Refractive-index sensing based on terahertz cavity-modal confinement of Bragg metal grating structure

Abstract Integrated sensing platforms, which possess active layers of local electric-field oscillation in the terahertz (THz) frequency, can sensitively detect analytes via electromagnetic (EM) wave guidance, resonance, and spatial confinement in the system of THz time-domain spectroscopy. A Bragg grating structure of periodically perforated metal slits (PPMSs) is presented here as an active layer to generate the local electric-field oscillation of metal-cavity EM wave resonance that is coupled from 0.1–1 THz metal surface waves along the wavelength-scaled segments of metal slabs. The cavity modal confinement property, which is characterized from device transmittance or loss spectra, leads to the distribution of the metal-cavity wave resonance over a 35-pitch length and a 0.1–0.2 mm metal thickness of PPMSs. These two distribution dimensions of metal-cavity wave resonance represent longitudinal and transverse confinement performances, respectively. The PPMS-THz waves specified by 2D spatial confinement can realize the functional integration of a dielectric superstrate component and a sample adsorbing layer to adjust the dispersion responsivity and concentration of analytes, respectively. A refractive-index sensing application of glucose is presented by this integrated scheme for a molecular density range of 0–24 μg mm −2 and a resolution density of 1.6 μg mm −2 . The PPMS-based THz integrated sensing platform facilitates the developments of specific test papers and relevant molecular surface modification to target molecules.