Fig. 4

Ultra-wideband THz fingerprint retrieval and identification of chiral molecules under the synchronous voltage tuning scheme. An ultra-thin molecular coating with a thickness of 1 μm is introduced for analysis. a, d The complex refractive index of chiral drug molecules L-carnitine and D-carnitine. The insets give their molecular formulas and chiral structural features. b, e The first-order absorption fingerprint envelopes of trace L-carnitine and D-carnitine molecules obtained by broadband retrieval based on graphene metasurface. The inset shows the molecular coatings deposited on the metasurface. The yellow mask is the non-applicable (N/A) area. The black frame marks the calculation range of the areas under the peaks to be obtained. c, f The second-order absorption fingerprint envelopes, the natural absorption spectra and the calculated signal enhancement factors of trace L-carnitine and D-carnitine molecules. g The developed nonlinear fingerprint spectral inversion model. h, i Extracted the ratios of absorption peak amplitudes and the ratios of areas under the absorption peak in the first-order absorption fingerprint envelopes corresponding to trace L-carnitine and D-carnitine molecules