Terahertz Perfect Absorption Based on Ultrathin Conductive Films

Abstract

Electromagnetic absorption is one of the most fundamental forms of light-matter interaction, spanning a broad spectral range from radio frequency to the ultraviolet band. In the terahertz (THz) regime, however, conventional materials typically exhibit weak absorption, which limits the performance of high-efficiency THz devices such as detectors, modulators, and sensors. The development of ultrathin conductive films (UTCFs), especially two-dimensional materials, offers new opportunities for enhancing THz absorption due to their unique optical and electrical characteristics. Yet, the extremely small thickness of these UTCFs often restricts their ability to sustain strong interaction with incident THz waves, making additional strategies necessary to achieve substantial absorption. In this work, using graphene as a representative UTCF, we demonstrate that near-perfect THz absorption can be realized through a total internal reflection (TIR) configuration. When the refractive index of the superstrate exceeds that of the substrate, TIR occurs once the incident angle surpasses the critical angle, enabling the confined evanescent field to interact strongly with the ultrathin film. This mechanism leads to highly efficient THz absorption in UTCF-based structures and supports a broadband and wide-angle near-perfect absorption region around the optimal absorption point. A detailed parameter-dependence analysis is conducted for both the UTCF and the surrounding dielectric environment, illustrating how the absorption peak evolves with material conductivity, film thickness, refractive-index contrast, and incidence angle. The findings provide a refined physical understanding and a practical design pathway for developing strong THz absorbers based on ultrathin conductive materials, contributing to future applications in THz sensing, imaging, and integrated photonic systems.