Enhancment Of Light Absorption By Large-scale Subwavelength Engineering Meta-structures

Enhancement of light or electromagnetic wave?EM?has long been one of the most attractive research issues.High performance EM absorbers are of great importance in various applications,such as,energy harvesting,photo-detection,Raman enhancement,stealth technology,and non-linear optics.As we know,the absorption efficiency of traditional absorbers are relying on the nature of material itself,e.g.device thickness,and optical parameters.In order to achieve high absorption,traditional absorbers are required to be almost the same size or larger than the working wavelength which limits its development of miniaturization and integration.Taking advantages of sub-wavelength scale,polarization-independent and omni-directional absorption performance,metamaterial perfect absorber?MPA?provide us with new opportunities and a wider stage.In recent years,attributing to the development of micro-nanofabrication technologies,great progresses have been made in the researches of metamaterial perfect absorber.However,MPA still suffers several obstacles,e.g.fabrication throughput,limitations on sample size,and bandwidth.It is a primary problem faced by artificial metamaterial that how to precisely control the size/spacing of the subwavelength periodic metal array structure with a low-cost,large-scale fabrication approach.Meanwhile,the noble metal that commonly adopted in MPA are chemically stable,of which carrier concentration and dielectric constant are difficult to regulate,and hence limiting their application in achieving tunable working frequency.Moreover,though many theoretical efforts have been made to improve the MPA performance,it is still remained as a big challenge to fabricate ultra-narrow/broad band nearly perfect absorption in experimental.Aiming at breaking through the above limitations of MPA,we have systematically studied the subwavelength engineering meta-structures enhanced absorbers following the principle from simple to complex,from two/three planar stack structure to multilayer spherical shell structure.In first chapter,we have briefly introduced the demands and significances of enhanced optical absorption,and also reviewed the research background of different types of electromagnetic absorbers.From chapter 2 to chapter 5,specific contents are discussed in detail through the process of the absorber structure changing from simple to complex,and the absorption peak evolving from ultra-narrow peak to ultra-broad band.In chapter 2,a simple two-layer metal/dielectric film stack structure was studied to realize nearly perfect absorption.Under the guidance of Coupled Mode Theory?CMT?,we constructed a thin planar stack structure with SiC film deposited on an opaque gold substrate.By taking advantage of SiC material that exhibits high permittivity mode within a very small frequency range in long-wave infrared region,an ultra-narrow band absorber was achieved?Q>100?.The result was further confirmed by Transfer Matrix Method?TMM?,two theoretical absorbance spectra were in consistent with each other.The two-layer planar stack structure is simple in construction,which provides us a feasible strategy to realize subwavelength,large-scale,ultra-narrow band absorber,in avoid of complex micro-nanofabrication technologies.In chapter 3,three-layer Metal/Insulator/Metal?MIM?film stack structure was adopted,and the top metal film was replaced with copper sulfide film(Cu_2-xS,carrier concentration is as high as¹0²22 cm^-3),so as to achieve a novel type of sub-wavelength F-P resonance absorber.In addition,we have clarified the physical mechanism by which loss medium implemented a novel F-P resonance mode.Copper sulfide film with high carrier concentration and copper oxide film with low carrier concentration(CuO,carrier concentration is about 10¹⁶¹0¹88 cm^-3)can be mutually converted under external conditions.Therefore,the designed absorber is easily to be switched from a high reflectivity state to high absorption state,or vice versa.In chapter 4,a large-scale low-cost infrared dual-band absorber was realized with MIM structure,in which disordered random gold nanoparticles structure substituted the periodic array as the top layer,and an ultra-thin monocrystalline silicon film was used as the dielectric layer.In this chapter,we have investigated the underlying mechanism of the random structure absorption performance,and realized the ability to tuning the absorption properties.Simulation results indicated that the short-wave absorption peak was triggered by F-P resonance,while the long-wave absorption peak was driven from magnetic resonance formed between the gold particles and the silver substrate.The resonance modes broke the limits of indirect energy bandgap and made the silicon absorption frequency redshift to longer wavelength range.The nearly perfect absorption of monocrystalline silicon in infrared band was realized and tunable within 1-10?m.In chapter 5,we proposed a periodic multilayer hemispherical shell structure to realize ultra-broadband absorber.A large-scale low-cost and multi-mode metamaterial absorber was fabricated by using self-assembled two dimensional colloidal microsphere array as template.In this chapter,the regulating effect of structural parameters on absorption peaks was studied in detail,and the internal mechanism of two resonance modes were investigated.The results indicated that the long-wave absorption peak was triggered by the localized surface plasmon resonance on the adjacent outer gold hemispherical shells,while the short-wave absorption was driven from the magnetic resonance formed between the inner gold hemispherical shell and the gold substrate.In order to overcome the defect of narrow absorption peak of MPA,two independent resonance absorption peaks were superimposed together to realize an ultra-broadband absorption band ranging from2.8-6?m?mid-infrared atmospheric window?,and the broad absorption band was tunable within 1-20?m.