Theoretical And Experimental Studies Of Optical Absorption Enhancement In Nanoparticles And Metamaterials

Totally absorption of solar energy has attracted widespread attention all over the world for its potential applications to provide abundant clean and renewable energy.Recently,it has been demonstrated that solar vapor generation could be enabled by nanoparticles in water,and such a phenomenon extnds the investigation of both two fields.Among them,even though there are comprehensive studies on the optical properties of nanoparticles and nanostructures,the solar energy absorption efficiency and photothermal conversion efficiency,together with the heat transfer between nanoparticle and surrounding medium,and the nucleation phenomenon on the surface of the nanoparticles have not been well known yet.For these reasons,this thesis will concentrate on the solar energy harvesting and the photothermal processes,including the optical absorption efficiency enhancement by using nanoparticles and some other kinds of nanostructures and the effects of parameters on the solar absorption efficiency.We employ three kinds of research approaches:numerical simulation,theoretical analysis and experimental exploring,to study the topics mentioned above.The numerical work is based on finite-difference time-domain?FDTD?,ragirous coupled wave analysis?RCWA?and finite element method?FEM?.Besides,Mie theory and equivalent dielectric permittivity model are used to give a detailed analysis about the results.The monochromator with integrating sphere are used to measure the absorptance of the metamaterials manufactured based on the numerical results in the experiments.The detailed research contents are described below:1.Theratical study of absorption property of anisotropic nanoparticles.We focus on the optical properties of nanoparticles made of graphene and Bi₂Te₃,which are rotationally symmetric and uniaxial anisotropic materials,respectively.The calculating results from Mie theory has been discussed based on a quasi-static approximation with equivalent dielectric permittivity model.The mainly underline mechanisms for the results are that,the absorption efficiency factor of the graphene-coated silver nanoparticle is mainly affected by the dielectric function of silver because of the low volume fraction of graphene,while the absorption efficiency factor of graphene nanoshells is greatly influenced by the dielectric function of graphene.All the results can be used as the guidance for the experiments related to graphene.As for the optical properties of Bi₂Te₃ nanoparticle calculated by using Mie theory,there is magnetic dipole response of such kind of nanoparticles.The results show that the unique absorption property of such kind of nanoparticles comes from the simultaneous excitation and mutual interference of electric and magnetic dipole resonances inside of the Bi₂Te₃ nanoparticle.The combination of the electric and magnetic dipoles in both of the x and z directions enhances the absorption peaks and broadens the absorption band of Bi₂Te₃ nanoparticles;2.Theoretical and numerical study of factors affecting the solar absorption efficiency factor of nanoparticles.Effects of surrounding medium,materials?including core-shell configuration?and geometry?size and shape?of nanoparticles on solar energy absorption are studied numerically by employing Mie theory and finite-difference time-domain method.It is shown that the resonance wavelength,the absorption peak,and the absorption band of nanoparticles play important roles in enhancing the solar absorption efficiency factor of single nanoparticles,a single nanoparticle who has high absorption peak and broad absorption band in the wavelength region with high solar energy density has high solar absorption efficiency factor.A nanoparticle with either high absorption efficiency peak or broad absorption band may have low solar absorption efficiency factor.In particular,the optical property and solar energy absorption efficiency factor of nanoparticles consisting carbon and gold has been studied.The combination of LSPR of gold and intrinsic absorption of carbon can greatly enhances the absorption peak of the core-shell nanoparticles and broadens its absorption band.Besides,the solar energy absorption efficiency factor of C-Au core-shell nanoparticle can be as high as155%for spherical nanoparticle,while it can even reach 233%for rectangular core-shell nanoparticle made of those two materials;3.Theratical and experimental study of a perfect absorber for solar energy harvesting.Based on finite-difference time-domain method for solving Maxwell equations,a numerical study is carried out on optical properties of an absorber made of a natural anisotropic hyperbolic material.Numerical results show that such an absorber has a perfect absorptance for solar energy by combining the slow-light effect and gradient index effect together.The hyperbolic optical constant mainly controls the optical property of the proposed absorber in hyper-II region,while the high absorption coefficient of Bi₂Te₃ is the priority for the perfect absorption in the long wavelength region.Optical properties of this absorber are affected by geometric sizes of the nanostructures,in specifically,the height and cross sections of top and bottom of the pyramidal structure domain the slow-light effect,while the distance between two adjacent nanostructures,as well as by the material and thickness of the substrate mainly affect the gradient index effect.Besides,a roughed Bi₂Te₃ surface with a roughness of 400 nm are fabricated and the optical property of it has been measured,the results show that it has a perfect absorption in the broadband like the perfect absorber designed before,and the experimental results with different roughnesses and distances meet with the theoretical predictions;4.The adjustment of the plasmon resonance wavelength of metamaterials with a single resonance wavelength.Base on the requirement of the thermal experiments heated by a certain laser,a numerical study is carried out for the adjustment of the resonance wavelength about a 3-layer plasmonic metamaterial by using finite-difference time-domain method.The unique property of core-shell nanoparticles with an adjustable plasmon resonance wavelength is employed in the present study.The results show that,first,the distance between two adjacent nanostructures determines the perfect absorption of incident light,second,the unabsorbed core's material in the nanoshell has little effect on the absorptance or the resonance wavelength of the metamaterials.The only choice to change the resonance wavelength is to varify the thickness of the nanoshells.To be specifically,changing the thickness of nanoshells in one direction can only slightly shifts the resonance wavelength,while the resonance wavelength of the metamaterial can be greatly changed by varifying all of the thickness of the wall in three directions.Theoretical,numerical and experimental methods have been employed to not only provides some new ideas on the enhancement of the optical properties of nanoparticles and metamaterials,but also studies the factors affecting the solar absorption efficiency of nanostructures.To totally absorb the solar energy and to enhance the absorption efficiency of a single nanoparticle have been achieved and the underline mechanisms have also been revealed.All the results in this thesis open a route effectively harvesting solar energy in photothermal conversion processes in water,especially the experimental and theoratical basis for solar vapor generation enabled by nanoparticles and nanostructures.