Broadband terahertz half-wave plate based on an all-silicon anisotropic effective medium

We present an all-silicon anisotropic effective-medium half-wave plate (HWP) that provides broadband, low-loss polarization control in the terahertz (THz) band. A two-dimensional anisotropic effective medium was created by etching elliptical air holes on a rhombic lattice through high-resistivity silicon using standard techniques based on micro-electromechanical systems. Finite element simulations predict a phase difference between orthogonal polarizations of <i>π</i> ± 0.08<i>π</i> across 1.2-2.0 THz for a single 200 µm-thick layer. The results of terahertz time-domain spectroscopy are presented to demonstrate this HWP performance. Moreover, they show that stacking two, three, and four identical layers to effective thicknesses of 400, 600, and 800 µm shifted the operating band to 0.6-1.0, 0.4-0.67, and 0.3-0.5 THz, respectively, without degrading the phase response. Experimental spectra agree closely with the theory, showing that arbitrary effective thicknesses-and thus target frequency windows-can be achieved by simply stacking thin, easily processed wafers. Parametric sweeps indicated that performance was preserved under standard photolithography tolerances and lateral misalignments of up to 10% of the lattice period. By combining high transmission, wide bandwidth, and straightforward fabrication, the proposed device offers a scalable route for compact THz-band polarization optics for spectroscopy, imaging, and next-generation (6G) wireless communications.