Multifunctional integrated devices have become the mainstream of nanophononics research in recent years as optical devices continue to evolve towards high capacity, multichannel, low loss and integration. Additionally, the arbitrary manipulation of circularly polarized (CP) electromagnetic (EM) waves is significant for a wide range of applications such as chiral molecule manipulation, imaging, and optical communication. However, conventional optical devices composed of natural materials cannot realize multiplexing with only one optical device because they rely on the body properties to change the propagation phase so as to modulate electromagnetic waves. As a result, conventional optical devices are not conducive to the diversification, integration, miniaturization, and efficiency improvement of optical devices due to the single function, system complexity, large size, and low efficiency.

In recent years, metasurfaces consisting of a series of ultra-thin subwavelength artificial atoms arranged in a specific manner in the plane have demonstrated powerful modulation of electromagnetic waves, providing a good platform for realizing multifunctional integration. Researchers have discovered a series of exotic physical phenomena and powerful planar optical devices by exploiting the advantages of metasurfaces, such as lightness and thinness, large degree of modulation freedom, low loss, and easy conformality and integration. The mechanisms for modulating the phase of EM waves based on metasurfaces can be classified into three main types including resonant phase, geometric phase, and propagation phase. The resonant phase modulation mechanism is usually achieved by changing the geometry of the constituent artificial atoms to shift their resonant frequencies under arbitrarily polarized incident light. The propagation phase is realized by accumulating the phase of an EM wave as it propagates within the artificial atoms of the medium. The geometric phase is achieved by rotating the artificial atoms to change the phase of the outgoing light, while the polarization state of the outgoing light is opposite to the circular polarization state of the incident light. Among them, the resonance phase and the propagation phase do not depend on the polarization state of the incident light, while the geometric phase relies on the CP light. Typically, the three types of metasurfaces realize a single function, and it is vital to extend the integration of device functions as the application and device integration requirements continue to increase. By changing the geometry of the two orthogonal directions of the artificial atoms and employing the resonance phase or propagation phase to design the resonance frequency of the artificial atoms, researchers can realize multifunctional integration of different lines of polarized incident light under irradiation from the device, which can be completely different in free space. This type of device is complicated by the fact that the two main axes of artificial atoms are not completely independent, resulting in crosstalk and complex design. Due to the strong controllability of CP waves, geometric phase metasurfaces have caught enormous research interest. However, these meta-devices exhibit locked functionalities under illuminations of CP light with different chirality. Meanwhile, such metasurfaces for modulating CP light are also employed to achieve multifunctional integration. This is yielded by combining several sets of geometric phases with different functions in the same device, and thus several different functions are formed in free space under irradiation from the same CP light, which is referred to as a “merge phase metasurface”. However, as it does not fully decouple different chirality of light, there is still function binding and low efficiency. More recently, researchers have found that the combination of the spin-dependent geometric phase with the resonance or propagation phase can unlock the fixed function. Such metasurfaces often referred to as composite phase metasurfaces have been adopted to further improve device performance and integration in response to the growing demand for integrated optics applications. Starting from the three different phase mechanisms for electromagnetic wave manipulation by metasurfaces, we present a brief overview of resonant phase metasurfaces, geometric phase metasurfaces, propagation phase metasurfaces, and composite phase metasurfaces, with their operating principles, design strategies, and experimental implementations included, and recent research advances in this field briefly discussed.

Finally, we study spin-decoupled composite phase metasurfaces. Today’s multifunctional devices are still at the laboratory stage, but in the future, they can be integrated with research in other fields to solve some bottlenecks, such as directing incident light of different chirality to different regions on a chip for biomonitoring. Additionally, most polarization multiplexing devices to date can only perform passive and static functions. Therefore, the study of multifunctional devices with active tunable operation of incident waves of different polarization states will play a vital role in future practical applications. We hope that this brief review will help readers deepen their understanding of geometric phase metasurfaces and composite phase metasurfaces, and provide guidance for designing their components in the future.