Fano resonance asymmetry for reliable characterization of dielectric film over a broad thickness range

A sub-THz non-contact platform for thin-film dielectric characterization was developed using high-contrast gratings (HCGs) to excite Fano resonances. By precisely designing and tuning the grating geometry, the interference between discrete resonant modes and the continuum background was effectively controlled, producing high-Q, asymmetric Fano spectra. These spectra enable quantitative retrieval of the film's complex dielectric constants from contour maps based on the resonance wavelength and effective quality factor. Calibration methods were implemented to mitigate system uncertainties, including air gaps, thereby reducing retrieval errors. Even under extreme conditions (air gap thickness t_air/Λ = 10^-4, exceeding the film thickness t_f/Λ = 10^-5), the retrieval errors for the real and imaginary parts of the dielectric constant remain below 4.5% and 10%, respectively, demonstrating high reliability. Furthermore, analysis of spectral asymmetry using the Fano parameter q defines a reliable operating range for thin-film thicknesses (t_f/Λ < 3.5 × 10^-3), making it suitable for nanoscale to microscale films. As a reliable non-contact characterization technique, this platform can be applied to thin-film materials research, optoelectronic device design, and high-frequency component development, with promising potential for future high-frequency wireless communication technologies such as 6G.