Study On Terahertz Tunable Metamaterial Absorbers

Metamaterial absorbers are artificially designed sub-wavelength wave-absorbing structures,which have many advantages,such as high absorption,thin thickness and designable electromagnetic characteristics.Metamaterial absorbers can effectively improve the sensitivity of terahertz functional devices and play an important role in the development of sensing,radar and stealth technology.However,due to the resonance mechanism of metamaterial,the absorption bandwidth of absorbers is usually narrow.Furthermore,once the structure is fabricated,its performance is often fixed and cannot meet the flexible absorbing requirements in various scenarios.On the other hand,most of the broadband absorbers reported previously only operate in a specific frequency band,which limits their multi-band application.In this work,we proposed several kinds of tunable terahertz broadband absorbers and multi-broadband absorbers in which the characteristics and physical mechanism of the absorption are investigated deeply.Besides,a terahertz wide-angle metamaterial absorber is fabricated and verified.The primary contents and achievements are as follows:(1)Based on the tunable characteristics of graphene,a single-band and dual-band tunable terahertz metamaterial absorbers are proposed,respectively.The single-band absorber is a three-layer structure consisting of patterned graphene,dielectric slab and metal ground plane.In the frequency range of 7-9.25 THz,the absorber achieves a broadband absorption with amplitude greater than 90%,and the relative bandwidth is 27.9%.By combining two similar rings with different sizes in one unit cell,the bandwidth over 90%absorption can be further increased to 3.2 THz,while the relative bandwidth reaches 39.3%.The dual-band absorber is a four-layer structure consisting of dielectric resonators,monolayer graphene,dielectric slab and metal ground plane,which exhibits both multi-band and broadband absorbing characteristics.The relative bandwidths over 80%absorption reach 97.8%and 31%,respectively.By changing the chemical potential of graphene,the state of the absorber can be flexible switched from absorption(>80%)to reflection(>90%)over the two bands.In addition,the structure has great advantages in fabricating and controlling due to the use of unpatterned monolayer graphene.(2)Based on the phase transition of vanadium dioxide,a terahertz switchable broadband absorber is proposed.The absorber is a three-layer structure consisting of metallic resonators,dielectric slab and vanadium dioxide ground plane.The absorption exceeds 89.7%in the frequency range of 1.116-1.564 THz,and the relative bandwidth reaches 33.8%.By changing the temperature of vanadium dioxide,the state of the absorber can be quickly switched from "on"(absorption>89.7%)to“off'(absorption<27%)over the whole operating band.Besides,the location of resonators has little effect on the broadband absorption.The proposed absorber shows similar performance when the arrangement of resonators is randomly changed,which greatly reduces the requirement of machining accuracy.(3)To meet the application requirements of near-infrared technology,a triple-broadband infrared metamaterial absorber is proposed.The absorber is a three-layer structure consisting of metallic resonators,dielectric slab and metal ground plane.The absorption of each band exceeds 80%in the three frequency ranges of 142-159 THz,183-200 THz and 233-245 THz.The 3dB relative bandwidths are 14.5%,13.1%and 9.9%,respectively.Due to the symmetrical design of the structure,the absorption is polarization-independent and good performance can be maintained at different incident angles.Physical mechanism of the absorption is well explained by using impedance matching theory and wave-interference theory.(4)Based on the superposition principle,a terahertz double-cross absorber is designed and then fabricated by printed circuit board technique.The reflection spectra of the sample at oblique incidence are directly obtained using a vector network analyzer in conjunction with a quasi-optical test bench.When the incident angle is 0° and 10°,the experimental results are in good agreement with the simulation results.When the incident angle is 30° and 40°,the reflection in experimental results is much lower than that of simulation results,which may be caused by the machining error.Therefore,a better performance is obtained that validates the feasibility and reliability of the absorber.