Terahertz dual-band polarization control and wavefront shaping over freestanding dielectric binary gratings with high efficiency

Metasurfaces for multi-band or wideband polarization control are usually achieved by cascading or nesting several elements in a period, which complicate the structure and introduce crosstalk. In this work, independent polarization control is achieved around two terahertz frequencies, 0.14 THz and 0.3 THz, through a simple binary one-dimensional grating. The correlation of the phase retardation with frequency is broken, since it is determined by the propagation of a single mode at the lower frequency and by the interference of two modes at the higher frequency. Four gratings capable of different types of dual-band polarization control, a half-half wave plate, a half-quarter wave plate, a quarter-half wave plate and a quarter-quarter wave plate, are designed and experimentally tested. All the samples are made of polymers and 3D-printed as freestanding lightweight structures. The experimental results of all the samples validate polarization conversion with state-of-the-art high efficiency. Simulation results indicate their favorable bandwidth of more than 30%. The half-half wave plate gratings are further applied to build a metalens for efficient dual-band beam focusing based on the geometric phase. They show great potential in developing multifunctional terahertz elements using simple configurations.