An Optically Transparent Near-Zero-Index Grating Metamaterial for Enhanced On-Glass Millimeter-Wave Radiation

An optically transparent antenna (OTA) based on a grating metamaterial is proposed for launching high-gain millimeter-wave radiation on glasses. This design incorporates a glass-based dielectric image line with a grating made of a transparent conductive film (TCF), which provides an effective near-zero-index response through precise manipulation of spatial harmonic dispersion. To effectively overcome the intrinsic loss issue of TCF and control the power leakage, the field confinement in the grating metamaterial is engineered for optimizing the purely leakage-induced and total modal attenuations at the zero phase-progress state. The prototyped grating metamaterial antenna features an average optical transmittance of 76% over the visible spectrum. The antenna is validated to generate a near-broadside directive beam, achieving a realized gain exceeding 23 dBi and a total efficiency higher than 60% from 21.1 to 22.3 GHz. Through leaky mode confinement control and complex dispersion relationship engineering, this work offers a promising solution to realizing OTAs with a substantially improved gain and aperture efficiency.