Magnetic skyrmions form in chiral multilayers from the shrinking or fission of elongated stripe textures. Here we report an experimental and theoretical study of the temperature dependence of this stripe-to-skyrmion transition in $\mathrm{Co}$/$\mathrm{Pt}$-based multilayers. Field-reversal magnetometry and Lorentz microscopy experiments over 100–350 K establish the increased efficacy of stripe-to-skyrmion fission at higher temperatures—driven primarily by the thermal evolution of key magnetic interactions—which enhances the resulting skyrmion density. Atomistic calculations elucidate that the energy barrier to fission governs the thermodynamics of skyrmion formation. Our results establish a mechanistic picture of the stripe-to-skyrmion transition and advance the use of thermal knobs for efficient skyrmion generation.

Spin waves are promising chargeless information carriers for the future, energetically efficient beyond CMOS systems. Among many advantages are the ease of achieving nonlinearity, the variety of possible interactions, and excitation types. Although the rapidly developing magnonic research has already yielded impressive realizations, multimode nonlinear effects, particularly with propagating waves and their nanoscale realizations, are still an open research problem. We theoretically study the dynamic interactions of spin waves confined to the edge of a thin ferromagnetic film with the spin-wave beam incident at this edge. We find inelastically scattered spin-wave beams at frequencies increased and decreased by the frequency of the edge spin-wave relative to the specularly reflected beam. We observe a strong dependence of the angular shift of the inelastic scattered spin-wave beam on the edge-mode frequency, which allows us to propose a magnonic demultiplexing of the signal encoded in spin waves propagating along the edge. Since dynamic magnetostatic interactions, which are ubiquitous in the spin-wave dynamics, are decisive in this process, this indicates the possibility of implementing the presented effects in other configurations and their use in magnonic systems.

We propose the design of a flat active wall panel and demonstrate its effectiveness in modulating the wall impedance, which can lead to the total absorption of incident sound waves in the low-frequency range of 50–120 Hz, or alter its reflection phase. Such an acoustic frequency range is usually the most difficult to handle for passive acoustic components, owing to the long wavelength, and consequently, the bulky size of the materials required. The active panel is actuated by a piezoelectric transducer, with its vibrational displacement amplified by a specially designed mechanical amplifier. Through both simulations and impedance-tube experiments, we show that simply tuning the active panel’s displacement amplitude can vary its impedance from that of the hard wall, to impedance matching with air (leading to total absorption), to zero (leading to reflection phase reversal). No feedback loop is necessary for the active-panel device; only an initial adjustment during installation is necessary. We simulate the room-acoustics application for a 1 × 1 ${\mathrm{m}}^2$ wall panel that occupies only 1/48 of the total wall area of the room and show that it can introduce switchable elements to the audio experience.

We investigate dissipative nonlinear dynamics in graphene-based active metamaterials composed of randomly dispersed graphene nanoflakes embedded within an externally pumped gain medium. We observe that graphene saturable nonlinearity produces a subcritical bifurcation of nonlinear modes, enabling self-organization of the emitted radiation into several dissipative soliton structures with distinct topological charges. We systematically investigate the existence domains of such nonlinear waves and their spatial dynamics, finding that soliton vortices are unstable, thus enabling self-organization into single dissipative structures with vanishing topological charge, independently of the shape of the graphene nanoflakes. Our results shed light on self-organization of coherent radiation structures in disordered systems and are relevant for future cavity-free lasers and amplifier designs.

Periodic structures composed of quarter-wavelength or Helmholtz resonators have been widely used in the design of acoustic metamaterials. An interesting phenomenon achievable through hybridization in such structures is the slow sound, which results from the strong dispersion produced by the local resonances. It gives rise to many applications such as deep subwavelength sound absorbers or metadiffusers. All the applications proposed so far have been analyzed only in the frequency domain (steady state). In this work, we propose a passive treatment that can be used in room acoustics, which requires considering the time domain and all multiple reflections. We analytically design a delay line from a metasurface made of Helmholtz resonators, using slow-sound propagation. We prove numerically and experimentally that such structures can delay a pulse and thus reproduce the sound perception of a propagation over a given distance, larger than the actual size of the treatment. The limitations of real-time pulse propagation, dispersion, and losses on audio fidelity are discussed.

Recent research on hybrid magnonics has been restricted by the long magnon wavelengths of the ferromagnetic resonance modes. We present an experiment on the hybridization of 250-nm-wavelength magnons with microwave photons in a multimode magnonic system consisting of a planar cavity and a magnetic bilayer. The coupling between magnon modes in the two magnetic layers, i.e., the uniform mode in Permalloy and the perpendicular standing spin waves (PSSWs) in yttrium iron garnet, serves as an effective means for exciting short-wavelength PSSWs, which is further hybridized with the photon mode of the microwave resonator. The demonstrated magnon-photon coupling approaches the superstrong coupling regime, and can even be achieved near zero bias field.

Elusive features in photonic and electronic devices can be detected by means of advanced, time-domain spectroscopic techniques. In this paper we introduce a modulation spectroscopy based on the optomechanical interaction of photonic and mechanical modes. Applying the technique to a $\mathrm{Si}$ metasurface and its drumlike mechanical modes, we detect narrow-band quasi-bound-state-in-the-continuum (quasi-BIC) modes close to normal incidence, where their measurement can be hindered by a high-symmetry protection and undesired background modes. Showing a visibility enhancement of more than one order of magnitude, the optomechanical modulation spectroscopy can be an innovative tool for precise spectroscopy of a wide set of photonic devices, including the goal of measuring purely symmetry protected BIC resonances.

Lattice structures can exhibit unusual mechanical properties by appropriate design and arrangement of the microstructures, and have already found applications in areas such as tissue engineering, stretchable electronics, and soft robotics. However, the designed microstructures generally follow simple geometric patterns with constant curvatures and theoretical models are developed for each geometry particularly. It lacks a universal method to model and design the periodic lattice constructed from microstructures with varying curvatures. In this paper, we develop a universal approach to design polygonal lattices with a wide range of microstructures with varying curvatures using instant curvature and parametric functions. A finite deformation model is proposed for the microstructures and the corresponding lattices, which considers the wavy or curled microstructure, hierarchical geometry, and large deformation. Experiments and finite-element simulations are conducted to validate the theoretical model. All of Young’s modulus, stretchability, and Poisson’s ratio can be inversely designed using the developed theoretical model. Results show that the stretchability of the highly stretchable lattice can be further increased by more than 4 times using densely curled microstructure. The signs of Poisson’s ratio can also change with the axial strain by rationally designing the microstructures. By combining curve fitting and curvature and parametric functions, the mechanical behaviors of lattices constructed by bioinspired microstructures can also be predicted. The lattice structures are then demonstrated to reinforce a hydrogel with modulus increased by around 2 orders and act as a biomimetic soft-strain sensor. This work could aid the design of a hierarchical lattice and have potential applications in tissue engineering, stretchable electronics, and soft robotics.

${90}^{\circ }$ pinned magnetic domain walls can be observed in thin magnetic layers attached to a ferroelectric substrate. The main stabilization mechanism of the noncollinear magnetic texture is the strain transfer, which is responsible for imprinting of the ferroelectic domains into the uniaxial anisotropy of the ferromagnet. Here, we investigate by means of micromagnetic simulations how the interfacial Dzyaloshinskii-Moriya interaction influences the ${90}^{\circ }$ domain-wall structure. It is shown that Dzyaloshinskii-Moriya interaction induces a large out-of-plane magnetization component, strongly dependent on the domain wall type. In particular, it is shown that this out-of-plane magnetization component is crucial for the transport of the in-plane magnetic skyrmions, also known as bimerons, through the magnetic domain walls. Based on the results of micromagnetic simulations, a concept of in-plane magnetic skyrmion valve based on two ${90}^{\circ }$ pinned magnetic domain walls is introduced.

We explore and systematically compare the morphological, structural, and optical properties of $\mathrm{Ga}\mathrm{N}$/($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ multiple quantum wells (MQWs) grown by plasma-assisted molecular beam epitaxy (PA MBE) on freestanding $\mathrm{Ga}\mathrm{N}$$(0001)$ and $\mathrm{Ga}\mathrm{N}$$(000\overline{1})$ substrates. Samples of different polarity are found to be comparable in terms of their morphological and structural perfection and exhibit essentially identical quantum well widths and $\mathrm{Al}$ content. Regardless of the crystal orientation, the exciton decay in the MQWs at $10$ K is dominantly radiative and the photoluminescence (PL) energy follows the quantum confined Stark effect for different quantum well widths. A prominent free-to-bound transition involving interface shallow donors is, however, visible for the N-polar MQWs. At room temperature, in contrast, the exciton decay in all samples is dominated by nonradiative recombination taking place at point defects, presumably $\mathrm{Ca}$ or $V_{\mathrm{N}}$ located at the bottom QW interface. Remarkably, the N-polar MQWs exhibit a higher PL intensity and longer decay times than the $\mathrm{Ga}$-polar MQWs at room temperature. This improved internal quantum efficiency is attributed to the beneficial orientation of the internal electric field that effectively reduces the capture rate of minority carriers by interface trap states.

We present metal-dielectric metasurfaces fabricated from metal periodical nanograting deposited on a dielectric substrate. The metasurface consists of a modulated dielectric, which is covered by a thin silver layer. The metasurface operates as an open plasmon resonator. The theory of plasmons excited in the open resonator formed by a metal nanograting is presented. The large local electromagnetic field is predicted for optical frequencies. The excitation of plasmons is experimentally demonstrated in the metasurface designed on a 4-in. $\mathrm{Si}$ wafer. The enhancement of the local electric field results in surface-enhanced Raman scattering (SERS). To investigate the SERS effect, the metasurface is covered with a thin layer of the 4-mercaptophenylboronic acid that molecules form covalent bonds with the silver nanolayer. The enhancement of the Raman scattering serves as a proof of concept. We obtain a detection limit of 230 nM for molecules of 4-mercaptophenylboronic acid.

Over the years, an enormous effort has been made to establish nitrogen vacancy (N-$V$) centers in diamond as easily accessible and precise magnetic field sensors. However, most of their sensing protocols rely on the application of bias magnetic fields, preventing their usage in zero- or low-field experiments. We overcome this limitation by exploiting the full spin $S=1$ nature of the N-$V$ center, allowing us to detect nuclear spin signals at zero and low field with a linearly polarized microwave field. As conventional dynamical decoupling protocols fail in this regime, we develop robust pulse sequences and optimize pulse pairs, which allow us to sense temperature and weak ac magnetic fields and achieve an efficient decoupling from environmental noise. Our work allows for much broader and simpler applications of N-$V$ centers as magnetic field sensors in the zero- and low-field regime and can be further extended to three-level systems in ions and atoms.

The effective modulation of acoustic fields is the most important property of acoustic metasurfaces. The realization of full-space wavefront control can significantly enhance the functionality of metasurfaces; however, the existing solutions to this problem are limited by the coupled modulations of the transmitted and reflected wavefronts. In this study, we demonstrate the possibility of controlling transmitted and reflected acoustic wavefronts in a decoupled manner with a passive structure. Simulated analyses of the parameter dependences of the transmission and reflection phases reveal that these phases can be combined arbitrarily within a range of structural parameters. Meanwhile, tunable designs increase the flexibility and simplicity of the modulation of acoustic waves. Using such a tunable structure, a transmission-reflection-integrated (TRI) metasurface is designed. By applying a single TRI metasurface, multiple independent functions are simultaneously realized in the transmitted and reflected regions, which is further confirmed by pancratic multifocal focusing (performed both experimentally and theoretically) and holographic imaging simulations. The simulated, calculated, and experimental data obtained demonstrate efficient wavefront control and excellent functional-integration performance of the TRI metasurface. In this paper, we propose a decoupled method for the simultaneous manipulation of reflected and transmitted acoustic waves, which can enhance the spatial utilization and functionality of acoustic devices.

Electrical control of conjugate degrees of freedom in multiferroics provides the advantage of reducing energy consumption to femto- and even attojoules per switch in spintronics and memory devices. This is achieved through the development of technologies that make it possible to fabricate artificial materials with constantly improving properties. Here, we present the design, physics, and characteristics of a composite multiferroic spintronic emitter, which provides electrical control of the emitted terahertz (THz) wave polarization. The effect is due to electrical control of the magnetization in a high-quality magnetostrictive superlattice, ${\mathrm{Tb}\mathrm{Co}}_2/\mathrm{Fe}\text{−}\mathrm{Co},$ deposited on an anisotropic piezoelectric substrate. In our approach, several mechanisms are realized in the system simultaneously: the strain-mediated coupling of the magnetic and piezoelectric subsystems, which operate in the range of the spin-reorientation transition of the magnetic superlattice, and THz-wave generation in the superlattice by an optical femtosecond pulse. This provides flexibility and control of the set of parameters. We determine the magnetoelectric parameter, which is responsible for THz polarization control. Our results offer a significant fundamental insight into the physics of composite multiferroic systems that can be used for applications of multiferroicity, primarily for THz spintronic emitters. We believe that our findings represent a decisive step towards technologies for other types of spintronic and memory devices.

Atomic force microscopy is a tool for characterizing surface acoustic waves, in particular for high frequencies, where the wavelength is too short to be resolved by laser interferometry. A caveat is, that the cantilever deflection is not equal to the amplitude of the surface acoustic wave. We show that the energy transfer from the moving surface to the cantilever instead leads to a deflection exceeding the surface modulation. We present a method for an accurate determination of surface acoustic wave amplitudes based on comparing force-curve measurements with the equation of motion of a driven cantilever. We demonstrate our method for a standing surface acoustic wave on a $\mathrm{Ga}\mathrm{As}$ crystal confined in a focusing cavity with a resonance frequency near 3 GHz.

This work presents a theoretical investigation of an active photonic grating of the parity-time- ($PT$) symmetric architecture. The analytical study of the free-space mode propagation from the grating structure indicates the unique bifurcation property due to the $PT$-symmetry modulation. It is shown that both the gain-loss contrast and the lattice constant parameters are critical factors to modulate the active photonic system in between the $PT$-symmetry to the symmetry-broken phases. Furthermore, numerical simulations via the rigorous coupled-wave analysis (RCWA) method discover the existence of a unique spectral singularity phenomenon in this $PT$ grating structure, which corresponds to a nontrivial single mode and near-zero bandwidth photonic resonant emission. Also, the guiding procedure for fulfilling spectral singularity modes is found to be related to the unique formation of the scattering matrix applied in the $PT$-symmetric photonic gratings. This theoretical work takes a fresh look into the active $PT$-symmetric photonic gratings focusing on the discovery of nontrivial free-space emission modes, which could contribute to the development of high-performance surface-emitting semiconductor devices.

We study the negative couplings amid local resonances of photonic metasurfaces and their radiation polarizations. In our analysis, we discover fully circularly polarized, wave-vector variant, radiational eigenstates that are attributed to interorbit coupling-induced negative couplings. Our theoretical model is exemplified via a guided resonance dielectric metasurface that possesses type-II non-Hermitian Dirac points, from where the circular polarization lines emanate. The designed metasurface carries circularly polarized radiation eigenstates in both the zeroth and the first diffraction orders, allowing spin-selective light deflections. The high quality factor and field enhancement of the designed metasurface could lead to applications for spin-selective sensing, beam control, and nonlinear optics. Our findings open a gateway for the design of near-field coupling-assisted metasurfaces.

Modern optoelectronic devices rely on cavity electrodynamics concepts for improved performances, embedding the active medium in an optical cavity to enhance the light-matter coupling. This coupling rate is usually small compared to the electronic and photon energies. Despite several demonstrations, devices operating with much larger light-matter coupling strength, in the so-called strong light-matter coupling regime, are yet to be demonstrated as viable, practical candidates. One of the main technological obstacles that hamper their dissemination is the comprehension of the carrier current extraction and injection from and into strongly coupled light-matter states. Here, we study this fundamental phenomenon in midinfrared quantum cascade detectors (QCDs) operating in the moderate to strong light-matter coupling regime. They operate around $\lambda =10\mu \mathrm{m}$ with a minimum Rabi splitting of 9.3 meV. A simple model based on the usual description of transport in QCDs does not reproduce the polaritonic features in the photocurrent spectra. On the contrary, a more refined approach, based on the semiclassical coupled modes theory, is capable of reproducing both optical and electrical spectra with excellent agreement. By correlating absorption and photoresponse with the simulations, we demonstrate that—in this system—resonant tunneling from the polaritonic states appears to be the predominant extraction mechanism. The dark intersubband states do not have a significant role in the process, contrary to what happens in electrically injected polaritonic emitters.

We realize and model an internal-state Rydberg atom interferometer for measurement of phase and intensity of radio-frequency (rf) electromagnetic waves. A phase reference is supplied to the atoms via a modulated laser beam, enabling atomic measurement of the rf wave’s phase without a rf reference wave. The rf and optical fields give rise to closed interferometric loops within the atoms’ internal Hilbert space. In our experiment, we construct interferometric loops in the state space $\{6P_{3/2},90S_{1/2},91S_{1/2},90P_{3/2}\}$ of cesium and employ them to measure phase and intensity of a 5-GHz rf wave in a room-temperature vapor cell. Electromagnetically induced transparency on the $6S_{1/2}$ to $6P_{3/2}$ transition serves as an all-optical interferometer probe. The rf phase is measured over a range of $\pi$, and a sensitivity of 2 mrad is achieved. Measurements of rf phase and amplitude at submillimeter optical spatial resolution are demonstrated.