The Study Of Several Optical Resonances At The Nanoscale

Thanks to rapid developments of nanofabrication techniques and numerical simulation ca-pabilities,the past decades have witnessed an explosive expansion of the field of nanophotonics.It aims at unveiling novel phenomenon of interactions between electromagnetic waves and ma-terial objects at the nanoscale.The resulting discoveries are benefiting our society and believed to contribute to a sustainable future.Surface plasmons,which are characteristic oscillations of induced charges at metal surfaces that can interact strongly with light are a key topic in nanopho-tonics.Surface plasmons can tightly confine the optical electric energy in the vicinity of metallic nanostructures,resulting in a largely enhanced field intensity.These appealing properties have been exploited to break diffraction limit,boost optical nonlinear effects,and improve solar cell performance,among other feats.Plasmons can be engineered to interact strongly with external light,which is oftentimes illus-trated by a large absorption cross section of the host structure compared with its projected physical area.When such nanostructures are arranged into a periodic array,it can even totally absorb the energy of an incident light wave,a phenomenon that is known as perfect absorption.Controlling the full with at half maximum(FWHM)of the spectrum,especially realizing perfect absorption with ultranarrow bandwidth,is desirable for sensitive photodetection among other appealing po-tential applications.In the first part of this thesis,we present a grating-based absorber with FWHM smaller than 1 nm.This very small bandwidth results from the low dissipation rate of the delocal-ized resonance supported by the structure.Commonly,ultranarrow band absorbers rely on various delocalized resonances,which require the absorber to have a relatively large spatial extension.In this context,we further propose a general method to guide a rational design of ultranarrow band absorbers that are based instead on localized resonances,which make it atfordable to minimize the size of the absorbers.The basic idea is to utilize some high-order localized mode raher than the customarily used fundamental mode of a resonator to reduce the radiative decay rate.Additionally,we consider dielectric elecents doped with gain impurities to compensate for the inelastic decay rate.We implemented this method in three kinds of absorbers that are based on different absorption mechanisms.All absorbers display dramatically improved performances compared with previous designs based on the use of fundamental modes.Metallodielectric coreshell nanospheres constitute a classical and thoroughly studied struc-ture in nanoplasmonic community.In the second part of this thesis,we firstly investigate the use of localized plasmon resonances supported by a metal shell to enhance the emission intensity of an upconversion nanoparticle embedded in the center of the dielectric core.To this end,a theoretical model accounting for absorption and emission processes of the system is established.Based on this theory,optimized coreshell structures are found under different pump intensity regimes.In the same chapter,we extend the simple coreshell nanoparticle structure to more complex multilay-er ones,which consist of alternate metal/dielectric shells.We reveal a cascade effect of the field enhancement in the structure.This can lead to huge intensity in the core under moderate light illu-mination.We further study its photothermal performance by computing the resulting temperature distribution.It is interesting to find that the temperature increase can be very spatially inhomo-geneous with the highest temperature in the center.The reason lies in the high inhomogeneity of the field enhancement and considerable thermal boundary resistance provided by multiple met-al/dielectric interfaces.Finally,the thermally induced internal pressure lift is also calculated.Interaction between light and particle arrays is a popular topic with great potential for practical applications.The collective behavior of the particles can be very different from the isolated ones.For example,a regular array of tiny nanoparticles is able to totally reflect the impinging light Recently,it has been realized that a regular array of two-level atoms holds the same capability.In the third part of this thesis,we take a step further to explore light scattering on three-level atom arrays.Unlike the two-level atom,which elastically interacts with light,the three-level atom can either dissipate,perfectly reflect,or amplify the probed light.Our investigations demonstrate these effects vividly,and how they can be controlled through pump light intensity.