Graphene-Based Plasmonic Detection of Magnetic Field and Gaseous Medium with Photonic Spin Hall Effect in a Broad Terahertz Region

In this work, we propose and analyze a graphene-based plasmonic sensor with four layers (germanium, dielectric, graphene, and gaseous medium) for detection of refractive index (RI) of a gas medium and magnetic field applied to the graphene layer. The main idea is to exploit the strong plasmonic properties of the graphene monolayer in the THz frequency region and their strong dependence on applied magnetic field (B) for achieving high-performance sensor design. The transverse spin-dependent shift (SDS) of the horizontal photonic spin Hall effect (PSHE) at a given frequency (5 THz) is considered. The sensor structure is analyzed for two applications. First, in the conventional weak measurements, for a magnetic field B = 0 T and for an amplified angle ∆ = 0.1°, an extremely fine refractive index resolution of 1.22×10⁻¹¹ RIU is obtained for gas medium in the range 1-1.1 RIU. The above resolution can be further improved under modified weak measurements, where for an amplified angle ∆ = 5.730° (i.e., 0.1 rad), a refractive index resolution of 6.24×10⁻¹⁷ RIU can be achieved. Second, for magnetic field detection, in the conventional weak measurements and for ∆ = 0.1°, a magnetic field resolution of 0.0146 μT is achievable. Also, in modified weak measurements, for ∆ = 5.730°, the magnetic field resolution of as fine as 3.59×10⁻⁶ μT can be achieved with the proposed sensor scheme. Our results are significantly finer than the current stae-of-the-art sensors (5×10⁻⁹ RIU and 0.7 μT). Further, the sensing performance for gas detection has the inverse dependence while that of magnetic field sensor has direct dependence on applied magnetic field. The resolution achieved by the proposed sensor design gets finer for smaller THz frequency, where the real and imaginary parts of the graphene RI are large.