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.

Yttrium orthosilicate (${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$, or YSO) has proved to be a convenient host for rare-earth ions used in demonstrations of microwave quantum memories and optical memories with microwave interfaces and shows promise for coherent microwave-optical conversion owing to its favorable optical and spin properties. The strong coupling required by such microwave applications could be achieved using superconducting resonators patterned directly on ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ and hence we investigate here the use of ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ as an alternative to sapphire or silicon substrates for superconducting-hybrid-device fabrication. A $\mathrm{Nb}\mathrm{N}$ resonator with frequency 6.008 GHz and low-power quality factor $Q\approx 400000$ is fabricated on a ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ substrate doped with isotopically enriched ${}^{145}\mathrm{Nd}$. Measurements of dielectric loss yield a loss tangent $\tan \delta =4\times {10}^{-6}$, comparable to that of sapphire. Electron spin resonance (ESR) measurements performed using the resonator show the characteristic angular dependence expected from the anisotropic ${}^{145}\mathrm{Nd}$ spin and the coupling strength between the resonator and the electron spins is in the high-cooperativity regime ($C=30$). These results demonstrate that ${\mathrm{Y}}_2{\mathrm{Si}\mathrm{O}}_5$ is an excellent substrate for low-loss, high-$Q$ microwave resonators, especially in applications for coupling to optically accessible rare-earth spins.

We propose a simple theory of photoelectron yield spectra (PYS) on the basis of a density-of-states approximation and universal transmission and loss functions. The theory reproduces the known threshold behavior of metals and that of semiconductors in the indirect-transition regime. We show that a Gaussian-shaped density of states leads to a PYS spectrum that increases very nearly as the third power of energy from threshold, indicating that the empirical cube law of PYS of organic matter reflects the Gaussian-broadened highest occupied molecular orbital peak. We show how the peak position and width can be extracted from the PYS spectrum. The results obtained for several organic compounds agree well with ultraviolet photoemission data. We further show that the density of occupied states is approximately proportional to the second derivative of the PYS spectrum in the near-threshold region.

The presence of a laser-intensity-related carrier-envelope-phase (CEP) artifact in CEP-dependent nonlinear photoemission experiments using a single-shot CEP tagging technique has obscured the detection of weak intrinsic CEP effects. The artifact is found to be correlated with random laser-intensity fluctuations in the experimental setup. We, therefore, remediate this issue by implementing single-shot tagging of the laser pulse energy in addition to the present CEP tagging technique in order to assign a corresponding (pulse-averaged) laser intensity to each detected photoelectron for every laser shot. After applying intensity filtering and an appropriate intensity correction, we demonstrate the capability of detecting weak CEP-dependent photoemission from a bulk tungsten surface with a modulation depth as low as approximately 1% within a measurement time of approximately 30 min at a 10-kHz repetition rate. Our approach has proven successful in increasing the sensitivity of the single-shot CEP tagging technique, which allows the studies of weak CEP effects in atoms, molecules, and solids.

Utilizing single-carrier devices for mobility determination, via space-charge-limited current measurements, has become commonplace in the organic, inorganic, and hybrid semiconductor communities. However, since there is often a difference in contact work functions of the single-carrier devices used for the measurements, a built-in voltage is commonly present, which must be accounted for during the analysis of the current density-voltage curves. We here present a direct and convenient method for determining the built-in voltage directly from the ratio of the forward and reverse currents, without any prior knowledge of the charge-carrier mobility, eliminating the need for complementary measurements.