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All-dielectric metamaterials and metasurfaces have been demonstrated as ideal platforms to manipulate electromagnetic waves and enhance light–matter interaction. Analogous to guided modes in a dielectric waveguide, high refractive index dielectric metasurface supports leaky modes (also known as quasi-normal modes), including electric and magnetic resonant modes, which are represented by a complex eigenfrequency. Such leaky modes play a dominant role in governing the optical properties of dielectric nanoparticles and metasurfaces (i.e., absorption, scattering and emission). These unique properties enable researchers to design dielectric metasurfaces with desired functionalities without resorting to computationally expensive parameter scanning with full-wave simulation. Moreover, leaky modes underpin many exciting topics, such as Fano resonances, bound states in the continuum (leaky modes with Q=+ ∞), coherent perfect absorption, parity-time symmetry, and exceptional points. This review provides an overview of the recent progress on leaky modes of all-dielectric-based metasystem, from fundamental physics to their applications. We start with surveying the fundamental physics of leaky modes and discussing leaky mode engineering in either a passive or active way. Then, we discuss the recent progress on the leaky modes' application in the perfect light absorber, solar cells, photodetectors, enhanced light emission and lasing, strong coupling, enhanced nonlinear harmonic generation, and structural color. Next, we present an overview of recent advances in bound states in the continuum and their application in enhanced light–matter​ interactions. Also, we review other interesting topics: parity-time symmetry and exceptional points, and discuss their application in manipulating light in an unprecedented way. Finally, we present our vision of the challenges and opportunities in this rapidly developing field of research. [ABSTRACT FROM AUTHOR]

I review the four mechanisms of bound states in the continuum (BICs) in the application of microwave and acoustic cavities open to directional waveguides. The most simple are symmetry-protected BICs, which are localized inside the cavity because of the orthogonality of the eigenmodes to the propagating modes of waveguides. However, the most general and interesting is the Friedrich–Wintgen mechanism, when the BICs are the result of the fully destructive interference of outgoing resonant modes. The third type of BICs, Fabry–Perot BICs, occurs in a double resonator system when each resonator can serve as an ideal mirror. Finally, the accidental BICs can be realized in the open cavities with no symmetry like the open Sinai billiard in which the eigenmode of the resonator can become orthogonal to the continuum of the waveguide accidentally due to a smooth deformation of the eigenmode. We also review the one-dimensional systems in which the BICs occur owing to the fully destructive interference of two waves separated by spin or polarization or by paths in the Aharonov–Bohm rings. We make broad use of the method of effective non-Hermitian Hamiltonian equivalent to the coupled mode theory, which detects BICs by finding zero-width resonances. [ABSTRACT FROM AUTHOR]

The quark model was formulated in 1964 to classify mesons as bound states made of a quark–antiquark pair, and baryons as bound states made of three quarks. For a long time all known mesons and baryons could be classified within this scheme. Quantum Chromodynamics (QCD), however, in principle also allows the existence of more complex structures, generically called exotic hadrons or simply exotics. These include four-quark hadrons (tetraquarks and hadronic molecules), five-quark hadrons (pentaquarks) and states with active gluonic degrees of freedom (hybrids), and even states of pure glue (glueballs). Exotic hadrons have been systematically searched for in numerous experiments for many years. Remarkably, in the past fifteen years, many new hadrons that do not exhibit the expected properties of ordinary (not exotic) hadrons have been discovered in the quarkonium spectrum. These hadrons are collectively known as X Y Z states. Some of them, like the charged states, are undoubtedly exotic. Parallel to the experimental progress, the last decades have also witnessed an enormous theoretical effort to reach a theoretical understanding of the X Y Z states. Theoretical approaches include not only phenomenological extensions of the quark model to exotics, but also modern non-relativistic effective field theories and lattice QCD calculations. The present work aims at reviewing the rapid progress in the field of exotic X Y Z hadrons over the past few years both in experiments and theory. It concludes with a summary on future prospects and challenges. [ABSTRACT FROM AUTHOR]