperature of 5K is used, a cw cold I2 molecular beam with a transverse temperature of 1.9mK and a longitudinal one of 0.5K will be generated by using a S-shaped hollow optical fiber with a 2cm radius-of- curvature for two bends, and the corresponding guiding efficiency and output flux are about 0.126% and the order of 108～109s-1,respectively.We propose a new scheme to generate a cw cold molecular beam by optically guiding molecules around an S-shaped integrated fiber bundle. We calculate the optical potential for I2 molecules inside the two-color evanescent field around the fiber bundle, and theoretically estimate the guiding efficiency of the guided molecular beam as well as its transverse and longitudinal temperatures based on a classical model. We also perform numerical simulations for the process of guiding molecules through the bent guide, and obtain the guiding efficiency of the output molecular beam as well as its spatial and velocity distributions in both transverse and longitudinal directions. When the power of two input lasers are 10W (1064nm) and 3.58W (266nm), the radius of the curvature of the fiber bundle is 0.2cm, and the transverse and longitudinal temperatures of the input I2 molecular beam are 0.5K and 5K, a cw cold I2 molecular beam with a transverse temperature of 0.29mK and a longitudinal one of 57mK will be produced, and the corresponding guiding efficiency and output flux are about 4.1×10-7 and the order of 105～106s-1,respectively.We propose a U-shaped laser guiding and velocity filtering scheme for cold molecules by using the evanescent waves inside four planar waveguides. We theoretically calculate the distribution of evanescent fields and the optical potential for I2 molecules inside the four planar waveguides, and numerically investigate the final guiding efficiency as well as the transverse and longitudinal temperatures of the guided I2 molecular beam. Our research shows that when the power of each incident laser is 200W, and the curvature radius of the bent guide is 2cm, a cw cold molecular beam with a transverse temperature of 0.34mK and a longitudinal one of 15.3mK can be obtained, and the corresponding output flux is about 104～105s-1We also propose a laser guiding scheme for cold molecules by using a cavity enhanced Gaussian beam. We calculate the light field distribution and the optical potential for I2 molecules inside the optical cavity, and study the dynamic behavior of laser guided molecules by classical Monte-Carlo simulation. Our study shows that when the input laser power is 100W and the angle between the input molecular beam and the axis of the optical cavity isθ=60°, a cw cold molecular beam with a flux of 108～109s-1, a transverse temperature of 4mK and a longitudinal one of 134mK will be generated.