Optical Dipole Force In Cascade Three-level Atomic Systems

In this thesis,we simulated the distribution of optical dipole force,optical potential and population transfer induced by Laguerre-Gaussian pulses of different orders in a cascade three-level molecular system by solving numerically the Bloch equations and the optical dipole force using the central difference method and the predictor-corrector method.It is shown that for the base transverse mode Gaussian beam LG00,the optical potential is attractive and the direction of optical dipole force points to the optical axis for positive detuning,which may lead to focusing of atoms;the optical potential is the repulsive and the direction of optical dipole force is perpendicular to the optical axis for negative detuning,which may lead to defocusing of atoms.The transverse dipole force has nodal circles in the radial direction induced by the Laguerre-Gaussian beam in LG00 mode,and the number of nodal circles is twice that of the order n,but the direction of the dipole force is opposite on both sides of the nodal circles due to the change of electric field gradient,therefore,the atom will split and be trapped in the optical potential wells at different positions.With the increase of the radial order n of Laguerre-Gaussian beams,the number of optical potential wells/barriers increases to n+1.In addition,the depth of the main potential well/barrier remains constant,but the spatial range of the main potential well/barrier becomes narrower,and the dipole force becomes stronger due to the radial gradient of the potential increases.Under the action of Laguerre-Gaussian beam in LGm0 mode,both optical force and optical potential have the characteristics of symmetry distribution about x-axis,y-axis and center point.The transverse dipole force has one radial nodal circle and m angular nodal lines,and the direction of the dipole force is opposite on both sides of the nodal circle.The optical potential has zero nodal circles and m angular nodal lines,which are the same as the mode distribution of the light field.With the increase of the angular order m,the number of potential wells/barriers is equal to 2m,the spatial angle range of a single potential well/barrier decreases but the depth of the well/barrier increases.Therefore,the particles are bound in the deeper and narrower optical potentials for high-order Laguerre-Gaussian beams,which is more conducive to the capture and accurate manipulation of particles.The research in this thesis can provide theoretical guidance for the optical manipulation of micro-particles.