A simple but efficient approach for the synthesis of transmission-type wideband polarization converters is presented. The proposed configuration comprises multilayer metasurfaces including resonant particles that are progressively rotated layer by layer. The progressive rotation of the particles allows for a polarization conversion over a large frequency band. The polarizing structure is efficiently designed and optimized through a transmission-line-model approach handling the cascade of anisotropic impedance layers and dielectrics. An optimized eight-layer design based on gradually rotated dipole resonators is presented as a proof of concept. The results obtained through the efficient transmission-line model are compared with full-wave simulations once the structure is optimized, showing satisfactory agreement. A prototype of the wideband polarization converter is fabricated and measured.

We use Fourier-plane imaging microscopy (FIM) to determine the transition-dipole-moment orientation in doped organic emissive thin films. The use of FIM enables precise, sensitive, and rapid measurement of dipole orientation in the emission layer of an organic light-emitting device (OLED). An optical model of a stratified birefringent multilayer is introduced for interpreting results obtained by FIM. Using the model, we determine the average orientation of transition-dipole-moment vectors of three phosphorescent dopant emitters. The dipole alignment measured by FIM quantitatively explains the difference in OLED efficiencies using these archetype dopant molecules. FIM provides a nondestructive tool to measure and ultimately improve the outcoupling efficiency of OLEDs and other light-emitting devices.

We study current-induced deterministic magnetization switching and domain-wall motion via polar Kerr microscopy in all-amorphous ${\mathrm{W}}_{66}{\mathrm{Hf}}_{34}/\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}(\mathrm{CFB})/{\mathrm{Ta}\mathrm{O}}_x$ with perpendicular magnetic anisotropy and a large spin Hall angle. Investigations of magnetization switching as a function of the in-plane assist field and the current-pulse width yield switching-current densities as low as $3\times {10}^9{\mathrm{A}/\mathrm{m}}^2$. We accredit this low switching-current density to a low depinning-current density, which is obtained from measurements of domain-wall displacements upon current injection. This correlation is verified by investigations of a $\mathrm{Ta}$/$\mathrm{CFB}$/$\mathrm{Mg}\mathrm{O}$/$\mathrm{Ta}$ reference sample, which shows critical-current densities that are at least one order of magnitude larger.

We develop an analytical model for the photocurrent of organic solar cells based on carrier drift and including second-order charge recombination, in which the figure of merit ${\theta }_o$ naturally appears. This approach provides expressions for the fill-factor–${\theta }_o$ and α-${\theta }_o$ relations, where α is the exponent of the function of the short-circuit current versus light intensity ($J_{\mathrm{SC}}$ ∝ $I^{\alpha }$). A correlation between the reduction factor of Langevin recombination and the dissociation probability of charge transfer states (P) is also discussed. Photocurrent curves of a polymer device obtained at different temperatures (100–300 K) are fitted by the model, whose fitting parameters are in agreement with the literature.

Here we provide a physical and mathematical framework for the description of flow-driven oscillators. These oscillators, differently from frequency-driven harmonic systems, are based on countercurrent mass flows and thermal-energy exchange. We describe this class of oscillators through two countercurrent fluids separated by a heated conductive medium. We show how this configuration embeds the essential elements of harmonic oscillators, such as resonance condition, periodic orbits, and quality factor or decay time. The key advantage of recognizing flow-driven systems as oscillators lies in the possibility to engineer them according to the properties of resonant systems and utilize them to control their temperature, maximize the stored energy, or coupling them in networks. We report examples of simple configurations at their resonant condition, enhancing both thermal energy and decay time by factors larger than 10. We finally show two flow-driven oscillators coupled in series, featuring a reconfigurable internal temperature distribution, depending on the selected resonant condition.

Graphene plasmons provide a compelling avenue toward chip-scale dynamic tuning of infrared light. Dynamic tunability emerges through controlled alterations in the optical properties of the system defining graphene’s plasmonic dispersion. Typically, electrostatic induced alterations of the carrier concentration in graphene working in conjunction with mobility have been considered the primary factors dictating plasmonic tunability. We find here that the surrounding dielectric environment also plays a primary role, dictating not just the energy of the graphene plasmon but so too the magnitude of its tuning and spectral width. To arrive at this conclusion, poles in the imaginary component of the reflection coefficient are used to efficiently survey the effect of the surrounding dielectric on the tuning of the graphene plasmon. By investigating several common polar materials, optical phonons (i.e., the Reststrahlen band) of the dielectric substrate are shown to appreciably affect not only the plasmon’s spectral location but its tunability, and its resonance shape as well. In particular, tunability is maximized when the resonances are spectrally distant from the Reststrahlen band, whereas sharp resonances (i.e., high-$Q$) are achievable at the band’s edge. These observations both underscore the necessity of viewing the dielectric environment in aggregate when considering the plasmonic response derived from two-dimensional materials and provide heuristics to design dynamically tunable graphene-based infrared devices.

Passive detection of special nuclear material (SNM) is challenging due to its inherently low rate of spontaneous emission of penetrating radiation, the relative ease of shielding, and the fluctuating and frequently overwhelming background. Active interrogation, the use of external radiation to increase the emission rate of characteristic radiation from SNM, has long been considered to be a promising method to overcome those challenges. Current active-interrogation systems that incorporate radiography tend to use bremsstrahlung beams, which can deliver high radiation doses. Low-energy ion-driven nuclear reactions that produce multiple monoenergetic photons may be used as an alternative. The ${}^{12}\mathrm{C}(\mathrm{p},{\mathrm{p}}^{\prime }{)}^{12}\mathrm{C}$ reaction is one such reaction that could produce large yields of highly penetrating 4.4- and 15.1-MeV gamma rays. This reaction does not directly produce neutrons below the approximately 19.7 MeV threshold, and the 15.1-MeV gamma-ray line is well matched to the photofission cross section of ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$. We report the measurements of thick-target gamma-ray yields at 4.4 and 15.1 MeV from the ${}^{12}\mathrm{C}(\mathrm{p},{\mathrm{p}}^{\prime }{)}^{12}\mathrm{C}$ reaction at proton energies of 19.5, 25, and 30 MeV. Measurements are made with two $3$-in. EJ-309 cylindrical liquid scintillation detectors and thermoluminescent dosimeters placed at $0^{\circ }$ and ${90}^{\circ }$, with an additional $1.5$-in. $\mathrm{Na}\mathrm{I}$($\mathrm{Tl}$) cylindrical scintillation detector at $0^{\circ }$. We estimate the highest yields of the 4.4- and 15.1-MeV gamma rays of $1.65\times {10}^{10}$ and $4.47\times {10}^8{\mathrm{sr}}^{-1}$ $\mu {\mathrm{C}}^{-1}$ at a proton energy of 30 MeV, respectively. The yields in all experimental configurations are greater than in a comparable deuteron-driven reaction that produces the same gamma-ray energies—${}^{11}\mathrm{B}(\mathrm{d},\mathrm{n}\gamma {)}^{12}\mathrm{C}$. However, a significant increase of the neutron radiation dose accompanies the proton energy increase from 19.5 to 30 MeV.

To develop low-moment, low-damping metallic ferromagnets for power-efficient spintronic devices, it is crucial to understand how magnetic relaxation is impacted by the addition of nonmagnetic elements. Here, we compare magnetic relaxation in epitaxial $\mathrm{Fe}$ films alloyed with light, nonmagnetic elements of $\mathrm{V}$ and $\mathrm{Al}$. $\mathrm{Fe}\text{−}\mathrm{V}$ alloys exhibit lower intrinsic damping compared with that of pure $\mathrm{Fe}$, reduced by nearly a factor of 2, whereas damping in $\mathrm{Fe}\text{−}\mathrm{Al}$ alloys increases with $\mathrm{Al}$ content. Our experimental and computational results indicate that reducing the density of states at the Fermi level, rather than the average atomic number, has a more significant impact on lowering damping in $\mathrm{Fe}$ alloyed with light elements. Moreover, $\mathrm{Fe}\text{−}\mathrm{V}$ is confirmed to exhibit an intrinsic Gilbert damping parameter of about 0.001, which is among the lowest ever reported for ferromagnetic metals.

We investigate the physical mechanisms for achieving an electrical control of conventional spin-singlet superconductivity in thin films by focusing on the role of surface orbital polarization. Assuming a multiorbital description of the metallic state, due to screening effects the electric field acts by modifying the strength of the surface potential and, in turn, yields nontrivial orbital Rashba couplings. The resulting orbital polarization at the surface and in its close proximity is shown to have a dramatic impact on superconductivity. We demonstrate that, by varying the strength of the electric field, the superconducting phase can be either suppressed, i.e., turned into normal metal, or undergo a 0-$\pi$ transition with the $\pi$ phase being marked by nontrivial sign change of the superconducting order parameter between different bands. These findings unveil a rich scenario to design heterostructures with superconducting orbitronics effects.

Metaphotonics, combining metamaterial (MM) and nanophotonic concepts, offers a unique platform for obtaining an unusual and/or advantageous electromagnetic response on a scale much less than the wavelength, although several practical applications are strongly hampered by the complexity of fabrication of MM devices and the intrinsic subwavelength inhomogeneous response of the MM composite. Recently, much research effort has been focused on the study of photonic devices for magnetic resonance imaging (MRI), one of the cornerstone diagnostics techniques in life science. In the MRI context, we introduce the use of magnetic surface excitations (magnetic localized surface plasmons) supported by a negative-magnetic-permeability MM sphere that results in a significant local enhancement of the MRI signal-to-noise ratio. Using the Mie resonance theory, we show that an increase in the signal-to-noise ratio boost can be obtained by replacing the MM sphere with a homogeneous high-permittivity one of the same radius. Overcoming some of the main MM limiting factors, our results suggest a simple approach for the realization of radio-frequency magnetic metadevices.

This report is on a model and experiment on the nature of mechanical-strain-mediated converse magnetoelectric (CME) effect in a composite of single-crystal iron borate, a canted antiferromagnet with a weak ferromagnetic moment, and ferroelectric lead zirconate titanate (PZT). The piezoelectric strain generated in PZT by an electric field E manifested as a shift in the quasiferromagnetic resonance (FMR) field in iron borate due to strong magnetoelastic interactions. The CME interaction strength determined from data on field shift in FMR versus E is 46–54 MHz cm/kV at 5.5–6.5 GHz. The strength of the CME is comparable to values reported for composites of ferrimagnetic oxides and PZT. A model that considers the effect of piezoelectric deformation on magnetic order parameters and magnetic resonance in iron borate is proposed for the CMEs in the composite and estimated ME coupling coefficients are in good agreement with data. The E tunability of the high-frequency AFMR mode at about 300 GHz is estimated to be on the order of 1.7 MHz kV/cm and is very small relative to the quasi-FMR mode. Composites of iron borate and ferroelectrics are very attractive for use in dual electric field and magnetic field tunable signal processing devices due to strong CME interactions and the need for a rather small bias magnetic field compared with traditional ferrimagnetic oxide based devices.

High-fidelity and robust quantum manipulation is the key for scalable quantum computation. Therefore, due to its intrinsic operational robustness, quantum manipulation induced by geometric phases is one of the promising strategies. However, the longer gate time for geometric operations and more physical difficulties with regard to implementation hinder its practical and wide application. Here, we propose a simplified implementation of universal holonomic quantum gates on superconducting circuits with experimentally demonstrated techniques, which can remove these two main challenges by introducing time-optimal control into the construction of quantum gates. Notably, our scheme is also based on a decoherence-free subspace encoding and requires minimal physical-qubit resources, which can be partially immune to error caused by qubit-frequency drift, one of the main sources of error for large-scale superconducting circuits. Meanwhile, gate error caused by unwanted leakage can also be eliminated by our deliberate design of quantum evolution paths. Finally, our scheme is numerically shown to be more robust than the conventional ones and thus provides a promising strategy for scalable solid-state fault-tolerant quantum computation.

Thermoelectric (TE) devices enable robust solid-state conversion of waste heat to electricity, but their wide-spread adoption is still limited by relatively modest efficiency. A widely used approach to improve efficiency is to enhance the power factor through confinement of carriers or energy filtering by potential barriers. However, their relative influence and the resulting improvement in the power factor in two-dimensional (2D) materials is not well understood. Here we study single-layer 2D ${\mathrm{Mo}\mathrm{S}}_2$ with lateral potential barriers to introduce either energy filtering or carrier confinement by changing the direction of the electric field, with confinement resulting when the electric field is parallel and energy filtering when the electric field is perpendicular to the potential barriers. We implement a Wigner-Rode model with electronic structure calculated from first principles to simulate the effect of the shape and size of potential barriers on parallel and perpendicular transport. Our results show that the power factor can be doubled, from $25{\mathrm{mWm}}^{-1}{\mathrm{K}}^{-2}$ without barriers to over $50{\mathrm{mWm}}^{-1}{\mathrm{K}}^{-2}$ for parallel transport in sharp, narrow potential wells. Perpendicular transport in smooth barriers results in a higher power factor compared to sharp barriers, while sharp barriers perform better in the case of transport parallel to the barriers, especially at small barrier widths. Our results aid in improving TE power factors and further the development of efficient waste-heat scavenging, flexible 2D TE converters, and Peltier cooling of nanoelectronics.

Optical networks implementing single-qudit quantum gates may exhibit superior properties to those for qubits as each optical element in the network can work in parallel on many optical modes simultaneously. We present a class of such networks that implements in a deterministic and efficient way the quantum Fourier transform (QFT) in an arbitrarily high dimension. These networks redistribute the initial quantum state into the orbital angular momentum (OAM) and path degrees of freedom and offer two modes of operation. Either the OAM-only QFT can be implemented, which uses the path as an internal auxiliary degree of freedom, or the path-only QFT is implemented, which uses the OAM as the auxiliary degree of freedom. The resources for both schemes scale linearly $O(d)$ with the dimension $d$ of the system, beating the best known bounds for the path-encoded QFT. While the QFT of the orbital-angular-momentum states of single photons has been applied in a multitude of experiments, these schemes require specially designed elements with nontrivial phase profiles. In contrast, we propose a different approach that utilizes only conventional optical elements.

In this paper, we demonstrate integrated photonic chips for multichannel quantum frequency conversion (QFC) based on reverse-proton-exchange periodically poled lithium niobate waveguides. The integrated chip is fiber coupled with two 34-channel fiber arrays, showing an average fiber-to-fiber sum-frequency generation (SFG) conversion efficiency of about 60% for multichannels. Then the same integrated chip is used to perform multichannel differential-frequency generation (DFG), showing the potential application in long-distance multimode quantum memory. Moreover, the integrated chip is utilized to construct a multichannel up-conversion detector, which has extensive applications including deep-space laser communication, high-rate quantum-key distribution, and single-photon imaging.

Quantum information processing is likely to have a far-reaching impact in the field of artificial intelligence. Noisy, intermediate-scale quantum devices provide a platform for exploring the possibility of attaining a quantum advantage through hybrid quantum-classical machine-learning algorithms. One example of such a hybrid algorithm is “quantum kitchen sinks,” which builds upon a classical algorithm known as “random kitchen sinks” to leverage a gate model quantum computer for machine-learning applications. We propose an alternative algorithm called “analog-quantum kitchen sinks” (AQKSs), which employs an analog-quantum computer for mapping data features into new features in a nonlinear manner. The new features can then be used by a classical algorithm to perform machine-learning tasks. We show the effectiveness of our algorithm for performing binary classification on both a synthetic dataset and a real-world dataset by simulating the operations of a quantum annealer. We demonstrate that the AQKS algorithm reduces the classification error of a linear classifier from $50\mathrm{\%}$ to $0.6\mathrm{\%}$ for the synthetic dataset and from $4.4\mathrm{\%}$ to $1.6\mathrm{\%}$ for the other dataset. Our proposed AQKS algorithm presents the possibility to use current quantum annealers for solving practical machine-learning problems.

We show that an individual droplet of liquid metal can be a source of coherent terahertz radiation when it is excited by two femtosecond laser pulses of the same frequency. Under certain delays between these pulses, the intensity of terahertz radiation increases by more than 3 orders of magnitude. We describe the experimental results with the model of dynamic gain control, which considers the interaction of both laser pulses with the droplet and explains the terahertz-generation process by taking into account the dynamics of electrons and ions after photoionization of the metal droplet. The spatial distribution of terahertz radiation has a forward-directed contribution, whose polarization properties are well described by a nonlinear susceptibility of the second order. Our theoretical estimations based on the experimental data show that under the dynamic gain control the observed terahertz output can be considerably increased. Joint generation of x-ray, ultraviolet, and, as shown in the present work, terahertz radiation allows one to forecast that a free-falling photoexcited droplet of liquid metal is a promising source of multifrequency electromagnetic radiation for extreme nonlinear laser science.

Elastic and acoustic metamaterials are man-made structures designed to control and manipulate waves through band gaps. The generalized bandwidth of metamaterials with locally resonant (LR) band gaps is usually narrow, limiting their applications in noise and vibration control. On the other hand, nonlinear metamaterials with dipolar resonances can present a broad chaotic band with low amplitude resonances due to bridging coupling, which is only observed under sufficiently large excitation amplitudes. In this work, we show that bridging-coupling phenomenon can also be observed in linear metamaterials with dipolar resonance, producing a special pass band with low amplitude resonances at low frequencies without excitation amplitude dependence. Such phenomenon emerges in a modified resonant metachain with zigzag interconnections and is based on the isomerism of traditional metamaterials. Although this modified isomer preserves the static stiffness and mass density (i.e., static performance) of the original metamaterial, its dispersive behavior presents interesting properties (i.e., branches with negative mass features and overlapping of opposite group velocity), resulting in an overall negative effective mass between the LR band gaps that is responsible for the remote interaction and the enhanced wave performance. In addition to the theoretical discussions, the bridging coupling is experimentally observed in a three-dimensional printed elastic metamaterial rod supporting longitudinal waves. The design can be extended to elastic bending, acoustic and electromechanical media, as well as to systems with periodicity in two and three dimensions (i.e., plates and solids). Therefore, the findings of this work open avenues to investigate the bridging-coupling phenomenon in linear systems as well as to explore the properties of metamaterial isomers.

Electric field control of magnetization dynamics is fundamentally and technologically important for future spintronic devices. Here, based on electric field control of both magnetic anisotropy and spin-orbit torque, two distinct methods are presented for switching the magnetization in topological-insulator– (TI) magnetic-TI hybrid systems. The magnetic anisotropy energy in magnetic TIs is formulated analytically as a function of the Fermi level, and it is confirmed that the out-of-plane magnetization is always favored for the partially occupied surface band. Also proposed is a transistorlike device with the functionality of a nonvolatile magnetic memory that uses voltage-driven writing and the (quantum) anomalous Hall effect for readout. For magnetization reversal, by using parameters of $\mathrm{Cr}$-doped $({\mathrm{Bi}}_{1-x}{\mathrm{Sb}}_x{)}_2{\mathrm{Te}}_3$, we find the estimated source-drain current density and gate voltage are on the order of ${10}^4$–${10}^5{\mathrm{A}/\mathrm{cm}}^2$ and 0.1 V, respectively, below 20 K and the writing requires no external magnetic field. Also discussed is the possibility of magnetization switching by the proposed method in TI–ferromagnetic-insulator bilayers with the magnetic proximity effect.