Preparation Of Metal Micro-nano Structures And Their Optical Properties

Due to its unique surface plasmon characteristics,plasmonic nanostructures can produce novel optical phenomena such as extraordinary optical transmission,enhanced light absorption,and optical chirality.Extraordinary optical transmission can be applied in the fields of filters,polarizing devices,and near-field optics.Researchers have investigated the optical transmission characteristics of circular and triangular nanopore nanostructures and nanoslit nanostructures,but the transmission spectra of these structures show a narrow transmission peaks,which limits its application in broadband filters;the enhanced light absorption has important research value for photovoltaic cells,super lenses,and thermal imaging applications.Nanorod array,metal/dielectric laminated structures,and multi-layer graphene nanostructures have been used to enhance light absorption,but research on composite structures composed of metals and graphene in enhancing light absorption is still scarce;optical chirality is useful in circular polarizers,negative refractive,biological sensing and other fields,but the chiral signals of biomolecules in nature are very weak,so researchers have proposed chiral plasmonic nanostructures to enhance their signals,such as double-layer Greek cross nanostructures,nanohelixes,and double-helix nanostructures based on DNA assembly,but the preparation of these structures have problems such as complex methods and high cost.Aiming at the above problems,this dissertation designs nanostructures with wide and narrow slits to obtain the broadband extraordinary optical transmission;designs metal-graphene composite nanostructures to achieve optically enhanced absorption;proposes and prepares different chiral nanostructures through simple experimental steps,the strong circular dichroism(CD)signal and its generation mechanism are obtained,and the chiral signal of biomolecules is enhanced.The main contents are summarized as follows:1.A multilayer structure with periodic nanoslit arrays is proposed and the optical transmission properties of such structure are studied by the finite-element method.Simulation results show that greatly enhanced transmissions over a broad spectral range are observed in near-infrared wavelengths,which results from the excitation of the electric resonances and magnetic resonances and the close resonance wavelength.The surface current distribution show that the massive concentrated electrons around the slits form strong oscillation and highly enhanced electric fields occur in the narrow slit.In addition,the effects of structural parameters on transmission properties are also investigated.All these findings could help the understanding of extraordinary optical transmission phenomenon and guide the design of devices with broadband-enhanced transmission and high electric-field concentration.2.A sandwich nanostructure(Ag nanoparticles-SiO2 film-Ag nanoparticles)is designed,then graphene is coated on the nanostructure to achieve enhanced absorption.The nanostructure is fabricated via oblique angle deposition and then a monolayer graphene by wet transfer method is coated on the nanostructure experimentally.Results of composite nanostructure indicate that graphene coverage leads to enhanced light absorption in the visible range.The simulation results show that the interaction between graphene and nanoparticles plasmons increase the light absorption originating from the strong localized electric field of graphene.In addition,the light-graphene interaction can be tuned via plasmon resonance by changing the size of nanoparticles and the thickness of the SiO2 film.These results promote the understanding of the light-matter interaction mechanism of graphene-covered nanostructures.3.Twisted Z-shaped nanostructure(TZN)composed of three vertical and twisted nanorods is proposed and its CD properties are investigated by the finite-element method.The absorption spectra exhibit that a dip and peak for different light clearly occur at the same wavelength,which leads to the giant CD effect approaching 88%.For right circularly polarized light,the strong effective current densities are concentrated in the top and bottom nanorods,leading to the antibonding mode;but for left circularly polarized light,the pronounced off-resonance dip is due to the destructive interference between the narrow bright mode and the broad dark mode.The CD properties can be tuned by the length of nanorods and the gap between them.These results would guide the design of plasmonic chiral nanostructures for achieving giant CD effect.4.A method to design chiral plasmonic nanostructures with CD effect is proposed.Under the framework of the transfer matrix method,the elements of linear polarization in different plasmonic nanostructures are selectively controlled to achieve different CD responses.Considering the near-field coupling of adjacent layers of trilayer nanostructure,the coupling matrix,which shows the relevance of the coupling status to the relative position of the adjacent layers,is analytically derived.Through the Jone matrix derivation of different nanostructures and the CD simulation of relevant nanostructure arrays,results show that the fitting functions of simulated CD signals are consistent with the theoretical formulas based on transfer matrix method.The addition of the coupling matrix to the transfer matrix is the necessary and valid condition for a multilayer nanostructure with a relative offset.The proposed method can supply beneficial information for prediction of the CD spectra.5.An L-shaped plasmonic nanostructure consisting of two metallic slices is designed,by covering a dielectric in one arm of the nanostructure to achieve enhanced CD signal.The nanostructure is prepared on self-assembled polystyrene nanosphere by glancing angle deposition,and then an SiO2 slice is deposited on one arm of the L-shaped nanostructure experimentally.Experimental results reveal that by introducing dielectric to one arm of the L-shaped nanostructure,CD can be enhanced and tuned by the thickness of the SiO2 slice.Simulated charge density distributions show that the increased optical phase difference induced by the cross electric diploes of two slices results in the enhanced CD due to the introduction of the dielectric SiO2 slice upon one arm of the nanoslices.These findings contribute to a better understanding of the CD physical mechanism.6.A chiral conic nanoshell plasmonic nanostructure composed of three nano shells with different heights is proposed and its CD properties are investigated.By varying the incidence and orientation angles of glancing angle deposition,the chiral nanostructure is fabricated on achiral tapered-nanopore anodic aluminum oxide template.Experiment and simulation results show that CD is amplified with increased height difference of the nanoshells and period of the nanopore.The dissymmetry factor of the nanostructure is up to 0.45 and the simulated enhancement of the chiral near-fields reaches 155 times,which results from the helix-like electron oscillation characteristics on the surface of the three nanoshells.The chiral signal is enhanced by about two orders of magnitude by using the nanostructure to detect chiral molecules.This study offers a concise and large-area regular method for fabricating plasmonic chiral nanostructures with tunable chiroptical response and provides an effective and convenient idea to control chiral near-fields for sensitive biomolecule detection.