Optical Pulling Force Based On Guided Modes Conversion

The optical field has a momentum,so it can give a force on the object,namely light pressure.The direction of light pressure is usually forward along the direction of light propagation.In recent years,people have proposed a very unusual optical pulling force,that is,the direction of the optical force on the object is opposite to the direction of propagation.This interesting phenomenon and the profound physical mechanism quickly attracted attention.Compared with the optical pushing force,the optical pulling force can pull the object towards the light source,so it provides a new degree of freedom for optical manipulation in some special circumstances that objects cannot be pushed.It has important theoretical and practical significance.However,in the current research,in order to generate optical pulling force,people have to use very complex light sources or extremely cumbersome structures,which makes the conditions of this pulling force very severe and has great difficulties in its application.The purpose of this dissertation is to find a simpler,more operable method in optical and fluidic channels to achieve optical pulling.In this dissertation,we first study the features of optical pulling force in a twodimensional multi-layer slab waveguide structure,in which the core layer of this waveguide structure is water,the high refractive index silicon on both sides,and the air on the outermost layer.The eigenmodes in the five-layer slab waveguide can be obtained by using the analytical method.The object is placed in the core of this waveguide and the influence of the object on the propagation mode in the waveguide when the object is incident in different order modes is studied by using the finitedifference time-domain method.The results show that the scattering effect of the object leads to modes conversion between different waveguide modes.According to the law of conservation of momentum,when the waveguide mode converts from the low momentum mode to the high momentum mode after passing through the object,the object may obtain the pulling force,and vice versa.Maxwell's tensor analysis method was used to quantitatively calculate the values of the optical force received by the object under different conditions,and the result was completely consistent with the law of conservation of momentum.Based on the study of two-dimensional multilayer slab waveguide structures,we will extend it to three-dimensional rectangular waveguides.The core of thiswaveguide is filled with water,the outer layer is high refractive index silicon,and the air cladding.Using the combination of numerical simulation and theoretical analysis,we can obtian the eigenmodes in a threedimensional rectangular waveguide.The finite-difference time-domain method was used to solve the basic conditions for the appearance of optical pulling force,and the condition to trap the object in lateral direction,thus achieving stable optical pulling in three-dimensional structures.Finally,for the shortcoming of small pulling force intensity in the dielectric structure,we have proposed the study of optical pulling force enhancement in the metal waveguide structure,which makes the value of the optical pulling force increase by 5 times.The tractor beam theory and implementation methods which are studied in this dissertation provides new methods and means for optical manipulation in the microfluidic system,which has important value and significance.