Fig. 1. (a) Diagram of chiral molecules. (b) Interaction between chiral light field and chiral substances. (c) THz circular dichroism spectra of L- and D-glucose were detected without metamaterials^[23]. (d) Different spectroscopic modalities, vibrational modes, and characteristic excitation energies of chiroptical spectroscopies^[11]. TCD, terahertz circular dichroism; CPL, circularly polarized luminescence.

Fig. 2. (a) Schematic diagram of transmitted THz-TDPS system^[61]. (b) The light path of the reflected THz-TDPS system and the schematic diagram of the experimental measurement principle^[62].

Fig. 3. (a) Metal metasurfaces: arrow-shaped (left) and symmetry-breaking double-splits ring-shaped (right)^[63,64]. (b) All-dielectric metasurfaces: Fano-resonant metasurface (left) and angle-multiplexed metasurface (right)^[65,66]. (c) Flexible metasurface^[67]. (d) Metasurface based on 3D printing technology^[68].

Fig. 4. (a) Detection of thermal denaturation of the BSA, WP, and OVA^[70]. (b) Hydrolysis detection of the BSA under papain^[71]. (c) Detection of the crystallization process of the WP from a dissolved to a crystalline state^[72].

Fig. 5. (a) Quantitative detection and qualitative identification of cell proliferation HepG2, Huh7, and H7402 hepatoma cells^[73]. (b) Measurement of cell proliferation under aspirin inhibition^[74]. (c) The comprehensive detection technique is based on polarization-state sensing of distinguishing H9N2, H1N1, and H5N2 viruses with closely resembling optical properties^[75].

Fig. 6. (a) Quantitative detection and the qualitative discrimination between lactose, glucose, and galactose with the ultra-thin flexible metasurface^[61]. (b) The highly sensitive identification between amino acids and their chiral enantiomers^[76]. (c) The highly sensitive chiral biochemical identification of the tyrosine with the PB metasurface^[77].

Fig. 7. (a) Enhancement of the chirality response of the CYT with the chiral metamaterial prepared by the lithography-free fabrication^[78]. (b) The label-free discrimination of D- and L-proline with the chiral metamaterial^[79]. (c) The highly-sensitive quantitative detection and the qualitative identification of amino acid D- and L-enantiomers with a metasurface-excited chirality at oblique incidence^[62].

Fig. 8. (a) Schematic demonstration of the ZIKV envelope protein binding with the respective antibody on the metasurface (left). SEM images of the metasurface covered with antibody and ZIKV envelope proteins attached to the antibody, respectively (middle). Transmission spectra for the presence of different concentrations of the ZIKV envelope protein (right)^[93]. (b) Schematic diagram of the manufacturing flow for the CASR-graphene TMFC (left). Measured THz transmission spectra of the target DNA at different concentrations (middle). Specificity of the CASR-graphene TMFC biosensor for detecting samples at the concentration of 100 µmol/L (right)^[95]. (c) Schematic of the all-dielectric metasurface consisting of double-ring arrays (left). Enantiomer identification results for L-Arg and D-Arg at the concentration of 40 mg/mL on the functionalized metasurface (right)^[96].

Fig. 9. (a) Schematic diagram of the metasurface and reflection module (left). Transmission spectra for L-phe and D-Phe at the same concentration (right)^[48]. (b) Experimental procedure of the proposed strategy and metasurface (left). Sensing performance of the THz biosensor (right)^[99]. (c) Graphene composite nano slot-based THz metamaterials for the recognition of single-stranded DNA (left). Experimental transmission spectra for different samples^[85].

Fig. 10. (a) Schematic of the all-dielectric metasurface consisting of a split half-cylinder array (left). Experimental transmission spectra for the BSA, OVA, WP, and HA antigen (middle). Experimental transmission spectra of y-polarization for HA antigen (right)^[102]. (b) An artistic rendering of the designed multipixel toroidal unit cell (left). Measured transmission spectra of the THz meta-sensor device for different concentrations of SARS-CoV-2 spike protein (right)^[103]. (c) Schematic diagram of the GNP modification and binding processes for EGFR detection (left). The transmission amplitude spectra of EGFR were detected with different sized GNP-Ab functionalized sensors (right)^[104].

Fig. 11. (a) Schematic of the fabrication and operating principle of the proposed THz metamaterial biosensor for bacterial DNA detection (left). Evaluation of the specificity of the synthetic S. aureus DNA detection (middle). Evaluation of the specificity of the S. aureus genomic DNA detection (right)^[106]. (b) Schematic diagram of the manufacturing flow for the CASR-graphene TMFC (left). Biosensing of the Eae gene sequences of E. coli O157:H7 (right)^[107]. (c) Schematic of the fabrication and operating principle of the proposed THz metamaterial biosensor for bacterial DNA detection (left). Detection of synthetic S. aureus DNA by the proposed biosensor (middle); detection of S. aureus genomic DNA by the THz metamaterial biosensor (right)^[108].