Resolving Chiral Biomolecule Mixtures via Terahertz Eigenmode‐Fingerprint Circular Dichroism Spectroscopy Empowered by Achiral Gradient Metasurfaces

Terahertz (THz) spectroscopy, a label-free and noninvasive tool that is used to resolve the characteristic information of biomolecules, has attracted extensive attention in both physics and biochemistry. However, the traditional THz time-domain spectroscopy (THz-TDS) systems are constrained by linear polarization, disregarding the analyte chirality information that is vital for conducting biomedical analyses in the food and pharmaceutical industries. Here, we develop a technique called THz eigenmode-fingerprint chiroptical spectroscopy (TEFCS) on an achiral gradient metasurface (AGM) platform, enabling the unambiguous resolution of heterogeneous chiral biomolecule mixtures (constituents, chiralities, and ratios). The THz chiral phonon signals can be spectrally consistent and significantly enhanced due to the optimized spectral overlap between the broadband AGM resonances and the biomolecule chiral phonons, resulting in augmented eigen circular dichroism (CD) spectra. The enhanced photon‒phonon coupling scheme in the AGM‒biomolecule system is understood via the anisotropic coupled oscillator theory and gives rise to a substantial sensitivity improvement. These findings provide new insights into integrated bioanalyses and pharmaceutical applications.