| The continuous development of terahertz radiation sources and detectors has greatly promoted the research of terahertz radiation theory and application.Especially the emergence of terahertz radar makes the demand for terahertz absorbing materials for stealth technology more urgent.In addition to the military field,terahertz absorbing materials are also widely used in sensitive detectors,energy collectors and imaging spectral systems.Traditional terahertz absorbing materials have many shortcomings,such as low absorption rate,narrow absorption band and large thickness,which limit their application.The absorbing performance of terahertz Absorbers Based on metamaterials mainly depends on the unit size of the structure.They have the characteristics of wide absorption bandwidth,high absorption rate and thin thickness.Based on this,we propose to design and fabricate broadband terahertz metamaterial absorbers using semiconductor materials.In order to solve the bottleneck problem of integrating the functions of transparent and broadband absorbing,we combined the preparation technology of transparent conductive thin film with the preparation technology of metamaterials.The main achievements of this dissertation are as follows:(1)The geometric parameters of two broadband terahertz absorber structures based on sandwich structures are optimized by transmission line theory.Two polarization-insensitive broadband terahertz absorbing structures are obtained: one is a single ITO square-ring absorbing structure,whose absorption is greater than 90 % in the range of 1.7 – 5.3 THz,and has two absorption peaks at 2.07 THz and 4.8 THz;the other is a tri-ring absorbing structure,whose absorption depth is more than 90 % at 2.76 – 10 THz,and has two resonance peaks at 3.51 THz and 9.28 THz.(2)The geometric parameters of two kinds of broadband terahertz metamaterial absorbers based on the resonator structure are optimized by using the resonant theory of resonator.One is a single-layer binary grating structure,which can absorb more than 90 % in the range of 3.0 – 10 THz.In order to further broaden the absorption band,we also designed a double-layer binary grating structure.After optimization,its absorption depth is more than 90 % in the range of 1.7 – 10 THz,and there are many resonance absorption frequencies.(3)The preparation of ITO thin films as conductive layers in sandwich absorbers and their performance in terahertz band are systematically studied.The visible light transmittance of ITO films increases with the increase of crystallinity and oxidation degree,up to 90 %.ITO films with specific resistance can be prepared by controlling oxygen flow rate.The reflection of ITO films in terahertz band generally satisfies HR relation,and increases with the increase of conductivity.To obtain ITO films with reflectivity greater than 90 %,the square resistance should be less than 30 ?/sq.(4)We systematically studied the preparation and the terahertz performance of poly(methyl methacrylate(PMMA)thin films as dielectric layers in sandwich absorbers.The visible light transmittance of PMMA films is up to 90 % and the bandgap is about 3.7 e V;The terahertz reflection decreases from 80% to 58% at 3.2 THz when the thickness increases from 11 to 17 ?m;The dielectric constant of PMMA films increases from 2.88 to 4.04 at specific frequencies with the thickness from 9 to 14 ?m.And the dielectric constant decreases with the increase of applied frequencies due to the polarization mechanism of electric dipoles.But it will be less than 2.8 in terahertz band.(5)We have completed the preparation of RING1 and RING2 absorbers based on sandwich structures.Broadband transparent terahetz metamaterial absorber with absorbing depth greater than 80 % in 4.1 – 7.4 THz was fabricated by RING2/Glass structure,and its visible light transmittance reaches 70 %.Flexible broadband terahertz metamaterial absorber with absorbing depth greater than 80 % in 4.0 – 7.1 THz was fabricated using RING2/PET structure. |
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