Graphene/Au/SiC Optical Waveguide-Based Surface Plasmon Resonance Sensor for Chemical and Biomedical Sensing in Aqueous Medium

Abstract

In chemical and biomedical sensing, identification and quantification of analytes in aqueous medium such as water, blood or other carriers are very important. It requires the core sensors to have the properties of low cost, high sensitivity, on-chip integration capability, etc. Various sensing methods have been developed such as prism coupler and grating coupler, etc. and among them, waveguide-based surface plasmon resonance (SPR) sensors, with sensitivity of the optical waveguide significantly improved by SPR, have been identified as a powerful tool for detection and quantitative characterization of chemical and biological species or biomolecular reactions in aqueous medium. For sensing in aqueous medium, the optical waveguide sensors are desirable to operate in the visible light wavelength to overcome the large absorption coefficient of water in the near infrared range. This high absorption coefficient causes optical energy in the near infrared incident light be absorbed by water rather than by the sensing medium. Conventional silicon (Si) is not the choice of material for such sensors due to the strong absorption below the wavelength of 1.1 μm due to its narrow bandgap (Eg=1.12 eV, 1.1 µm wavelength).As a wide bandgap semiconductor, silicon carbide (SiC) is more desirable for such sensors with advantages of: (1) bandgap energy of 3.2 eV in 4H and 6H polytypes and 2.2 eV in 3C polytype with transparent wavelength in visible and near-infrared range, (2) excellent material properties especially the chemical inertness and biocompatibility, and (3) device fabrication compatibility with standard Si device fabrication for manufacturing and on-chip integration. Therefore, SiC is a desirable material candidate for SPR sensors for chemical and biomedical sensing in aqueous medium, and approaches to enhance its sensitivity have been explored. 2-D materials such as graphene, TMDCs (MoS2, WS2, MoSe2, WSe2), etc. and their applications in sensing technology has aroused wide attention because of their distinctive electrical and optical properties. SPR sensors with 2-D and Si to enhance the sensitivity have been studied by publications. In this paper, we investigated graphene/Au/SiC waveguide-based SPR sensors (Fig. 1) for aqueous medium sensing. The resonance frequency shift with different numbers of graphene layer were studied (Fig. 2). Comparison of resonance wavelength shift from conventional and proposed sensor were performed (Fig. 3). At the incident light of 633 nm, the confinement factor above 0.9 can be achieved at the refractive index of n=1.3~1.5. With the change of refractive index, the shift of resonant wavelength showed linear characteristics, and a sensitivity of 2580 nm/RIU (refractive index unit) was obtained in a refractive index range of n=1.34~1.36 (Fig. 4). Figure 1