Nonperturbative Near-Field Mapping of Terahertz High- Q Resonator Modes

Abstract We present a nonperturbative near-field imaging technique to measure the electromagnetic fields of high-quality-factor (high-Q) terahertz (THz) resonators. The method combines a continuous-wave terahertz spectrometer with a near-field photoconductive antenna (PCA) and Hilbert transform analysis, providing an unprecedented capability to access unperturbed evanescent fields with high spatial and spectral resolution. Our method also enables precise experimental measurements of the resonator’s effective refractive index, which is key for a wide range of terahertz device designs. The presented technique is broadly applicable to photonic and electromagnetic resonant systems and is relevant for terahertz sensing, spectroscopy, antennas, and metasurfaces. Overall, the approach establishes a versatile platform for studying and engineering high-Q THz resonators, quintessential building blocks for light–matter interaction in the THz domain.