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Metasurfaces are ushering in an era of multifunctional control over optical wavefronts realized with ultrathin planarized devices. Recent advances have been enabling unprecedented control over the frequency response of these surfaces, suggesting that the future of flat optics may tailor both spectral and spatial degrees of freedom in highly multispectral and multifunctional devices. Diffractive nonlocal metasurfaces are opening new opportunities in this direction: they leverage symmetry‐protected scattering from quasi‐bound states in the continuum and, by spatially manipulating controlled geometric perturbations, they support ultrasharp optical responses with wavefront‐manipulating features. Encoded in nonlocal (i.e., spatially extended) resonant modes, the resulting response is observed exclusively within the bandwidth of the resulting Fano resonance, affording ideal features for a wide range of applications. In this perspective, this novel class of metasurfaces are discussed in the broader context of flat optics, highlighting their peculiar operation in contrast to relevant predecessors, and highlighting the opportunities for future advancement and applications. In particular, it is emphasized that nonlocality and selectivity are inherently related, but that spectral and spatial selectivity can be independently tuned in suitably tailored metasurfaces. In turn, this freedom allows the design and implementation of both wavefront‐shaping and wavefront‐selective devices. The novel optical responses, combined with the compatibility with rational design, herald new prospects for active, nonlinear and quantum metasurfaces, ultrathin devices for augmented reality, and compact tailored optical sources. [ABSTRACT FROM AUTHOR]

B Diffractive Nonlocal Metasurfaces b In article number 2100633, Adam Overvig and Andrea Alù present a perspective on the emerging field of diffractive nonlocal metasurfaces, which are able to tailor both the spatial and spectral properties of light through deliberately engineered perturbations in high symmetry lattices. Highly customizable scattering rate, phase, and polarization properties, tailored over subwavelength scales, enable highly selective optical responses, offering new avenues for rationally designing ultrathin photonic devices. [Extracted from the article]

Optical metalenses offer a compact approach for the development of nanoscale optical devices with various imaging functionalities. Realizing high‐quality images using metalenses with large numerical aperture (NA) requires eliminating unwanted optical aberrations. Existing strategies have largely focused only on correcting chromatic aberrations, and a metalens capable of simultaneously correcting chromatic and spherical aberrations has not yet been developed. Here, this issue is addresed by realizing an aplanatic phase profile for a red–green–blue (RGB) achromatic metalens, thereby demonstrating an RGB‐achromatic aplanatic metalens. This device consists of crystalline silicon nanostructures and it enables RGB‐achromatic aplanatic focusing with an NA of 0.635. The focal length of metalens remains unchanged at the three RGB wavelengths, whereas the associated focal spots are significantly smaller than those of an RGB‐achromatic metalens with spherical aberrations. An enhanced RGB imaging resolution is demonstrated, offering exciting opportunities to power up various imaging and display applications using flat metalenses. [ABSTRACT FROM AUTHOR]

Since the late 19th century, enormous endeavors have been made in extending the scope and capability of optical interferometers. Recently, plasmonic vortices that strongly confine the orbital angular momentum to surface have attracted considerable attention. However, current research interests in this area have focused on the mechanisms and dynamics of polarization‐dependent single plasmonic vortex generation and evolution, while the interference between different plasmonic vortices for practical applications has been unexplored. Here, a method for flexible on‐chip spin‐to‐orbital angular momentum conversion is introduced, resulting in exotic interferograms. Based on this method, a new form of interferometers that is realized by the interference between customized plasmonic vortices is demonstrated. Within wavelength‐scale dimension, the proposed plasmonic vortex interferometers exhibit superior performance to directly measure the polarization state, spin and orbital angular momentum of incident beams. The proposed interferometry is straightforward and robust, and can be expected to be applied to different scenarios, fueling fundamental advances and applications alike. [ABSTRACT FROM AUTHOR]

Coupled resonances in non‐Hermitian systems can lead to exotic optical features, such as bound states in the continuum (BICs) and exceptional points (EPs), which have been recently emerged as powerful tools to control the propagation and scattering of light. Yet, similar tools to control diffraction and engineer spatial wavefronts have remained elusive. Here, it is shown that, by operating a metagrating around BICs and EPs, it is possible to achieve an extreme degree of control over coupling to different diffraction orders. Subwavelength metallic slit arrays stacked on a metal‐insulator‐metal waveguide, enabling a careful control of the coupling between localized and guided modes are explored. By tuning the coupling strength from weak to strong, the overall spectral response can be tailored and the emergence of singular features, like BICs and EPs can be enabled. Perfect unitary diffraction efficiency with large spectrum selectivity is achieved around these singular features, with promising applications for selective wavefront shaping, filtering, and sensing. [ABSTRACT FROM AUTHOR]

Confining electromagnetic energy is crucial to enhance light-matter interactions, with important implications for science and technology. Here, the opportunities offered by trapping and confining light in open structures, based on the concept of embedded eigenstates within the radiation continuum enabled by zero-index metamaterials, are discussed. Building upon the physical insights offered by the analysis, a general platform is put forward that allows the realization of extremely high field enhancements in open structures under external illumination. Structures supporting embedded eigenstates represent a rare example of physical systems in which extreme-in principle unbounded-responses can be tamed. The proposed design recipe to realize bound states in the continuum also offers a simple model that allows testing of important questions that surround the concept of embedded eigenstates, such as their effect on the local density of photonic states. The findings help clarify which nano-optical and radio-wave applications may benefit from this unusual and singular response. [ABSTRACT FROM AUTHOR]

Tailoring of electromagnetic spontaneous emission predicted by E. M. Purcell more than 50 years ago has undoubtedly proven to be one of the most important effects in the rich areas of quantum optics and nanophotonics. Although during the past decades the research in this field has been focused on electric dipole emission, the recent progress in nanofabrication and study of magnetic quantum emitters, such as rare-earth ions, has stimulated the investigation of the magnetic side of spontaneous emission. Here, we review the state-of-the-art advances in the field of spontaneous emission enhancement of magnetic dipole quantum emitters with the use of various nanophotonics systems. We provide the general theory describing the Purcell effect of magnetic emitters, overview realizations of specific nanophotonics structures allowing for the enhanced magnetic dipole spontaneous emission, and give an outlook on the challenges in this field, which remain open to future research. [ABSTRACT FROM AUTHOR]

All-optical ultrafast signal modulation and routing by low-loss nanodevices is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron-hole plasma with ultrafast relaxation rate. On the basis of this concept, we demonstrate the effect of beam steering by up to 20 degrees through simple variation of the intensity of incident light. The effect, which is suitable for ultrafast light routing in an optical chip, is demonstrated both in the visible and near-IR spectral regions for silicon- and germanium-based nanoantennas. We also reveal the effect of electron-hole plasma photoexcitation on the local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50%, whereas modification of the projected LDOS can be even more dramatic, almost five-fold for transverse dipole orientation. Moreover, our analytical model sheds light on the transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the suggested ultrafast nanodevice. The proposed concept paves the way to the creation of low-loss, ultrafast, and compact devices for optical signal modulation and routing. [ABSTRACT FROM AUTHOR]

On-Chip Plasmonic Vortex Interferometers (Laser Photonics Rev. 16(10)/2022) B Optical Interferometers b In article number 2200242, Yuan-Hao Lang, Quan Xu, and colleagues have developed a new form of optical interferometers by customizing the plasmonic vortex generation from two spin channels. In several square wavelengths, plasmonic vortex interferometer constructs interference field with subwavelength fine structure, demonstrating the measurement of polarization state, spin, and orbital angular momentum of the incident light beam. [Extracted from the article]

Spontaneous emission of light from materials is typically associated with electric dipole transitions. However, nanoscale emitters with strong magnetic dipole transitions also exist in nature, and they have been intensively studied in recent years. Similarly to electric dipole transitions, the spontaneous decay rate of magnetic dipole emitters can be controlled led by properly engineering the local density of optical states. This paper reviews the recent progress in enhancing magnetic dipole spontaneous emission using various nanophotonic approaches, ranging from planar systems to nanoantennas and artificially engineered metamaterials. (Picture: Denis G Baranov et al., article number 1600268, in this issue) [ABSTRACT FROM AUTHOR]