We report on the generation and confinement of short-wavelength spin waves in a continuous film with periodically modulated magnetic anisotropy. The concept, which is demonstrated for strain-coupled ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/{\mathrm{BaTiO}}_3$ heterostructures, relies on abrupt rotation of magnetic anisotropy at the boundaries of magnetic stripe domains. In combination with an external bias field, this modulation of magnetic anisotropy produces a lateral variation of the effective magnetic field, leading to local spin-wave excitation when irradiated by a microwave magnetic field. In domains with small effective field, spin waves are perfectly confined by the spin gap in neighboring domains. In contrast, standing spin waves in domains with large effective field radiate into neighboring domains. Using micromagnetic simulation, we show that the wavelength of emitted spin waves is tunable from a few micrometers down to about 100 nm by rotation of the bias field. Importantly, the orientation of the wave front remains fixed. We also demonstrate that dynamic fluctuations of the effective magnetic field produce exchange-dominated spin waves at single-anisotropy boundaries. The multiferroic heterostructures presented here enable the use of global excitation fields from a microwave antenna to emit tunable spin waves from a nanometer-wide line source at well-defined locations of a continuous ferromagnetic film.

We demonstrate terahertz chiral metamaterials that achieve resonant transmission and strong optical activity. This response is realized in a metasurface coupled to its Babinet complement, with additional twist. Uniquely, the optical activity achieved in this type of metamaterial is weakly dispersive around the resonant transmission maxima, but it can be highly dispersive around the transmission minima. It has recently been shown that this unique optical activity response is closely related to zeros in the transmission spectra of circular polarizations through the Kramers-Kronig relations and strong resonant features in the optical activity spectrum corresponding to the Blaschke phase terms. Here we demonstrate how modifying the meta-atom geometry greatly affects the location and magnitude of these Blaschke phase terms. We study three different meta-atoms, which are variations on the simple cross structure. Their responses are measured using terahertz time-domain spectroscopy and analyzed via numerical simulations.

While classical Rayleigh streaming, whose circulations are perpendicular to the transducer radiating surfaces, is well known, transducer-plane streaming patterns, in which vortices circulate parallel to the surface driving the streaming, have been less widely discussed. Previously, a four-quadrant transducer-plane streaming pattern has been seen experimentally and subsequently investigated through numerical modeling. In this paper, we show that by considering higher-order three-dimensional cavity modes of rectangular channels in thin-layer acoustofluidic manipulation devices, a wider family of transducer-plane streaming patterns are found. As an example, we present a transducer-plane streaming pattern, which consists of eight streaming vortices with each occupying one octant of the plane parallel to the transducer radiating surfaces, which we call here eight-octant transducer-plane streaming. An idealized modal model is also presented to highlight and explore the conditions required to produce rotational patterns. We find that both standing and traveling wave components are typically necessary for the formation of transducer-plane streaming patterns. In addition, other streaming patterns related to acoustic vortices and systems in which traveling waves dominate are explored with implications for potential applications.

A thermodynamic model for light-emitting diodes (LEDs) is developed by considering energy and entropy flows in the system. Thermodynamic constraints have previously been considered separately for the reversible process of electroluminescence in LEDs and for light extraction and collimation in other optical systems. By considering both processes in the LED model, an irreversible upper bound for the conversion of electrical energy to optical energy is derived and shown to be higher than unity, but tighter and more realistic than the reversible case. We also model a LED as an endoreversible heat engine where the carrier-transport processes can be directly connected to the elements of a thermodynamic cycle.

Harnessing the unique properties of quantum mechanics offers the possibility of delivering alternative technologies that can fundamentally outperform their classical counterparts. These technologies deliver advantages only when components operate with performance beyond specific thresholds. For optical quantum metrology, the biggest challenge that impacts on performance thresholds is optical loss. Here, we demonstrate how including an optical delay and an optical switch in a feed-forward configuration with a stable and efficient correlated photon-pair source reduces the detector efficiency required to enable quantum-enhanced sensing down to the detection level of single photons and without postselection. When the switch is active, we observe a factor of improvement in precision of 1.27 for transmission measurement on a per-input-photon basis compared to the performance of a laser emitting an ideal coherent state and measured with the same detection efficiency as our setup. When the switch is inoperative, we observe no quantum advantage.

The zinc nitride ${\mathrm{Zn}}_3{\mathrm{N}}_2$ is composed of inexpensive and earth-abundant Zn and N elements and shows high electron mobility exceeding $100{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$. Although various technological applications of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ have been suggested so far, the synthesis of high-quality ${\mathrm{Zn}}_3{\mathrm{N}}_2$ samples, especially single crystals, is still challenging, and therefore its basic properties are not yet well understood. Indeed, the reported band gaps of as-grown ${\mathrm{Zn}}_3{\mathrm{N}}_2$ widely scatter from 0.85 to 3.2 eV. In this study, we investigate the large gap variation of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ in terms of the Burstein-Moss (BM) effect and point-defect energetics using first-principles calculations. First, we discuss the relation between electron carrier concentration and optical gaps based on the electronic structure obtained using the Heyd-Scuseria-Ernzerhof hybrid functional. The calculated fundamental band gap is 0.84 eV in a direct-type band structure. Second, thermodynamic stability of ${\mathrm{Zn}}_3{\mathrm{N}}_2$ is assessed using the ideal-gas model in conjunction with the rigid-rotor model for gas phases and first-principles phonon calculations for solid phases. Third, carrier generation and compensation by native point defects and unintentionally introduced oxygen and hydrogen impurities are discussed. The results suggest that a significant BM shift occurs mainly due to oxygen substitutions on nitrogen sites and hydrogen interstitials. However, gaps larger than 2.0 eV would not be due to the BM shift because of the Fermi-level pinning caused by acceptorlike zinc vacancies and hydrogen-on-zinc impurities. Furthermore, we discuss details of peculiar defects such as a nitrogen-on-zinc antisite with azidelike atomic and electronic structures.

We report a fully integrated interface delivering efficient, reflection-free, single-mode, and self-aligned coupling between a step-index fiber and a gas-filled hollow-core photonic crystal fiber. The device offers a universal solution for interfacing solid and hollow cores and can be sealed to allow operation either evacuated or at high pressure. Stimulated Raman scattering and molecular modulation of light are demonstrated in a ${\mathrm{H}}_2$-filled hollow-core photonic crystal fiber using the device.

Efficient sources of individual pairs of entangled photons are required for quantum networks to operate using fiber-optic infrastructure. Entangled light can be generated by quantum dots (QDs) with naturally small fine-structure splitting (FSS) between exciton eigenstates. Moreover, QDs can be engineered to emit at standard telecom wavelengths. To achieve sufficient signal intensity for applications, QDs have been incorporated into one-dimensional optical microcavities. However, combining these properties in a single device has so far proved elusive. Here, we introduce a growth strategy to realize QDs with small FSS in the conventional telecom band, and within an optical cavity. Our approach employs ‘‘droplet-epitaxy’’ of InAs quantum dots on (001) substrates. We show the scheme improves the symmetry of the dots by 72%. Furthermore, our technique is universal, and produces low FSS QDs by molecular beam epitaxy on GaAs emitting at $\sim 900\mathrm{nm}$, and metal-organic vapor-phase epitaxy on InP emitting at $\sim 1550\mathrm{nm}$, with mean FSS $4\times$ smaller than for Stranski-Krastanow QDs.

Utilizing discrete phase distribution to fit continuous phase distribution has been a primary routine for designing metasurfaces. In the existing method, the validation of the discrete designs is guaranteed only by using the subwavelength condition of unit cells, which is insufficient—especially for arbitrary phase distribution. Herein, we propose an analytical method to design metasurfaces by estimating the width of the source in a unit cell. Also, by calculating field patterns in both real and momentum space, we provide four guidelines for directing future applications of metasurfaces, such as an arbitrary multifocal lens with the same strength of each focus, a convex-concave double lens, and a lens with a large numerical aperture that can precisely prevent undesired diffraction orders. In addition to metalenses, this methodology can provide a wide platform for designing tailored and multifunctional metasurfaces in the future, especially large-area ones in practical applications.

We perform a combined first-principles and micromagnetic study on the strain effects in Nd-Fe-B permanent magnets. First-principles calculations on ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ reveal that magnetocrystalline anisotropy ($K$) is insensitive to deformation along the $c$ axis, and that $a\text{−}b$ in-plane shrinkage is responsible for $K$ reduction. The predicted $K$ is more sensitive to lattice deformation than the previous phenomenological model suggests. The biaxial and triaxial stress states have a greater impact on $K$. Negative $K$ occurs in a much wider strain range in the $a\text{−}b$ biaxial stress state. Micromagnetic simulations of Nd-Fe-B magnets using first-principles results show that a 3% to 4% local strain in a 2-nm-wide region near the interface around the grain boundaries and triple junctions leads to a negative local $K$ and thus, remarkably, decreases the coercivity by about 60%, or 3 to 4 T. The local $a\text{−}b$ biaxial stress state is more likely to induce a large loss of coercivity. In addition to the local stress states and the strain levels themselves, the shape of the interfaces and the intergranular phases also makes a difference. Smoothing the edge and reducing the sharp angle of the triple regions in Nd-Fe-B magnets would be favorable for a coercivity enhancement.

We show that constant-Fermi-level ab initio molecular dynamics can be used as a computer-based tool to reveal and control relevant defects in semiconductor materials. In this scheme, the Fermi level can be set at any position within the band gap during the defect generation process, in analogy to experimental growth conditions in the presence of extra electrons or holes. The scheme is illustrated in the case of GaAs, for which we generate melt-quenched amorphous structures through molecular dynamics at various Fermi levels. By a combined analysis which involves both the atomic structure and a Wannier-function decomposition of the electronic structure, we achieve a detailed description of the generated defects as a function of the Fermi level. This leads to the identification of As—As homopolar bonds and Ga dangling bonds for Fermi levels set in the vicinity of the valence band. These defects convert into As dangling bonds and Ga—Ga homopolar bonds, as the Fermi level moves toward the conduction band. This demonstrates a computer-aided procedure to identify semiconductor defects in an unbiased way.

Efficient suppression of reflection is a key requirement for perfect absorption of light. Recently, it has been shown that reflection can be effectively suppressed utilizing a single ultrathin film deposited on metals or polar materials featuring phonon resonances. The wavelength at which reflection can be fully suppressed is primarily determined by the nature of these substrates and is pinned to particular values near plasma or phonon resonances—the former typically in the ultraviolet or visible and the latter in the infrared. Here, we explicitly identify the required optical properties of films and substrates for the design of absorbing antireflection coatings based on ultrathin films. We find that completely suppressed reflection using films with thicknesses much smaller than the wavelength of light occurs within a spectral region where the real part of the refractive index of the substrate is $n\lesssim 1$, which is characteristic of materials with permittivity close to zero. We experimentally verify this condition by using an ultrathin vanadium dioxide film with dynamically tunable optical properties on several epsilon-near-zero materials, including aluminum-doped zinc oxide. By tailoring the plasma frequency of the aluminum-doped zinc oxide, we are able to tune the epsilon-near-zero point, thus achieving suppressed reflection and near-perfect absorption at wavelengths that continuously span the near-infrared and long-wave midinfrared ranges.

Solid-state thermionic power generators are an alternative to thermoelectric modules. In this paper, we develop an analytical model to investigate the performance of these generators in the nonlinear regime. We identify dimensionless parameters determining their performance and provide measures to estimate an acceptable range of thermal and electrical resistances of thermionic generators. We find the relation between the optimum load resistance and the internal resistance and suggest guidelines for the design of thermionic power generators. Finally, we show that in the nonlinear regime, thermionic power generators can have efficiency values higher than the state-of-the-art thermoelectric modules.

To achieve spin transport in heavily doped $n$-type Ge ($n^{+}$-type Ge) in the high-temperature range ($T\ge 130\mathrm{K}$), we examine the growth of highly spin-polarized ${\mathrm{Co}}_2{\mathrm{FeAl}}_{0.5}{\mathrm{Si}}_{0.5}$ (CFAS) films on Ge(111). Using lateral spin valves with the $\mathrm{CFAS}/\text{Ge}$ Schottky-tunnel contacts, we can observe giant enhancement in the nonlocal spin signals at low temperatures and get a spin signal of approximately $1\mathrm{m}\mathrm{\Omega }$ at room temperature. Since nonlocal Hanle-effect curves can be seen up to about 250 K, we experimentally clarify that the spin-relaxation mechanism in $n^{+}$-type Ge in $8\mathrm{K}\le T\le 250\mathrm{K}$ is attributed to impurity- and phonon-induced spin-flip scatterings. This study experimentally shows the availability of the CFAS spin injector and detector for getting important factors of the spin relaxation up to near room temperature, even in Ge.

Reconfigurable phononic circuits can be created by the selective fluid filling of holes in a solid phononic crystal. For frequencies within a complete band gap of the bare phononic crystal, the filled holes become cavities that sustain acoustoelastic defect modes. Those cavities couple evanescently with a strength that depends on their separation. We investigate the dispersion relation and the transmission properties of coupled-resonator acoustoelastic waveguides formed by a chain of cavities. While the dispersion relation is strongly dependent on the separation between cavities, transmission properties are only weakly dependent on the details of the phononic circuit for a fixed separation. Furthermore, depending on the polarization of the source of waves, defect modes can be excited selectively. As a result, rather arbitrary phononic circuits can be created, such as multiply bent waveguides or wave splitters.

A transformation-optics approach is used to derive a general method for designing electromagnetic devices able to manipulate the wave vectors in the specific manner required by the functionality of the device. While the wave paths inside the device remain of secondary importance, the wave vectors are gradually changed in the desired way by choosing the appropriate coordinate transformation. The proposed method is applied to designing both converging and diverging flat lenses. Computer simulations reveal the focusing ability of a converging flat lens designed using the proposed technique.

Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux qubits with short coherence times limited primarily by the use of large persistent currents $I_{\mathit{p}}$. Here, we examine an alternative approach using qubits with smaller $I_{\mathit{p}}$ and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (approximately 50-nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.

Temperature-sensitive hydrogels based on polymers such as poly($N$-isopropylacrylamide) (PNIPAM) undergo a volume phase transition in response to changes in temperature. During this transition, distinct changes in both thermal and mechanical properties are observed. Here, we illustrate and exploit the inherent thermodynamic link between thermal and mechanical properties by showing that the compressive elastic modulus of PNIPAM hydrogels can be probed using differential scanning calorimetry. We validate our approach by using conventional osmotic compression tests. Our method could be particularly valuable for determining the mechanical response of thermosensitive submicron-sized and/or oddly shaped particles, to which standard methods are not readily applicable.