Flexible Broadband Terahertz Wave Absorber Based On Ultra-thin Artificial Materials

Terahertz wave is a segment of electromagnetic wave located between highfrequency infrared and low-frequency microwave in the electromagnetic wave spectrum.Its absorption spectrum contains very rich information about materials.Terahertz wave has broader bandwidth and higher signal-to-noise ratio than microwaves,and it has lower photon energy,making it safer for bio-body than high-energy x-rays.In addition,terahertz waves also have fingerprint frequency to identify chemicals.These advantages grant terahertz technology great potentials in areas such as short-range communications,medical imaging,non-destructive testing,security screening systems,thermal imaging and stealth cloaks.For these applications,terahertz absorbers are indispensable.Due to its unique electromagnetic properties,terahertz absorbers based upon metamaterials have been rapidly developed in recent decades.However,the current terahertz metamaterial absorber has drawbacks such as narrow bandwidth and direction dependent absorption,which make it difficult for practical application.Therefore,how to effectively expand the absorption bandwidth and remove direction dependence has become the current research hotspot of terahertz metamaterial absorber.To use the absorber on uneven surface,metallic metamaterials was deposited on flexible substrate to make flexible terahertz absorber.In this thesis,terahertz metamaterial absorbers are designed and fabricated using thin metals,which utilize the principle of skinning depth to achieve broadband absorption.Absorbance of over 90% was achieved from 0.46 THz to 1.07 THz with 90% relative absorption bandwidth of 79.74%.The main research of this thesis includes following two parts.(1)In the regime of penetration depth,terahertz absorption was compared at different thickness of metal metamaterials.And the mechanism was studied to understand the trends with metamaterial thickness.By comparing the penetration depth,chemical and physical properties of several metals,the metal was chosen for the metamaterial fabrication.Here the absorber with platinum metal disk metamaterials was compared with different thickness.The plasmonic oscillation in the platinum disk was studied as the Pt thickness varies.(2)Terahertz metamaterial broadband absorbers were designed using 5 nm platinum metamaterials.The absorption mechanism and energy loss in the broadband metamaterials were explored in this thesis.It is found that their broadband absorption performance is due to the superposition of the electric dipole resonance at low frequencies and the F-P resonance absorption at high frequencies in the metamaterial array.The conductivity of thin metals is closely related to their thickness.In general,the lower the thickness of the metal,the lower the conductivity and the higher the loss per unit lengh for electromagnetic waves.Due to the low Q factor by using metal below penetration depth,a broadband absorber designed with a metallic platinum metamaterial can therefore obtain a high absorption over a wide range by superimposing only two absorption peaks.Based on the absorption mechanism,the optimization of the broadband absorbers is prospected.The research in this thesis extends the application of ultra-thin metals by designing flexible broadband terahertz wave absorbers based on ultra-thin artificial materials with the advantages of wide absorption bands,insensitivity to polarization angles,the ability to operate over a wide range of oblique incidence angles and simplicity of fabrication.