Design of Refractive Index Sensors Based on Valley Photonic Crystal Mach-Zehnder Interferometer

The refractive index is an important optical property of materials which can be used to understand the composition of materials. Therefore, refractive index sensing plays a vital role in biological diagnosis and therapy, material analysis, (bio)chemical sensing, and environmental monitoring. Conventional optical refractive index sensors based on optical fibers and ridge waveguides have relatively large sizes of a few millimeters, making them unsuitable for on-chip integration. Photonic crystals (PCs) have been used to significantly improve the compactness of refractive index sensors for on-chip integration. However, PC structures suffer from defect-introduced strong scattering, resulting in low transmittance, particularly at sharp bends. Valley photonic crystals (VPCs) can realize defect-immune unidirectional transmission of topological edge states, effectively reducing the scattering loss and increasing the transmittance. However, optical refractive index sensors based on VPC structures have not been demonstrated. This paper proposes a refractive index sensor based on a VPC Mach-Zehnder interferometer (MZI) structure with a high forward transmittance of 0.91 and a sensitivity of 1534%/RIU at the sensing wavelength of λ = 1533.97 nm within the index range from 1.0 to 2.0, which is higher than most demonstrated optical refractive index sensors in the field. The sensor has an ultracompact footprint of 9.26 μm × 7.99 μm. The design can be fabricated by complementary metal-oxide semiconductor (CMOS) fabrication technologies. Therefore, it will find broad applications in biology, material science, and medical science.