Fundamental Limitations of Terahertz Quarter-Wave Plates Based on High-Contrast Dielectric Gratings
We present a systematic study of high-contrast dielectric gratings operating as broadband quarter-wave plates in the terahertz range. Using higher-order effective medium theory, we identify an achromatic operating regime characterized by a parabolic-like frequency dependence of the phase retardation. This framework reveals a fundamental bandwidth limit imposed by single-mode operation in one-dimensional rectangular gratings. We show that Fabry-Perot resonances and the higher-order modes set intrinsic constraints on achievable performance. The theoretical predictions are quantitatively validated by rigorous full-wave simulations and terahertz time-domain spectroscopy experiments. Our results reveal clear physical design rules and intrinsic performance limits for broadband terahertz dielectric grating waveplates.