Fabrication And Electromagnetic Properties Of Broadband Metamaterials

Metamaterials,artificially designed with periodic subwavelength meta-atoms,can interact with electromagnetic waves to achieve much distinct electromagnetic performance,which doesn't exist in natural materials.Through designing the artificial nanostructures,it is possible to effectively modulate the inherent properties of electromagnetic waves,such as amplitude,polarization,and phase.However,due to the inherent dispersion of materials and the resonance characteristics of most metamaterials,many peculiar physical properties only exhibit at specific frequencies.The narrow-band resonances greatly limit the applications of metamaterials in many fields,such as perfect absorption,polarization conversion,and wavefront control.Therefore,a series of metamaterials with wide-band resonances are designed and fabricated in this dissertation.These metamaterials can realize perfect absorption,linear polarization conversion,and electromagnetic wavefront control in a wide band by regulating the amplitude,polarization,and phase of electromagnetic waves.Besides,the mechanism and characteristic of broadband resonances are analyzed and studied.Accordingly,the studies in this dissertation are as follows:Firstly,a ZnO-Au-stacked truncated-cone-type multi-layer metamaterial prepared by the masked etching process exhibits broadband,polarization-insensitive,and wideangle perfect absorption.The fabrication process is composed of lithography and ion beam etching techniques,which have the advantages of high throughput,flexibility,and various structural morphologies.The experimental results indicate that ZnO-Au stacked metamaterials can achieve broadband perfect absorption in different bands by adjusting the geometric parameters.The multi-layer truncated-cone-type nanostructure can be decomposed into a series of metal-dielectric-metal(MIM)structures,which can be regarded as a Fabry-Perot-like cavity of gap surface plasmon polaritons and trap the incident light with specific wavelength.Therefore,the tapered multi-layered structure can be regarded as a series of Fabry-Perot-like cavities with different sizes which can realize multiwavelength absorption,thus achieving broadband perfect absorption.The proposed broadband metamaterial absorber has extensive application prospect in the fields of solar cells,thermal emitter,and biological sensing.Secondly,a single-layer metamaterial with broken symmetry is proposed and prepared to achieve broadband linear polarization conversion.The unit cell of this metamaterial is composed of an asymmetric split-resonant ring and a square metallic hole.The simulated and experimental results indicate that over 70% of linear polarization conversion rate is realized in a broad frequency range(the simulated frequency range is 200.1 THz-240.6 THz and the experimental frequency range is 195.3 THz-236.4 THz).Therefore,the symmetry-breaking metamaterial can achieve broadband and highly efficient linear polarization conversion.The analysis of transmitted polarization states shows that the symmetry-breaking metamaterial can convert linear polarized light into circularly polarized light at specific frequencies.The electric field distributions at two resonant frequencies indicate that the symmetry breaking of SRR structure can excite the electric field distribution perpendicular to the incident polarization,and that the partial symmetry causes the suppression of copolarization component,which leads to the improvement of linear polarization conversion efficiency.The superposition of two resonances results in a wide-band linear polarization conversion.This high-efficiency and broadband metamaterial-based polarization converter has broad applications,such as polarization control and wavefront modulation.Finally,a ZnO-Ag-stacked hyperbolic metamaterial is proposed to realize broadband and high-efficiency circular polarization conversion as well as geometric phase control.Based on this hyperbolic metamaterial,a switchable metamaterial image,a broadband circular polarization beam splitter,and a 3D integrated metamaterial device simultaneously fulfilling circular polarization conversion and prism functions are proposed.By adjusting the geometric parameters,hyperbolic metamaterials with different circular polarization conversion performance are obtained.Based on different polarization conversion performance,a switchable metamaterial image controlled by incident wavelength and reflection polarization is proposed.For hyperbolic metamaterials with wide-band circular polarization conversion,the circular polarization conversion rate higher than 85% can be achieved in the bandwidth between 400 nm and 750 nm.By introducing the geometric phase arrangement,the hyperbolic metasurface can realize a circular beam splitter with high efficiency and wide operating band.With the circular polarization beam splitter and a filter metamaterial in series,a 3D integrated metamaterial is proposed,which can achieve the separation of crosspolarized light by space and wavelength.In other words,this 3D integrated metamaterial can be regarded as a miniaturized prism with circular polarization conversion function.The proposed geometric-phase hyperbolic metamaterial can operate in the entire visible band,which facilitates the realization of broadband wavefront control.Besides,the designed 3D integrated metamaterial device,which can simultaneously fulfil circular polarization conversion and prism functions,enriches the optical functions of metamaterials.