Electromagnetic metamaterials are composed of sub-wavelength artificial unit cells with periodic and aperiodic arrangements, which can achieve peculiar properties that natural materials do not have. As the two-dimensional metamaterials, metasurfaces have the advantages of low profile, easy integration and low cost. With the introduction of active elements, sensing elements and intelligent algorithms, metasurfaces further realize real-time programmable and intelligent control of electromagnetic waves. At present, most electromagnetic metasurfaces researches are devoted to the manipulation of reflecting waves and transmitting waves. In fact, electromagnetic metasurfaces also have the strong regulation ability for radiating waves. This paper will introduce the research progress of metasurfaces in regulating the amplitude, phase, polarization of radiating waves systematically. Based on the integration of metasurfaces and feeds and the regulation principle of metasurfaces on radiating electromagnetic waves, this paper focuses on folded array metasurfaces, Fabry-Perot metasurfaces, leaky wave metasurfaces and radiation-type metasurfaces, corresponding to space wave feeding, surface wave feeding, gap coupling feeding, coaxial feeding. The regulation mechanism and applications of these four types of metasurfaces on radiating waves are introduced from the perspectives of passive and active. Finally, the future research directions of electromagnetic metasurfaces in regulating radiating waves are prospected.

All-metal metasurfaces are structural arrays composed of sub-wavelength metal units, which exhibit high efficiency and large bandwidth in phase manipulation of electromagnetic waves. Compared with metal-dielectric hybrid metasurfaces, all-metal metasurfaces have excellent thermal and mechanical properties, such as high-temperature resistance, high strength, and good ductility, which enable them to be applied in extremely complex environments such as high temperature and high pressure. In this paper, we briefly summarize the recent research progress based on all-metal metasurfaces. We mainly introduce their applications in the construction of highly efficient and multi-functional planar optical devices as well as multi-spectrum electromagnetic stealth, and provide an outlook of the future direction of its development.

The quasi-bound state in the continuum (quasi-BIC) is a special resonant mode in a metasurface with a very high quality factor that can greatly enhance the light-matter interaction and has important applications in fluorescence enhancement, nanolaser, optical sensing, and nonlinear optics. In this paper, we study the application of quasi-BIC dielectric metasurface for refractive index sensing based on the theory generated by our previous quasi-BIC. The basic structure of the sensing device is given, the preparation of the sample optofluidic structure is completed by using electron beam lithography combined with injection molding process, and the performance is initially tested. The results showed that the metasurface has two high-Q quasi-BIC resonance peaks (1.523 μm and 1.570 μm, with quality factors of 3069 and 4071, respectively), thanks to the new strategy of quasi-BIC generation. The test experiments with four refractive index solutions (n=1.450/1.462/1.470/1.480, respectively) as samples showed that both resonance peaks could complete the refractive index detection with the sensitivity of 452 nm/RIU and 428 nm/RIU, respectively, and the performance evaluation indexes FOM were 376.7 and 372, respectively, which are better than the existing literature. The linearity between resonance wavelength and refractive index is good, showing the potential of quasi-BIC metasurface in refractive index sensing.

With the high-speed development of mobile communication and the increasingly complex communication environment, multibeam antennas are widely required in the application fields of multi-target radar, satellite communication, multi-point wireless communication, etc. Orbital angular momentum is one of the fundamental properties of electromagnetic waves. It has a spiral wavefront and is independent of basic properties such as amplitude, phase, and polarization. It can provide a new multiplexing dimension for electromagnetic waves. Based on the excellent electromagnetic control capability of the sub-wavelength metal waveguide array, we designed a multibeam rotatable terahertz (THz) array antenna. By adjusting the phase distribution of two orthogonal polarization components of the incident wave, they can be transformed into two vortex beams with the same intensity distributions and opposite orders. The interferometric patterns in the 45° polarization direction can be rotated by changing the phase difference between the two components. Moreover, the array antenna also shows the performance of high gain (31 dBi) and wide bandwidth (up to 61 GHz). This work can provide a new way for azimuth measurement based on multibeam array antennas and is of great significance to enrich the design of array antennas in the THz band.

Metasurfaces play an important role in controlling the amplitude, phase, polarization, and complex wavefront of electromagnetic waves. Dynamic tunable devices can be realized by combining various active modulation means. However, most of the existing reconfigurable devices have volatile properties that require a constant stimulus to maintain. The chalcogenide phase-change material Ge2Sb2Te5 (GST) has the characteristics of non-volatility, reconfigurability, and large optical contrast, which can be used to achieve tunable metasurface devices. In this review, we review the recent research progress of GST-based terahertz (THz) metasurface devices and introduce the spectral characteristics and reversible phase transition conditions of GST in the THz band. Furthermore, we systematically summarizes the relevant works on non-volatile, reconfigurable, and multi-level manipulation of THz amplitude, polarization, and wavefront by combining GST with metasurfaces. Finally, the future development prospects and challenges are discussed. The non-volatile nature of GST provides a new path to achieve non-volatile reconfigurable THz devices with low energy consumption, while its ultra-fast volatility can be used for next-generation high-speed communication.

Electromagnetic metasurfaces, as a class of planar electromagnetic materials consisting of single-layer or multilayer subwavelength artificial micro-structures, can precisely control the amplitude, phase, wavefront, dispersion, polarization, and angular momentum of electromagnetic waves in the subwavelength scale. In particular, reflectionless electromagnetic metasurfaces provide new theories and schemes for realizing high-efficiency electromagnetic devices. In this review, we elaborate the underlying mechanism of reflectionless metasurfaces from the perspective of the Huygens principle, electromagnetic resonances and the Brewster effect. We also discuss the important applications including anomalous refraction, polarization manipulation, meta-antireflection coatings, and perfect electromagnetic absorption, and point out the challenges and potentials of this field.

Optical analog computing avoids the photoelectric conversion in various application scenarios by directly modulating the optical input in the spatial domain. Therefore, it has become a research focus in many applications such as image processing. In this paper, a polarization-multiplexed optical analog computing metasurface structure based on the Green's function method is designed using topologal optimization. Under different linearly polarized light incidence, this topological metasurface can independently tailor the amplitude and phase of the transmitted light field. It achieves bright-field imaging and one-dimensional second-order differentiation operations in orthogonal polarization states, as well as a polarization- controlled differentiation direction for a multiplexed differential system. These polarization-multiplexed designs can play a vital role in more optical computing application scenarios.