The Study Of Optical Torque And Optical Chirality Of Plasmonic Nanostructures

The surface plasmon refers to a collective oscillation of the free electrons on the metal surface.When the electrons in a metal structure resonate under the excitation of light,the amplitude of the electromagnetic field on the metal surface may be more than ten times larger than the excitation amplitude,that is,the so-called optical near field enhancement.In the past two decades,significant progress has been made in the study of surface plasmon,which finds a wide application in the fields of nanoscale light manipulation,biological detection at the single molecular level,light transmission enhancement of subwavelength aperture,and highresolution optical imaging that breaks through diffraction limits.That is due to the flexible regulation of the amplitude,phase,and polarization of the light field by the geometric properties of the metal micro-/nano-structure,which enables us to manually design the optical properties of metallic materials,so that they have unique electromagnetic properties that are not available in natural materials,the so-called electromagnetic metamaterials.Recently,the optical manipulation of nano-objects has attracted the interest of researchers,because the laser trap and light-driven rotation of nanoparticles have the high precision,noncontact and non-invasive characteristics.Due to the effect of surface plasmon resonance,metal nanostructures interact strongly with light,so that the energy carried by light can be effectively converted into the mechanical energy of nanostructures.Light-driven,high-speed rotating nanoparticles can be used as nanomotors,which may find applications in future biological experiments carried out in the micron scale,including targeted drug and cargo transport,biosensing,diagnosis,and cell classification.Not only can plasmonic nanostructures greatly aggregate light and form near-field enhancement,but also effectively enhance the optical signal of molecules,improve the chiroptical response of molecules,and contribute to the chiral sensing of molecules.This property is called optical chirality,and the field with stronger optical chirality than circularly polarized light is called a superchiral field,which may provide a method to detect the chirality of a single molecule.The main contents of this thesis are as follows:Chapter 1: The research background and significance of surface plasmon,optical torque and optical chirality are introduced.Moreover,the recent literature on optical torque and optical chirality is reviewed,and the structure of this thesis is arranged.Chapter 2: The dielectric constant model of metal materials(Drude model),the numerical computational method(finite difference time domain(FDTD)method),and the software(CST Microwave Studio)are introduced,respectively.Finally,the calculation formulas of optical force and optical torque are given.Chapter 3: Firstly,we study the optical torque of two kinds of symmetry structures,disk and cross structures,by circularly polarized light.Then,the phenomenon of negative optical torque on 2D chiral nanostructures excited by circularly polarized light is studied,and the physics mechanism of negative optical torque is analyzed by near-field Poynting vector,helicity conversion and scattering/absorption interaction.Finally,the optical torque of 2D chiral nanostructures is regulated by elliptically polarized light,which realizes a wide range of rotation rate regulation.Chapter 4: Two kinds of nanoarrays are designed,whose unit cells are two-nanoslits staggered structure and 2D chiral nanocavity structure,respectively.We study their near-field enhancement,optical chirality and circular dichroism.Finally,the effect of environmental refractive index on the transmission resonance of 2D chiral nanocavity array is analyzed.Chapter 5: Summarize and prospect about the research results above.