Superfocusing plate of terahertz waves based on a gradient refractive index metasurface

Abstract

A new scenario to realize superfocusing of terahertz waves based on a gradient index (GRIN) metasurface is proposed. To design the GRIN material, a robust algorithm is presented and numerically demonstrated to retrieve the constitutive effective parameters (permittivity and permeability) of the metasurface from the measurement of S parameters. In addition, a new kind of unit cell with a simple structure is designed to verify the theory of effective parameters. By computing the effective refractive index of the unit cell using the algorithm, we design a plate with a thickness of approximately 0.11λ0 (free-space wavelength) at 0.967 THz to superfocus terahertz waves in one dimension. We also revise the theory of superfocusing to make full use of the coupling effect among the cells in the plate and are able to demonstrate it. Comprehensive simulations of focusing are provided, and the full width at half-maximum beam width reaches up to 0.167λ0 at a distance of 0.2λ0. The focusing depth can be further enhanced by decreasing the cell size of the metasurface. This work is beneficial for metasurface design, super-resolution imaging, and other applications in the near field.