Near‐Field Terahertz Scattering Spectroscopy for Preparation‐Free Powder Sensing Enhanced by Quasi‐Bound State in the Continuum

ABSTRACT The capacity of metasurfaces for significantly enhancing light‐matter interactions via strong field confinement has driven widespread research into terahertz sensing based on structures like bound states in the continuum (BIC) in recent years. However, most reported works rely heavily on complex sample preparation and pixelated metasurfaces to reconstruct quasi‐continuous spectra. This study introduces a sample‐preparation‐free approach: terahertz near‐field scattering spectroscopy for powder detection. Lactose powder was directly sieved and dispersed onto a quasi‐BIC (QBIC) metasurface, forming a thin layer that effectively preserves the intrinsic material properties. A photoconductive probe was employed for near‐field detection and a differential analysis algorithm was proposed for sensing the quantity of lactose powder. By varying the probe height, the optimal detection distance is identified, yielding a sensitivity enhancement of 251% compared to far‐field measurements. Moreover, it is demonstrated that field localization and quality factor Q are pivotal in strengthening metasurface‐analyte interactions, resulting in a 470% sensitivity improvement of the QBIC sensing over the dipole‐mode‐based sensing. This study proposes a facile, single‐pixel strategy for the terahertz fingerprint detection of powdered materials, promising to expand the application scope of terahertz spectroscopy and advance the development of metasurface‐based sensing.