The conventional approach in transformation optics (TO) starts with a virtual space and determines a complicated material profile in physical space to achieve unconventional phenomena, such as invisibility cloaks. We demonstrate that complicated materials can be effectively fabricated by virtual space based on the reversed process of TO in two dimensions. We first show that a conformal singularity of a refractive-index profile, with either zero or infinity resulting from a power conformal mapping $w=z^{\alpha }$, is equivalent to a topological defect with positive charge or negative charge. The splitting effect and illusion effect are induced by the conformal singularities and related to the two-dimensional topological defects. Based on the equivalence, we fabricate a device of such a topological defect with a positive charge in experiments. Moreover, we observe its related light-bending functionality with laser beams. It has potential important applications in designing on-chip optical devices.

Scanning tunneling microscopy (STM) enables the fabrication of two-dimensional $\delta$-doped structures in $\mathrm{Si}$ with atomistic precision, with applications from tunnel field-effect transistors to qubits. The combination of a very small contact area and the restrictive thermal budget necessary to maintain the integrity of the δ layer make developing a robust electrical contact method a significant challenge to realizing the potential of atomically precise devices. We demonstrate a method for electrical contact using ${\mathrm{Pd}}_2\mathrm{Si}$ formed at the temperature of silicon overgrowth (250 °C), minimizing the diffusive impact on the δ layer. We use the transfer length method to show our ${\mathrm{Pd}}_2\mathrm{Si}$ contacts have very high yield (99.7% +0.2% −1.5%) and low resistivity (272 ± 41 Ω µm) in contacting mesa-etched $\mathrm{Si:P}$ δ layers. We also present three terminal measurements of low contact resistance (<1 kΩ) to devices written by STM hydrogen depassivation lithography with similarly high yield (100% +0% −3.2%).

A self-assembled two-dimensional (2D) film of tetra-phenyl-porphyrin-4-ferrocene molecules on $\mathrm{Au}(111$) is studied by STM for the presence of intra- and intermolecular correlations in the configurations of the four-pendant ferrocenyl moieties. A statistical analysis of STS images exploits the Pearson’s linear correlation coefficient derived from changes in the molecular electron density across lateral positions in the molecular network as a measure of the intra- and intermolecular coupling and/or conjugation between adjacent equivalent molecular components. Density functional theory (DFT) calculation shows that these electron density changes can be assigned to conformational changes of the ferrocenyl units of the molecules. The methodology presented here can be extended to measure correlations in other 2D systems.

We introduce a scanning probe technique derived from scanning gate microscopy (SGM) to investigate thermoelectric transport in two-dimensional semiconductor devices. Thermoelectric scanning gate microscopy (TSGM) consists in measuring the thermoelectric voltage induced by a temperature difference across a device while scanning a polarized tip that locally changes the potential landscape. We apply this technique to perform interferometry of the thermoelectric transport in a quantum point contact (QPC). We observe an interference pattern in both SGM and TSGM images, and evidence large differences between the two signals in the low-density regime of the QPC. In particular, a large phase jump appears in the interference fringes recorded by TSGM, which is not visible in the interference fringes recorded by SGM. We discuss this difference of sensitivity using a microscopic model of the experiment, based on the contribution from a resonant level inside or close to the QPC. This work demonstrates that combining scanning gate microscopy with thermoelectric measurements offers new information as compared with SGM alone, and provides direct access to the derivative of the device transmission with respect to energy, both in amplitude and in phase.

High energy electron probes, with better penetrating power than those used in conventional transmission electron microscopes, are suitable for imaging of thicker objects and expected to be promising in the diagnostics of high energy density physics. Pushing the spatial resolution of high-energy-electron radiography (HEER) from tens of microns at present to micron or even submicron levels requires technical innovations both in electron sources and imaging optics. In this paper, we report the demonstration of single-shot high-energy-electron radiography with micron-level spatial resolution via the combination of a state-of-the-art rf photoinjector and a cascaded-strong-permanent-magnet-quadrupoles-(PMQs) based imaging system. Successful implementation of cascaded or multistage high-energy-electron radiography is necessary for obtaining a large magnification factor in order to achieve improved resolving power. In addition, the ultimate resolution of the high energy electron imaging system is analyzed and future plans toward high-resolution HEER is discussed.

A spectrally uncorrelated biphoton state generated from the spontaneous nonlinear optical process is an important resource for quantum information. Currently, such a spectrally uncorrelated biphoton state can only be prepared from limited kinds of nonlinear media, thus limiting their wavelengths. In order to explore a wider wavelength range, here we theoretically study the generation of a spectrally uncorrelated biphoton state from 14 isomorphs of potassium dihydrogen phosphate (KDP) crystal. We find that 11 crystals from the “KDP family” still maintain similar nonlinear optical properties of KDP, such as KDP, DKDP, ADP, DADP, ADA, DADA, RDA, DRDA, RDP, DRDP, and KDA, which satisfy three kinds of the group-velocity matching conditions for spectrally uncorrelated biphoton-state generation from near-infrared to telecom wavelengths. Based on the uncorrelated biphoton state, we investigate the generation of a heralded pure-state single photon by detecting one member of the biphoton state to herald the output of the other. The purity of the heralded single photon is as high as 0.98 without using a narrow-band filter; the Hong-Ou-Mandel interference from independent sources can also achieve a visibility of 98%. This study may provide more and better single-photon sources for quantum-information processing at near-infrared and telecom wavelengths.

We demonstrate room-temperature stabilization of dipolar magnetic skyrmions with diameters in the range of 100 nm in a single ultrathin layer of the Heusler alloy ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Al}$ (CFA) under moderate magnetic fields. Current-induced skyrmion dynamics in microwires is studied with a scanning nitrogen-vacancy magnetometer operating in the photoluminescence quenching mode. We first demonstrate skyrmion nucleation by spin-orbit torque and show that its efficiency can be significantly improved using tilted magnetic fields, an effect that is not specific to Heusler alloys and could be advantageous for future skyrmion-based devices. We then show that current-induced skyrmion motion remains limited by strong pinning effects, even though CFA is a magnetic material with a low magnetic damping parameter.

We report broadband spectroscopy and numerical analysis by which we explore propagating spin waves in a magnetic bilayer consisting of a 23-nm thick permalloy film deposited on 130-nm thick ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$. In the bilayer, we observe a characteristic mode that exhibits a considerably larger group velocity at a small in-plane magnetic field than both the magnetostatic and perpendicular standing spin waves. Using the finite element method, we confirm the observations by simulating the mode profiles and dispersion relations. They illustrate the hybridization of spin-wave modes due to exchange coupling at the interface. The high-speed propagating mode found in the bilayer can be utilized to configure multifrequency spin-wave channels enhancing the performance of spin-wave-based logic devices.

${\mathrm{VO}}_2$ undergoes metal-insulator transition (MIT) at the transition temperature (τ_c) of 68 °C, accompanied by a sharp optical response change. A unique $\mathrm{Au}/{\mathrm{VO}}_2$ half-sphere core-shell structure is fabricated to study the temperature-dependent plasmonic effects and the thermochromic response for smart window applications. By increasing the $\mathrm{Au}$ core size, the surface plasmonic resonance wavelength of the ${\mathrm{VO}}_2$ high-temperature rutile phase is tuned from 600 to 720 nm and confirmed by simulations. Meanwhile, ${\tau }_c$ of ${\mathrm{VO}}_2$ is reduced by up to 10 °C, which is thought to be associated with plasmon-induced or percolation-enhancement-related effects. Modification of ${\tau }_c$ occurs on the optical domain only and not in the electrical domain. The in-depth analysis suggests that the interaction of the plasmon between $\mathrm{Au}$ and ${\mathrm{VO}}_2$ in the near field triggers different mechanisms, which reduce the ${\tau }_c$ of ${\mathrm{VO}}_2$, in particular under high irradiation levels. However, under normal irradiation levels the main contributing effect is found to be thermal strain at the nanoparticles‘ surface.

A fundamental limitation to the ability to transport sessile droplets is frictional forces arising from surface adhesion. This can be overcome by using the Leidenfrost effect on a heated substrate to levitate the droplet on a cushion of vapor. By structuring the surface under the droplet, the flow of vapor below the droplet can be controlled and this can be used to induce preferential droplet propulsion in a particular direction. However, while propulsion can be induced, the dramatic reduction in frictional forces leads to instability and it is difficult to control droplet motion when transporting droplets along a defined path. Here, we present a self-propulsion and self-centering concept using the principles of negative feedback to enable a droplet to be transported along a defined path. In our implementation, we use a combined herringbone and ratchet design, which provides the ability to control droplet position without compromising on speed. This intrinsic self-centering and correction via negative feedback offers the potential to design paths and tracks for droplets to follow, without the need for walls.

We analyze the origin of the intrinsic timing jitter in superconducting nanowire single-photon detectors in terms of fluctuations in the latency of the detector response, which is determined by the microscopic physics of the photon-detection process. We demonstrate that fluctuations in the physical parameters, which determine the latency, give rise to the intrinsic timing jitter. We develop a general description of latency by introducing the explicit time dependence of the internal detection efficiency. By considering the dynamic Fano fluctuations together with static spatial inhomogeneities, we study the details of the connection between latency and timing jitter. We develop both a simple phenomenological model and a more general microscopic model of detector latency and timing jitter based on the solution of the generalized time-dependent Ginzburg-Landau equations for the 1D hotbelt geometry. While the analytical model is sufficient for qualitative interpretation of recent data, the general approach establishes the framework for a more quantitative analysis of detector latency and the fundamental limits of intrinsic timing jitter. These theoretical advances can be used to interpret the results of recent experiments measuring the dependence of detection latency and timing jitter on photon energy to the few-picosecond level.

Manipulating acoustic waves beyond the diffraction limit offers alternative possibilities to many applications in wave physics. Here, we show that a gradient-index acoustic metasurface is able to achieve this purpose by modulating airborne sound at subwavelength scales via the local-resonance-controlled spoof surface acoustic waves. From the dispersion relation of such a surface mode, a design approach of explicitly correlating the effective refractive index with the geometrical unit-cell parameters can be obtained. With one carefully designed gradient-index acoustic metasurface constructed using locally resonant elements, we demonstrate the experimental realizations of both subwavelength plane-wave sound focusing and super-resolution acoustic imaging. This work provides a feasible pathway to the subwavelength manipulation of airborne sound. Moreover, it offers an ideal experimental platform to directly observe how acoustic energy flow is governed by refractive-index modulation. The proposed design approach can be easily applied to various spoof surface acoustic wave devices that demand gradient index distribution.

We find an unusually optimal near-field heat transfer, where the maximum heat transfer is reached at experimentally feasible gap separation. We attribute this to the localized zero-energy electronic edge states, which also substantially change the near-field behaviors. We demonstrate these anomalous behaviors in two typical carbon-based nanostructures: zigzag single-walled carbon nanotubes and graphene nanotriangles. For the system of carbon nanotubes, the maximal heat flux in this work surpasses all the previous results reported so far. The underlying mechanism for the peculiar effects is uncovered from a simple Su-Schrieffer-Heeger model. Our findings also offer a route to achieve an active near-field thermal switch, where the heat flux can be modulated through tuning the presence or absence of edge states.

Previously, we developed a set of photon-free quantum random-number generators (QRNGs) using $\mathrm{In}\text{−}\mathrm{Ga}\text{−}\mathrm{As}$ single-photon avalanche diodes. We exploited the stochastic property of the quantum tunneling effect. Here, we utilize tunneling signals in $\mathrm{Si}$ diodes to generate random numbers. In our experiment, instead of applying periodic pulses to diodes, we apply a fixed voltage and detect the random time intervals between adjacent tunneling signals. This $\mathrm{Si}$ QRNG generates raw data with its ratio of min-entropy over ideal entropy reaching 0.98, which is higher than many other QRNG configurations. The final data rate is 6.98 MB/s and could reach 23 MB/s with a more efficient cooling system.

We design a one-dimensional chain of two different alternating three-dimensional elastic chiral unit cells. The chain’s topological band gap, a result of the alternation of unit cells combined with their chirality and an effective mirror symmetry, guarantees a protected edge state, corresponding to a localized twist mode with an eigenfrequency inside the one-dimensional band gap. A small axial modulation at the one end of the beam can excite this resonant twist mode at the other end of the beam, via evanescent modes in the gap. The topological robustness of the edge state allows us to add a micromirror to the other end of the beam, turning the arrangement into a resonant mechanical laser-beam scanner. Its scalable operation frequency makes it attractive for applications.

The field- and current-induced magnetization reversal of a $\mathrm{Co}$-$\mathrm{Fe}$-$\mathrm{B}$ layer in a perpendicular magnetic tunnel junction (pMTJ) is studied at room temperature. The magnetization switching probability from the parallel (P) state to the antiparallel (AP) state and from AP to P is found to be asymmetric. We observe that this asymmetry depends on the magnetic configuration of the synthetic antiferromagnetic (SAF). The state diagram and the energy landscape are compared for two SAF configurations. We conclude that the asymmetry is due to the inhomogeneity of the fringe field generated by the SAF. Our study highlights the role of the reference-layer fringe field on the free layer’s energy barrier height, which has to be carefully controlled for memory applications.

We study a special class of mechanical metamaterials, namely lattices, based on beams (struts) with nonuniform cross sections, of which pentamode mechanical metamaterials are a special case. Five symmetric beam types including simple cylinder, concave double cone, convex double cone, concave hyperbolic, and convex hyperbolic are considered. Three types of loads including lateral force, axial force, and moment are applied to the free end of cantilever beams with the five aforementioned geometries and their responses are compared in terms of displacement, normal stress variation along the beam length, and shear stress variation along the beam length. Using the displacement diagrams obtained for different strut geometries and loading conditions, semi-analytical relationships are derived for the displacement at the end of cantilever beams. The semi-analytical relationships are then used to calculate the elastic modulus and yield strength of lattice structures based on two types of unit cells, namely diamond and cube. We also build numerical models and perform experiments to benchmark our semi-analytical results. Comparison of the analytical, numerical, and experimental results demonstrate the accuracy of the semi-analytical relationships presented for the mechanical properties of this class of mechanical metamaterials. Moreover, in both the cube- and diamond-based structures, increasing the $\alpha$ value (i.e., the ratio of the largest to the smallest cross-section radius in each strut) at a constant relative density decreases the elastic modulus, yield strength, the initial maximum stress, and the plateau stress.

Almost all thermal metamaterials are essentially achieved by tailoring asymmetric interaction between matrices and embedded particles. However, the asymmetric interaction results in the noncommutability of matrices and particles, which may reduce the flexibility for heat management. To solve this problem, here we propose a different mechanism by tailoring symmetric interaction between particles arranged in periodic lattices, thus called “periodic interparticle interaction.” For practical application, we refer to the representative thermal transparency, which, however, is realized by tailoring periodic interparticle interaction. Theoretical analysis, finite-element simulations, and laboratory experiments all validate the proposed mechanism. Moreover, we reunderstand the Maxwell-Garnett theory and the Bruggeman theory from their scope and relation by using theoretical analysis and finite-element analysis. Compared with the existing thermal transparency, our scheme is more feasible to handle many-particle systems. This work opens a gateway to exploring periodic interparticle interaction, and further work can be expected: (i) exploring periodic interparticle interaction with different lattice types and relative positions for particle arrangement; (ii) applying periodic interparticle interaction to achieve other functions, such as thermal camouflage.

We investigate “sharp bending”—localized, extreme bending deformation—of corrugated nanoscale plates as well as their ability to fully recover from such bending. Unlike planar films, these mechanical metamaterials with nanoscale thickness exhibit surprising mechanical robustness and recoverability due to their periodic honeycomb corrugation and high aspect ratios (e.g., height-to-thickness ratio). An analytical model that fully characterizes one-dimensional bending of these corrugated plates at large deformations is developed and validated by comparing the predictions to experimental and finite element results. The sharp bending is enabled by a nonmonotonic moment-curvature relation that allows for the coexistence of regions of high and low curvatures despite the complete linearity of the underlying material. Systematic studies are performed to investigate the effects of the corrugation geometry parameters on the sharp bending properties, identifying the parameter space where the structures exhibit complete recovery from sharp bending. Optimized plate mechanical metamaterials could be employed for applications requiring rigid lightweight plates that can recover their shape after extreme bending deformation, for example, wings of microflyers or expandable aerospace components that need to be deployed after take off.

We demonstrate a variety of precessional responses of the magnetization to ultrafast optical excitation in nanolayers of galfenol ($\mathrm{Fe}$,$\mathrm{Ga}$), which is a ferromagnetic material with large saturation magnetization and enhanced magnetostriction. The particular properties of galfenol, including cubic magnetic anisotropy and weak damping, allow us to detect up to six magnon modes in a 120-nm layer, and a single mode with effective damping ${\alpha }_{\text{eff}}=0.005$ and frequency up to 100 GHz in a 4-nm layer. This is the highest frequency observed to date in time-resolved experiments with metallic ferromagnets. We predict that detection of magnetization precession approaching THz frequencies should be possible with galfenol nanolayers.