Response Regimes in On-Chip THz Spectroscopy.

On-chip THz spectroscopy enables quantitative measurements of the optical conductivity of subwavelength 2D materials by tightly confining THz fields in metallic transmission line structures interfaced to the material. However, because the probed structures are smaller than the THz wavelength, finite-size and environmental effects can strongly influence the measured response. Here, we identify the conditions under which the near-field conductivity of a metallic sample exhibits a genuine Drude response and when finite-size and environmental effects must be considered. We introduce and characterize an unexplored regime, the Phantom-Drude response, which mimics Drude behavior but instead originates from the superposition of multiple finite-momentum plasmonic resonances. If unrecognized, this regime can lead to the misinterpretation of intrinsic material properties. We systematically show how the Phantom-Drude response can emerge and demonstrate its sensitivity to sample dimensions, transmission line geometry, material shape, and gate properties, providing practical guidelines for avoiding this regime in on-chip THz measurements.