Distance-multiplexed terahertz absorption enhancement via dielectric metagrating for trace analyte detection

Terahertz (THz) spectroscopy offers substantial potential for the rapid and non-destructive detection of organic macromolecules, owing to its sensitivity to characteristic fingerprint absorption features arising from their unique vibrational and rotational modes. However, practical applications are hindered by the difficulty of detecting trace amounts of analytes. In this study, a novel approach is proposed to enhance the THz fingerprint spectrum by tuning the spacing between a structurally inverted dielectric metagrating and a metallic substrate. By varying the gap between the metagrating and the substrate, the absorption resonance peaks are shifted to overlap with the characteristic absorption band of α-lactose in the 0.48-0.57 THz range. Subsequently, an 0.2 μm α-lactose film is applied to the flat backside of the metagrating, resulting in an absorption envelope that is 411 times stronger than that of the free-standing film. These findings demonstrate that the proposed distance-tuning strategy offers a simple yet effective means to significantly enhance THz spectral responses across a broad frequency range, thereby improving the sensitivity of THz-based molecular detection.