Recent Advances in Polarization‐Encoded Optical Metasurfaces

Similar to frequency, phase, and amplitude, polarization is an intrinsic property of light. Generally, polarization describes the oscillation direction of the electric field and indicates the transverse nature of light. As polarization is uncorrelated with frequency, phase, and amplitude, it could extend the information channels and has be widely used in practical applications. However, the majority of light sources, such as the sun, emit unpolarized light with randomly oriented oscillation directions. With polarization optics, such as polarizers and wave plates, unpolarized light with randomly oriented vibrations is transformed into either a linear, circular, or elliptical light wave. However, conventional optical components that allow to manipulate the state of polarization (SOP) are usually based on the accumulated phase retardation between two orthogonally polarized electric fields when light propagates a distance much larger than its wavelength. As a consequence, traditional polarization optics are bulky, thereby limiting the potential of miniaturization and dense integration in advanced photonic devices and systems. Therefore, it is highly desired to fully manipulate the SOP with a truly compact device that possesses excellent performance and multiple functionalities. In recent years, optical metasurfaces, the 2D inhomogeneous interface composed of planar meta-atoms, have shown their unprecedented capabilities of controlling the optical fields with a subwavelength spatial resolution at the single meta-atom level. Specifically, the phase, amplitude, and even frequency of light can be tailored at will through judiciously designed metaatoms with particular responses. As a result, optical metasurfaces have gained much attention and started to replace conventional bulky optics with planar, compact, and high-performance metadevices, especially for polarization optics. In particular, polarization-encoded metasurfaces exhibit the advantages of flexibility, versatility, ease of fabrication and integration. Herein, we review the basic principles and emerging applications of polarization-encoded functional metasurfaces in the optical regime. We mainly focus our attention on the optical metasurfaces although there are many fantastic metasurface-enabled polarization converters and polarization-multiplexing devices in the microwave and terahertz regimes. After the description of polarization (Section 2), we introduce how to convert and detect the SOP of light with metasurfaces (Section 3 and 4). Then we summarize the main achievements in polarization-multiplexed metadevices (Section 5). Finally, we end up this review with a short conclusion and personal outlook on potential directions in this fast-growing research area (Section 6).