Generalized Parity–Time Symmetric Metasurfaces

Parity–time (PT) symmetric metasurfaces, which rely on balanced loss and gain, enable exceptional optical phenomena but face practical limitations due to stringent material gain requirements. Here, we propose a generalized PT symmetry scheme that significantly reduces the needed gain while preserving non-Hermitian wave phenomena. We establish a generalized PT-symmetric metasurface system by exploiting a metasurface with near-zero imaginary permeability (|Im(μ)| ∼ 0), which exploits a magnetic-field enhancement effect to exactly emulate the wave behaviors of a conventional gain metasurface (|Im(ε)| ≫ 1). Remarkably, this approach reduces the imaginary component of gain material parameters by over 5 orders of magnitude while maintaining key non-Hermitian effects, including exceptional points and unidirectional reflectionless refraction. Full-wave simulations of a practical implementation validate our design. This work presents a feasible strategy to overcome the bottleneck of high-gain material requirements in non-Hermitian wave control.