Highly sensitive simultaneous measurement of refractive index and its changes using a photonic crystal biosensor with coupled resonators

• Proposed novel highly sensitive biosensors based on 1D photonic crystal with two defect layers. • Simultaneous measurement of RI and its changes, ascertain the precise value of the analyte RI using coupled modes. • Appearance of coupled modes due to introducing defect layers and using them for sensing. • Determination of the threshold separation between defect layers to form coupled modes. • Highly effective sensor for sensing the analytes with an RI close to reference materials. The paper proposes a novel optical sensor based on one-dimensional binary (SiO2/Si) photonic crystals (PC) with two defect layers to measure the refractive index of the sensing analyte directly and in real-time. Two coupled modes are observed in the bandgap of the PC which is affected by the defect layer separation and the difference between the refractive index of analyte and reference materials. The sensitivities of the coupled modes increase as the two defect layers get closer and the threshold distance of dual defect layers for using the advantage of coupled modes is determined in the paper. The sensitivities and Q factors of these two coupled modes are different in most conditions which highlights the potential of utilizing one of the coupled modes with higher sensitivity for sensing applications of the specific material. Furthermore, accurate and real-time measurements of the analyte refractive index are possible with these coupled modes. This capability enhances the performance of optical sensors based on photonic crystals, allowing for precise measurements in various environments. The proposed optical sensor demonstrates promising features for industrial, diagnostic, and medical applications due to its simplicity in production and capability for multiple simultaneous analyte measurements on a single chip.