Equivalent Circuit Modeling of Pyrolytic Carbon Film as a Metal-Free Terahertz Absorber

Abstract

This paper presents an analytical study of pyrolytic carbon (PyC) thin film as a metal-free terahertz (THz) absorber using an equivalent circuit model (ECM). The complex permittivity of PyC is modeled via the Drude-Lorentz formulation with experimentally validated parameters yielding ε′ = −12.87 and ε″ = 14,380 at 0.1 THz, confirming metallic behavior across 0.1–3 THz under normal incidence (θ = 0°). A Salisbury-screen structure—PyC (25 nm) /  (50 µm) / PEC—is analyzed using the transfer matrix method (TMM). The ECM, formulated as a shunt RL branch (R = 500 Ω/□, L = 1 pH) in parallel with a lossless transmission-line STUB, reproduces the TMM response with maximum deviation of 1.73% and RMS error of 0.72%. A peak absorbance of 98.02% at 0.758 THz. The absorber exhibits two distinct absorption bands with >90% absorption: from 0.63 to 1.35 THz (Band I) and from 1.75 to 2.40 THz (Band II), with a deep absorption dip at ~1.5 THz. The cumulative bandwidth (sum of both bands) is 1.37 THz. Parametric studies confirm robustness across  and spacer thickness variations. The design is metal-free, ultra-thin (50.025 µm), and requires no full-wave simulation, offering a process-compatible, passive alternative to graphene-based THz absorbers.