We study optical and magneto-optical (MO) properties of epitaxially grown modulated $\mathrm{Fe}$-$\mathrm{Pt}$ multilayer metamaterials by means of generalized MO ellipsometry (GME). Large Kerr rotation, ellipticity, and MO coupling parameters in the visible and near-infrared regions are obtained for modulated metamaterials, in which the stacking sequences follow an inverse Fibonacci series. In contrast, periodic multilayers show small MO parameters, although they otherwise exhibit magnetic and optical properties that are very similar to those for the modulated metamaterials. The present study demonstrates that specific thickness modulation can have a significant materials design potential for one-dimensional magnetic metamaterials.

The prediction of ultralow magnetic damping in ${\mathrm{Co}}_2\mathrm{Mn}Z$ Heusler half-metal thin-film magnets is explored in this study and the damping response is shown to be linked to the underlying electronic properties. By substitution of the Z elements in high-crystalline-quality films (${\mathrm{Co}}_2\mathrm{Mn}Z$ with Z = $\mathrm{Si}$, $\mathrm{Ge}$, $\mathrm{Sn}$, $\mathrm{Al}$, $\mathrm{Ga}$, $\mathrm{Sb}$), electronic properties such as the minority-spin band gap, Fermi-energy position in the band gap, and spin polarization can be tuned and the consequences for magnetization dynamics analyzed. The experimental results allow us to directly explore the interplay of spin polarization, spin gap, Fermi-energy position, and the magnetic damping obtained in these films, together with predictions from ab initio calculations. The ultralow magnetic damping coefficients measured in the range from 4.1 × ${10}^{-4}$ to 9 × ${10}^{-4}$ for ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Si}$, ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Ge}$, ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Sn}$, and ${\mathrm{Co}}_2\mathrm{Mn}\mathrm{Sb}$ are the lowest values obtained on a conductive layer and offer a clear experimental demonstration of theoretical predictions on half-metal magnetic Heusler compounds and a pathway for future materials design.

$\mathrm{L}{1}_0$ $\mathrm{Mn}\mathrm{Al}$ permanent-magnet alloys are promising candidates to plug the gap in performance and applications between rare-earth permanent magnets and ferrites; doping with a third atom is essential to achieve decent magnetic properties and good $\mathrm{L}{1}_0$-phase stability. In this work, the influence of substitutional atoms on intrinsic magnetic properties in $\mathrm{L}{1}_0$ $\mathrm{Mn}\mathrm{Al}$ alloys is investigated by combined experimental and theoretical studies. A highly pure $\mathrm{L}{1}_0$ phase is synthesized in ${\mathrm{Mn}}_{55}{\mathrm{Al}}_{45-x}{M}_x$ ($M$ = $\mathrm{Co}$, $\mathrm{Cu}$, $\mathrm{Ga}$) alloys by regulation of the heat treatment, on the basis of ternary phase diagrams. It is demonstrated by experimental measurements and calculations that $\mathrm{Cu}$ and $\mathrm{Co}$ tend to enter 1a (0, 0, 0) sites, where they degrade the intrinsic magnetic properties of the alloys, and $\mathrm{Ga}$ prefers the 1d (1/2, 1/2, 1/2) sites, where it improves them. The excess valence electrons of $\mathrm{Cu}$ or $\mathrm{Co}$ compared with $\mathrm{Mn}$ lead to stronger bonding with $\mathrm{Al}$, resulting in the preference of occupying 1a (0, 0, 0) sites. The similar valence-electron structure of $\mathrm{Ga}$ compared with $\mathrm{Al}$ makes $\mathrm{Ga}$ tend to occupy 1d (1/2, 1/2, 1/2) sites. On the basis of this understanding, we propose a strategy of replacing the antiferromagnetic $\mathrm{Mn}$ atoms by 3d atoms with fewer valence electrons than $\mathrm{Mn}$, which may effectively enhance the intrinsic magnetic properties. Moreover, the stability of the alloyed $\mathrm{L}{1}_0$ phase is investigated; $\mathrm{Co}$ is found to degrade the stability of the phase, but the addition of $\mathrm{Cu}$ or $\mathrm{Ga}$ can stabilize it. These results provide guidance for further performance optimization and composition design of $\mathrm{Mn}$-based rare-earth permanent magnets.

One of the most promising metasurface architectures for the microwave and terahertz frequency ranges consists of three patterned metallic layers separated by dielectrics. Such metasurfaces are well suited to planar fabrication techniques and their synthesis is facilitated by modeling them as impedance sheets separated by transmission lines. We show that this model can be significantly inaccurate in some cases, due to near-field coupling between metallic layers. This problem is particularly severe for higher frequency designs, where fabrication tolerances prevent the patterns from being highly subwavelength in size. Since the near-field coupling is difficult to describe analytically, correcting for it in a design typically requires numerical optimization. We propose an extension of the widely used equivalent-circuit model to incorporate near-field coupling and show that the extended model can predict the scattering parameters of a metasurface accurately. Based on our extended model, we introduce an improved metasurface synthesis algorithm that gives physical insight to the problem and efficiently compensates for the perturbations induced by near-field coupling. Using the proposed algorithm, a Huygens metasurface for beam refraction is synthesized showing a performance close to the theoretical efficiency limit despite the presence of strong near-field coupling.

Recently, for the first time we discovered that thermoacoustic imaging (TAI) can the map density of polar molecules, which play major roles in reflecting physiological functions [Luo et al. Phys. Rev. Appl. 10, 024044 (2018)]. A dual-modality imaging system is established that combines TAI with Doppler ultrasound imaging used for simultaneously acquiring polar-molecules’ density and flow velocity. Meanwhile, combining both types of information, a parameter called nutritional polar perfusion velocity (q) can be obtained, which can be used to assess the supply of nutrient molecules (NMs). NMs in the blood are biomolecules that maintain normal function, keep growth, and offer self-maintenance. In living bodies, a tumor consumes more NMs than normal tissue, thus, the q of the tumor is larger than normal tissue and this can be quantitatively measured by the proposed system. Thus, the q could be a potential indicator to estimate a tumor’s physiological status and the proposed system may promote the clinical application of TAI.

A wave plate consisting of dielectric subwavelength gratings (SWGs) with a certain thickness is typically used for polarization control. Since the phase retardation strongly depends on the wavelength, the wave plates made from dielectric SWGs usually operate in narrow or limited bandwidths, which severely hinders their device applications. In this paper, we demonstrate theoretically and experimentally that broadband quarter- and half-wave plates may be achieved in a single layer of high-index-contrast SWG by tailoring the dispersion relations of two orthogonal waveguide-array modes supported by the structure. The broadband SWG-based wave plates demonstrated in this work could be an important step forward in the development of functional polarization conversion devices for practical applications.

This paper pertains to a long-standing debate about the quantification criteria and mechanisms of optical superresolution in microsphere-based nanoscopy. Its super resolution capability has been closely linked in the literature to the width of photonic nanojets. A comprehensive and systematic comparison of the resolution obtained from the point-spread function (PSF) and the nanojet width is provided here. In both cases, the Maxwell equations are rigorously solved in a two-dimensional geometry using similar microsphere parameters. By carrying out the analysis in a rather broad range of parameters (microsphere size, object-to-microsphere separation), the assumption that the standard resolution defined by the PSF can be deduced from the nanojet focusing properties is disproved. The resolution values that result from the PSF and magnification are studied in two regimes: with and without the excitation of whispering-gallery-mode resonances in the microparticle. Unlike previous studies of nanojet focusing, the presented results summarize the true resolution capability of microspheres stemming from the classical point-emission geometry. It is also argued that a higher resolution capability can result from the coherence of illumination or the resonantly increased role of near fields in nanoplasmonic arrays.

Efficient optical read-out of single, solid-state electronic spins at room temperature is a key challenge for nanoscale quantum sensing. Nitrogen-vacancy color centers in diamond have a fast optical spin-state read-out mechanism, but it provides little information in a single shot, because the spin state is destroyed before many photons can be collected. Recently, a technique based on spin-to-charge conversion (SCC) was demonstrated that circumvents this problem by converting the spin state to a long-lived charge state. Here, we study how the choice of spin read-out technique impacts the performance of a single nitrogen-vacancy center in bulk diamond for quantum-sensing applications. Specifically, we show that the SCC technique results in an order-of-magnitude reduction in spin read-out noise per shot and a factor of 5 increase in ac-magnetometry sensitivity compared with the conventional optical read-out method. Crucially, these improvements are obtained for a low collection efficiency and bulk diamond geometry, which opens up the SCC technique to a wide array of sensing applications. We identify applications where single-shot spin read-out noise, rather than sensitivity, is the limiting factor (e.g., low duty cycle pulsed sequences in biomagnetometry involving long dead times).

The transformation of an electromagnetic wave into a propagating plasmon in a periodical dual-grating-gated graphene structure is theoretically investigated. The electromagnetic wave is incident at normal direction upon the structure. The most effective transformation appears in two cases: (i) a combined excitation of the radiative and “nonradiative” gated plasmon modes and (ii) excitation of the plasmon modes with the wavevectors in multiples of the structure inverse period. We find a 70% transformation of the incident power into a propagating plasmon for a 1-ps carrier momentum relaxation time in graphene when canceling the transmission of the electromagnetic wave through the graphene structure by a metallic ground plate and blocking the reflection with a Fabry-Perot resonator.

As an emerging stable two-dimensional (2D) semiconductor, monolayer (ML) antimonene is of great interest for future electronic and optoelectronic device applications. However, correlative research has been hindered by the lack of high-quality electrode contact. In this paper, on the basis of ab initio electronic calculations and quantum-transport simulations, the interfacial contact properties of ML antimonene-metal contacts are studied comprehensively for the first time. ML antimonene forms n-type Schottky contacts with $\mathrm{Sc}$, $\mathrm{Cr}$, and $\mathrm{Ti}$ electrodes, with an electron Schottky-barrier height (SBH) of 0.44, 0.49, and 0.52 eV, respectively, while it forms a p-type Schottky contact with $\mathrm{Ag}$, bulk $\mathrm{Sb}$, $\mathrm{Au}$, $\mathrm{Pt}$, and $\mathrm{Pd}$, with a hole SBH of 0.56, 0.44, 0.4, 0.31, and 0.29 eV, respectively. The Fermi pinning factor S is calculated to be 0.29, indicating a strong pinning effect. By contact with graphene or 2D $\mathrm{Hf}2\mathrm{N}(\mathrm{OH})2$ or by insertion of monolayer $\mathrm{BN}$ or graphene between ML antimonene and bulk metal, the Fermi-level pinning effect can be weakened because of the suppression of metal-induced gap states. ML antimonene forms a p-type contact with graphene with a small lateral hole SBH of 0.12 eV. In a ML antimonene device with 2D $\mathrm{Hf}2\mathrm{N}(\mathrm{OH})2$ and graphene-$\mathrm{Pt}$ as electrodes n-type Ohmic contact and p-type Ohmic contact are realized, respectively, indicative of a potentially high device performance. Our study provides a theoretical basis for the selection of favorable electrodes in future ML antimonene devices. Namely, the optimal n-type electrode is 2D ${\mathrm{Hf}}_2\mathrm{N}(\mathrm{OH}{)}_2$, followed by $\mathrm{Sc}$, $\mathrm{Cr}$, and $\mathrm{Ti}$ electrodes, and the optimal p-type electrode is graphene-$\mathrm{Pt}$ or graphene, followed by $\mathrm{Pd}$, $\mathrm{Pt}$, $\mathrm{Au}$, bulk $\mathrm{Sb}$, and $\mathrm{Ag}$ electrodes.

Surface recombination has a major impact on the open-circuit voltage ($V_{\mathrm{OC}}$) of organic photovoltaics. Here, we study how this loss mechanism is influenced by imbalanced charge transport in the photoactive layer. As a model system, we use organic solar cells with a 2 orders-of-magnitude-higher electron than hole mobility. We find that small variations in the work function of the anode have a strong effect on the light-intensity dependence of $V_{\mathrm{OC}}$. Transient measurements and drift-diffusion simulations reveal that this is due to a change in the surface recombination rather than the bulk recombination. We use our numerical model to generalize these findings and determine the circumstances under which the effect of contacts is stronger or weaker compared to the idealized case of balanced charge transport. Finally, we derive analytical expressions for $V_{\mathrm{OC}}$ in the case that a pile-up of space charge is present due to highly imbalanced mobilities.

We report the precise control of tunneling magnetoresistance (TMR) in devices of self-assembled core-shell ${\mathrm{Fe}}_3{\mathrm{O}}_4$/${\mathrm{Co}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles ($0\le x\le 1$). Adjusting the magnetic anisotropy through the content of ${\mathrm{Co}}^{2+}$ in the shell, provides an accurate tool to control the switching field between the bistable states of the TMR. In this way, different combinations of soft-hard and hard-soft core/shell configurations can be envisaged for optimizing devices with the required magnetotransport response.

Lithium titanate (${\mathrm{Li}}_2{\mathrm{Ti}\mathrm{O}}_3$) is an attractive ceramic material for various industrial applications, particularly as one of the most promising breeder blanket materials in future nuclear-fusion reactors. Previously reported studies mainly focus on sintered polycrystalline samples of ${\mathrm{Li}}_2{\mathrm{Ti}\mathrm{O}}_3$. Surface structure of the single-crystal form is rarely reported, although the information of surface structures and stabilities can be critical for further understanding the surface-associated processes. In this work, we perform extensive first-principles density-functional-theory (DFT) calculations to obtain the surface energies of ${\mathrm{Li}}_2{\mathrm{Ti}\mathrm{O}}_3$ (001) with different surface terminations. For four perfect (defect-free) $\mathrm{Li}$-, $\mathrm{O}$-, or $\mathrm{Li}\mathrm{Ti}$-terminated (001) surfaces, $\mathrm{Li}$- or $\mathrm{O}$-terminated (001) surfaces can be most stable in limited chemical-potential ranges corresponding to certain experimental conditions, while a $\mathrm{Li}\mathrm{Ti}$-terminated (001) surface is always unfavorable relative to $\mathrm{Li}$ or O terminations. By calculating the total energies of various possible configurations with surface vacancies, we determine the energetically most favorable vacancy-defected surface terminations. From the corresponding ternary phase diagram, we analyze the stability of a specific surface termination with vacancies as well as the possible formation of oxides. Our stability analysis together with DFT-simulated STM images reveals that a 1/3-monolayer-$\mathrm{Li}$-terminated surface most likely corresponds to the ordered hexagonal-like pattern observed previously in STM experiments. For a 1/2-monolayer-$\mathrm{Li}$-terminated surface, the most stable surface structure from our DFT calculations contrasts with previous results from an empirical-potential model.

Microwave kinetic inductance detectors (MKIDs) are thin-film, cryogenic, superconducting resonators. Incident Cooper pair-breaking radiation increases their kinetic inductance, thereby measurably lowering their resonant frequency. For a given resonant frequency, the highest MKID responsivity is obtained by maximizing the kinetic inductance fraction $\alpha$. However, in circuits with $\alpha$ close to unity, the low supercurrent density reduces the maximum number of readout photons before bifurcation due to self-Kerr nonlinearity, therefore setting a bound for the maximum $\alpha$ before the noise-equivalent power (NEP) starts to increase. By fabricating granular aluminum MKIDs with different resistivities, we effectively sweep their kinetic inductance from tens to several hundreds of pH per square. We find a NEP minimum in the range of $30\mathrm{aW}/\sqrt{\mathrm{Hz}}$ at $\alpha \approx 0.9$, which results from a trade-off between the onset of nonlinearity and a nonmonotonic dependence of the noise spectral density versus resistivity.

Nanomagnetic logic, which makes use of arrays of dipolar-coupled single-domain nanomagnets for computation, holds promise as a low-power alternative to traditional computation with CMOS. Beyond the use of nanomagnets for Boolean logic, nanomagnets can also be exploited for nondeterministic computational schemes such as edge detection in images and for solving the traveling salesman problem. Here, we demonstrate the potential of arrangements of thermally active nanomagnets based on artificial spin ice for both deterministic and probabilistic computation. This is achieved by engineering structures that follow particular thermal relaxation pathways consisting of a sequence of reorientations of magnet moments from an initial field-set state to a final low-energy output state. Additionally, we demonstrate that it is possible to tune the probability of attaining a particular final low-energy state, and therefore the likelihood of a given output, by modifying the intermagnet distance. Finally, we experimentally demonstrate a scheme to connect several computational building blocks for complex computation.

Pushing a uniaxial magnetic anisotropy ($K_u$) up to an order of meV per atom in bulk structures composed of only $3d$ magnetic elements with large saturation magnetization (${\mu }_0{M}_s$) and Curie temperature ($T_c$) has been a long-term goal of hard permanent magnet and spintronics studies. Herein, using ab initio calculations, we predict an unexpectedly large $K_u$ up to 1/2 meV/formula unit in B2-ordered tetragonal $\mathrm{Fe}\mathrm{Co}$, which is even comparable to or more than 50% of those for the currently known permanent magnetic materials with $4d$, $5d$, and $4f$ orbitals. This large $K_u$ is associated with an interplay mechanism of the Jahn-Teller-like tetragonal distortion induced by doping with an interstitial $2p$-electron element, herein nitrogen as an example, and the strong orbital hybridization between $3d$ and $2p$ states. Our findings demonstrate the feasibility of significant enhancements on the magnetic anisotropy and energy product of $3d$ magnetic metals through the interstitial doping with $2p$-electron nonmetal elements.

Methods to measure the electromagnetic field intensity on a large scale should be improved to meet the growing demand in civil and military applications. The recent development and an application of a microwave electromagnetic field imaging method using a thermofluorescent device and detection in the visible-light range are described. The sensor is a flexible film sensitive to either an electric field (slightly conductive film) or a magnetic field (insulating film with ferromagnetic particles) coated with a polymer doped with fluorescent molecules displaying temperature-dependent emission. We use a lock-in thermography protocol consisting of modulation of the field at low frequency, which allows us to suppress the noise due to heat-transfer phenomena and the part of the signal not directly related to field absorption. Two applications of the method are presented: (i) imaging the microwave magnetic field emitted by a resonator antenna and (ii) the local probe measurement of the electric field inside a multibeam antenna. The process to develop the sensitive film, its detailed characterization, and the thermofluorescence-image demodulation postprocessing are presented in detail. This approach is much cheaper than one based on infrared thermometry and could permit democratization of microwave- and radio-frequency electromagnetic field imaging.

We present a multiscale model that simulates optically induced spin currents in metallic bilayer structures that emit terahertz radiation after optical pulse excitation. We describe hot-electron transport in a metallic bilayer by a Boltzmann transport equation, which is solved numerically by a particle-in-cell approach. Optical excitation and propagation effects are taken into account by our determining the emitted terahertz waves from the excited-carrier dynamics. We apply this approach to an $\mathrm{Fe}$/$\mathrm{Pt}$ bilayer and show in detail how microscopic scattering effects and transport determine the emitted signal. The versatility of the approach presented here allows it to be readily adapted to a wide spectrum of spintronic-terahertz-emitter designs. As an example, we show how the terahertz generation efficiency, defined as the output-power-to-input-power ratio, can be increased and optimized with use of serially stacked layers in conjunction with terahertz antireflective coatings. We derive an analytical expression for the terahertz emission of a single layer that allows us to determine the relationship between the emitted field and the current profile that generates it.

A theoretical model for describing the emission spectra of microsphere cavities is presented and its predictions of detailed line shapes of emission spectra associated with whispering gallery modes (WGMs) of various orders in $\mathrm{Zn}\mathrm{O}$ microspheres (MSs) are verified experimentally by photoluminescence (PL) spectroscopy. The interplay of the Purcell effect, quality factor, and leaky modes in spontaneous and stimulated emission spectra related to WGMs of all orders is revealed. The key success of the theory is based on the expansion of the full Green’s function of the MS in terms of all possible resonance modes in complex frequency space, which allows incorporation of contributions from leaky modes, stimulated emission processes, and the Purcell effect. We show that the spontaneous emission spectrum calculated according to Mie theory (without the Purcell effect) is dominated by the contribution of leaky modes, while the spontaneous and stimulated emission enhanced by the Purcell effect is responsible for the main WGM resonance peaks observed experimentally. It is found that the stimulated emission peaks are doubly enhanced by their respective mode quality factor $Q$: one factor from the Purcell effect and the other factor from the photon number derived from the rate equation. After combining all these effects, the theory can provide a quantitative description of fine features of both TE and TM modes (including higher-order modes) observed in the PL spectra of $\mathrm{Zn}\mathrm{O}$ MSs. Surprisingly, it is found that for $\mathrm{Zn}\mathrm{O}$ MS with a diameter larger than 5 $\mu \mathrm{m}$, the PL emission spectrum is dominated by higher-order modes. The quantitative understanding of the interplay of these emission mechanisms should prove useful for optimizing the performance of light-emitting devices based on microresonators.