Electromagnetic (EM) wave front modulation has significance for both scientific studies and industrial applications. However, traditional components for wave field manipulations are often bulky and heavy, which restricts their utilization in miniaturized optical devices and compact detection systems. Metasurfaces are usually composed of arrays of artificial microstructures, also known as meta-atoms, and arranged in a uniform or non-uniform spatial pattern. They can arbitrarily manipulate the amplitude, phase, and polarization of light with sub-wavelength resolution. As a result, metasurfaces have caught much attention in the research on next-generation optical systems. The metasurface design and fabrication has dramatically boosted the employment of optical field modification in compact optical equipment. Metasurfaces are anticipated to break through the bottleneck of conventional optical components and systems, paving the way for miniaturization, integration, and multifunctional processes. While traditional optical elements primarily regulate the optical field by phase accumulation of light along propagation, metasurfaces provide a novel method for controlling the optical field at subwavelength ranges via the interaction between light and meta-atoms. As a two-dimensional planar material with a thin depth profile, a metasurface can generate non-classical phase distributions for transmitted and reflected electromagnetic waves at its interface. Therefore, more flexible control over the wavefront can be realized. Polarization is an inherent property of light waves and electromagnetic waves. It comprises abundant information on substances, which is essential for target detection and identification. The efficient polarization state control is the core content of electromagnetic wave manipulation, which is vital for imaging, communication, display, optical encryption, and optical force manipulation. Due to its ability to manipulate the polarization state on the subwavelength scale, the metasurface serves as a powerful tool for polarization regulation and vector beam generation. Conventional polarization regulators such as those found in natural materials and three-dimensional superstructural materials typically control the global polarization of light by manipulating the amplitude and phase delay of the electric field in the orthogonal polarization components. In contrast, polarization and wavefront modulation by metasurface are the results of“abrupt phase changes”in anisotropic reflection/transmission at the interface.“Abrupt phase changes”mean a surface effect that depending on the mechanism can originate from the geometric (Pancharatnam Berry) phase, propagation phase, and generalized geometric phase. Artificially created meta-atoms can overcome the limitations of natural materials, like limited birefringence and polarization sensitivity. Acting as birefringent elements, the meta-atoms with specific designs can significantly enhance the polarization modulation capability, and be adopted to realize subwavelength pixelated polarization control for polarization conversion, polarization-dependent multiplexing and even generating complex vector beams. In recent years, polarization-modulated metasurfaces have drawn a lot of attention both in theory and applications, with continuously evolving new principles and applications. Thus, it is important to outline present research and emerging applications and thus better guide future development.

We describe the basic principle and typical structure of metasurface design for polarization control, and then introduce and discuss the representative applications of metasurfaces, including polarization conversion along the propagation direction, vector vortex beam generation, vector holography and encryption, polarimeter, and dynamic control. In classic optics, the light polarization is commonly expressed mathematically by the Jones vector. Thus, the polarization control mechanism of metasurfaces is explained by solving Jones matrix. The roadmap of electromagnetic polarization manipulations with metasurfaces is summarized in Fig. 1. The two important mechanisms for metasurfaces to control polarization are the geometric phase and the propagation phase. The geometric phase for anisotropic materials is solely determined by the rotation angle θ as it results from the photonic spin-orbit interaction (PSOI). The propagation phase is related to the shape and size of the structure. Recently, our research group has proposed a novel principle of metasurfaces to break the PSOI symmetry by merging the geometric phase and propagation phase. We also summarize some emerging applications of polarization modulation by metasurfaces, involving polarization conversion along propagation direction, vector vortex beam generation, vector holography and encryption, polarimeter, and dynamic control. Meanwhile, we elaborate on the ideas of each study, analyze their advantages and limitations, and discuss their prospective implementations. For example, in addition to controlling polarization in the transverse plane, a new class of metasurfaces achieves parallel polarization transformations along the optical path. In this case, a single metasurface can mimic an arrangement of multiple polarization optics cascaded in series. By employing the polarization-dependent phase optimization concept, our group has reported a crosstalk-free broadband achromatic full Stokes imaging polarimeter composed of polarization-sensitive dielectric metalenses. The average crosstalk under incident light with arbitrary polarization has been largely reduced to ensure a more precise measurement of the polarization state. Finally, the challenges and future development direction of polarization-modulated metasurfaces and the areas are also prospected.

In summary, we highlight the promising polarization-related applications for metasurfaces and serve as up-to-date references for researchers in metasurface and metamaterial fields. As a new generation of transformative optical devices, polarization-modulated metasurfaces will provide a broad platform for polarization conversion, vector vortex beam generation, vector holography and encryption, polarimeter, etc.