The latest trends in nanophotonics

Recent explosive growth in nanophotonics, ignited by the rapidly developing nanotechnologies, has demonstrated that light exhibits extraordinary light-matter interactions with subwavelength-scale structures.Such exotic light behaviors not only manifest the importance of searching for unprecedented optics but also suggest the possibility of real-world applications in the visible range.Indeed, recent progress in nanophotonics has revealed that nanophotonic-based devices and applications can be powerful candidates for replacing the conventional bulky optical components in a compact manner.The International Conference on Metamaterials, Photonic Crystals, and Plasmonics (META) is an annual conference for nanophotonics research.It especially covers a variety of research in metamaterials, photonic crystals, plasmonics, and nanophotonic devices and applications.The latest conference, META'21, was held online due to the COVID-19 pandemic (July 20-23, 2021), where the latest developments in nanophotonics, metamaterials, and related topics were livestreamed across the world.This special issue "The latest trends in nanophotonics" introduces a collection of invited and selected research and review articles from the conference.Plasmonics is one major branch of nanophotonics, dealing with the surface plasmons, i.e., collective oscillation of electrons at metal-dielectric interfaces.Kim et al. [1] review plasmonics in ultrashort time scale (∼femtoseconds or less), the so-called ultrafast plasmonics.Fundamentals and recent achievements in ultrafast plasmonics are reviewed extensively in two selected examples: strong-field physics and ultraprecision spectroscopy.Menabde et al. [2] present a comprehensive review on image polaritons, a new type of the polaritonic modes coupled with its mirror image when the material is near a highly conductive material, formed in van der Waals crystals.The authors describe the dispersion of the image polaritons and those in a variety of van der Waals crystals including the hyperbolic and nonlocal characteristics and experimental breakthroughs.Xu et al. [3] propose an equilibrium-thermodynamic computational method to generalize the previously reported theories to compute plasmoelectric potential.To improve the application scope and the accuracy of the previous model, the authors introduce an equivalent wavelength method to estimate absorption crosssection and incorporate the plasmonic local heating.The generalized method can quantify the plasmoelectric potential in non-Mie-resonant plasmonic systems whereas the conventional method is only applicable to the Mie-resonant systems.Two main features of plasmonics are tight field confinement and field enhancement.The former enables a hidden light-matter interaction [4].Sakai et al. demonstrate that plasmonic nanostructures consisting of gold tetramers can squeeze structured light with a quadrupolar profile within a nanoscale area.This structured light tightly confined in plasmonic nanostructures allowed the authors to access multipolar transitions that were otherwise forbidden due to the length scale mismatch.Meanwhile, the latter, i.e., the field enhancement near plasmonic nanostructures, is known to accelerate the spontaneous emission of nearby emitters but simultaneously exhibit a quenching effect.