Nowadays Terahertz (THz) radiation is actively used for imaging applications medical diagnostics, security inspection [1] or food and pharmaceutical industry [2]. Components of optical system should be compact and high efficiency. Diffractive components such as a metal zone plates and phase zone plates reduce the size of the optical systems [3] while meta-materials increase the efficiency of individual components and allow to control the parameters of the radiation.
The main goal of this work was to define the best meta-material design for focusing the 0.6 THz radiation. Dependency of focus efficiency and phase quantization level was also determined. Simulations were done using CST microwave studio program.
To achieve the focusing functionality, the split ring metasurface with various opening angles $\theta$ ware used. We have created two different designs (see Fig. 1) to identify the maximum transmission coefficient at 0.6 THz.
Different opening angles $\theta$ were used in order to achieve the phase shift covering $2\pi$ range and nearly constant transmission amplitude simultaneously. The required phase distribution at the appropriate place in diffractive elements was obtained by cylindrical lens formula. Both designs of the diffractive element let to focus 0.6 THz radiation in the focal point of 4 mm is shown in Fig. 2.
To conclude, the focusing characteristics of the diffractive element consisting of ring-shaped metamaterials were theoretically evaluated and optimized design was estimated.