We report the realization of a coherent perfect absorber, using a pair of passive resonators coupled to a microwave transmission line in the background, which can completely absorb light in its parity-time (PT-)symmetric phase but not in its broken phase. Instead of balancing material gain and loss, we exploit the incident waves in the open system as an effective gain so that ideal PT symmetry can be established by using only passive materials. Such a route will be effective to construct PT-symmetric metamaterials and also tunable PT-symmetric optical elements in general. It also provides a flexible platform for studying exceptional-point physics with both electric and magnetic responses.

We achieve reflection-type bound states in the continuum (BICs) in the compound grating waveguide structure by tuning the excitation of the guided modes. Assisted by the quasi-BICs with ultrahigh Q factors, the Goos-Hänchen (GH) shift can be greatly enhanced to larger than or equal to four orders of wavelength. In addition, different from the large GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift assisted by the reflection-type quasi-BIC is located at the reflectance peak with unity reflectance, which can be more easily detected in the experiment. Based on the quasi-BIC-assisted giant GH shift, we propose an ultrasensitive temperature sensor with a minimum resolution in the order of ${10}^{-4}$ °C. Our work provides a route, under the current experimental conditions, to design ultrasensitive sensors, light information storage devices, wavelength division de/multiplexers, optical switches, and polarization beam splitters based on this giant GH shift together with high reflectance.

This Letter reports on the experimental observation of a topologically protected edge state and exceptional point in an open and non-Hermitian (lossy) acoustic system. Although the theoretical underpinning is generic to wave physics, the simulations and experiments are performed for an acoustic system. It has nontrivial topological properties that can be characterized by the Chern number provided that a synthetic dimension is introduced. Unidirectional reflectionless propagation, a hallmark of exceptional points, is unambiguously observed in both simulations and experiments.

Manipulating the properties of the isofrequency contours (IFCs) of materials provides a powerful means of controlling the interaction between light and matter. Hyperbolic metamaterials (HMMs), an important class of artificial anisotropic materials with hyperbolic IFCs, have been intensively investigated. Because of their open dispersion curves, HMMs support propagating high-k modes and possess an enhanced photonic density of states. As a result, HMMs can be utilized to realize hyperlenses breaking the diffraction limit, metacavity lasers with subwavelength scale, high-sensitivity sensors, long-range energy transfer, and so on. Aimed at those who are about to enter this burgeoning and rapidly developing research field, this tutorial article not only introduces the basic physical properties of HMMs but also discusses dispersion manipulation in HMMs and HMM-based structures such as hypercrystals. Both theoretical methods and experimental platforms are detailed. Finally, some potential applications associated with hyperbolic dispersion are introduced.

New structures with richer electromagnetic properties are in high demand for developing novel microwave and optic devices aimed at realizing fast light-based information transfer and information processing. Here we show theoretically that a topological photonic state exists in a hexagonal LC circuit with short-range textures in the inductance, which is induced by a band inversion between p- and d-like electromagnetic modes carrying orbital angular momentum, and realize this state experimentally in planar microstrip arrays. Measuring both amplitude and phase of the out-of-plane electric field accurately using microwave near-field techniques, we demonstrate directly that topological interfacial electromagnetic waves launched by a linearly polarized dipole source propagate in opposite directions according to the sign of the orbital angular momentum. The open planar structure adopted in the present approach leaves much room for including other elements useful for advanced information processing, such as electric/mechanical resonators, superconducting Josephson junctions and SQUIDs.

We use a color-magnetic interaction model (CMIM), a traditional constituent quark model and a multiquark color flux-tube model to systematically investigate the properties of the states $[Q_1{Q}_2][{\overline{Q}}_3{\overline{Q}}_4]$ ($Q=c$, $b$). The dynamical investigation indicates that the CMIM can not completely absorb QCD dynamical effects through the effective constituent quark mass and overestimates the color-magnetic interaction in the states under the assumption of the same spatial configurations. The Coulomb interaction plays a critical role in the dynamical model calculations on the heavy hadrons, which induces the fact that none of bound states $[Q_1{Q}_2][{\overline{Q}}_3{\overline{Q}}_4]$ can be found in the dynamical models. The color configuration ${[[Q_1{Q}_2{]}_{{\mathbf{6}}_c}[{\overline{Q}}_3{\overline{Q}}_4{]}_{{\overline{\mathbf{6}}}_c}]}_{\mathbf{1}}$ should be taken seriously in the ground states due to the strong Coulomb attraction between the $[Q_1{Q}_2{]}_{{\mathbf{6}}_c}$ and $[{\overline{Q}}_3{\overline{Q}}_4{]}_{{\overline{\mathbf{6}}}_c}$. The color configuration ${[[Q_1{Q}_2{]}_{{\overline{\mathbf{3}}}_c}[{\overline{Q}}_2{\overline{Q}}_4{]}_{{\mathbf{3}}_c}]}_{\mathbf{1}}$ is absolutely dominant in the excited states because of the strong Coulomb attraction within the $[Q_1{Q}_2{]}_{{\overline{\mathbf{3}}}_c}$ and $[{\overline{Q}}_2{\overline{Q}}_4{]}_{{\mathbf{3}}_c}$. The $J/\mathrm{\Psi }$-pair resonances recently observed by LHCb are difficult to be accommodated in the CMIM. The broad structure ranging from 6.2 to 6.8 GeV can be described as the ground tetraquark state $[cc][\overline{c}\overline{c}]$ in the various dynamical models. The narrow structure $X(6900)$ can be identified as the excited state $[cc][\overline{c}\overline{c}]$ with $L=1$ ($L=2$) in the constituent quark model (color flux-tube model).