Special issue: Metamaterials and plasmonics in Asia, a tribute to Byoungho Lee

This special issue is a development of the 6th A3 Metamaterials Forum held from June 27 to 29, 2022, at Seoul National University, Korea. Namkyoo Park, from Seoul National University, served as the conference chair on behalf of Byoungho Lee due to his health issue at that time. Unfortunately, Byoungho Lee passed away on November 7, 2022, which was acknowledged by obituaries in theNature Photonics journal [1] and the societies Optica [2] and SPIE [3]. His dedication to the A3Metamaterials Forum (2016–2022) and theKorea-JapanMetamaterials Forum (2011–2015) contributed tomaintaining the high quality of invited talks. Byoungho Lee himself was a top researcher in nanophotonics (plasmonics andmetamaterials), holography, and 3D display. He left us with a legacy of relentless passion for scientific research and devoted leadership in professional societies such as the Optical Society of Korea, Optica, and SPIE. It is with deep respect that we dedicate this special issue to Byoungho Lee with a memorandum [4]. The capability to design and fabricate sub-wavelength artificial structures as inclusions has led to research on metamaterials with tailored effective-medium properties operating within a frequency range from microwave and terahertz to the visible spectral region, depending on the size of sub-wavelength inclusion. This special issue presents four review articles and 30 research articles. The ease of implementing artificial gain and loss in optical processes renders nanophotonics a straightforward platform for exploring non-Hermitian physics in artificial optical structures. In addition, the concept of a topological insulator in condensed matter physics can be extended to optics platforms for a variety of advantages in optical device applications, the presence of a robust topological edge state being one prominent example. The advances and application of non-Hermitian topological photonics are elegantly presented in ref. [5], while ref. [6] covers the construction and manipulation of photonic topological states, and touches uponnon-Hermitian topology. Ametasurface is a two-dimensional metamaterial, where the Huygens wavefront of the outgoing wave is controlled bymanipulating the amplitude, phase, and polarization of the incoming wave. This topic is extensively reviewed in ref. [7]. Additionally, ref. [8] reviews microscopic tools for super-resolution, fast volumetric imaging, and a large imaging field of view. The emergence of exceptional points is characteristic of a non-Hermitian system, which can be extended to a photonic crystal structure with negative index media to realize a 2D symmetry-protected exceptional ring as shown in ref. [9]. Research articles on metasurfaces cover applications of angle-multiplexing in ref. [10], multilayer metalenses in ref. [11], high numerical aperture metalenses in ref. [12], anomalous diffraction in ref. [13], spatial filters in ref. [14], and boundstate-in-continuum based on nanohole array in ref. [15]. Active beam control is examined in schemes of deep-learning-assisted reconfigurable metasurface antenna in ref. [16], on-chip integration for optical phase array in ref. [17], and a digital and programmable metasurface in ref. [18]. An important feature of metamaterials is the versatility of energy-momentum dispersion engineering for broadband absorption, as well as epsilon-near-zero response. This is explored in a broadband absorber covering the entire microwave X-band in ref. [19], in cermet films with a nano-cone structure for a perfect absorber in ref. [20],