Design And Characteristic Analysis Of Multiband Terahertz Metamaterials Antireflection Coating

Most functional devices use silicon as the interface in terahertz band.Because the impedance of silicon and free space does not match,which leads to the reflectivity of 30%and unnecessary fabry-perot interference fringes,a layer of antireflection coating with good performance should be added to the interface of silicon.Traditional quarter antireflection coatings are made of ultra-thin metal,which can achieve a good low reflectivity effect.But it has disadvantages such as too thick coating,high loss and fixed reflection frequency band.Metamaterials have unique electromagnetic properties,which can overcome the disadvantages mentioned above.Therefore,metamaterials are applied in the field of making terahertz antireflection coating.Currently,the research on the terahertz metamaterials antireflection coating is limited and focuses on the realization of single-band low reflectivity.However,there is a need for multiband simultaneous low reflectivity in practical applications.Based on it,two kinds of double band terahertz metamaterials antireflection coating with different structures are proposed in this paper as follows:(1)A double band terahertz metamaterials antireflection coating is designed,which adopts the multi-layer structure,i.e.,two layers of metal-polyimide.By numerically calculation of the electric field distribution on two metal surfaces,the causes for the two bands of the reduced reflectivity are analyzed.By adjusting the thickness of the polyimide and the size of the metal structures,the ultra-low reflectivity with the lowest percentage of 0.28%and 0.25%can be realized at the two bands of f=0.47 THz and f=1.56 THz respectively,and the bands width with the reflectivity below 10%reach 0.26 THz and 0.21 THz,and the tolerance characteristic is±4%.Compared with single-metal structure,the advantages of using multi-layer structure are analyzed.(2)A double band terahertz metamaterials antireflection coating based on single-layer structure is designed.By numerically calculation of the electric field distribution on the metal surface,the mechanism of the double band generation is analyzed,and by numerical calculation of the current on the metal surface,the equivalent circuit at the second reflectivity suppression valley is obtained,so the resonant frequency is calculated.Based on the resonance frequency and the basic theory of antireflection coating,under the condition of considering reflectivity and bandwidth,by optimizing the thickness of the polyimide and the size of the metal structures,the ultra-low reflectivity with the lowest percentage of 0.49%and 0.096%can be realized at the two bands of f=0.57 THz and f=2.37 THz respectively,and the band width with the reflectivity below 10%reaches 0.33 THz and 0.19 THz,loss as low as 20%,tolerance characteristic as high as±6%,polarization insensitivity.The double band realization mechanism of this structure is different from the multi-layer structure,which provides more design ideas for the multi-band terahertz metamaterials antireflection.