We investigate the modifications of the exchange bias, effective ferromagnet-antiferromagnet (FM-AFM) coupling, and coercivity in annealed or ion-irradiated $\mathrm{Ir}\text{−}\mathrm{Mn}$/spacer layer (SL)/$\mathrm{Co}$ trilayers. A ferromagnetic, either $\mathrm{Fe}$ or permalloy (Py), thin SL or a nonmagnetic $\mathrm{Ru}$ one with different thicknesses ($t_{\mathrm{SL}}$) is used. The magnetic characterization is performed at room temperature via conventional magnetometry and, partly, via soft-x-ray magnetic circular dichroism. The latter shows that at the FM-AFM interface there is small uncompensated $\mathrm{Mn}$ magnetization coupled, preferentially, antiferromagnetically to $\mathrm{Fe}$ moments. This indicates the formation of small $\mathrm{Fe}\mathrm{Mn}$ clusters that reverse their magnetizations together with those of $\mathrm{Co}$, $\mathrm{Ni}$, and the rest of $\mathrm{Fe}$. Neither annealing nor ion irradiation of the films with $t_{\mathrm{Fe}}\ge 0.5$ nm changes significantly the pinning part of the FM-AFM interface. In the Py-spacer films, however, the great tendency of $\mathrm{Mn}$ to interdiffuse with $\mathrm{Ni}$ leads to a decrease of the $\mathrm{Ir}\text{−}\mathrm{Mn}$ anisotropy at the interface, lowering its pinning capacity. While defects created in the bulk of the AFM are mainly responsible for the changes of the magnetic characteristics of the $\mathrm{Ir}\text{−}\mathrm{Mn}$/$\mathrm{Fe}$/$\mathrm{Co}$ films, interdiffusion and defect creation at the FM-AFM interface are the respective mechanisms determining the behavior of the Py-spacer series. These conclusions are reinforced by results for the $\mathrm{Ru}$-spacer series.

We investigate refractive-index changes caused by femtosecond photoionization in a gas-filled hollow-core photonic-crystal fiber. Using spatially-resolved interferometric side-probing, we find that these changes live for tens of microseconds after the photoionization event — eight orders of magnitude longer than the pulse duration. Oscillations in the megahertz frequency range are simultaneously observed, caused by mechanical vibrations of the thin-walled capillaries surrounding the hollow core. These two nonlocal effects can affect the propagation of a second pulse that arrives within their lifetime, which works out to repetition rates of tens of kilohertz. Filling the fiber with an atomically lighter gas significantly reduces ionization, lessening the strength of the refractive-index changes. The results will be important for understanding the dynamics of gas-based fiber systems operating at high intensities and high repetition rates, when temporally nonlocal interactions between successive laser pulses become relevant.

We demonstrate a new phononic crystal (PnC) platform with wideband hypersonic phononic bandgaps (PnBGs) for surface acoustic waves (SAWs). These SAW PnCs are fabricated on a CMOS-compatible substrate and constructed by a two-dimensional periodic array of piezoelectric aluminium nitride pillars on silicon to achieve a low-loss all-dielectric PnC platform. Our experimental PnBG results acquired with integrated wideband SAW filters (i.e., two slanted interdigital transducers as an emitter and a receiver) show a surface PnBG from 1.6 to 1.75 GHz for the fabricated surface PnC, enabling the formation of low-loss hypersonic PnC-based devices for a wide range of ultrahigh-frequency applications, including wireless communications.

We develop a compact model for the THz plasmonic unstable structures with tunable narrow-channel regions of an increased width (called plasmonic “stubs”) using the transmission line analogy and derive the dispersion equations describing unstable plasmons. The solutions of the dispersion equations in the plasmonic systems with the electron drift, which are derived using the hydrodynamic model and generalized to account for the stubs, illustrate the device physics and can be used for design and characterization of THz plasmonic-electronic sources. Our results show that adding stubs allows one to control the Dyakonov-Shur instability in plasmonic field-effect transistors by optimizing the boundary conditions, controlling the plasma velocity, and making it possible to drive periodic plasmonic structures wirelessly, thus avoiding the contact problems.

A discrete element method (DEM) implementation is developed to study the micromechanics of liquefaction in granular materials. In a liquefaction event, the pore water acts as a cushion between the grains, reducing the contact and friction forces and the overall soil strength. The proposed model reproduces this phenomenon by introducing the effect of pore water as a constraint over the DEM particles’ mechanics. The DEM particles will suffer resistance to any displacement changing the pore volume, which takes into account the very small compressibility of water. It is found that this constraint is enough to simulate soil liquefaction under quasistatic deformation. Finally, it is shown that the initial density of the granular skeleton, defined by the number of contacts between grains, plays a critical role in determining if the soil will liquefy or not. This critical value opens the possibility of treating liquefaction in soils as a bifurcation problem.

The distributed absorption of photons in photodiodes induces an excess noise in continuous-wave photodetection above the transit-time roll-off frequency. We show that it can be treated as a frequency-dependent excess optical loss in homodyne detection. This places a limit on the bandwidth of high-accuracy homodyne detection, even if an ideal photodetector circuit is available. We experimentally evaluate the excess loss in two ways: a comparison of signal gain and shot-noise gain of one-port homodyne detection and a balanced homodyne detection of squeezed light at a 500-MHz sideband. These results agree with an analytic expression we develop, where the randomness of the photoabsorption is directly modeled by an intrusion of vacuum field. At 500 MHz, we estimate the excess loss at 14% for a $\mathrm{Si}$ photodiode with 860-nm incident light, while the numerical simulation predicts much smaller excess loss in $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ photodiodes with 1550 nm light.

We present analytical formulae for light propagation through a multilayer of dielectric media as a function of the “total charge” and the “accumulated charge” that we propose by the transfer-matrix method. We redefine the concept of the total charge introduced by Liu et al. [J. Phys. Condens. Matter. 29, 455303 (2017)] and newly define the accumulated charge to determine the magnitude of electric field inside the multilayers analytically. In addition, we derive an analytical expression for the group velocity of light in each layer. The purpose of this work is to predict which kind of sequence would provide the strongest amplification of the electric field inside the multilayer as well as to find the sequence that gives the lowest group velocity. We find that the symmetric alternating sequence gives the best amplification of the electric field and the lowest mean group velocity compared with the other sequences.

Magnetic domain wall motion driven by an electric current is an important phenomenon in the development of memory and logic devices because the domain wall acts as an information carrier. To conserve the information, unidirectional motion of multiple domain walls is required. Recent works show that a chiral texture of the magnetic domain wall preferred by the Dzyaloshinskii-Moriya interaction (DMI) is an essential condition for unidirectional motion. Thus, measuring DMI is an important issue for device development, but this is not an easy task when the magnitude of the DMI is weak. Here, we report that domain walls with weak DMI can form unusual domain shapes, triangular and sawtooth, because the electric current and the external magnetic field cancel each other out at certain domain wall angles. These angles strongly depend on DMIs that show the possibility of characterizing weak DMIs.

The magneto-optical polarization rotation effect has myriad applications in many research areas spanning the scientific spectrum, including space and interstellar research, nanotechnology, material science, biomedical imaging, and subatomic particle research. In the nonlinear magneto-optical rotation (NMOR) effect, the angle of rotation of a linearly polarized optical field in a magnetized medium is dependent upon its intensity. However, typical NMOR signals of conventional single-beam $\mathrm{\Lambda }$-scheme atomic magnetometers are peculiarly small, requiring sophisticated magnetic shielding and high-frequency phase-sensitive detection. Here, we show the presence of an energy-symmetry-based propagation growth blockade that undermines the NMOR effect in conventional single-beam $\mathrm{\Lambda }$-scheme atomic magnetometers. We further demonstrate, both experimentally and theoretically, an inelastic wave-mixing technique that breaks this NMOR blockade, resulting in more-than-2-orders-of-magnitude enhancement of the NMOR signal power amplitude that cannot be achieved with conventional single-beam $\mathrm{\Lambda }$-scheme atomic magnetometers. This technique, demonstrated here with substantially reduced light intensities at near-room temperatures, may lead to many applications, especially in the field of biomagnetism and high-resolution low-field magnetic imaging.

Self-organization of large-area nanoscale patterns employing a single-step inexpensive process can be crucial in the fabrication of low-cost but high-performance devices. In the present study, we employ the spin dewetting of a conductive polymer to fabricate an array of micro-to-nanoscale ordered-heterojunctions (OHJ) to demonstrate the improvements in the key performance indicators of organic photovoltaic (OPV) devices in ambient conditions. For this purpose, the surface of a hole-collector polymer film [e.g., (poly-(2,3-dihydrothieno-1, 4-dioxin):poly-(styrene sulfonate) (PEDOT:PSS)], coated on a transparent conducting substrate, is decorated with physicochemical patterns of a self-assembled monolayer. Afterward, the electron donor polymer [e.g., poly (3-hexylthiophene-2,5-diyl) (P3HT)] is spin-dewetted into a large collection of digitized micro- and nanodroplets. A theoretical analysis of the governing equations with appropriate boundary conditions uncovers that the imbalance of centripetal, capillary, and van der Waals forces plays a major role in deciding the droplet spacing of the spin-dewetted morphologies. Further, simulations are performed to understand the effect of size and periodicity of the donor droplets inside the device architecture, which could lead to an enhanced current flow when compared with a planar heterojunction (PHJ) device composed of thin films. Subsequently, a detailed experimental analysis is performed to uncover the role of spin speed and the initial loading of the electron donor polymer into the solvent during spin casting on the size, periodicity, and density of the electron donor droplets on the hole-collector surface. Capping the optimally discretized P3HT droplet arrays with the electron-acceptor layer [e.g., ([6,6]-phenyl-${\mathrm{C}}_{61}$ butyric acid methyl ester (PCBM)] led to the formation of a highly corrugated donor-acceptor interface suitable for higher photon absorption, facile exciton generation, and improved exciton separation. The self-organized-large-scale-integration (SOLSI) of the spin-dewetted droplets at the charge-carrier donor-acceptor interface of the OPV-OHJ assemblage enables the enhancement by approximately 40% as compared to similar OPV-PHJ configurations. The enhanced photoconversion efficiency takes place via optimal separation of photon absorption and carrier collection pathways. The study uncovers the importance of developing high-density and large-area nanopatterns employing spin dewetting to develop process-intensified OPV-OHJ cells with improved performance at a lower fabrication cost.

We present a single-phase double-sided pillared metamaterial with effective mass density and elastic modulus simultaneously negative. The negative effective mass density is achieved owing to the combination of the symmetric bending resonance and the antisymmetric compressional resonance of both pillars, whereas the negative effective elastic modulus results from their symmetric compressional resonance. Comparisons with the single-sided pillared structure are made. The dependence of the width of the double-negative band against the geometric parameters of the unit cell is also investigated. In addition, the negative refraction and the zero-index refraction of the symmetric Lamb wave in the deep-subwavelength scale are observed, while at the same frequency, the propagation of the antisymmetric Lamb wave is forbidden.

The growth rate of infinitesimal perturbations in electrified jets exhibits different behavior at the viscid and inviscid limits. Using Saville’s approach to estimate the growth rate for perturbations in the long-wavelength limit and by ignoring the effects of gravity, we derive two scaling laws for the jet breakup length in two regimes of the Taylor-cone mode. Our experimental measurements show clear dependency of the jet length on the flow rate; however, change of the applied voltage appears to affect only the cone angle but not the jet itself. The experimental data agree excellently with our theoretical model in both cases. On the basis of the jet diameter, the transition between viscid and inviscid limits appears to occur at an electric Reynolds number of approximately 5. Finally, we show how to enhance the quality and the resolution in electrostatic-inkjet-printing applications by setting the printing distance less than the jet length and predict the line width as a function of the operational parameters.

We report on an experimental and numerical study of the fragmentation mechanisms of a micrometer-sized metal droplet irradiated by ultrashort laser pulses. The results of the experiment show that the fast one-side heating of such a droplet may lead to either symmetric or asymmetric expansion followed by different fragmentation scenarios. To unveil the underlying processes leading to fragmentation, we perform simulations of liquid-tin droplet expansion produced by initial conditions similar to those in an experiment using the smoothed particle hydrodynamics (SPH) method. Simulations demonstrate that a thin heated surface layer generates an ultrashort shock wave propagating from the frontal side to the rear side of a droplet. Convergence of such a shock wave followed by an unloading tail to the droplet center results in cavitation of material inside the central zone, caused by the strong tensile stress. Reflection of a shock wave from the rear side of a droplet produces another region of highly stretched material where spallation may occur, producing a thin spall layer moving with a velocity higher than the expansion rate of the liquid shell around the central cavity. It is shown both experimentally and numerically that the threshold laser intensity necessary for the spallation is higher than the threshold required to induce cavitation close to the droplet center. Thus, the regime of asymmetrical expansion is realized if the laser intensity exceeds the rear-side spallation threshold. The transverse and longitudinal expansion velocities obtained in SPH simulations of different expansion regimes agree well with our experimental data.

We investigate alignment and electro-optical properties of a room temperature dye-doped antiferroelectric liquid-crystal mixture. We achieve uniform alignment on a macroscopic scale of thin cells with the combination of proper surface alignment and electric field treatment. We also successfully demonstrate that two films of dye-doped antiferroelectric liquid crystals in their anticlinic chiral smectic C phase can be used to switch the transmitted light intensity between dark and bright states without the need for polarizer filters. We also show that one can get either normally dark or bright states. Normally dark states can be useful in a number of applications such as in privacy windows or smart refrigerators. A normally transparent display has applications in a plethora of other areas, such as navigation systems, built-in windshields, in goggles, or smart windows.

We present a compact atomic clock using cold rubidium atoms based on an isotropic light cooling, a Ramsey microwave interrogation, and an absorption detection. Its technology readiness level is suitable for industrial transfer. We use a fiber optical bench, based on a frequency-doubled telecom laser. The isotropic light cooling technique allows us to cool down the atoms in 100 ms and works with a cycle time of around 200 ms. We carry out measurements in simulated microgravity and obtain narrow fringes, up to 1.25 Hz.

Luminescence (nano)thermometry is an important technique for remote temperature sensing. The recent development of lanthanide-doped nanoparticles with temperature-dependent emission has expanded the field of applications, especially for ratiometric methods relying on the temperature variation of relative emission intensities from thermally coupled energy levels. Analysis and calibration of the temperature dependence is based on a Boltzmann equilibrium for the coupled levels. To investigate the validity of this assumption, we analyze and model thermal equilibration for ${\mathrm{Eu}}^{3+}$ ${}^5{D}_1$ and ${}^5{D}_0$ emission in $\mathrm{Na}{\mathrm{YF}}_4$. The results show that for low ${\mathrm{Eu}}^{3+}$ concentrations, temperature-dependent multiphonon relaxation can accurately explain both the intensity ratio and emission decay dynamics. The analysis also reveals that a Boltzmann equilibrium is not realized in the temperature regime investigated (300–900 K). By increasing the ${\mathrm{Eu}}^{3+}$ concentration, cross relaxation between neighboring ${\mathrm{Eu}}^{3+}$ ions enhances ${}^5{D}_1-{}^5{D}_0$ relaxation rates and extends the temperature range in which emission intensity ratios can be used for temperature sensing (500–900+ K). The results obtained are important for recognizing, understanding, and controlling deviations from Boltzmann behavior in luminescence (nano)thermometry. By varying the dopant concentration, the range for accurate temperature sensing can be adjusted. These insights are crucial in the development and understanding of reliable temperature sensors.

The enhancement of stimulated Raman scattering (SRS) with a $\mathrm{Ga}\mathrm{As}$/$\mathrm{Al}\mathrm{As}$ intermixed superlattice that works as a ${\chi }^{(3)}$-quasi-phase-matched structure is studied, where such Kerr-induced effects as four-wave mixing (FWM), self-phase-modulation (SPM), cross-phase-modulation (XPM), and two-photon absorption (TPA) are included. In particular, the efficiency of anti-Stokes generation is enhanced here; anti-Stokes generation inherently has an extremely small efficiency due to a phase mismatch in the interaction of the pump, Stokes, and anti-Stokes waves (while the efficiency of Stokes generation is sufficiently large because of no such phase mismatch). The superlattice enhances the anti-Stokes efficiency up to the order of ${10}^3$ when compared with that without the superlattice, particularly at a small pump intensity. In this enhancement, it is seen that there is an efficiency boost via simultaneous FWM. In this situation, it is shown how much SPM and XPM degrade the efficiency enhancement. Furthermore, an optimal superlattice length is identified that provides the highest efficiency. The degradation of the efficiency at the optimized length due to TPA is also analyzed. Finally, to gain more anti-Stokes efficiency (or control the sizes of the Stokes and anti-Stokes efficiencies), a photonic-band-gap cavity structure is proposed.

The Rayleigh law describes the linear dependence of the permittivity of a ferroelectric on the applied ac electric field amplitude due to irreversible motions of domain walls. We show that this gives rise to a new equivalent-circuit element predestined to fit the impedance spectra of ferroelectrics based on an accepted physical model. Such impedance spectroscopy is a powerful tool to obtain a dielectric and resistive representation of the entire sample structure. The superiority of the Rayleigh analysis based on impedance spectroscopy compared to the common single-frequency approach is demonstrated for a ferroelectric $\mathrm{Si}:{\mathrm{Hf}\mathrm{O}}_2$ thin film.

We investigate the magnetization dynamics of a pair of ferromagnetic insulators (FMIs) deposited on the surface of a topological insulator (TI). Because of the nonlinear nature of the underlying physics and intrinsic dynamics, the FMIs can exhibit oscillatory behaviors even under a constant applied voltage. The motion of the surface electrons of the TI, which obeys relativistic quantum mechanics, provides a mechanism of direct coupling between the FMIs. In particular, the spin-polarized current of the TI surface electrons can affect the magnetization of the two FMIs, which in turn modulates the electron transport, giving rise to a hybrid relativistic quantum and classical nonlinear dynamical system. We find robust phase and antiphase locking between the magnetization dynamics. As driving the surface electrons of a TI requires only extremely low power, our finding suggests that nanoscale FMIs coupled by a spin-polarized current on the surface of a TI have the potential to serve as the fundamental building blocks of unconventional, low-power computing paradigms.

We propose to achieve wideband, high-efficiency polarization conversions of transmission waves with the synergy of the waveguide and spoof surface plasmon polariton (SSPP) modes. High-efficiency transmissions based on SSPP mode coupling and waveguide mode coupling are designed in two orthogonal directions, respectively. By designing the nonlinear dispersions of the SSPP and waveguide modes, the required phase differences for polarization conversion can be obtained in a wide frequency range. To verify this principle, two different polarization conversion metamaterials (PCMs) are designed and fabricated for linear-to-circular (LTC) and circular-to-circular (CTC) polarization conversions, respectively. It is found that the LTC polarization conversion transmittivity of the LTC PCM is greater than −0.5 dB from 7 to 12.6 GHz, and the cross-polarization transmittivity of the CTC PCM for a circularly polarized incidence wave is greater than −0.5 dB in the frequency range of 7–11 GHz, showing a wide band feature.