Multi-dimensional terahertz detection for methamphetamine using microporous metal Metasurfaces and multi-wavelength...

To enhance the sensitivity and information dimension of terahertz (THz) detection for methamphetamine, this study proposes a THz time-domain spectroscopy (THz-TDS) detection method combining microporous metal metasurfaces with multi-wavelength optical pumping. This method uses a 0.1 mm-thick aluminum sheet as the substrate to fabricate a microporous metasurface structure with a pore diameter of 100 μm and a pore spacing of 200 μm via laser processing. By taking advantage of the local field enhancement effect, the interaction between THz waves and the sample is significantly enhanced. During the experiment, the methamphetamine standard solution coated on metasurface was irradiated by 405 nm, 450 nm and 780 nm lasers, and the THz absorption spectra, refractive indices and dielectric constants under different laser wavelengths were measured. Under dark conditions, the changes in refractive index, absorption coefficient and dielectric constant primarily stem from intrinsic electronic transitions, molecular vibrations and dipole orientation within the sample. Under single wavelength laser pumping at 405 nm, 450 nm and 780 nm, the overall trends of the refractive index and dielectric constant resemble those observed under dark conditions, though the specific values vary with the wavelength. The absorption coefficient exhibits a multi-peak-and-valley variation phenomenon, with the peak-and-valley morphology altering with wavelength, reflecting its dual dependence on frequency and wavelength. This phenomenon arises from the laser excitation of specific electronic transitions or molecular vibrations. Under combined pumping of 405 nm, 450 nm and 780 nm lasers, the trends in the refractive index and absorption coefficient persist as described above. However, the imaginary part of the dielectric constant increases with the rise of photon energy, reflecting the enhanced scattering due to the increase in carrier concentration and the increased energy dissipation of THz waves. This aligns with the Drude model's description of the free charge carrier behaviors. Multi-wavelength co-excitation induces multiple electronic transitions and molecular vibrations, triggering the complex carrier dynamics and structural changes, thereby enabling the synergistic regulation of spectral parameters. The method exhibits an excellent linear response in the range of 10 μg/mL ∼ 100 μg/mL for methamphetamine quantitative detection, with correlation coefficients R² > 0.997 at the two characteristic absorption peaks. The limit of detection (LOD) is 10 μg/mL in this paper, which is five orders of magnitude lower than that of the conventional method. Theoretical analysis reveals that the laser wavelength influences the THz response by modulating the molecular energy level transitions and carrier concentration, while the local surface plasmon resonance (LSPR) effect of the metasurface enhances the regulatory effect. Based on the results in this paper, the "metasurface enhancement + multi-wavelength optical pumping" recognition scheme in this study provides the theoretical basis and experimental support for the rapid detection of methamphetamine.