The performance of an on-chip Terahertz (THz) source based on the Dyakonov-Shur (DS) instability is analytically and numerically investigated. The impact of nonideal termination impedances at the source and the drain of a III-V semiconductor-based HEMT-like plasmonic cavity is first studied in the linear approximation of the hydrodynamic model. Then, a multiphysics simulation platform that self-consistently solves the full hydrodynamic model and Maxwell’s equations is developed and utilized to numerically investigate the effects of the finite termination impedances on the DS instability, the generated plasmonic current, and the radiated THz electromagnetic signals in the steady state. The results show that nonideal boundary conditions at the cavity terminations can drastically impact the DS instability and reduce the generated and radiated THz em signals.

The capability to tailor the dispersion and the cut-off frequency of waveguides is of importance, as these essential parameters govern the operating frequency range and the waveguide dimension. Here, we propose the concept of substrate-integrated impedance surface (SIIS) that enables arbitrary control of propagation characteristics of closed-shape waveguides. Specifically, we develop a theoretical framework for the simplest form of SIIS constituted by a one-dimensional array of blind vias, which is equivalent to a homogenized surface capacitance embedded in the waveguide. We theoretically and experimentally demonstrate that loading a substrate-integrated waveguide (SIW) with a capacitive SIIS can effectively reduce its cut-off frequency, regardless of the transverse dimension of the SIW. In addition, a SIIS-loaded SIW exhibits several intriguing phenomena, such as the slow-wave guiding properties and the local field concentration. This SIIS-loading technique may open up new possibilities for miniaturization of various waveguide-based components and for enhancement of their uses in microwave sensing and nonlinear functions.

We propose a diagonal approximation for the self-energy that describes the interaction between electrons and polar optical phonons in the framework of nonequilibrium Green’s function transport simulations. Our model is based on the definition of a scaling factor, which renormalizes the local electron-phonon coupling, to take into account the nonlocality of the interaction and provide the correct scattering rates. While previous studies relied on empirical values of this factor, we derive, from basic physical relationships, analytical expressions in the presence of the one- and two-dimensional confinement of phonons. We apply our model to the self-consistent simulation of double-gate $p$-type transistors made of technologically relevant III-V materials ($\mathrm{In}\mathrm{As}$, $\mathrm{In}\mathrm{Sb}$, and $\mathrm{Ga}\mathrm{Sb}$). Their performance is benchmarked, for different crystallographic orientations and strain constraints, against the corresponding $\mathrm{Si}$ and $\mathrm{Ge}$ devices. We find that the electron-polar optical phonon scattering plays a major role in degrading the performance of the III-V devices and typically results in a widening of the performance gap existing between III-V and $\mathrm{Si}$ or $\mathrm{Ge}$ devices in ballistic transport conditions.

In conventional all-dielectric one-dimensional photonic crystals (1DPCs), photonic gaps will shift toward short wavelengths (blueshift) as the incident angle increases for both transverse magnetic (TM) and transverse electric (TE) polarizations. We theoretically and experimentally achieve redshift gaps in 1DPCs composed of alternative hyperbolic metamaterials (HMMs) and dielectrics for TM polarization based on the anomalous wavevector dispersion of HMMs. The HMM is mimicked by layered titanium dioxide and silver with a subwavelength unit cell. However, the gaps remain to be blue-shifted for TE polarization. Therefore, the edges of such gaps can be utilized for high-efficiency wide-angle polarization selection. We achieve efficient polarization selection for incident angles ranging from 50° to 80° at the wavelength of 365 nm.

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.