From reactive impedance surfaces and embedded circuit metamaterials to reflectionless terahertz metasurfaces: two decades of research in design and applications of artificial electromagnetic materials [Invited]

This paper reviews two decades of research in my research group on reactive impedance surfaces (RIS), embedded-circuit metamaterials, electromagnetic bandgap (EBG) structures, and miniaturized-element frequency-selective surfaces (MEFSSs) recently known as metasurfaces, highlighting the theoretical foundations, design methodologies, fabrication techniques, and emerging applications of these structures. Beginning with the development of RIS substrates for antenna miniaturization and bandwidth enhancement, the work evolved toward embedded resonant-circuit metamaterials capable of synthesizing artificial permeability, tunable bandgap responses, and bi-anisotropic behavior. These concepts subsequently enabled the introduction of MEFSS architectures in which spatial filtering characteristics are achieved through strongly localized subwavelength interactions rather than conventional resonant-array phenomena. The review further discusses the emergence of high-order and multipole MEFSS structures, electronically tunable and reconfigurable surfaces, and ultra-thin spatial filters with low sensitivity to angle of incidence and structure harmonic suppression. Recent advances in millimeter-wave and terahertz implementations are presented, including membrane-supported MEFSS structures, reflectionless planar lenses, and reconfigurable orbital angular momentum (OAM) phase plates operating above 200 GHz. The article concludes with a discussion of future research directions involving programmable metasurfaces, integrated sensing and communications, adaptive wavefront engineering, and sub-terahertz imaging systems.