Broadband Terahertz Wavefront Engineering Based on Topology Optimization of Refractive Index in Non-Dielectric Structures

In recent years, the use of terahertz waves with frequencies ranging from 100 GHz to 1 THz has been considered for ultrafast wireless communications and high-resolution sensing. There, it is important to control the wavefront of the transmitted signals. However, the use of dielectric materials faces a challenge that dielectric losses increase with the frequency. To circumvent this challenge, it has recently been demonstrated to implement distributed effective refractive index inside a pair of metal plates by spatially varying the distance between the plates, thereby realizing a terahertz lens without using dielectrics [1]. However, that design is valid only at a certain frequency, and it works just approximately in other frequencies. Therefore, it is difficult to ensure broadband capability. In this study, we try to reconcile dielectric-free refractive index distribution and broad bandwidth by numerically optimizing the effective refractive index between the metal plates based on topology optimization [2].