Optical Properties Of Nonlocality And Nonlinearity In Plasmonics

In this thesis,we investigate nonlocality and nonlinearity in plasmonic core-shell nanoparticle systems,which are very important issues in the modern plasmonic optics.The main results of our works are listed as follows:1.Nonlocal effects on surface enhanced Raman scattering from bimetallic coated nanoparticlesWe study the surface enhanced Raman scattering(SERS)from bimetallic core-shell nanoparticles by taking into account the nonlocal effect.The Gersten-Nitzan model is applied to investigate SERS from a molecule adsorbed on the nonlocal bimetallic nanoparticle.Numerical results show that there are two enhanced SERS peaks for bimetallic coated nanoparticles,and nonlocal effects will lead to less enhancement and blue-shift of SERS peaks.In addition,unusual resonant electric field patterns are found in the nonlocal gold core in comparison with those in the local case which arises from the excitation of longitudinal mode.Our investigation is helpful for understanding some details of SERS schemes in nano-optics and plasmonics when nonlocal effects are considered.2.Equivalent permittivity and permeability and multiple Fano resonances for nonlocal metallic nanowires(sphere)Equivalent permittivity and permeability for metallic nanowires with spatial dispersion are derived analytically.The extinction cross section based on the local fullwave theory with our equivalent permittivity and permeability is found to be in agreement with the one based on the nonlocal full-wave theory.When the electromagnetic wave is incident on the nanocylinder with nonlocal permittivity,multiple Fano resonances exist because of the interference of different longitudinal modes with the dipole moments,and the nonlocal nanocylinder can be designed to yield an abrupt transition between full transparency and strong scattering.Interestingly,both the frequencies at which the transparency and strong scattering occur can be described well by the equivalent permittivity.Our study may be helpful in the design of optical nanoswitches and nanomemories.3.Generation conditions for Nonlocal core-shell nanoparticle lasers and their low thresholdsWe establish the spaser generation conditions for nonlocal plasmonic core-shell lasers based on the full-wave nonlocal Mie theory.Two different kinds of core-shell structure are considered,one consists of nonlocal metallic core and gain medium shell,the other one has gain core and nonlocal metallic shell.Numerical results show that,for the former case the required gain threshold and the gain refractive index become large generally when the nonlocality or spatial dispersion is taken into account.This tendency can be understood by the analysis with the proposed equivalent permittivity for nonlocal metallic nanoparticles.In the latter case,there exist two lasing states arising from the hybridized antibonding and bonding modes for this coated nanolaser.By varying the surrounding medium and the gain materials,we can achieve low gain threshold for each mode with flexible radii ratios on the purpose of realistic easy fabrication.Numerical results show that nonlocal effects have different influences on the required gain threshold and gain refractive index of these two lasing modes,which may be of great importance in the design of such kind of ultrasmall nanoparticle lasers 4.Superscattering of light from core-shell nonlocal plasmonic nanoparticlesWe study the scattering properties of coated nanoparticles with high-permittivity core and nonlocal plasmonic shell with full-wave nonlocal Mie theory.Both electric and artificial magnetic resonances of different orders are supported simultaneously for such coated particles.By properly engineering the aspect ratio,we can achieve not only the overlapping of electric dipole(ED)and magnetic dipole(MD)modes but also that of electric quadrupole(EQ)and magnetic quadrupole(MQ).We numerically demonstrate that there are two overlapping ways to obtain the superscattering state for core-shell nanoparticles.Furthermore,we show that the nonlocality becomes less important in the scattering of coated nanoparticles with ultrathin shell thickness but large total size due to the fact that nonlocality is essentially a surface effect.5.Optical bistability in nonlinear nano-composite consisting of core-shell nanoparticles.In three dimension,we propose a new nonlinear composite composed of graphenewarpped nanoparticles embedded in dielectric host.Within the quasi-static approximation,we derive the effective third-order nonlinear coefficient and nonlinear equation for the field intensity inside and outside the composite.Then theoretically study the third-order nonlinear response and optical bistability at terahertz frequencies.Physical parameters are investigated in such composite to achieve low switching-threshold optical bistability.We further propose a scheme to study the transmittance of this nonlinear composite slab.These results reveal novel regime of the optical bistability of the transmittance of light.On the other hand,in two dimension,we proposed the composite consisting of Kerr-type nonlinear core and nonlocal metallic shell to investigate the nonlocal effects on its optical bistability.We demonstrate the tunable Fano resonances in the scattering spectra of the core-shell cylinder based on different degrees of nolocality in the linear limit.It is found that nonlocality would enhance the Fano resonant peak and the near field strength,thus can further boost the nonlinear response of the core-shell cylinder composite in both near field and far field,indicating the nonlocality-enhanced optical bistability.We show that nonlcaility can efficiently reduce the biatable switching threshold of the nonlinear composite.