Optical Metamaterials and Information Metamaterials

Electromagnetic metamaterials, which consist of subwavelength-scale artificial meta-atoms, have revolutionized the design concepts of materials in the past two decades. By judiciously designing the geometrical parameters, lattice, and the constituent materials of the meta-atoms, one can easily manipulate the polarization, phase, amplitude and frequency of electromagnetic waves which interact with metamaterials. In recent years, information metamaterial, a new branch of artificial electromagnetic materials, emerges as a promising platform for implementing multi-dimensional information processing. By integrating the electromagnetic physics with digital information, various novel devices and systems have been developed to control the electromagnetic fields and process the digital information simultaneously. In recognition of the great breakthroughs and increasing importance of electromagnetic metamaterials, we organize this special issue and focus on two sub-fields: optical metamaterials and information metamaterials. These two kinds of electromagnetic metamaterials have been widely explored to meet the grand challenges of information processing at optical, THz and GHz frequencies, etc. This special issue includes 1 review article and 14 research articles. The topic covers a broadband range of spectrum, including advanced optical imaging and metalens, information metamaterials, wavefront engineering and detection, optical holography and polarization control, THz and nonlinear metamaterials. The first section of the special issue is on advanced optical imaging and metalens. In the review paper (see article 202203149), Xiujuan Zou et al. summarize the fundamental principle of optical metasurface, as well as the forward and reverse design approaches in various imaging applications. Jin Yao et al. propose a design strategy based on phase compensation mechanism to continuously tune the properties of water-based meta-lenses at 5.0 GHz (see article 202300130). The integrated-resonant units are carefully designed to tailor the polarization conversion efficiency and phase compensation. This work may open new avenue for developing underwater meta-imaging systems working in the microwave frequency regime. The second section discusses the recent progress of the information metamaterials. Jingcheng Liang et al. demonstrate an optically transparent 2-bit reconfigurable intelligent metasurface with low angular sensitivity. This is achieved by utilizing a transparent substrate and the metal-mesh-based patterns. The designed device has an optical transmittance close to 50% and is insensitive to the incident angles in the wireless communication applications (see article 202202081). Zhengxing Wang et al study a long-range and nearly-passive RFID-controlled information metasurface. The proposed device consists of 4 RFID tags and 8 × 8 elements, each of which is integrated with a single pole double throw switch chip to achieve the 1-bit phase reconfigurability. With remote control of the RFID system, this intelligent metasurface can be programmed to switch between three functionalities (see article 202203114). Zhuo Wang et al. explore the concept of multi-task multiscale intelligent sensing with algorithm-driven distributed multi-frequency reprogrammable metasurfaces. The authors experimentally demonstrate the continuously monitoring of the action behavior and healthy status of the target acting in real world (see article 202203153). Peng Xu et al. show that the reconfigurable coding metasurface can be used to dynamically regulate the transmission and reflection states and realize independent phase control of the orthogonally polarized electromagnetic waves (see article 202203117). The proposed strategies in this section may pave novel avenues for large-scale applications of the information metamaterials in 6G wireless communications and sensing. The main focus of the third section is wavefront engineering and detection. Mingke Jin et al. report on the design, fabrication and characterization of modal optical vortex beam shaping of paraxial light in the visible regime (see article 202202149). Pu Peng et al. develop a deep-learning-based platform for designing metasurfaces, which can intelligently generate the pre-designed multifunctional vortex beams. This intelligent design strategy may lay a solid foundation for the high-performance application of optical metasurfaces (see article 202300158). Runzhe Zhang et al. demonstrate an angular super-oscillatory metalens by combining the super-oscillatory phase and angular metalens phase in a single device (see article 202300009). Therefore, one can map the received orbital angular modes into specific patterns in the focal plane with different azimuthal angles and is capable of determining the orbital angular momentum spectrum with a single-shot measurement. In article 202203121, the "flatland" analog of the leaky-wave radiation is introduced to describe a new mechanism of the in-plane radiation leakage that can occur in artificial or natural low-dimensional materials. The proposed methodology provides a new tool for controlling surface waves at the nanoscale and may find interesting applications in the emerging field of polaritonics. In the fourth section, three research groups share their recent progress in the field of optical holography and polarization control. Jiawei Xi et al. utilize an integrated neural network method to design metasurfaces directly from independent holograms with multiple polarization conversion channels (see article 202202663). Such an information-driven approach enables designing complex polarization holograms from an existing metamaterial library. Qi Hu et al. demonstrate the vectorial multi-beam patterning with on-demand polarizations by using space-time-varying programmable metasurface (see article 202300093). This is achieved by independently applying periodic space-time-varying modulation signals to the tunable components of the metasurface. The combination of arbitrary amplitude and phase responses is then obtained for orthogonally polarized waves. Yuttana Intaravanne et al. experimentally realize polarization digital numbers at multiple wavelengths. In this work, one metasurface is used to simultaneously realize wavelength and phase multiplexing, polarization rotation, and wavelength-selective digital numbers (see article 202203097). The approaches developed in this section may find important applications in virtual reality, anti-counterfeiting, and information storage. The fifth section reports the advances in the field of THz and nonlinear metamaterials. Through the incorporation of sequential and inverse deposition of W-Co20Fe60B20-Pt heterostructure films, Sai Chen et al. succeed to control the spin currents in the multilayer device and the waveforms of the generated THz waves. The authors experimentally realize a 180° phase shift across a broad frequency spectrum while efficiently radiating THz waves (see article 202300899). Sai Shradha et al. investigate the enhancement mechanism of the second harmonic generation of monolayer MoS2, which is embedded in a monolithic microcavity to simultaneously enhance the light-matter interaction at both the excitation and the second harmonic wavelengths (see article 202300907). In summary, we would like to thank the great contributions from all authors and expect that the readers will enjoy the contents in this special issue and find them inspiring. Last but not least, we want to thank the editors Dr. Jipei Yuan and Dr. Anja Wecker from Advanced Optical Materials for their great support to make the success of this special issue.