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Rapidly changing electromagnetic fields are the basis of almost any photonic or electronic device operation. We report how electron microscopy can measure collective carrier motion and fields with subcycle and subwavelength resolution. A collimated beam of femtosecond electron pulses passes through a metamaterial resonator that is previously excited with a single-cycle electromagnetic pulse. If the probing electrons are shorter in duration than half a field cycle, then time-frozen Lorentz forces distort the images quasi-classically and with subcycle time resolution. A pump-probe sequence reveals in a movie the sample's oscillating electromagnetic field vectors with time, phase, amplitude, and polarization information. This waveform electron microscopy can be used to visualize electrodynamic phenomena in devices as small and fast as available.

The discovery of topological photonic states has revolutionized our understanding of electromagnetic propagation and scattering. Endowed with topological robustness, photonic edge modes are not reflected from structural imperfections and disordered regions. Here we demonstrate robust propagation along reconfigurable pathways defined by synthetic gauge fields within a topological photonic metacrystal. The flow of microwave radiation in helical edge modes following arbitrary contours of the synthetic gauge field between bianisotropic metacrystal domains is unimpeded. This is demonstrated in measurements of the spectrum of transmission and time delay along the topological domain walls. These results provide a framework for freely steering electromagnetic radiation within photonic structures. Topologically protected states at the interface of magnetic domain walls in a parallel plate waveguide with adjustable rods, are shown to be directed along different paths, as the waveguide geometry changes.

Short, intense laser pulses can be used to access the transition regime between classical and quantum optical responses in dielectrics. In this regime, the relative roles of inter- and intraband light-driven electronic transitions remain uncertain. We applied attosecond transient absorption spectroscopy to investigate the interaction between polycrystalline diamond and a few-femtosecond infrared pulse with intensity below the critical intensity of optical breakdown. Ab initio time-dependent density functional theory calculations, in tandem with a two-band parabolic model, accounted for the experimental results in the framework of the dynamical Franz-Keldysh effect and identified infrared induction of intraband currents as the main physical mechanism responsible for the observations.

Intense light pulses in the visible and adjacent spectral ranges with their energy mostly confined to a half wave cycle—optical attosecond pulses—are synthesized and used to measure the time it takes electrons to respond to light. Sub-femtosecond control of bound electrons A fundamental speed limit for controlling matter through the electromagnetic force of light arises from the time it takes bound electrons to respond. Experiments have shown that this response is not instantaneous, but the lack of sufficiently fast probes has prevented direct measurements. Eleftherios Goulielmakis and colleagues have now produced intense light pulses in the visible and nearby spectral ranges and with energy largely confined to a half wave cycle, and show that these so-called optical attosecond pulses can control and measure the dynamics of bound electrons in krypton atoms. Proof-of-principle measurements establish the value of optical attosecond pulses for probing and manipulating bound electrons in atoms, molecules or solids, and suggest they may also find use in light-based nonlinear photonics operating on sub-femtosecond time scales and petahertz rates. The time it takes a bound electron to respond to the electromagnetic force of light sets a fundamental speed limit on the dynamic control of matter and electromagnetic signal processing. Time-integrated measurements of the nonlinear refractive index 1 of matter indicate that the nonlinear response of bound electrons to optical fields is not instantaneous; however, a complete spectral characterization of the nonlinear susceptibility tensors 2 —which is essential to deduce the temporal response of a medium to arbitrary driving forces using spectral measurements—has not yet been achieved. With the establishment of attosecond chronoscopy 3 , 4 , 5 , the impulsive response of positive-energy electrons to electromagnetic fields has been explored through ionization of atoms 6 and solids 7 by an extreme-ultraviolet attosecond pulse 8 or by strong near-infrared fields 9 , 10 , 11 . However, none of the attosecond studies carried out so far have provided direct access to the nonlinear response of bound electrons. Here we demonstrate that intense optical attosecond pulses synthesized in the visible and nearby spectral ranges allow sub-femtosecond control and metrology of bound-electron dynamics. Vacuum ultraviolet spectra emanating from krypton atoms, exposed to intense waveform-controlled optical attosecond pulses, reveal a finite nonlinear response time of bound electrons of up to 115 attoseconds, which is sensitive to and controllable by the super-octave optical field. Our study could enable new spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids 12 , as well as light-based electronics operating on sub-femtosecond timescales and at petahertz rates 13 , 14 , 15 .

Surface plasmon polaritons (SPPs) are localized surface electromagnetic waves that propagate along the interface between a metal and a dielectric. Owing to their inherent subwavelength confinement SPPs have a strong potential to become building blocks of a type of photonic circuitry built up on 2D metal surfaces; however, SPPs are difficult to control on curved surfaces conformably and flexibly to produce advanced functional devices. Here we propose the concept of conformai surface plasmons (CSPs), surface plasmon waves that can propagate on ultrathin and flexible films to long distances in a wide broadband range from microwave to mid-infrared frequencies. We present the experimental realization of these CSPs in the microwave regime on paper-like dielectric films with a thickness 600-fold smaller than the operating wavelength. The flexible paper-like films can be bent, folded, and even twisted to mold the flow of CSPs.

The ACES-China 2022 symposium was successfully held in Xuzhou, China on July 28-31 2022. The conference chairs along with a dedicated team of guest editors edited this special issue to provide the whole technical community the opportunity to further explore the most significant contributions to the symposium. Seventeen papers are presented in this special issue. All have been carefully peer reviewed and we hope that you find this special issue a valuable and inspiring contribution to the development of applied computational electromagnetics.

This letter proposes a super‐resolution (SR) neural network model for high‐contrast electromagnetic scattering problems. The model is designed to predict fine‐grid field distributions based on low‐cost coarse‐grid simulations. By integrating a spatial channel attention mechanism, the model enhances accuracy in capturing field discontinuities induced by strong scatterers. Additionally, a residual‐in‐residual architecture is incorporated to provide the network with sufficient depth for effective correction of dispersion errors. The efficiency and accuracy of the proposed model have been validated through numerical experiments. Comparative evaluations with a recently proposed electromagnetic SR network, supplemented by rigorous ablation studies, further demonstrate the superior performance of our approach in high‐contrast scenarios. This letter proposes a super‐resolution neural network model for high‐contrast electromagnetic scattering problems. The model is designed to predict fine‐grid field distributions based on low‐cost coarse‐grid simulations.

Surface enhanced Raman spectroscopy (SERS) is an attractive analytical technique, which enables single-molecule sensitive detection and provides its special chemical fingerprints. During the past decades, researchers have made great efforts towards an ideal SERS substrate, mainly including pioneering works on the preparation of uniform metal nanostructure arrays by various nanoassembly and nanotailoring methods, which give better uniformity and reproducibility. Recently, nanoparticles coated with an inert shell were used to make the enhanced Raman signals cleaner. By depositing SERS-active metal nanoislands on an atomically flat graphene layer, here we designed a new kind of SERS substrate referred to as a graphene-mediated SERS (G-SERS) substrate. In the graphene/metal combined structure, the electromagnetic “hot” spots (which is the origin of a huge SERS enhancement) created by the gapped metal nanoislands through the localized surface plasmon resonance effect are supposed to pass through the monolayer graphene, resulting in an atomically flat hot surface for Raman enhancement. Signals from a G-SERS substrate were also demonstrated to have interesting advantages over normal SERS, in terms of cleaner vibrational information free from various metal-molecule interactions and being more stable against photo-induced damage, but with a comparable enhancement factor. Furthermore, we demonstrate the use of a freestanding, transparent and flexible “G-SERS tape” (consisting of a polymer-layer-supported monolayer graphene with sandwiched metal nanoislands) to enable direct, real time and reliable detection of trace amounts of analytes in various systems, which imparts high efficiency and universality of analyses with G-SERS substrates.

Previous demonstrations of cloaking, where objects are rendered invisible at certain frequencies, have been limited to the microwave regime. Moving us a significant step closer to invisibility in a region that can been seen by humans, a cloaking device has now been demonstrated for a broad range of frequencies in the near-infrared. Invisibility devices have captured the human imagination for many years. Recent theories have proposed schemes for cloaking devices using transformation optics and conformal mapping 1 , 2 , 3 , 4 . Metamaterials 5 , 6 , with spatially tailored properties, have provided the necessary medium by enabling precise control over the flow of electromagnetic waves. Using metamaterials, the first microwave cloaking has been achieved 7 but the realization of cloaking at optical frequencies, a key step towards achieving actual invisibility, has remained elusive. Here, we report the first experimental demonstration of optical cloaking. The optical ‘carpet’ cloak is designed using quasi-conformal mapping to conceal an object that is placed under a curved reflecting surface by imitating the reflection of a flat surface. The cloak consists only of isotropic dielectric materials, which enables broadband and low-loss invisibility at a wavelength range of 1,400–1,800 nm.

Chiral molecules with opposite handedness exhibit distinct physical, chemical, or biological properties. They pose challenges as well as opportunities in understanding the phase behavior of soft matter, designing enantioselective catalysts, and manufacturing single-handed pharmaceuticals. Microscopic particles, arranged in a chiral configuration, could also exhibit unusual optical, electric, or magnetic responses. Here we report a simple method to assemble achiral building blocks, i.e., the asymmetric colloidal dimers, into a family of chiral clusters. Under alternating current electric fields, two to four lying dimers associate closely with a central standing dimer and form both right- and left-handed clusters on a conducting substrate. The cluster configuration is primarily determined by the induced dipolar interactions between constituent dimers. Our theoretical model reveals that in-plane dipolar repulsion between petals in the cluster favors the achiral configuration, whereas out-of-plane attraction between the central dimer and surrounding petals favors a chiral arrangement. It is the competition between these two interactions that dictates the final configuration. The theoretical chirality phase diagram is found to be in excellent agreement with experimental observations. We further demonstrate that the broken symmetry in chiral clusters induces an unbalanced electrohydrodynamic flow surrounding them. As a result, they rotate in opposite directions according to their handedness. Both the assembly and propulsion mechanisms revealed here can be potentially applied to other types of asymmetric particles. Such kinds of chiral colloids will be useful for fabricating metamaterials, making model systems for both chiral molecules and active matter, or building propellers for microscale transport. Significance Although colloids have been used as molecular analogues for understanding how simple building blocks can assemble into functional materials, they are mostly spherical with isotropic properties. We are still far from truly accessing the diversity of structures desired for either fundamental understanding or technological application. Here, we report the electric-field–directed assembly of asymmetric colloids into clusters that exhibit a ubiquitous type of symmetry in nature: the chirality. We further demonstrate that the chirality induces unbalanced hydrodynamic flow, which causes rotational propulsion of chiral clusters that are fully dictated by their handedness. Both the assembly and propulsion mechanisms discovered can be universal and applied to other types of asymmetric particles. They are also useful in modeling active matter and making microengines.