Design And Performance Study Of Broadband Terahertz Polarization Conversion And All-metal Visible Absorption Metamaterials

In recent years, the artificial metamaterial with special electromagnetic(EM) properties which not exist in natural materials has attracted people's highly attention and developed various novel applications. Especially by designing and regulating the structure carefully, the micro/nano structured metamaterial can raise the surface plasma effect, that realize unique optical phenomenon with terahertz(THz) and solar EM waves. For example, to achieve the polarization conversion of THz EM wave and the absorption of visible light with high efficiency and broadband in a small volume, the metamaterial will have a good application prospect in detecting, communication, photo-thermal, photovoltaic, etc. However, the terahertz wave polarization conversion metamaterials are still narrow band, and Poor thermal conductivity restricted the application range of the visible light absorption metamaterials. In this article, we propose a novel polarization transformation metamaterials that has long gradient leaf shape meta-surface to realize broadband linear polarization conversion for THz EM waves, and an all-metal structured metamaterial to realize broadband and efficiently absorption for visible light, respectively. The mechanism and performance of them are also studied. Specific research contents and conclusions are as follows:First of all, to break through the limit that the ? phase difference of the reflected electric field components in orthogonal direction, which is one of the necessary conditions to realize EM wave polarization conversion, only arise in one plasmon resonance band of existing metematerials, we designed a “cut-line” type metamaterial with circle arc shape long edge. The circle arc shape long edge can regulate the fundamental and three times frequency plasmon resonance link together, and meet the broadband polarization conversion condition spanning two resonant band. We studied the influences of the length of major and minor axis, the thickness and dielectric constant of medium to the polarization conversion properties of the metamaterials, and analyzed the mechanism. The results show that when the dimension of unit cell is 260?m,the length of major and minor axis is 165 ?m and 57 ?m, the thickness of dielectric layer is 48 ?m, a more broadband of line polarization transformation be gained. There are three standard phase difference of ? in the working band of designed metamaterial other than only two in other shape metamaterials. The polarization converter rate(PCR) reach 0.98 from 0.55 THz to 1.15 THz, and the vertical polarization conversion magnitude is over 0.8. The measured data and the simulated value proved our conclusion. More interesting is by reducing the unit cell size, length of axis and the thickness of dielectric, the designed metamaterials can serve in other THz bands to realize more broadband line polarization conversion. For example, when the metamaterial structures reduce the 2.6 times, namely the size of unit cell is 100?m, the working bandwidth of PCR = 0.98 reach 1.5 THz(from 1.25 THz to 2.75 THz).Secondly, based on the high melting point and high thermal conductivity properties of the metal Ni, we designed an all-metal metamaterial to absorb visible light. The metamaterial is composed of a Ni cylinder stand on the Ni film. The structure parameters are as follow: unit cell size of structure is 300 nm, the inner and outer diameter of the tube is 100 nm and 150 nm, and the height of the metal tube is 140 nm. We studied the influence of the size, height and thickness of metal structure to the performance of absorption. The results show the optimized metamaterial has an absorbing rate of more than 95% in the whole visible light band, and band reached 98% is of 85%. The broadband of absorption can be explained by the three loops current that the localized plasmon resonance occurred in metal structure. We also found that the allmetal metamaterial exist wide working incidence angle, namely under range 0 ~ 50° of the light incident angle, the absorbing rate is always above 90% in total visible light band. Finally, by studied the heat output properties of metamaterials, we found that a high temperature of 216 will output under irradiation of natural solar light, and the temperature distribution of metamaterial is uniform means small thermal stress. Those above indicate the metamaterial we designed has a good potential application in the field of photo-thermal.