Recently, it was shown that dissipatively coupled laser arrays simulate the classical $XY$ model. We show that phase locking of laser arrays can give rise to the spontaneous formation of vortex and antivortex phase patterns that are analogous to topological defects of the $XY$ model. These patterns are stable although their formation is less likely in comparison to the ground-state lasing mode. In addition, we show that small ratios of photon to gain lifetime destabilize vortex and antivortex phase patterns. These findings are useful for studying topological effects in optics as well as for designing laser array devices.

Orientation-dependent electronic properties of wurtzite zinc oxide nanorods are characterized by aloof-beam electron energy-loss spectroscopy (EELS) carried out in a scanning transmission electron microscope (STEM). The two key crystal orientation differentiating transitions specific to the in-plane (13.0 eV) and out-of-plane (11.2 eV) directions with respect to the wurtzite structure are examined by first-principles density-functional theory calculations. We note some degree of orientation dependence at the onset of direct band gap transition near 3.4 eV. We demonstrate that good polarization selectivity can be achieved by placing the electron probe at different locations around the specimen with increasing impact parameter while keeping the beam-specimen orientation fixed. The observed results are qualitatively elucidated in terms of the perpendicular electric fields generated by the fast electron (60 kV) used in the microscope. The fact that good polarization selectivity can be achieved by aloof-beam EELS without the requirement of sample reorientation is an attractive aspect from the characterization method point of view in the STEM-EELS community.

We measure and analyze the light emission from a room-temperature n-type unipolar-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}/\mathrm{Al}\mathrm{As}$ double-barrier resonant-tunneling diode (RTD) that occurs just above the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ band edge and peaks around 1631 nm. The emission is attributed to electron-hole recombination emission made possible by holes generated in the high-field region on the collector side of the device by interband tunneling and impact ionization, which contribute comparable hole densities, according to our analysis. Although the external quantum efficiency (EQE) in our experimental configuration is rather low (≈2 × ${10}^{-5}$ at 3.0-V bias), limited by suboptimal output coupling, the internal quantum efficiency (IQE) is much higher (≈6% at 3.0-V bias), as derived from the experimental EQE and a radiometric analysis. To check this value and better understand the transport physics, we also carry out an independent estimate of the IQE using a combined interband-tunneling impact-ionization transport model, which yields an IQE of 10% at 3.0-V bias. The satisfactory agreement of theory with experimental data suggests that a RTD designed for better hole transport and superior optical coupling could become a useful light-emitting device, while retaining the intrinsic functionality of high-speed negative differential resistance, and all without the need for resistive p-type doping.

The interaction of light and electrons has been one of the important frontiers for generating ultrashort electron pulses in free-electron lasers, ultrafast science, and dynamical analysis of matter. However, the generation of a relativistic attosecond electron beam remains a challenge. In a photocathode radio-frequency (rf) gun, this work identifies a regime for obtaining an isolated or a train of relativistic attosecond electron pulses. A photoelectron beam is generated from the cathode under the illumination of a driving laser pulse in the electron gun, which is subsequently microbunched to optical wavelength scale by the field of a synchronized radially polarized laser (RPL) pulse that is focused near the cathode surface. The rf field in the gun cavity simultaneously accelerates the electron pulse to several MeV. The rf field causes a velocity differential inside the electron beam as it passes into the region near the focal point of the RPL (microbunching is processed), and the microbunch at the head is substantially faster than that at the tail. Using this knowledge, the spacing between consecutive microbunches, referred to as bunch spacing, may be controlled across a wide range by regulating the velocity difference, which can be accomplished by tuning the phase and amplitude of the rf field. Numerical simulations show that a train of attosecond bunches with tunable bunch spacing can be generated using realistic laser parameters corresponding to current GW-power-level laser systems, and an isolated attosecond pulse can be obtained when the driving laser pulse is about 50 fs. This regime may result in opportunities in ultrafast electron diffraction and microscopy, free-electron lasers, and other applications that require high-energy electrons with the temporal structure of single-cycle light.

Flexible oxide thin films with uniaxial magnetocrystalline anisotropy $(K_u)$ not only have many applications in flexible electronics, but also provide an ideal low-crystal-symmetry material for a fundamental understanding of the competitive interplay among magnetocrystalline anisotropy, shape anisotropy, and stress anisotropy. However, an understanding of the mechanical tuning of magnetic parameters in flexible oxide films with $K_u$ has not been realized up to now. In this work, epitaxial flexible hexaferrite ${\mathrm{Ba}\mathrm{Fe}}_{12}{\mathrm{O}}_{19}$ thin films with a room temperature out-of-plane $K_u$ of 1.1 × ${10}^5$ J/m${}^3$ are fabricated. The continuous and controllable tuning of magnetic parameters is found to be realizable by different bending radii of the film. The changes in magnetic parameters are symmetrical under tensile and compressive bending due to the dominance of film geometry and bending-stress directions, which are perpendicular to the easy axis of $K_u$. The micromagnetic simulations further show that the magnitude of $K_u$ plays a critical role in the mechanical tuning performance of the film, where high $K_u$ drastically reduces the role of stress anisotropy and increases the magnetic bending repeatability of the film by suppressing bending-defect-related anisotropy degradation. This work not only provides an understanding of the role of the direction and magnitude of $K_u$ in the flexible thin films, but also demonstrates that the flexible hexaferrite film is one of the best materials for mechanical tunable devices up to the millimeter-wave region.

In the coming era of precision medicine, there is a high demand for the development of approaches for in vivo tracking of biomolecules and measurements of their local environments. In this work, we present an approach for the simultaneous imaging of magnetic nanoparticle (MNP) concentration, temperature, and viscosity. The proposed approach allows one to determine the spatial distribution of the MNPs, while the MNPs are used as nanosensors for simultaneous measurements of the spatial distributions of temperature and viscosity. This technique has great potential for providing not only tools for disease diagnostics and therapy, but also insights into further understanding of the biophysical mechanisms involved in tumor tissues during cancer development and tumor metastasis.

In this report, a rectifying surface acoustic wave (SAW) device is proposed and simulated based on a simple inclined phononic crystal (PnC) heterojunction, consisting of monolithic pillars on $\mathrm{Si}$ substrate. The designed nonreciprocal operation principle is initially based on the frequency alignment of the surface-coupled guiding bands in the first half of the $\mathrm{Pn}\mathrm{C}$ with the local-surface-resonance (LSR) band gap in the second half of the $\mathrm{Pn}\mathrm{C}$, along two different equivalent incident directions. Benefiting from flexible LSR band-gap engineering, we tune the band-gap central frequency by optimizing the structural dimensions of the pillars in a small chip area, which is not achievable in conventional Bragg band gaps without varying the lattice constant. The other physical principle that dominantly affects the broken reciprocity in our proposed structure is the induced SAW shear-to-sagittal mode conversion at a limited frequency range in the trapezoidal $\mathrm{Pn}\mathrm{C}$ half, which depends on the incident direction with respect to the inclined cut line of the $\mathrm{Pn}\mathrm{C}$. Moreover, we optimize the spacing gap between the PnCs to modulate the elastic coupling strength between the half $\mathrm{Pn}\mathrm{Cs}$, and prove a significant SAW nonreciprocity of 34 dB at a frequency of 6.9 GHz, in addition to an acceptable rectified transmission of about −10.68 dB, by the proposed $\mathrm{Pn}\mathrm{C}$-based operation principles. The presented design benefits from a simple $\mathrm{Si}$-based structure and a CMOS-compatible fabrication process, without the need for any external excitation, and it is a promising miniature and efficient SAW rectifying candidate for wireless-communication applications.

We develop a homogenization theory for a spatiotemporally modulated wire medium. We first solve for the modal waves that are supported by this composite medium, and we show peculiar properties such as extraordinary waves that propagate at frequencies below the cutoff frequency of the corresponding stationary medium. We explain how these unique solutions give rise to an extreme Fresnel drag that exists already with weak and slow spatiotemporal modulation. Next we derive the effective material permittivity that corresponds to each of the first few supported modes, and write the average fields and Poynting vector. Nonlocality, nonreciprocity, and anisotropy due to the spatiotemporal modulation are three inherent properties of this medium, and are clearly seen in the effective material parameters. Lastly, we validate that homogenization and spatiotemporal variation are not necessarily interchangeable operations. Indeed, in certain parameter regimes the homogenization should be performed directly on the spatiotemporally modulated composite medium, rather than the stationary medium being homogenized first and then the effect of the space-time modulation being introduced phenomenologically.

Propagation of easy-plane magnetic precession can enable more efficient spin transport than conventional spin waves. Such easy-plane spin transport is typically understood in terms of a hydrodynamic model, partially analogous to superfluids. Here, using micromagnetic simulations, we examine easy-plane spin transport in magnetic strips as the motion of a train of domain walls rather than as a hydrodynamic flow. We observe that the motion transitions from diffusive to fluidlike as the density of domain walls is increased. This transition is most evident in notched nanostrips, where the domain walls are pinned by the notch defect in the diffusive regime but propagate essentially unimpeded in the fluidlike regime. Our findings suggest that spin transport via easy-plane precession, robust against defects, is achievable in strips based on realistic metallic ferromagnets and hence amenable to practical device applications.

Current-induced spin polarization in magnets has recently attracted much attention because out-of-plane spin polarization is allowed, as opposed to polycrystalline nonmagnetic heavy metals. We investigate the $\mathrm{Pt}$-impurity-doping dependence of current-induced out-of-plane spin polarization in $\mathrm{Fe}$ thin films with the use of magnetocircular dichroism. Current-induced out-of-plane spin polarization is clearly observed and attributed to anomalous spin-orbit torque, in which the spin-orbit torque induced by the spin Hall effect in $\mathrm{Fe}$ acts on the magnetic moments of $\mathrm{Fe}$ at the surfaces, generating out-of-plane spin polarization. Notably, current-induced out-of-plane spin polarization is significantly enhanced with increasing $\mathrm{Pt}$ concentration. $\mathrm{Pt}$ impurities with a large spin-orbit interaction induce the extrinsic spin Hall effect of $\mathrm{Fe}$, and consequently, the anomalous spin-orbit torques are enhanced. Our results provide a guiding principle for strengthening current-induced spin polarization in magnets.

Optical antennas made of low-loss dielectrics have several advantages over plasmonic antennas, including high radiative quantum efficiency, negligible heating, and excellent photostability. However, due to weak spatial confinement, conventional dielectric antennas fail to offer light-matter interaction strengths on par with those of plasmonic antennas. We propose here an all-dielectric antenna configuration that can support strongly confined modes ($V\sim {10}^{-4}{\lambda }_0^3$) while maintaining unity antenna quantum efficiency. This configuration consists of a high-index pillar structure with a transverse gap that is filled with a low-index material, where the contrast of indices induces a strong enhancement of the electric field perpendicular to the gap. We provide a detailed explanation of the operational principle of such photonic gap antennas (PGAs) based on the dispersion relation of symmetric and asymmetric horizontal slot waveguides. To discuss the properties of PGAs, we consider silicon pillars with air or the polymer CYTOP as the gap material. We show by full-wave simulations that PGAs with an emitter embedded in the gap can enhance the spontaneous emission rate by a factor of approximately $1000$ for air gaps and approximately $400$ for CYTOP gaps over a spectral bandwidth of $\mathrm{\Delta }\lambda \approx 300\mathrm{nm}$ at $\lambda =1.25\mu \mathrm{m}$. Furthermore, the PGAs can be designed to provide unidirectional out-of-plane radiation across a substantial portion of their spectral bandwidth. This is achieved by setting the position of the gap at an optimized off-centered position of the pillar so as to properly break the vertical symmetry of the structure. We also demonstrate that, when acting as receivers, PGAs can lead to a near-field intensity enhancement by a factor of approximately $3000$ for air gaps and approximately $1200$ for CYTOP gaps.

Conventional multiple vortex beams carrying different orbital-angular-momentum (OAM) modes propagating in different directions suffer from beam divergences along with the increasing transmission distance, which greatly limits their practical applications. This article presents a diffraction-resisting dual high-order Bessel-beam (DHOBB) generator based on an anisotropic holographic impedance metasurface (AHIM) operating at 30 and 30.5 GHz. By virtue of leaky-wave theory and the optical holographic principle, surface waves excited by a monopole antenna welded on the back of the metasurface can be modulated into nondiffractive DHOBBs. Different geometric parameters of the subwavelength meta-atoms can be mapped by the interference patterns between the reference surface waves and the desired beams. Furthermore, this designed single-layer anisotropic metasurface can be equivalent to the functionalities of spiral-phase-plate, refractive-hyperbolic, and axicon lenses synthetically utilized in the popular air-fed metasurface to generate HOBBs. Compared with the most popular air-fed metasurface, this design results in predominant performances of ultralow profile, easy fabrication, and integration with other electronic components and avoids alignment errors. Both simulated and measured results demonstrate that nondiffractive DHOBBs carrying different OAM modes can be generated by this designed AHIM. Nondiffractive DHOBBs with different Bessel orders with distinctive properties may open a window for wireless communications with multiarea coverage and multitarget radar detection.

In this work, we briefly overview various options for Josephson junctions, which should be scalable down to nanometer range for utilization in nanoscale digital superconducting technology. Such junctions should possess high values of critical current, $I_c$, and normal state resistance, $R_N$. Another requirement is the high reproducibility of the junction parameters across a wafer in a fabrication process. We argue that superconductor-normal metal-superconductor ($SN$-$N$-$NS$) Josephson junction of “variable thickness bridge” geometry is a promising choice to meet these requirements. Theoretical analysis of the $SN$-$N$-$NS$ junction is performed in the case where the distance between the $S$ electrodes is comparable to the coherence length of the $N$ material. The restriction on the junction geometrical parameters providing the existence of superconductivity in the $S$ electrodes is derived for the current flowing through the junction of an order of $I_c$. The junction heating, as well as available mechanisms for the heat removal, is analyzed. The obtained results show that a $SN$-$N$-$NS$ junction with a high (submillivolt) value of $I_c{R}_N$ product can be fabricated from a broadly utilized combination of materials like Nb/Cu using well-established technological processes. The junction area can be scaled down to that of semiconductor transistors fabricated in the frame of a 40-nm process.

Transition-edge sensors (TESs) are the selected technology for future spaceborne x-ray observatories, such as Athena, Lynx, and HUBS. These missions demand thousands of pixels to be operated simultaneously with high energy-resolving power. To reach these demanding requirements, every aspect of the TES design has to be optimized. Here we present the experimental results of tests on different devices where the coupling between the x-ray absorber and the TES is varied. In particular, we look at the effects of the diameter of the coupling stems and the distance between the stems and the TES bilayer. Based on measurements of the ac complex impedance and noise, we observe a reduction in the excess noise as the spacing between the absorber stem and the bilayer is decreased. We identify the origin of this excess noise to be internal thermal fluctuation noise between the absorber stem and the bilayer. In addition, we see an impact of the coupling on the superconducting transition in the appearance of kinks. Our observations show that these unwanted structures in the transition shape can be avoided with careful design of the coupling geometry. The stem diameter appears to have a significant effect on the smoothness of the TES transition. This observation is still poorly understood, but is of great importance for both ac and dc biased TESs.

The excellent optoelectronic and transport properties of halide perovskites have led to the rapid development of perovskite-based optoelectronic devices. A fundamental understanding of charge-carrier dynamics, as well as the implementation of physical models able to accurately describe their behaviour, is essential for further improvements in the field. Here, combining advanced modeling and characterization, a method for analyzing the short time dynamics of time-resolved fluorescence imaging (TRFLIM) decays is demonstrated. A theoretical scaling law for the time derivative of transient photoluminescence decays as a function of excitation power is extracted. This scaling law, computed from classical drift-diffusion equations, defines an innovative and simple way to extract quantitative values for several transport parameters, including the external radiative-recombination coefficient. The model is notably applied on a set of images acquired with a temporal shift of 250 ps to map the top-surface recombination velocity of a triple-cation mixed-halide perovskite thin film at the microscale. The development of high-time-resolution imaging techniques coupled with a scaling method for analyzing short time dynamics provides a solid platform for the investigation of local heterogeneities in semiconductor materials and the accurate determination of the main parameters governing their carrier transport.