Low-index and thin-film material characterization in the terahertz range via dielectric waveguide evanescent-field interaction

Terahertz (THz) and millimeter-wave (mmW) spectroscopy provide non-ionizing, label-free access to the electromagnetic response of a wide range of materials, enabling applications in quality control, biomedical diagnostics, and non-destructive evaluation. Conventional free-space THz spectroscopy, however, often suffers from limited interaction length and reduced sensitivity when the sample is thin, spatially localized, available only in small volume, or incompatible with beam-based fixtures. In this work, we present an integrated, non-contact approach for broadband material-property extraction based on the evanescent field of a low-loss high-resistivity silicon dielectric waveguide. The material under test (MUT) is placed in the cladding region, where it perturbs the guided-mode propagation constant and attenuation. By measuring the complex transmission through a reference waveguide and a waveguide loaded with the MUT and by combining these data with mode simulations and a perturbation-based inversion, we extract the complex permittivity over a continuous frequency band. The approach leverages an evanescent-field-based non-destructive method, a compact measurement setup, and feasibility to rescale toward higher frequencies for broadband material screening in the mmW to THz range.