Terahertz Asymmetric S-Shaped Complementary Metasurface Biosensor for Glucose Concentration

Abstract

In this article, we present a free-standing terahertz metasurface based on asymmetric S-shaped complementary resonators under normal incidence in transmission mode configuration. Each unit cell of the metasurface consists of two arms of mirrored S-shaped slots. We investigate the frequency response at different geometrical asymmetry via modifying the dimensions of one arm of the resonator. This configuration enables the excitation of asymmetric quasi-bound states in the continuum resonance and, hence, features very good field confinement that is very important for biosensing applications. Moreover, the performance of this configuration as a biosensor was examined for glucose concentration levels from 54 mg/dL to 342 mg/dL. This range covers hypoglycemia, normal, and hyperglycemia diabetes mellitus conditions. Two sample coating scenarios were considered, namely the top layer when the sample covers the metasurface and the top and bottom layers when the metasurface is sandwiched between the two layers. This strategy enabled very large resonance frequency redshifts of 236.1 and 286.6 GHz that were observed for the two scenarios for a 342 mg/dL concentration level and a layer thickness of 20 μm. Furthermore, for the second scenario and the same thickness, a wavelength sensitivity of 322,749 nm/RIU was found, which represents a factor of 2.3 enhancement compared to previous studies. The suggested terahertz metasurface biosensor in this paper could be used in the future for identifying hypoglycaemia and hyperglycemia conditions.

Keywords: biosensing; bound states in the continuum; glucose sensing; metasurfaces; terahertz technology.

Figure 1

(a) Unit cell dimensions of the asymmetric S-shaped complementary resonator with the related dimensions. (b) Three-dimensional representation showing the glucose sample top and bottom layers. The inset of part (a) illustrates the field polarization.

Figure 2

(a) Transmission and (b) reflection spectral map response of the S-shaped metasurface for l₂ values that swept between 120 μm and 140 μm with 1 μm step.

Figure 3

(a) Transmission and (b) reflection frequency response of the S-shaped complementary metasurface for the symmetric case when l₂ = 140 μm and the asymmetric case when l₂ = 130 μm.

Figure 4

Spatial distributions of electric fields (a) and (b), magnetic fields (c) and (d) at the surface of the metasurface for the asymmetric resonance (a) and (c), and symmetric dipole resonance (b) and (d) modes.

Figure 5

Asymmetric resonance frequency shifts when (a) the top layer is considered and when (b) the top and bottom layers are considered for different glucose concentration levels and a sweep of glucose sample thicknesses between 2 and 20 μm.

Figure 6

Asymmetric resonance frequency shift for two scenarios (i) top layer and (ii) top and bottom layers for different values of the refractive index between 1.2 and 2.0 with each sample layer thickness being 20 μm.