Fig. 1. (a) Schematic diagram of the proposed THz metasurface structure. (b) Unit cell of the metasurface with geometric parameters. (c) The photograph and (d) micrograph of the fabricated metasurface sample. (e) The measured thickness of the copper layer on the quartz substrate.

Fig. 2. (a) Simulated and (b) experimental transmission spectra of the metasurface structure with different values of L2 (L1=130  μm). The dashed lines trace the evolution of the resonances. (c) Two-dimensional transmittance spectra with different value of L2. (d) Q-factor of the simulated (sphere) and experimental (star) QBIC resonance as a function of asymmetry α (α=L1−L2).

Fig. 3. (a) Surface electric field and (b) magnetic field distribution at 0.64 THz (BIC mode with L2=130  μm). (c) Surface electric field and (d) magnetic field distribution at 0.64 THz (QBIC mode with L2=120  μm).

Fig. 4. (a) Q-factor and resonance intensity with different asymmetry α. (b) Q×I value with different asymmetry α. (c) Resonance frequency shift with the analyte thickness increasing, where refractive index was fixed as n=1.6. (d) Column chart of the resonance frequency shift (the refractive index changes from 1.0 to 2.0).

Fig. 5. (a) Experimental setup of the continuous-wave THz spectroscopy system. The inset shows the glass rotor flowmeter. (b) Schematic diagram of the THz spectroscopy system. (c) Schematic representation of the sensing measurement. (d) Frequency shifts of the QBIC resonance at different chlorpyrifos concentrations ranging from 0.1 to 1000 ppm.