Scattering Properties And Mechanical Effects Of Photonic Nanojet Using Discrete Dipole Approximation

In this thesis,the discrete dipole approximation(DDA)is used to study the scattering properties and mechanical effects of the photonic nanojet(PNJ).As we all know,photonic nanojets are narrow and high-intensity beams that are widely used in optical imaging,optical subwavelength detection,optical tweezers,nanolithography,optical data storage,and optical switching.Photonic nanojets have many scattering properties,including maximum intensity,effective length,focus position,and full width at half maximum(FWHM).The mechanical effects of photonic nanojets are more reflected in the force on tiny particles,including gradient force and scattering force.The beams used to generate photonic nanojets include plane waves,Gaussian beams,and vector Bessel beams.Compared with the former two beams,Bessel beams have the characteristics of self-reconstruction and no diffraction,as well as other adjustable characteristics,such as half cone angle,beam order,etc.,so this paper focuses on Bessel beam generation photonic nanojet.This will help enhance the application of Bessel beams in generating photonic nanojets.The discrete dipole approximation is a method that can calculate particle scattering problems,including any kind of beam and any kind of particle shape.Compared with other methods,it has the advantage of higher calculation accuracy.The systematic study of the influence of beam properties and particle properties on the scattering properties and mechanical effects of photonic nanojets is very important for improving the modulatability of photonic nanojets.The main work contents are as follows:1.Using the discrete dipole approximation method,the photonic nanojet generated by the vector Bessel beam irradiating ellipsoid particles is studied,and the influence of the beam characteristics and particle characteristics on the scattering characteristics of the photonic nanojet is analyzed.Regarding beam characteristics,the effects of wavelength,beam waist radius,half cone angle,beam order,beam center,and beam polarization are discussed.Regarding particle properties,the effects of particle equivalent radius,refractive index,and ellipticity are discussed.The distribution of the light field intensity of the photonic nanojet in the yz plane,the intensity distribution curves along the z and y axes,and the variation curves of each scattering characteristic value with the discussed factors are plotted.The changes of maximum intensity,effective length,focus position,FWHM,and quality standard value Q of photonic nanojets with beam characteristics and particle characteristics are mainly analyzed.2.Using the Rayleigh scattering model,the mechanical effect of the photonic nanojet on a Rayleigh particle was calculated and analyzed.The influencing factors considered are related to the factors that generate photonic nanojets.After photonic nanojets are generated by adjusting different factors,we analyze how the optical force acting on the Rayleigh particle changes.In this paper,the scattering force and gradient force of the photonic nanojet on the Rayleigh particle is studied.The factors discussed include beam properties and particle properties,the same as those considered in the section on scattering properties.In the results section,the intensity distribution of the gradient force and the scattering force in the yz plane and the intensity distribution curve along the y-axis are drawn,and the trends of the gradient force and the scattering force intensity with the discussed factors are discussed.3.On the basis of the previous two parts,the scattering properties and mechanical effects of curved photonic nanojets,namely photonic hooks and twin photonic hooks,are investigated.By changing the beam center and beam order of incident ellipsoid particles,curved photonic hook and twin photonic hooks can be generated.The effects of particle refractive index,equivalent radius,and beam polarization on the photonic hook are discussed.The effects of particle refractive index and equivalent radius on twin photonc hooks are also discussed.