By analogy with electromagnetic networks that connect multiple input-output ports, metasurfaces can be considered as multiport devices capable of providing different functionalities for waves of different polarizations illuminating the surface from different directions. The main challenge in the design of such multichannel metasurfaces is to realize independent and full control of the electromagnetic response for each channel incidence, ensuring the fulfilment of the boundary condition at the metasurface. In this work, we demonstrate that by properly engineering the evanescent fields excited at each port (that is, for all possible illumination directions), it is possible to independently control the reflection or transmission for all different illuminations. Using the mode-matching method, we analyze the scattering properties of generic space-modulated impedance metasurfaces. This method, combined with mathematical optimization, allows us to find a surface-impedance profile that simultaneously ensures the desired electromagnetic responses at each port. We validate the technique via the design of phase-controlled multichannel retroreflectors and multichannel perfect absorbers. In addition, we demonstrate that the method is rather powerful in the design of other functional metasurfaces, such as multifunctional reflectors.

Characterization of microstructures in living tissues is one of the keys to diagnosing early stages of pathology and understanding disease mechanisms. However, the extraction of reliable information on biomarkers based on microstructure details is still a challenge, as the size of features that can be resolved with noninvasive magnetic resonance imaging (MRI) is orders of magnitude larger than the relevant structures. Here we derive from quantum information theory the ultimate precision limits for obtaining such details by MRI probing of water-molecule diffusion. We show that currently available MRI pulse sequences can be optimized to attain the ultimate precision limits by choosing control parameters that are uniquely determined by the expected size, the diffusion coefficient, and the spin relaxation time $T_2$. By attaining the ultimate precision limit per measurement, the number of measurements and the total acquisition time may be drastically reduced compared to the present state of the art. These results are expected to open alternative avenues towards unraveling diagnostic information by quantitative MRI.

In this work, magnetization dynamics is studied in superconductor-ferromagnet-superconductor three-layered films in a wide frequency, field, and temperature ranges using the broad-band ferromagnetic resonance measurement technique. It is shown that in the presence of both superconducting layers and of superconducting proximity at both superconductor-ferromagnet interfaces a massive shift of the ferromagnetic resonance to higher frequencies emerges. The phenomenon is robust and essentially long-range: it has been observed for a set of samples with the thickness of ferromagnetic layer in the range from tens up to hundreds of nanometers. The resonance frequency shift is characterized by proximity-induced magnetic anisotropies: by the positive in-plane uniaxial anisotropy and by the drop of magnetization. The shift and the corresponding uniaxial anisotropy grow with the thickness of the ferromagnetic layer. For instance, the anisotropy reaches 0.27 T in experiment for a sample with a 350-nm-thick ferromagnetic layer, and about 0.4 T in predictions, which makes it a ferromagnetic film structure with the highest anisotropy and the highest natural resonance frequency ever reported. Various scenarios for the superconductivity-induced magnetic anisotropy are discussed. As a result, the origin of the phenomenon remains unclear. Application of the proximity-induced anisotropies in superconducting magnonics is proposed as a way for manipulations with a spin-wave spectrum.

We present proof-of-concept imaging measurements of a polycrystalline material that integrate the elements of conventional high-energy x-ray diffraction microscopy with coherent diffraction-imaging techniques, and that can enable in situ strain-sensitive imaging of lattice structure in ensembles of deeply embedded crystals over five decades of length scale upon full realization. We demonstrate that the combination of these two characterization methods enables a more complete picture of the crystal-lattice strain than either method is capable of providing individually. Such complementary imaging capabilities are critical to addressing questions in a variety of research areas such as materials science and engineering, chemistry, and solid-state physics. Towards this eventual goal, the following key aspects are demonstrated: (1) high-energy Bragg coherent diffraction imaging (HE BCDI) of submicron-scale crystallites at 52 keV at current third-generation synchrotron light sources, (2) HE BCDI performed in conjunction with far-field high-energy diffraction microscopy (FF HEDM) on the grains of a polycrystalline sample in a smoothly integrated manner, and (3) the orientation information of an ensemble of grains obtained via FF HEDM used to perform complementary HE BCDI on multiple Bragg reflections of a single targeted grain. The imaged structures are seen to be $477\pm 50$ nm or smaller in size, with an estimated strain resolution of $2.5\times {10}^{-4}$. These steps lay the foundation for integration of HE BCDI, which typically provides a spatial resolution tens of nanometers, into a broad suite of well-established HEDM methods, extending HEDM beyond the few-micrometer resolution bound and into the nanoscale, and positioning the approach to take full advantage of the orders-of-magnitude improvement of x-ray coherence expected at fourth-generation light sources presently being built and commissioned worldwide.

Luminescence arising from $\beta$ decay of radiotracers has attracted much interest recently as a viable in vivo imaging technique. The emitted Cerenkov radiation can be directly detected by high-sensitivity cameras or used to excite highly efficient fluorescent dyes. Here we investigate the enhancement of visible and infrared emission driven by $\beta$ decay of radioisotopes in the presence of a hyperbolic nanocavity. By means of a transfer-matrix approach, we obtain quasianalytic expressions for the fluorescence-enhancement factor at the dielectric core of the metalodielectric cavity, and report a 100-fold amplification in periodic structures. A particle-swarm optimization of the layered shell geometry reveals that enhancement of up to $10000$-fold is possible because of the hybridization and spectral overlap of whispering-gallery and localized-plasmon modes. Our findings may find application in nuclear-optical medical imaging, as they provide a strategy for the use of highly energetic gamma rays, Cerenkov luminescence, and visible and near-infrared fluorescence through the same nanotracer.

High-resolution scanning transmission x-ray microscopy (STXM) can provide quantitative access to local magnetization canting angles relative to the surface. We use this cutting-edge microscopy technique to probe the weak stripe magnetic texture hosted by a 180-nm-thick ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ layer. We report a full set of measurements of the weak stripe canting angle, as well as a method that uses only a single-impinging-direction x-ray beam to extract the angle. This method also allows the spatial profile to be extracted with 30 nm resolution, and the canting angle versus applied field can be measured. Combining these information, the macroscopic magnetization versus applied field loop on a complex spin texture can be reconstructed in an unprecedented way and shows undoubtedly that flux closure domains exist. These results show that the presence of flux closure domains, at the surface of such a thick film, is difficult to demonstrate by means of the STXM technique. Beyond the characterization of the weak stripe angle, this quantitative x-ray magnetic microscopy can be used to study the local textures hosted in all materials exhibiting some circular x-ray magnetic dichroism.

We investigate the performance of a microgap vacuum thermionic energy converter by considering loss mechanisms due to the space-charge effect and interelectrode radiative heat transfer. The dependencies of the space-charge effect and near-field radiative heat exchange on the interelectrode distance are derived based on established theories. The electrode temperatures are determined by solving the steady-state energy-balance equations in a numerical iterative process and considering a constant energy flux input to the emitter. The resultant behavior of different mechanisms of energy flow from the electrodes is studied for a wide range of interelectrode distances, which provides insights into the device operation. The maximum efficiency of the converter is obtained by optimizing the operating voltage and interelectrode distance. Considering the interplay between the space-charge effect and near-field radiative heat transfer, an optimal range is determined for the interelectrode distance. The optimal value of the distance and lower limit of this range are significantly higher than those previously reported, where constant electrode temperatures are assumed.

We calculate a spin-polarized conductance in a nanostructure with a composition of a “high-temperature ferromagnetic insulator-graphene-ferroelectric film,” with special attention paid to control of the electric polarization direction and value in a multiaxial ferroelectric film by a misfit strain. We propose a phenomenological model, taking into account the shift of the Dirac point due to the proximity of the ferromagnetic insulator and using the Landauer formula for the conductance of the graphene channel. We derive analytical expressions, which show that the strain-dependent ferroelectric polarization governs the concentration of two-dimensional charge carriers and Fermi level in graphene in a self-consistent way. The obtained results demonstrate the opportunity to control the spin-polarized conductance of graphene by a misfit strain at room and higher temperatures in the nanostructures ${\mathrm{Co}\mathrm{Fe}}_2{\mathrm{O}}_4$-graphene-${\mathrm{Pb}\mathrm{Zr}}_{0.4}{\mathrm{Ti}}_{0.6}{\mathrm{O}}_3$ and ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$-graphene-${\mathrm{Pb}\mathrm{Zr}}_{0.4}{\mathrm{Ti}}_{0.6}{\mathrm{O}}_3$, and thus, open up the possibilities for the application of ferromagnetic-graphene-ferroelectric nanostructures as nonvolatile spin filters and spin valves.

We present a general and convenient first-principles method to study near-field radiative heat transfer. We show that the Landauer-like expression of heat flux can be expressed in terms of a frequency- and wave-vector-dependent macroscopic dielectric function that can be obtained from the linear response density functional theory. A random phase approximation is used to calculate the response function. We compute the heat transfer in three systems: graphene, molybdenum disulfide (${\mathrm{Mo}\mathrm{S}}_2$), and hexagonal boron nitride ($h$-BN). Our results show that the near-field heat flux exceeds the black body limit by up to 4 orders of magnitude. With an increase of the distances between two parallel sheets, a $1/d^2$ dependence of heat flux is shown, consistent with Coulomb’s law. The heat transfer capacity is sensitive to the dielectric properties of materials. Influences from chemical potential and temperature are also discussed. Our method can be applied to a wide range of materials including systems with inhomogeneities, which provides solid references for applications of both physics and engineering.

Ultra-high-frequency nanomechanical resonators ($f_m>300$ MHz) can increase our capacity to study fast physical phenomena, for example by measuring forces. Their extreme stiffness is also a chance to access molecular forces in the subpicometer low amplitude of motion limit, but it makes them hard to drive and control. Here we analyze a method to optomechanically sense a force field with an ultra-high-frequency and stiff mechanical resonator, where back-action optical forces set the resonator into a self-sustained stable oscillator trajectory. After elucidating the experimental conditions to obtain optimal resolution, we carry out controlled experiments where the oscillator senses an optical force generated by a secondary laser. We analyze and model our results, and illustrate the concrete advantage of the method in the measurement of such a weak force, which would otherwise remain undetected by the undriven probe. We establish the thermodynamical limits of the approach, and finally connect it to the class of feedback-controlled problems, clarifying its assets and limitations.

We analyze the properties of femtosecond-laser-written waveguides in silicon. By using a mix of semianalytical and numerical methods, we find that the transverse index profile is accurately modeled by a W-shaped function supporting nonleaky guided modes. The maximum change in the refractive index is about $4\times {10}^{-3}$, whereas the central lobe is about 2.5 $\mu \mathrm{m}$ wide for a writing wavelength of 1.55 $\mu \mathrm{m}$. Waveguide shape and maximum index variation both saturate for input pulse energies larger than 100 nJ and a pulse duration of 860 fs. We find that the performance of the waveguides are limited by the presence of randomly distributed scattering centers situated in the waveguide core. To confirm our findings, we probed the waveguide response as the input beam is kept normal at the input interface but shifted perpendicularly to the waveguide axis, resulting in a variable offset with respect to the center of the waveguide. The behavior of the energy coupled in the waveguide versus the beam shift is in agreement with theoretical predictions. We also used a simple analytical model to describe the nonlinear propagation of the writing beam.

Combining micromagnetic simulation and analytical modeling, we perform an in-depth study on the correlation between current-driven transport characteristics of individual skyrmions and deformation under the influences of Dzyaloshinskii-Moriya interaction, perpendicular magnetic anisotropy, temperature, magnetic fields, and electrical currents. How the system parameters affect the configuration of skyrmions and further influence their transport characteristics (e.g., speed, Hall angle, and mobility) is revealed and dynamic “phase diagrams” of skyrmions are presented. Generally, the mobility of a skyrmion increases as its size increases, and the shapes and cores of skyrmions play important roles in their transport behavior. Beyond the small-driving-current regime, a remarkable deformation of skyrmions with expanding size and noncircular shape occur during transport. This leads to an increase of the driving forces and skyrmion mobility, manifesting a nonlinear feature of the skyrmion speed-current curve. Conversely, an increase of the driving forces causes a larger deformation of skyrmions and forms a positive feedback mechanism. Once the current exceeds a critical value, the driving forces are so large that skyrmions cannot maintain their topology and break down. Our work establishes the correlation between transport characteristics and deformation of skyrmions and predicts possible nonlinear and breakdown effects caused by dynamic deformation.

Stray magnetic fields contain significant information about the electronic and magnetic properties of condensed-matter systems. For two-dimensional (2D) systems, stray field measurements can even allow full determination of the source quantity. For instance, a 2D map of the stray magnetic field can be uniquely transformed into the 2D current density that gives rise to the field and, under some conditions, into the equivalent 2D magnetization. However, implementing these transformations typically requires truncation of the initial data and involves singularities that may introduce errors, artefacts, and amplify noise. Here we investigate the possibility of mitigating these issues through vector measurements. For each scenario (current reconstruction and magnetization reconstruction) the different possible reconstruction pathways are analyzed and their performances compared. In particular, we find that the simultaneous measurement of both in-plane components ($B_x$ and $B_y$) enables near-ideal reconstruction of the current density, without singularity or truncation artefacts, which constitutes a significant improvement over reconstruction based on a single component (e.g., $B_z$). On the other hand, for magnetization reconstruction, a single measurement of the out-of-plane field ($B_z$) is generally the best choice, regardless of the magnetization direction. We verify these findings experimentally using nitrogen-vacancy-center magnetometry in the case of a 2D current density and a 2D magnet with perpendicular magnetization.

Graphene antidot lattices (GALs) are two-dimensional (2D) monolayers with periodically placed holes in otherwise pristine graphene. We investigate the electronic properties of symmetric and asymmetric GAL structures having hexagonal holes, and show that anisotropic 2D GALs can display a dimensional crossover such that effectively one-dimensional (1D) electronic structures can be realized in two dimensions around the charge neutrality point. We investigate the transport and thermoelectric properties of these 2D GALs by using the nonequilibrium Green function method. Dimensional crossover manifests itself as transmission plateaus, a characteristic feature of 1D systems, and enhancement of thermoelectric efficiency, where thermoelectric figure of merit, $zT$, can be as high as 0.9 at room temperature. We also study the transport properties in the presence of Anderson disorder and find that mean free paths of effectively 1D electrons of anisotropic configuration are much longer than their isotropic counterparts. We further argue that dimensional crossover due to broken symmetry and enhancement of thermoelectric efficiency can be nanostructuring strategy virtually for all 2D materials.

Fast controllable optical passive devices containing intricate couplings of multiple physical fields, for instance, magneto-, electro-, and acousto-optic interactions, are frequently used as critical regulation tools in diverse optical systems. Their unpredictable transient spectral properties under high-speed modulation strongly influence the operational performance of the whole system, but are exhibited very little in conventional spectroscopies due to slow scans with a frame rate of the order of a kilohertz. Here, a dissipative soliton fiber laser assisted dispersive Fourier transform is built to achieve ultrafast spectroscopic characterization. An acoustically induced fiber grating (AIFG), as one typical case of a fast-response device, is studied under driving signals in frequency-switch and frequency-sweep modes. The experimental results reveal the continuous wavelength-sweeping property of the AIFG. Its transmission spectrum for a driving-frequency chirp is destroyed when the sweep time approaches the transit time of the acoustic wave in the grating region. This work opens up an avenue for the measurement of transient physical characteristics of passive devices, such as spectral aberration, transit time, and operational bandwidth.

We demonstrate voltage-tunable thermo-optic bi- and tristability in silicon photonic microring resonators with lateral $p$-$i$-$n$ junctions and present a technique for characterizing the thermo-optic transient response of integrated optical resonators. Our method for thermo-optic transient response measurement is applicable to any integrated photonics platform and uses standard equipment. Thermo-optic relaxation in encapsulated waveguides is found to be approximately logarithmic in time, consistent with the analytical solution for two-dimensional heat diffusion. We develop a model for thermo-optic microring multistability and dynamics that agrees with experimental data over a wide range of operating conditions. Our work highlights the fundamental connection in semiconductor waveguides between active free-carrier removal and thermo-optic heating, a result of particular relevance to Kerr soliton-state stability and on-chip frequency-comb generation. The devices studied here are fabricated in a CMOS foundry process and as a result our model is useful for the design of silicon photonic waveguide devices.

Freestanding silicon nitride ($\mathrm{Si}\mathrm{N}$) mechanical resonators are of central interest in applications such as temperature and mass sensing, and for fundamental optomechanical research. Understanding thermal coupling between a membrane resonator and its environment is required to predict thermal noise, frequency noise, and sensor responses to temperature changes. In this work, we provide closed-form derivations of intrinsic thermal-coupling quantities in freestanding thin films—namely, total thermal conductance with the surroundings, thermal response time, and the relative contribution of thermal radiation. Our model is valid for any freestanding thin film anchored on all sides when small temperature differences between the film and its environment are considered. We particularly emphasize the specific case of $\mathrm{Si}\mathrm{N}$, for which spectral emissivity is thoroughly investigated as a function of thickness and temperature. We find that radiative heat exchanges can play a non-negligible role, and can even dominate thermal coupling for membranes of sizes commonly used in optomechanics experiments. We find that our model is in agreement with preliminary experimental results on radiative heat transfer between a ceramic heater and a $3\times 3{\mathrm{mm}}^2$ membrane in a high vacuum.

The navigation of microrobots in complex flow environments is controlled by rheotaxis, the reorientation with respect to flow gradients. Here, we demonstrate how payloads can be exploited to enhance the motion against flows. Using fully resolved hydrodynamic simulations, the mechanisms are described that allow microrobots of different shapes to reorient upstream. We find that cargo pullers are the fastest at most flow strengths but that pushers feature a nontrivial optimum as a function of the counterflow strength. Moreover, the rheotactic performance can be maximized by tuning the microrobot shape or cargo size. These results may be used to control microswimmer navigation but they also apply to rheotaxis in microbial ecology and the prevention of bacterial-contamination dynamics.

Controllable interaction between superconducting qubits is desirable for large-scale quantum computation and simulation. Here, based on a theoretical proposal by Yan et al. [Phys. Rev. Appl. 10, 054061 (2018)] we experimentally demonstrate a simply designed and flux-controlled tunable coupler with a continuous tunability by adjusting the coupler frequency, which can completely turn off adjacent superconducting qubit coupling. Utilizing the tunable interaction between two qubits via the coupler, we implement a different type of controlled-phase (cz) gate with “dynamically decoupled regime,” which allows the qubit-qubit coupling to be only “on” at the usual operating point while dynamically “off” during the tuning process of one qubit frequency into and out of the operating point. This scheme not only efficiently suppresses the leakage out of the computational subspace, but also allows for the acquired two-qubit phase being geometric at the operating point. We achieve an average cz gate fidelity of $98.3\pm 0.6\mathrm{\%}$, which is dominantly limited by qubit decoherence. The demonstrated tunable coupler provides a desirable tool to suppress adjacent qubit coupling and is suitable for large-scale quantum computation and simulation.