Influence of sample preparation methods on the spatial overlap between analytes and metasurfaces in...

Terahertz spectroscopy, with its intrinsic molecular fingerprint characteristics, has become a powerful tool for qualitative and quantitative analysis of organic materials. Metasurfaces can significantly enhance terahertz signals through strong localized electromagnetic fields; however, the enhancement critically depends on the spatial overlap between the organic crystal and the metasurface resonance mode. To address the variation in coupling efficiency caused by different sample preparation conditions, this study employs absorption-induced transparency (AIT) as the characteristic enhancement mechanism and designs a terahertz metasurface for coupling-enhanced characterization of lactose thin films. Two AIT features including resonance redshift and resonance-depth reduction are used as quantitative indicators, together with uncertainty analysis, to evaluate the spatial coupling quality. Four preparation methods for forming lactose films were systematically compared: static convection heating, static conduction heating, static radiation heating, and spin-coating followed by radiation heating. Experimental analyses of film uniformity, crystal morphology, and vibration modes overlapping reveal that the spin-coating & radiation-heating method achieves the highest coupling efficiency. This method yields the strongest AIT response, with a resonance redshift of 133.9 ± 0.2 GHz and a transmission increase of 10.5 ± 0.3%, while also exhibiting the smallest uncertainties. Moreover, it requires only one-quarter of the solution amount used by the other three methods, providing the best balance between enhancement performance, reproducibility, and material utilization. This work clarifies the fundamental relationship between sample preparation and spatial coupling between metasurface and analyte, offering a reproducible experimental strategy for optimizing metasurface design and enabling highly sensitive terahertz detection of trace organic materials.