Percolative behavior in complex permittivity and permeability of silver/epoxy nanocomposite at terahertz by time-domain spectroscopy

Complex permittivity and permeability govern the electromagnetic (EM) behavior of materials and are critical for advanced high-frequency applications. In this work, we investigate silver/epoxy nanocomposites as a prototypical conductor-insulator system operating in the terahertz (THz) regime. Using terahertz time-domain spectroscopy (THz-TDS), we extract both complex permittivity and permeability across 0.2-2.0 THz. At low silver concentrations, classical effective medium theories (EMTs) capture the gradual enhancement in permittivity. However, as the volume fraction approaches the percolation threshold, classical EMTs fail, and percolation effects dominate—manifested by a rapid rise in permittivity and the emergence of negative magnetic susceptibility. We observe a nonmonotonic permeability response: it decreases due to percolation-induced diamagnetic loops and then recovers as electrons delocalize approaching the threshold. Power-law fitting yields critical exponents consistent with the two-dimensional percolation behavior, likely influenced by geometry and measurement configuration. These findings represent the first published experimental demonstration of percolation-induced diamagnetism in disordered composites at THz frequencies. The results provide new insights into THz composite design and highlight the necessity of incorporating both permittivity and permeability for accurate modeling near the percolation threshold.