Fig. 1. Common structures for implementing EP. (a) Single micro-cavity system^[24]; (b) coupled two-components system^[38]

Fig. 2. Eigenvalue evolution of PT symmetric system ^[13]. (a) Real part of eigenvalues versus normalized coupling strength κ/κ_PT, the inset shows that the PT-symmetric system consists of two coupled micro-cavities (i.e. the two components) with gain G and loss -G; (b) imaginary part of eigenvalues versus normalized coupling strength κ/κ_PT, the inset sketches the intra-cavity field intensities in the PT-sy

Fig. 3. Frequency splitting characteristics of sensors operating at an EP or a DP^[24]. (a) Frequency splitting of DP versus the perturbation strength ε; (b) frequency splitting of EP versus the perturbation strength ε

Fig. 4. EP sensing applied for nanoparticle detection. (a) Schematic of the micro-toroid with two nanofiber tips to achieve EP^[19]; (b) micro-disk with two particles ^[20]; (c) micro-disk with two holes^[22]; (d) DP sensor consisting of a single microcavity, and EP sensor consisting of a single microcavity and two local perturbations; (e) ratio of the freque

Fig. 5. Temperature sensing based on EP. (a) Schematic of the thermo-sensitive glass slide^[34]; (b) temperature response curves (the ordinates of the three curves from top to bottom are: half of the eigenfrequency splitting from EP, forward reflection coefficient from EP, and half of the eigenfrequency splitting from DP^[34]; (c) schematic of ternary PT-symmetric coupled micro-cavity; (d) frequency splitti

Fig. 6. Refractive index sensing based on EP. (a) Non-Hermitian multilayers dielectric structure based on GH shift^[31]; (b) SC versus real part of refractive index of the D layer^[31]; (c) quasi-PT-symmetry coupling structure^[32]

Fig. 7. Optical gyroscope based on EP^[27]. (a) Schematic of PT-symmetric laser gyroscope system; (b) beat frequency as a function of the rotation rate for single ring (dotted line) and PT-symmetric coupled ring systems with different coupling strengths

Fig. 8. Graphene-based biological or chemical sensing based on EP^[37]. (a) PT-symmetric system consisting of active graphene metal resistance wire connected by dielectric with a thickness of d; (b) two-port transmission-line model; (c) scattering spectra of the PT symmetric graphene sensor under different doping levels; (d) scattering spectra of the conventional passive graphene sensor unfer different doping levels; (e) frequency shift of PT symme