| Terahertz wave has the advantages of broadband,penetration and high speed,so it has broad application prospects in security inspection,and communication technologies.The functional devices in the terahertz band are often based on silicon,but due to the refractive index mismatch between silicon and air,the reflection amplitude at the air-silicon interface reaches 30%and it also leads to the Fabry-Perot interference fringes.Therefore,it is necessary to add an excellent antireflection coating between the air and silicon to increase the transmission and reduce the reflection.In this paper,based on the high design flexibility of metamaterials,which could effectively control the amplitude and phase of the terahertz wave,the properties of two THz metamaterial antireflection coatings were numerically studied.Split ring resonator(SRR)is a common structure in metamaterials design.Based on the theoretical research of SRR,two wide band THz metamaterial antireflection coatings are proposed:(1)A terahertz antireflection coating based on stand-up SRRs is designed.Employing the couple effect between two ' stand-up' SRRs to suppress the reflection in the terahertz range,the reflection minimum can be reduced to about 0.001,and the bandwidth with the reflectivity below 0.1 reaches 0.45 THz.The operating mechanism of the proposed antireflection coating is analyzed with the multiple reflections theory and the parameter retrieval method.Furthermore,we investigate the influences of the distance between the two split rings,the thickness of the polyimide substrate and the length of the SRR arm on the antireflection performances.The performance is also compared with that of the single 'stand-up' SRR.The numerical calculations and analytical results show that the designed metamaterial antireflection has excellent broadband reflection inhibition and large fabrication tolerance characteristics and has a wide application prospect for sensors,filters and other functional devices in the terahertz range.(2)A planar SRR-based terahertz antireflection coating is also proposed.Through the modeling and simulation calculations by the CST Microwave software,the antireflection coating based on the coin-type SRR is designed which could reduce the reflectivity to 0.0001 or lower,the reflection suppression bandwidth reached 0.49 THz,and this structure is polarization insensitive.The parameter retrieval algorithm proves that the designed antireflection coating could satisfy the reflection suppression condition at the reflection minimum and thus the antireflection coating could be assumed to be a homogeneous quarter-wave antireflection coating.After analyzing the influence of the size of the metal structure and the thickness of the polyimide further by the parameter scanning process,it is found that the bandwidth could be further optimized by adjusting the structural parameters,and the designed antireflection coating shows fabrication tolerance as well.The contrast between the results of the designed antireflection coating in this paper and the previous research results is shown,which displays the advantages and characteristics of the designed coin-type structure.The comparison of various performance indexes such as the minimum reflectance,the bandwidth,the fabrication tolerance and the polarization sensitivity shows that the two above antireflection coatings have their own advantages and characteristics.The standing SRR antireflection coating could be applied during the 1.35?1.80 THz band with a flat reflection suppression range,and could also be used as multi-band,high fabrication tolerance THz antireflection coatings.The designed coin-type SRR structure is suitable for 0.82?1.31 THz,which could be used as the THz antireflection coatings with low reflection suppression bandwidth of about 0.5 THz and non-polarization-sensitivity. |
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