The Fabrication And Optical Properties Of Hyperbolic Materials Based On The Three-dimensional Assembly Of Nanorod Arrays And Materials Templating

Optical metamaterials have attracted much attention owing to their unique electromagnetic and optical properties that are difficult to achieve in natural materials,including negative refraction,perfect absorbers,biosensing,and invisibility cloak.However,it is still a challenge to the fabrication of metamaterials in three dimensions（3D）,considering the complex structural requirement in subwavelength scale.The 3D metamaterials are fabricated by multilayer nanofabrication methods including electron beam lithography and focused ion beam milling.These nano-microfabrication methods have advantages including high-precision in geometric parameters and repeatability.But the cost of these method is extremely expensive,and the number of applicable layers in 3D is limited.It has always been an important topic to develop an alternative low-cost and large-scale route to fabricate the 3D metamaterial.The material chemistry method such as self-assembly and the following material templating method may be an option.Taking the above requirements into consideration,this paper focuses on the construction of optical hyperbolic metamaterials three-dimensional geometries.The 3D assembled arrays consisting of silver nanorods or 3D dielectric rod array structures with conformal metal coating presented the indefinite dielectric constant properties in optical wavelength.An electric field-assisted capillary channel method leading to the fabrication large-scale rod assemble film in 3D was developed;The method to conformal metal coating including silver and platinum towards to 3D rod arrays was explored.The basic optical properties and negative refraction optical transmission properties of our 3D hyperbolic metamaterials was discussed.The main results of our study are listed as follows:（1）It is clarified that the 3D assembled arrays consisting of silver nanorods and 3D close-packed dielectric array structures with conformal metal coating show the indefinite dielectric constant properties in the visible wavelengths which is the basic properties of hyperbolic metamaterials.Through the inversion under S-parameter electromagnetic simulation,the dielectric constant anisotropic properties of two types of 3D periodic rod arrays,the directional 3D array of silver nanorods in air and the conformal metal coating on the directional arranged arrays of silica nanorods,were calculated.The results shows that the two types of 3D array structures have indefinite dielectric constant properties in the visible and near-infrared frequency ranges,in which the positive and negative dielectric constants of the rods in the axial and radial directions are different.The 3D assembled arrays consisting of silver nanorods and 3D close-packed dielectric array structures with conformal metal coating show the strong anisotropic properties.The indefinite wavelength of structures can also by varied by changing the axial periodicities,the periodic width,rod length,and diameter of nanorods in the 3D assembled silver arrays.The indefinite interval of structures can also by varied by changing the thickness of metal and diameter of nanorods in 3D close-packed dielectric array structures with conformal metal coating.And negative refraction can be observed further in the 3D assembled arrays consisting of silver nanorods and 3D close-packed dielectric array structures with conformal metal coating in the indefinite intervals.Compared with the 3D assembled arrays consisting of silver nanorods,it is suitable to fabricate 3D optical metamaterial that close-packed dielectric array structures with conformal metal coating.And the indefinite intervals can be changed with the length of the silicon oxide nanorods and the thickness of metal.（2）Under the electric field-assisted capillary force,large-area 3D aligned silica nanorod arrays were fabricated.Monodisperse silica nanorods were synthesized by the hydrolysis-directed growth of ethyl orthosilicate controlled by emulsion.The monodisperse silica nanorods with different aspect ratios can be controlled and synthesized by changing the concentration of the reagents.A 3D periodic silica nanorod array structure can be formed by applying an electric field to ITO electrodes with a spacing of nearly 100microns,which is oriented perpendicular to the electrodes,but the structure area formed is small.In the process of electric field-assisted capillary force,the capillary force can provide continuous replenishment of the colloidal solution of nanorods for structure formation.And under appropriate conditions without affecting the 3D alignment of silica nanorods,this method is suitable for the preparation of large-area colloidal arrays.There is a competitive relationship between thermal motion and electric field induced force in this process.In order to obtain nanorods with a large area,assembly at 35°C is the optimal condition.When the effective value of the same voltage is 5-6 V,it is suitable for the alignment of silicon oxide nanorods.Excessive Joule heating is induced from high voltage.,which prevents the rod alignment in an orderly manner.（3）The coating of with metals onto the 3D arrays of silica nanorods was important to the construction of the 3D hyperbolic metamaterial was constructed.（COD）（hfac）Ag is used as the silver source.and he precursor decomposition temperature was set at 165℃during the self-made cyclic static CVD process.In order to effectively increase the amount of silver deposition,the temperature of the sample stage was reduced by circulating water cooling and the source injection was extended to 120 min with tens of circulating depositions cycles.The deposition of platinum atomic layers outside the 3D array of silica nanorods was achieved using Me Cp Pt Me₃ and ozone.Platinum was successfully deposited on the silica rod arrays with a controllable thickness.Compared with oxygen,ozone sources are more suitable for the deposition of three-dimensional nano porous structures because of the lower deposition temperature.Through the electroless plating process with silver nitrate and glucose as the basic reactants,silver loading on the inner surface of the 3D silica nanorod array can be achieved.（4）A test method based on optical microscope micro-area negative refraction transmission was developed in our paper for the determination of fabricated hyperbolic materials.The negative refraction transmission performance in visible and near-infrared wavelength in 3D arrays of metal-coated silica nanorods was verified.A microscopic optical path system by the usage of optical fiber assisted illumination and the conjugate plane of the aperture diaphragm of the optical microscope was successfully built.The system can discriminate the positive and negative transmission light using the relative transmission ratio using the semi shielding.In TM polarized mode,the relative transmission spectrum curve of the silver-coated 3D silica nanorods showed a valley in the relative transmission spectrum in related wavelength ranges,compared with the silver-coated the silica opal structures and the silver-coated silica nanorods in random arrangement.Therefore,the negative refraction light transmission was proved when light passed through the structure in the corresponding frequency range.In the TE polarization mode,none relative transmittance valley was observed.When the thickness of platinum coating was controlled,the vertical array of platinum-coated 3D silica nanorods presented negative refraction transmission.And negative refraction interval moved to the shorter wavelength with the increase of thickness,which is consistent with the thickness-dependent indefinite interval of the dielectric constant.