Parity-time symmetry based on resonant optical tunneling effect for biosensing

This paper proposes and analyzes a parity-time (PT) symmetry structure based on resonant optical tunneling effect (ROTE) by using two directly coupled ROTE resonators to achieve a balanced gain-loss distribution. The unbroken/broken states of the PT symmetric system are theoretically verified by coupled-mode theory (CMT), transmission matrix method (TMM) and finite-difference time-domain (FDTD). To demonstrate the application potential, we further propose a label-free biosensing scheme that takes advantages of the square-root dependence in frequency splitting near exceptional point (EP). The theoretical results show that the sensor has a maximum sensitivity of 1 × 10⁵ nm/IP unit (imaginary part unit of refractive index) and a theoretical detection limit of 5 × 10⁻¹⁰ IP unit (corresponds to 0.4 ng carcinoembryonic antigen (CEA)). Compared with the PT systems based on coupled waveguides or resonators, our design has some distinctive features. It is a multi-layer structure and does not need complicated nanoscale fabrication; the liquid samples “flow-through” the sensing region in the mid of PT structure and would greatly enhance the analyte binding efficiency as compared with the common “flow-over” manner. This simple yet highly sensitive platform would find applications in biomedical sensors, drinking water safety, and drug screening.