The presence of trap states plays an important role in the performance of optoelectronic devices. In the present investigation, charge-carrier dynamics in amorphous and crystalline ${\mathrm{In}}_2{\mathrm{Se}}_3$ thin films is studied using broadband femtosecond pump-probe spectroscopy in the temperature range of 5–300 K. Amorphous ${\mathrm{In}}_2{\mathrm{Se}}_3$ thin films (approximately 300 nm thick) are deposited using a thermal-evaporation method, and undergo a transition to a crystalline phase under vacuum annealing at $250{}^{\circ }\mathrm{C}$. Ultrafast transient absorption studies suggest that the charge-carrier dynamics is dominated by the presence of inter-band-gap defect states in both types of film, and it is found to be slower in the amorphous than in the crystalline material due to the presence of deep defect states in the amorphous material. The carrier dynamics in the amorphous film is faster at 5 K than at 300 K, as the probability of free carriers becoming trapped in defect states is much higher at high temperatures due to their higher mobility. The change in resistance under light and with temperature is directly correlated with ultrafast spectroscopic data.

We study, both experimentally and theoretically, the modification of ${\mathrm{Er}}^{3+}$ photoluminescence properties in $\mathrm{Si}$ dielectric nanoslots. The ultrathin nanoslot (down to 5-nm thickness), filled with $\mathrm{Er}$ in ${\mathrm{Si}\mathrm{O}}_2$, boosts the electric and magnetic local density of states via coherent near-field interaction. We report an experimental 20-fold enhancement of the radiative decay rate with negligible losses. Moreover, via modifying the geometry of the all-dielectric nanoslot, the outcoupling of the emitted radiation to the far field can be strongly improved, without affecting the strong decay-rate enhancement given by the nanoslot structure. Indeed, for a periodic square array of slotted nanopillars an almost one-order-of-magnitude-higher ${\mathrm{Er}}^{3+}$ PL intensity is measured with respect to the unpatterned structures. This has a direct impact on the design of more efficient CMOS-compatible light sources operating at telecom wavelengths.

There are diverse interdisciplinary applications for nanoscale-resolution electrometry of elementary charges under ambient conditions. These include characterization of two-dimensional electronics, charge transfer in biological systems, and measurement of fundamental physical phenomena. The nitrogen-vacancy center in diamond is uniquely capable of such measurements, however electrometry thus far has been limited to charges within the same diamond lattice. It has been hypothesized that the failure to detect charges external to diamond is due to quenching and surface screening, but no proof, model, or design to overcome this has yet been proposed. In this work we affirm this hypothesis through a comprehensive theoretical model of screening and quenching within a diamond electrometer and propose a solution using controlled nitrogen doping and a fluorine-terminated surface. We conclude that successful implementation requires further work to engineer diamond surfaces with lower surface-defect concentrations.

Second-order topological insulators, which exhibit capability of hosting topologically protected zero-dimensional corner states distinct from the well-studied topological edge states, unveil a horizon beyond the conventional bulk-edge correspondence. Motivated by recent experimental observation of Wannier-type second-order corner states in acoustic structures, we investigate numerically and demonstrate experimentally the in-plane edge and corner states in a mechanical kagome lattice. By manipulating simply lattice geometry and quantized characterization, we exploit that the emerging corner states are topologically robust against disorders. We further present a second-order topological insulator with multi-interfaces such that the mechanical energy can be localized in multiple locations, which provides the possibility of practical application in energy harvesting devices. The present study is the physical observation to extend the second-order topological insulator to in-plane elastic dynamics, and modal coupling of in-plane elasticity makes it more challenging to be measured experimentally.

Theoretical investigations of carrier transport in type-II superlattice detectors have been mostly limited to simplified semiclassical treatments, due to the computational challenges posed by quantum kinetic approaches. For example, interband tunneling in broken-gap configurations calls for a multiband description of the electronic structure, and spatially indirect optical transitions in superlattice absorbers require fully nonlocal carrier-photon self-energies. Moreover, a large number of iterations is needed to achieve self-consistency between Green’s functions and self-energies in the presence of strongly localized states not directly accessible from the contacts. We demonstrate an accurate, yet computationally feasible nonequilibrium Green’s function model of superlattice detectors by formulating the kinetic equations in terms of problem-matched maximally localized basis functions, numerically generated from few modes representing the main conductive channels of the nanostructure. The contribution of all the remaining modes is folded in an additional self-energy to ensure current conservation. Inspection of spatially and energetically resolved single particle properties offers insight into the complex nature of carrier transport in type-II superlattice detectors, and the connection to semiclassical approaches enables the interpretation of mobility experiments.

The Airy beam, known to present particular unique features, such as diffraction-free, self-healing, and self-bending, has attracted considerable interest in recent years. Here, an electronically controlled reflective coding metasurface is exploited to generate and dynamically manipulate the Airy beam over a wide frequency band in the microwave regime. By judiciously controlling the external dc bias voltage applied to the programmable metasurface, two distinct control states, “0” and “1”, are engineered as digital codes for −π/2 and +π/2 phase responses, respectively. The programmable binary-phase-coding metasurface enables the generation and active reshaping of the Airy beam to be numerically simulated and experimentally validated. Moreover, the metasurface allows the bandwidth limitation of passive counterparts to be overcome and is able to modulate electromagnetic waves over a broad frequency range, spanning from 9 to 12 GHz, to realize diffraction-free Airy beams.

The negative-capacitance effect in devices based on combined ferroelectric-dielectric gate oxides is thought to be a potential solution to break free from the so-called Boltzmann tyranny. To lower the power consumption in field-effect transistors, the subthreshold swing factor $S$ should be reduced below the thermodynamic limit of $60\mathrm{mV}$ per decade. Yet, despite numerous studies dedicated to this effect in the past decade, its origin in ferroelectric capacitors or ferroelectric-based superlattices remains unclear, being considered either a transitory product of polarization switching or an intrinsic phenomenon related to the presence of ferroelectric polarization. In this study it is shown, starting from simple electrostatic considerations, that negative capacitance is present during polarization switching and is accompanied by a significant increase of the current flowing through the ferroelectric capacitor. Coupled with piezo-force microscopy results, it is shown that the polarization orientation suddenly changes at the coercive voltage, accompanied by a complete reconfiguration of the potential barriers at the Schottky-like contacts present at the electrode-ferroelectric interfaces. A method to estimate the polarization-switching time, as the time associated with the presence of the negative-capacitance effect, is proposed. Values in the range from 100 to 1000 ns are obtained for epitaxial $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ films. These findings suggest that negative capacitance may be an intrinsic effect in ferroelectrics but that it is a transitory effect, present only when ferroelectric polarization passes through zero (switching).

We show how an exceptional point of degeneracy (EPD) is formed in a system composed of an electron beam interacting with an electromagnetic mode guided in a slow wave structure (SWS) with distributed power extraction from the interaction zone. Based on this kind of EPD, a regime of operation is devised for backward-wave oscillators (BWOs) as a synchronous and degenerate regime between a backward electromagnetic mode and the charge wave modulating the electron beam. Degenerate synchronization under this EPD condition means that two complex modes of the interactive system do not share just the wave number, but they rather coalesce in both their wave numbers and eigenvectors (polarization states). In principle, this condition guarantees full synchronization between the electromagnetic wave and the beam’s charge wave for any amount of output power extracted from the beam, setting the threshold of this EPD-BWO to any arbitrary, desired, value. Indeed, we show that the presence of distributed radiation in the SWS results in having high-threshold electron-beam current to start oscillations, which implies higher power generation. These findings have the potential to lead to highly efficient BWOs with very high output power and excellent spectral purity.

An understanding of the damping mechanism in finite-size systems and its dependence on temperature is a critical step in the development of magnetic nanotechnologies. In this work, nanosized materials are modeled via atomistic spin dynamics, the damping parameter being extracted from ferromagnetic resonance (FMR) simulations applied for FePt systems, generally used for heat-assisted magnetic recording media (HAMR). We find that the damping increases rapidly close to $T_C$ and the effect is enhanced with decreasing system size, which is ascribed to scattering at the grain boundaries. Additionally, FMR methods provide the temperature dependence of both damping and the anisotropy, which are important for the development of HAMR. Semianalytical calculations show that, in the presence of a grain-size distribution, the FMR line width can decrease close to the Curie temperature due to a loss of inhomogeneous line broadening. Although FePt has been used in this study, the results presented in the current work are general and valid for any ferromagnetic material.

In this work, we propose the use of power-flow–conformal metasurfaces for the creation of dual-physics devices that can simultaneously control waves of different natures. In particular, we introduce metasurface devices that perform specified operations on both electromagnetic and acoustic waves at the same time. Using a purely analytical model based on surface impedances, we introduce metasurfaces that perform the same functionality for electromagnetic and acoustic waves and, even more challenging, different functionalities for electromagnetics and acoustics. We provide realistic topologies for practical implementations of the proposed metasurfaces and confirm the results with numerical simulations.

Current paradigm suggests that the Ohmic electrical response of nanochannel-microchannel systems is determined solely by the nanochannel while the effects of the adjacent microchannels are negligible. However, recent works have challenged this paradigm and have shown that at low concentrations the microchannels contribute in a non-negligible manner. As such, the two favored models used to explain experiments are inadequate in describing realistic nanochannel-microchannel systems. To partially reconcile some of these issues, two newer nanochannel-microchannel models were derived and suggested as a suitable replacement for the nanochannel-dominant models. Unfortunately, these two models are limited to either very low or very high concentrations. In this work, we review these four leading models. We discuss their key assumptions, advantages, shortcomings, and present a knowledge gap between all models pertaining to the effects of the microchannel resistance for all concentrations. To overcome this gap, we derive an analytical solution that accounts for the effects of the microchannels and holds for all concentrations. This solution unifies three of the existing models where we show that they are limiting cases of our more general solution. We are also able to disqualify the fourth model. Our derived solution shows remarkable correspondence to simulations and experiments. The insights from this unifying model can be used to improve the design of any nanofluidic based systems.

A promising method to leverage near-field power densities without the use of nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When placed between a hot and cold reservoir, an ideal HMM can transmit surface waves across several microns, enabling an extension of near-field enhancements. However, when accounting for transmission loss due to realistic levels of absorption within the waveguide, previous studies have shown that the enhancements are significantly curtailed at wide separations. In our study, we investigate the role of internal sources within nonisothermal HMMs. We demonstrate that, in some cases, the emission from the HMM accounts for over 90% of the total radiative heat transfer to the receiver, and that these additional sources can largely compensate for optical losses associated with decreased transmission from the emitter to the receiver. Lastly, we investigate the spectral transport in a realistic three-body system that has mismatching optical properties between the boundaries (emitter and receiver) and the waveguide (HMM). Our model shows that the near-field thermal transport remains spectrally selective to the boundaries, even as major radiative contributions come from the waveguide. This work may enable the design of nonisothermal emitter-waveguide-receiver systems that transmit near-field power levels over wider separations.

Layer-dependent electronic and structural properties of emerging graphitic carbon-boron compound ${\mathrm{C}}_3\mathrm{B}$ are investigated using both density-functional theory and the GW approximation. We discover that, in contrast to a moderate quasiparticle band gap of 2.55 eV for monolayer ${\mathrm{C}}_3\mathrm{B}$, the calculated quasiparticle band gap of perfectly stacked bulk phase ${\mathrm{C}}_3\mathrm{B}$ is as small as 0.17 eV. Therefore, our results suggest that layered material ${\mathrm{C}}_3\mathrm{B}$ exhibits a remarkably large band-gap renormalization of over 2.3 eV due to the interlayer coupling and screening effects, providing a single material with an extraordinary band-gap tunability. The quasiparticle band gap of monolayer ${\mathrm{C}}_3\mathrm{B}$ is also over 1.0 eV larger than that of ${\mathrm{C}}_3\mathrm{N}$, a closely related two-dimensional semiconductor. Detailed inspections of the near-edge electronic states reveal that the conduction- and valence-band edges of ${\mathrm{C}}_3\mathrm{B}$ are formed by out-of-plane and in-plane electronic states, respectively, suggesting an interesting possibility of tuning the band edges of such layered material separately by modulating the in-plane and out-of-plane interactions.

Surface-acoustic-wave (SAW) aerosol generators are very promising systems for applications requiring small aerosol sources and adjustable droplet-size distributions at low aerosol flowrates. However, the droplet-generation mechanism by SAW is yet unclear and dedicated experiments are necessary to gain further insights. In this work, the atomization zone is investigated experimentally to gain more insight into the droplet-generation mechanism by SAW. Three regions are observed in the atomization zone showing different acoustofluidic effects, including the formation of liquid films and patterns in a standing SAW (SSAW) wavefield and droplet generation. In addition, the influence of the humidity of surrounding air on the measured droplet-size distribution of the aerosols generated by SSAW is investigated. Therefore, the air humidity is adjusted at constant air temperature in the sophisticated wind-tunnel Turbulent Leipzig Aerosol Cloud Interaction Simulator combined with an optical particle spectrometer with resolution down to the sub-µm scale. Besides the humidity influence on the droplet size in the measurement region, water-saturated air allowed us to estimate the initial droplet-size distribution at the atomization zone on the chip surface. This parameter is crucial for the determination of the underlying liquid-film disintegration mechanism, i.e., the droplets’ origin. Hereby, a bimodal log-normal droplet-size distribution is obtained. The mean diameter of the main droplet fraction decreases from 7.2 to 5.3 µm with increasing relative humidity from 8.6 to 97%, apparently due to evaporation of smaller droplets in unsaturated air. A second peak corresponds to a mean diameter as small as 600 nm at high humidity conditions, which is likely to correspond to a droplet-generation mechanism not reported so far and extremely difficult to measure or visualize with conventional techniques. The droplet-size measurement results from the aerosol spectrometer are compared and validated in respect to existing droplet-generation models with the results of a phase-Doppler anemometry, a high-resolution, nonintrusive local aerosol-measurement technique.

Here, we demonstrate a substantially increased near-field radiative heat transfer between asymmetric structures (i.e., doped $\mathrm{Si}$ and ${\mathrm{Si}\mathrm{O}}_2$) by using a thin $\mathrm{Ti}$ film as a plasmonic coupler. The measured near-field enhancement at vacuum gap of 380 nm is found to be 3.5 times greater than that for the case without the coupler. The enhancement mechanism is thoroughly elucidated for both polarizations, and a dimensionless parameter, which can quantify the coupling strength of the surface polaritons across the vacuum interfaces, is proposed.

The discovery of ferroelectric polarization in ${\mathrm{Hf}\mathrm{O}}_2$-based ultrathin films has spawned much interest due to their potential applications in data storage. In 2018, an R3m rhombohedral phase was proposed to be responsible for the emergence of ferroelectricity in the [111]-oriented ${\mathrm{Hf}}_{0.5}{\mathrm{Zr}}_{0.5}{\mathrm{O}}_2$ thin films, but the fundamental mechanism of ferroelectric polarization in such films has remained poorly understood. In this paper, we employ density-functional-theory calculations to investigate structural and polarization properties of the R3m ${\mathrm{Hf}\mathrm{O}}_2$ phase. We find that the film thickness and in-plane compressive-strain effects play a key role in stabilizing the R3m phase, leading to robust ferroelectricity of [111]-oriented R3m ${\mathrm{Hf}\mathrm{O}}_2$.