Fig. 1. Schematic of coupled-mode system^[38]

Fig. 2. Applications of EP points in the optical and microwave regimes. (a) PT micro-ring lasers^[32-33]; (b) three micro-ring structures with PT symmetry, where the two side cavities have equal gain and loss^[40]; (c) PT symmetric high-sensitivity displacement sensors^[41]; (d) PT symmetric LC wireless sensing systems^[42]

Fig. 3. Passive terahertz metasurface EP points sensing. (a) Bilayer metasurface composed of titanium (purple) and gold (yellow) on a silicon substrate^[51]; (b) two degenerate eigenvalues corresponding to the EP points^[51]; (c) aperture resonator ring metasurface^[52]; (d) phase discontinuity of circularly polarized wave at the EP points^[52]; (e) plasmonic metasurface composed of two layers of laterally displaced gold bars^[53]; (f) resonance splitting measured by DP and EP sensors at different concentrations of anti-immunoglobulin G^[53]

Fig. 4. Passive terahertz metasurface EP points sensing. (a) (b) Schematic and diagram of a “π”-shaped plasmonic metasurface structure^[54]; (c) multilayer aperture resonator ring metasurface^[55]; (d) red line represents resonance frequency splitting proportional to the square root of the perturbation^[55]

Fig. 5. Active terahertz metasurface EP points sensing. (a) Metasurface unit composed of two orthogonally arranged SRRs (yellow)^[56]; (b) theoretical calculation surface of the magnitude and phase of eigenvalues in the (ω,σ) parameter space^[56]; (c) metasurface unit composed of two orthogonally arranged SRRs, with one SRR embedded with a square graphene patch in the gap^[57]; (d) calculation surface of the magnitude and phase of eigenvalues in the (ω,Γ_x) parameter space^[57]; (e) schematic of a graphene-metal hybrid metasurface structure^[58]; (f) relationship between phase and energy level EF at different frequencies^[58]

Fig. 6. Active terahertz metasurface EP points sensing. (a) Schematic diagram of the structure^[59]; (b) mode frequencies in the absorption spectrum displayed as a Riemann surface self-intersecting as a function of E_f and θ^[59]; (c) application diagram of Ge hybrid non-Hermitian metasurface under optical excitation^[60]; (d) left is simulated eigenmode transmission amplitude, right is phase in the parameter space covered by frequency and Ge conductivity^[60]; (e) schematic of achieving mutual and unidirectional transparent without reflection^[61]; (f) dual-port transmission line network model^[61]; (g) schematic of a PT-symmetric terahertz system composed of tunable graphene metasurface and metal plate^[62]; (h) transmitted and reflected schematic of terahertz waves incident from the bottom (blue) and top (red) of the graphene-based PT-symmetric sensor before (left) and after (right) chemical doping^[62]