Significance In the rapidly developing field of modern photonics, the precise and reliable control of the light polarization state stands as a crucial and indispensable foundation that strongly supports a wide range of core technological applications, including high-speed optical communication, advanced quantum information processing, and high-resolution optical imaging systems. Conventional Hermitian optical systems typically manipulate the polarization of light through passive geometric structural design, relying on fixed physical configurations rather than active tuning mechanisms. Their inherent confinement by the fundamental law of energy conservation gives rise to notable and unavoidable drawbacks in terms of dynamic response performance, operational flexibility, and adaptability to complex working environments. Non-Hermitian physics, as an innovative and revolutionary theoretical framework, breaks through the inherent shackles of this traditional Hermitian system by introducing gain and loss as active and controllable tuning degrees of freedom into optical systems, thereby opening up new possibilities for optical manipulation.

Progress As a promising interdisciplinary product integrating non-Hermitian physics and nanophotonics, non-Hermitian metasurfaces have rapidly emerged as a prominent and highly focused research direction in the photonics community in recent years. The theoretical foundation of non-Hermitian polarization metasurfaces is mainly established on the principles of non-Hermitian Hamiltonians and the time-domain coupled-mode theory (TCMT), with their most prominent and distinctive characteristic being the existence of exceptional points (EPs), which are unique to non-Hermitian systems. When the key parameters of the optical system evolve to an exceptional point, two or more eigenvalues and their corresponding eigenstates undergo simultaneous degeneracy, a phenomenon that does not occur in traditional Hermitian systems. This degeneracy further triggers drastic and remarkable changes in the overall system responses, such as efficient chiral mode conversion, unidirectional polarization rotation, and enhanced light-matter interaction.

By fully taking advantage of the intrinsic and unique properties of exceptional points (EPs) and parity-time (PT) symmetry, such non-Hermitian metasurfaces can realize a series of advanced polarization manipulation effects that are extremely hard or even impossible to achieve with traditional optical methods. These effects include asymmetric transmission of polarized light, topological phase transitions in polarization states, and polarization-dependent mode switching, all of which have important implications for the development of next-generation optical devices.

Early research work in this emerging and promising field mainly focused on the design, fabrication, and basic performance testing of static geometric structures. Researchers successfully achieved various basic polarization control functions, such as the conversion from linear polarization to circular polarization and the precise selection of specific circular polarization states, through the rational, precise, and elaborate design of subwavelength meta-atoms with specific shapes, sizes, and orientations. By further integrating gain and loss functional materials (such as semiconductor materials and plasmonic materials) into these carefully designed geometric structures, PT-symmetric metasurfaces were successfully fabricated and characterized. A variety of novel and interesting physical effects, including polarization-dependent laser emission with high directionality and unidirectional invisibility of polarized light, were experimentally observed, measured, and verified on these specially designed PT-symmetric metasurfaces. In recent years, research efforts in this dynamic field have gradually advanced towards deeper theoretical exploration, more refined experimental manipulation, and more practical application-oriented directions. Researchers have successfully achieved precise full coverage control of arbitrary polarization states on the Poincaré sphere by introducing anisotropic meta-atoms with tailored optical properties and gradient phase distributions across the metasurface. The organic and effective combination of geometric phase effects and non-Hermitian optical phenomena has also provided strong and effective support for the implementation of practical optical functions, such as broadband polarization conversion with high efficiency, efficient vortex beam generation with controllable topological charge, and polarization-insensitive light absorption.

In the ongoing study of non-Hermitian polarization metasurfaces, a key and significant breakthrough lies in achieving independent, precise, and synchronous control of multiple polarization degrees of freedom. Specifically, based on structure-optimized coupled resonator metasurfaces with carefully designed resonant modes, researchers have successfully achieved synchronous and precise control of two key polarization parameters: polarization ellipticity and polarization orientation. After artificially introducing well-controlled non-Hermitian coupling effects between orthogonal polarization modes, polarization-dependent nonreciprocal transmission phenomena were also clearly observed and systematically studied in experiments. These phenomena are the core working mechanisms of essential photonic devices such as optical isolators and circulators in modern optical communication and quantum information systems.

To effectively overcome the inherent limitations of traditional static metasurface devices, such as their fixed performance and lack of dynamic adjustability, researchers have in recent years begun to actively explore and develop dynamically tunable non-Hermitian polarization metasurfaces. By integrating functional materials and components such as electro-optic crystals, thermo-optic polymers, and optomechanical structures into the overall metasurface design, real-time, flexible, and reversible control over gain/loss distribution and polarization response has been successfully achieved, greatly expanding the practical application scenarios of these metasurfaces.

Conclusions and Prospects This paper focuses on the core theme of polarization control, comprehensively reviewing the relevant theoretical foundations, key experimental methods, and representative research achievements of non-Hermitian polarization metasurfaces from three important aspects: system geometric parameters optimization, polarization degrees of freedom control, and dynamic regulation by external fields. Looking ahead to the future development prospects of this exciting field, the deep and seamless integration of non-Hermitian physics with emerging advanced technologies such as machine learning, artificial intelligence, and topological photonics is expected to bring entirely new innovative breakthroughs to the study of non-Hermitian polarization metasurfaces.