The Coupling Among Different Light Scattering Modes Of A System Of Nanoparticles

The coupling between a collective oscillation of free electrons on metal surface and incident electromagnetic field will form the surface plasmon polariton(SPP).Because the incident electromagnetic field must have the right frequency to drive the free elec-trons resonantly,this phenomenon is also called the surface plasmon resonance(SPR).In metal nanoparticles,it is called the localized surface plasmon resonance(LSPR).LSPR has many unique optical properties,including easily tailored resonance wave-length,large scattering and absorption cross sections,and strong field localization and near field enhancement.These unique properties endow it with great potential of ap-plication in many different fields,such as sensing,biomedicine,solar energy,meta-materials,and so on.However,metallic materials have high loss in the visible region,up to present only a small fraction of applications based on LSPR have been realized in practice.Recent research has found that high refractive index dielectric nanoparti-cles can also support strong optical resonances,including electric-and magnetic-type resonances,while metal nanoparticles can only support electric-type resonances.Mean-while dielectric materials have much lower loss in the visible region than metallic ma-terials.These advantages make the study of optical resonances in high refractive index dielectric nanoparticles attracting widespread concern recently.In this thesis we have studied the coupling among optical responses of different systems of nanoparticles ex-cited by incident light,and specifically our work can be summarized into the following aspects.1.The analytical solution to electromagnetic scattering by a system of spheroidal particles.To deal with the problem of electromagnetic scattering by a system of spheroidal particles,we have expanded all electromagnetic fields as infinite series sum of spheroidal vector wave functions.Utilizing the boundary conditions requiring the continuity of tangential components of electric and magnetic vectors on the surface of each spheroid,together with the transformation relation of spheroidal vector wave functions between different coordinate systems,a set of linear equations containing undetermined field expansion coefficients can be obtained.By solving the linear equations,electromag-netic fields in the whole space can be obtained.Based on this theory we have built the corresponding program,which can calculate a plane wave scattering by a system of spheroidal or spherical particles in arbitrary configurations and provide related in-formation about both near-and far-field.After optimization,compared with numerical methods such as finite-difference time-domain(FDTD),our program is faster and can provide more accurate results while requiring fewer computational resources.2.Directional light scattering in moderate refractive index dielectric nanoparticles and experimental verification.We have found that for dielectric nanospheres with re-fractive index larger than 3.0,the electric and magnetic dipole resonances are spectrally separated from each other,thus they cannot achieve effective coupling at the total scat-tering peak.In contrast,for dielectric nanospheres with refractive index in the range of 1.7-3.0,the electric and magnetic dipole resonances have spectral overlap in the visible and near-infrared regions.Hence effective coupling between electric and mag-netic dipole responses can be realized at the total scattering peak,which can cause the directional light scattering phenomenon.Compared with high refractive index dielec-tric nanospheres,moderate refractive index dielectric nanospheres are more suitable as directional nanoantennas.We have prepared Cu2O(the real part of refractive index is about 2.7)nanospheres to demonstrate the directional scattering feature of moderate re-fractive index dielectric nanospheres experimentally,and the experimental results are in good agreement with the theoretical ones.3.Anomalous scattering in dielectric gain nanoparticles.For nanospheres made from lossless or lossy dielectric materials,the electric and magnetic dipole responses cannot achieve complete destructive interference in the forward direction,thus forward scattering cannot be reduced to zero.We have found that through introducing suitable gain in dielectric nanospheres,the electric and magnetic dipole responses can realize to-tal destructive interference in the forward direction under certain incident wavelength.When the contribution of quadrupole and higher order terms is negligible,zero for-ward scattering can be obtained and simultaneously backscattering will get enhanced.However,if quadrupole and dipole terms are of the same order of magnitude,coupling between them may result in the scattering minimum and maximum deviating from for-ward and backward directions,respectively.4.The optimization design of electromagnetic nihility nanoparticles.Nihility ma-terial is a medium whose relative permittivity and permeability simultaneously tend to 0 while keeping impedance equal to 1.Nihility material has attracted considerable atten-tion due to its novel electromagnetic properties.Because of the lack of nihility material in nature,artificially prepared structures are needed to realize nihility material.We have found that a core-shell hybrid nanosphere constituted by commutative ε-negative andμ-negative materials can give the optimal approximation of electromagnetic scattering properties of perfect nihility nanospheres in a wide frequency range.The core-shell hy-brid nanosphere we designed has the same scattered electromagnetic field distribution as the perfect nihility nanosphere.5.Scaling laws of plasmonic cloaking for dielectric nanoparticles.Through en-wraping a plasmonic material with specific thickness on the surface of dielectric nanopar-ticles,the total scattering of the particle can be drastically reduced,thus cloaking can be achieved.There are two different types of plasmonic materials that can realize cloaking,one has a positive permittivity and the other has a negative permittivity.We have found that compared with negative permittivity materials,positive permittivity materials are more suitable for realizing plasmonic cloaking of dielectric nanoparticles in a wide fre-quency range.We have proposed two different kinds of scaling laws to describe the relation between the optimal relative cover thickness and size parameter(inversely pro-portional to the incident wavelength)when dielectric nanoparticles are coated with the two different types of materials.It is noticed that the complete difference between the behaviors of the two scaling laws stems from the different distributions of the electric polarization vector inside particles.6.The light absorption properties of sodium clusters.For sodium clusters contain-ing different numbers of sodium atoms,we have proposed a unified theory to describe their light absorption properties.We have treated a sodium atom as an isotropic ho-mogeneous nonmagnetic sphere,and a sodium cluster is regarded as an aggregate con-structed by many sodium atoms according to a certain spatial structure.Thus the optical absorption spectra of sodium clusters can be calculated using classical electromagnetic theory.Through comparing with experimental absorption spectra,we have found that there is a good correspondence between the positions of main absorption peaks in the-oretical results and those in experimental ones.This study indicates that at least for sodium cluster systems,classical electromagnetic theory still has certain applicability above the atomic scale.