Optimizing Aperture Geometry in THz-TDS for Accurate Spectroscopy of Quantum Materials

Terahertz time-domain spectroscopy (THz-TDS) provides a powerful platform for investigating low-energy excitations in quantum materials. Because these materials are often limited in size, experimental setups typically rely on tightly focused beams and metallic holders with small apertures. In this work, we perform a systematic study of how aperture geometry influences THz signal transmission in a standard free-space configuration. By analyzing time- and frequency-domain data for circular apertures of varying diameters and thicknesses, we quantify the spatial and spectral filtering effects imposed by aperture size. We show that small apertures progressively attenuate low-frequency components of the transmitted signal, while higher-frequency content remains comparatively unaffected. These effects become especially significant for apertures smaller than typical THz beam waists, resulting in amplitude suppression and phase distortions that compromise the accuracy of frequency-domain analysis and optical parameter retrieval. To validate these observations, additional measurements were performed on a representative quantum material, confirming the practical relevance of the identified aperture effects. The transmitted intensity as a function of aperture diameter also provides a straightforward method for estimating the beam waist at the focus. In contrast, standard aperture thicknesses do not introduce measurable distortions, confirming the adequacy of treating thick, non-resonant apertures as dielectric slabs. These findings establish practical guidelines for aperture selection in THz-TDS and underscore the importance of preserving low-frequency response for reliable characterization of quantum materials.