Dual-Mode THz-TDS for Accurate Electrodynamic Characterization of Phase-Change GeTe Thin Films

We experimentally validate a dual-mode terahertz time-domain spectroscopy (THz-TDS) approach for accurate extraction of the electrodynamic properties of subwavelength GeTe thin films in the terahertz frequency range. The method combines simultaneous reflection and transmission measurements, enabling reliable characterization of phase-change films with thicknesses well below the THz wavelength. The approach is applied to copper and silicon films, as well as GeTe films undergoing thermally induced phase transitions from an amorphous to a crystalline state accompanied by pronounced changes in dielectric permittivity, electrical and optical conductivity. The extracted electrodynamic parameters are further employed for the design and fabrication of a tunable THz metastructure incorporating GeTe as an active layer, demonstrating on–off switching of a Fano resonance. The optical reconfigurable potential of GeTe-based THz devices has been demonstrated experimentally by studying the reversible amorphous–crystalline phase transitions of GeTe films induced by a single nanosecond laser pulse. In situ Raman spectroscopy is used to monitor the structural phase state of GeTe, providing direct correlation between structural transformations and electromagnetic response. The proposed dual-mode THz-TDS framework offers a robust platform for the characterization of ultrathin phase-change materials and tunable THz photonic devices.