Understanding Quantum Plasmonics From Classical Electromagnetic Theory And Quantum-Corrected Model

When the incident light resonates with the surface plasmon oscillation of a metallic plasmonic nanostructure,the free electrons in the metal start to oscillate and localized sur-face plasmon resonance(SPR)takes place.Recently,SPR has evolved from a fairly esoteric physical phenomenon to an optical tool that is widely used in physical,chemical and bio-logical investigations where the characterization of an interface is of interest.SPR modes of the metal nanostructures not only depend markedly on their shape,size,and composi-tion,but also on their adjacent coupling with other nanoparticles(NPs).Metallic NPs can assemble to form aggregates.In NP assembles,their individual plasmons couple togeth-er so that the near-field in the gaps between NPs can be enhanced by several additional orders of magnitude due to the interaction between the particles.The local field intensity enhancement has been widely used in many applications,such as surface-enhanced Raman scattering for single-molecule detection,optical nanoantennas,high-harmonic generation,solar energy harvesting,and so on.Coupling multiple nanoparticles together in chainlike structures has been suggested as an approach to nanoscale optical wave-guiding and focus-ing.The tremendous progress in experimental techniques and fabrication of nanoplasmonic devices has stimulated theoretical development for addressing that complex physical phe-nomenon.Most of the existing theoretical studies employed either classical electromagnetic interaction or the simplified plasmon hybridization or dipole-dipole coupling models.Those classical electromagnetic approaches provide a simple way to understand the plasmon cou-pling,are particularly well suited for weakly coupled nanostructures.However,in closely plasmon of individual particles can be significantly distorted,due to the interparticle polar-ization effect and exchange electron interaction.The optimal electron distribution in closely spaced NPs can be considerably different with that in weakly coupled NPs.As a result,the simple plasmon hybridization model or dipole-dipole coupling model fail,and theoretical treatments taking into account the quantum effects,such as full quantum model based on the time-dependent density functional theory,the nonlocal hydrodynamical(NLHD)mod-el,and quantum corrected model(QCM),are required.Unfortunately,TDDFT can only be used to describe small size clusters with only a few thousand conduction electrons.The ac-tual plasmon system usually contains millions or even hundreds of millions of electrons and they cannot be treated by the first-principles method.NLHD cannot be used to explain the size effect of nanoparticles on the plasmon energy in free electron materials such as alkali metals or aluminum.Compared to the redshift phenomenon predicted by TDDFT,the NL-HD calculation produces a blue shift.However,when embedding a virtual medium in the nanoparticle gap,the results obtained by the quantum correction model(QCM)are in good agreement with the TDDFT calculations.This QCM inserts the quantum effect into a clas-sical electromagnetic model and can accurately describe the optical properties of densely packed nanoparticles without increasing the computational cost.In this thesis,the extinction spectra and local fields in the gap of NPs with differen-t sizes,number of NPs,and spatial separations will be calculated by QCM approach.A comparison with the results produced by the classical electrodynamic model,the finite-difference time-domain approach,is made.We seek an understanding on how the size and interparticle distance of NPs dictate the frequency and absorption intensity of the plasmon resonance,and elucidate the electron tunneling effect,which results in the spectral lineshape changes and the reduce of the local field intensity in the gap of Au nanoparticle dimers,symmetric and asymmetric trimers.The relative importance of nanoparticle size and gap distance in determining the plasmon resonance and local field enhancement will be quan-tified.This work provides a guide for the design of the nanostructures tailored for specific applications.