Influence Of The Imaginary Part Of Permittivity To The Conservative And Non-conservative Forces Exerted On Mie Particles

Permittivity is a characteristic quantity for material,which is influenced by the electric field and directly related to polarizability.Moreover,it has a influence on the optical force.The expression of permittivity is e e ×ep r i= +i,in which the real part is er,which reflects the ability to store energy,and the imaginary part is ie,which reflects the loss of energy.Optical force exerted on particles can be divided into conservative force(gradient force)and non-conservative force(scattering force and absorption force),which have different properties and applications.Numerical and analytical separations of conservative force and non-conservative force for Mie particles(comparable to wavelength,and are also common particles used in experiments)have just been realized until recent years.Based on this research,this paper numerically(based on fast Fourier transform and Mie theory)and analytically(based on multipole expansion)investigates the influence of the imaginary part of permittivity to conservative and non-conservative forces exerted on the particles.This work will offer new opportunities for more accurate and free manipulation on micro-particles by using different properties of these two forces.This thesis is organized as follows:The introduction of the background and the principle of the separation of conservative and non-conservative forces are presented in chapter one,which is further divided into three parts: the overview of the development of optical micromanipulation;the basic principle based on which optical force exerted on particles is divided into conservative force and non-conservative force;the main content of this paper.Chapter two also includes three parts.Firstly,parameters and corresponding symbols used in this paper are introduced.Then the methods to judge the correctness and accuracy of the numerical simulation program are presented.At last,the z component conservative and non-conservative forces exerted on small metallic or dielectric sphere(radius a(28)0.1mm)are calculated with the change of the imaginary part of permittivity under the illumination of linear polarized Gaussian beam by combining the two methods(fast Fourier transform and multipole expansion).The maximum values of the z-component non-conservative force always lies on the position of the focus,while the conservative force always lies on both sides of the focus,and the two forces both diminish quickly when the particle runs away from the maximum fields.In chapter three,we firstly compare the conservative and non-conservative forces exerted on Mie particles for polarized beams with opposite helicities(left or right circular polarization).Then we numerically and analytically investigate the trend of the conservative force exerted on dielectric or metallic Mie particles,with the influence of the imaginary part of permittivity.For 2.53re(28)considered the conservative force exerted on the small dielectric particles does not change its signs,with the increase of the imaginary part of permittivity.While the conservative force exerted on the larger particles exchanges the positions of positive and negative forces,with the increase of the imaginary part of permittivity.The conservative force exerted on metallic Mie particles(5re(28)-for example)has an opposite trend from that of dielectric(2.53re(28)considered)with the increase of the imaginary part of permittivity.Chapter four is the summary of this paper and prospects for the follow-up development.