High-sensitivity complex refractive index sensing using Fano resonance based on a side-coupled slotted photonic crystal nanobeam cavity

We propose an optical sensor comprising a straight waveguide integrated with a partial transmitting element and a side-coupled slotted photonic crystal nanobeam cavity (SPCNC) for complex refractive index sensing (CRI) in gaseous environments. The coherent interaction between the continuous state in the waveguide and the discrete state in the SPCNC generates a sharp and asymmetric Fano resonance lineshape in the transmission spectrum. Through systematic structural optimization and rigorous three-dimensional finite-difference time-domain simulations, the proposed sensor achieves exceptional sensitivities of 518 nm/RIU for the real refractive index and 16750<i>R</i><i>I</i><i>U</i>^-1 for the imaginary refractive index in CRI sensing. Furthermore, the impact of fabrication disorder on the sensing performance is investigated and evaluated. The proposed sensor demonstrates significant potential for applications in multi-parameter sensing and multi-component mixture detection.