The lack of coherent room-temperature sources in the whole terahertz spectral window (0.3–10 THz) has significantly hampered the growth of scientific and technological applications in this range. Among them, high-precision frequency measurements of molecular transitions play a central role but remain an open challenge. Here, room-temperature generation and detection of continuous-wave, broadly tunable, narrow-linewidth THz radiation are presented, and their application to high-resolution spectroscopy in the broad 1–7.5 THz spectral range is demonstrated. This result has been achieved by implementing a Cherenkov phase-matching scheme into a channel waveguide in a nonlinear crystal. This simple approach, entirely based on robust telecom technology, unprecedently merges in a single source an ultra-broad continuous-wave spectral coverage and a state-of-the-art accuracy (approximately ${10}^{-9}$) in molecular-transition-center determination.

Molecular-dynamics simulations are used to compute the threshold displacement energy ($E_d$) in a series of progressively damaged graphite structures. The $E_d$ values are obtained by a statistically robust probabilistic method using a large number of primary-knock-on-atom events at energies up to 100 eV. No sharp threshold for $E_d$ is observed, and a number of possible definitions are considered. For pristine graphite, the best estimate of $E_d$ is 24 eV. $E_d$ decreases with increasing irradiation damage, dropping by nearly a factor of 2 at a dose of one displacement per atom. For a fully disordered amorphous-carbon structure, $E_d$ is around 5 eV. This evolution of $E_d$ is an important missing ingredient in current estimates of radiation doses in nuclear reactors, which assume $E_d$ is constant over the reactor lifetime, despite substantial structural evolution.

We design two-dimensional mechanical metamaterials that may be deformed substantially at little or no energy cost. Examples of such deformable structures include assemblies of rigid isosceles triangles hinged at their corners on the macroscale and polymerized phenanthrene molecules forming porous graphene on the nanoscale. In these and in a large class of related structures, the Poisson ratio $\nu$ diverges for particular strain values. It also changes its magnitude and sign, and displays a shape-memory effect.

Active polarization rotators are desirable for many practical applications such as liquid crystal displays, fiber communication, etc. Ultrathin metasurfaces incorporating phase-change or magneto-optical materials provide active tuning of polarization, but suffer from a long response time or the requirement for a strong magnetic field. In this study, a tunable plasmonic polarization rotating system is proposed, which consists of two coupled metal screens perforated with subwavelength apertures. The polarization rotating mechanism can be attributed to the near-field coupling of metal screens and its dependence on the screen’s lateral displacement. The theory and experiment in the microwave band demonstrate that, by using the x polarized incident wave and displacing one metal screen at the millimeter scale, a continuous and efficient polarization rotation of the outgoing wave (with a nearly linear polarization state) from −90° to 90° can be achieved. This arbitrarily directional and tunable polarization rotating effect may be extended to the THz regime as well and find potential applications in constructing versatile polarization rotators and signal modulators.

We propose nonreciprocal phonon lasing in a coupled cavity system composed of an optomechanical resonator and a spinning resonator. We show that the optical Sagnac effect leads to significant modifications in both the mechanical gain and the power threshold for phonon lasing. More importantly, the phonon lasing in this system is unidirectional; that is, the phonon lasing occurs when the coupled system is driven in one direction but not the other. Our work establishes the potential of spinning optomechanical devices for low-power mechanical isolation and unidirectional amplification. This provides a new route, well within the reach of current experimental abilities, to operate cavity optomechanical devices for a wide range of applications such as directional phonon switches, invisible sound sensing, and topological or chiral acoustics.

Acoustic illusion devices are usually designed using transformation optics. An alternate method is proposed to achieve acoustic illusions without external devices by elaborately manipulating the acoustic scattering potential of an object. The proposed method is more of a “cosmetic operation” for an object, which modifies the scattered acoustic pressure distribution of the object to mimic another object by exchanging their scattering potentials in two symmetrical areas in the wave vector domain. The advantage of this method is the simplicity of material parameters: only positive isotropic mass density and bulk modulus are required, which is impossible with the conventional method of using transformation optics due to the complex material requirements (anisotropic and negative index parameters).

Finding the shortest path in a graph has applications in a wide range of optimization problems. However, algorithmic methods scale with the size of the graph in terms of time and energy. We propose a method to solve the shortest-path problem using circuits of nanodevices called memristors and validate it on graphs of different sizes and topologies. It is both valid for an experimentally derived memristor model and robust to device variability. The time and energy of the computation scale with the length of the shortest path rather than with the size of the graph, making this method particularly attractive for solving large graphs with small path lengths.

Bragg-edge strain imaging from energy-resolved neutron-transmission measurements poses an interesting tomography problem. The solution to this problem will allow the reconstruction of detailed triaxial stress and strain distributions within polycrystalline solids from sets of Bragg-edge strain images. Work over the last decade has provided some solutions for a limited number of special cases. In this paper we provide a general approach to reconstruction of an arbitrary system based on a least-squares process constrained by equilibrium. This approach is developed in two dimensions before it is demonstrated experimentally on two samples with use of the RADEN instrument at the Japan Proton Accelerator Research Complex spallation neutron source. Validation of the resulting reconstructions is provided through a comparison with conventional constant-wavelength strain measurements performed with the KOWARI engineering diffractometer of the Australian Nuclear Science and Technology Organisation. The paper concludes with a discussion on the range of problems to be addressed in a three-dimensional implementation.

Alloys with adjustable mechanical performance are of fundamental interest in material designs. Here, we investigate the magnetic- and chemical-ordering behavior of the ferromagnetic $\mathrm{Fe}\text{−}\mathrm{Cr}\text{−}\mathrm{Co}\text{−}\mathrm{Ni}\text{−}{\mathrm{Al}}_x$ $(1\le x\le 2.5)$ high-entropy alloys with the help of first-principle alloy theory. The lattice constants and the single- and polycrystalline elastic parameters for partially ordered and random structures are considered. In contrast to the trend found for the completely disordered phase, we demonstrate that ordering driven primarily by $\mathrm{Al}$ results in an enhanced Young’s modulus, especially at high-$\mathrm{Al}$ concentrations, which is in line with the observed increase of the hardness for systems with a body-centered-cubic underlying lattice. The results suggest that outstanding strength and ductility can be realized by proper control of the ordering level in single- and multiphase high-entropy alloys.

The afterpulse effect is an intrinsic characteristic of the single-photon avalanche photodiode that has been widely used in quantum key distribution (QKD). As QKD systems move into the gigahertz regime, the afterpulse effect is no longer ignorable, which will lead to a great deviation compared with the existing analytical model. Here we develop an analytical model to make QKD systems more afterpulse compatible. In addition, we obtain the secure key rate for our model with the analysis of statistical fluctuation using Hoeffding’s inequality and Azuma’s inequality. Our results show that the optimized parameters of the afterpulse-compatible model can provide a much higher key rate than the optimized parameters of the previous afterpulse-omitted model in the same situation.

We study the dynamics of a magnetic vortex in a permalloy disk, as it is switched between bistable pinning sites in the presence of room-temperature thermal noise. We use static and time-resolved magneto-optical Kerr-effect microscopy to characterize the equilibrium and nonequilibrium motion of the vortex in response to a train of magnetic field pulses. The limitations of this approach due to inhomogeneous averaging are overcome by using a proximal nitrogen-vacancy spin in diamond to probe the final position of the vortex core following a single magnetic field pulse. Repeating this single-shot measurement, we characterize the probability of switching between two bistable pinning sites as a function of pulse time. We develop a simple model of the stochastic vortex trajectory through the pinning sites’ basins of attraction to understand how the separate contributions of damping and dephasing result in the observed switching behavior.

We invoke the Huygens principle to derive dispersion characteristics of acoustic transformational metasurfaces that can deflect parallel wavefronts in a desired direction. We also propose a dual-Lorentz resonator whose aperture fields can be tuned by geometrical changes to implement a particular phase with unity reflection coefficient. The proposed metascreen is designed by our arranging slightly detuned Lorentz cavities that generate the necessary interference to compensate for the incident wavefronts. Since a complete 0–$2\pi$ range of the reflection phase is achieved, the metascreen can steer a beam across the full horizon. Moreover, since the proposed dual resonator has subwavelength dimensions, the metascreen is capable of high-resolution imaging, which is an essential trait in cloaking and deceptive acoustics. Successful demonstration of acoustic beam steering and carpet cloaking is done for a wavelength of 790 mm with the help of full-wave simulations.

Two-Dimensional Transition Metal Dichalcogenides (2D TMDCs) such as ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{Mo}\mathrm{Se}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{W}\mathrm{Se}}_2$ with van der Waal's type interlayer coupling are being widely explored as channel materials in a Schottky Barrier Field Effect Transistor (SB FET) configuration. While their excellent electrostatic control and high on/off ratios have been identified, a clear correlation between electronic transport and the low-frequency noise with different atomic-layer thickness is missing. For multilayer channels in ${\mathrm{Mo}\mathrm{S}}_2$ FETs, the effects of interlayer-coupling resistance on device conductance and mobility have been studied, but no systematic study has included interlayer effects in consideration of the intrinsic (channel) and extrinsic (total device) noise behavior. Here, we report the 1/f noise properties in ${\mathrm{Mo}\mathrm{Se}}_2$ FETs with varying channel thicknesses (3–40 atomic layers). Contributions of channel vs access/contact regions are extracted from current-voltage (transport) and 1/f noise measurements. The measured noise amplitude shows a direct crossover from channel- to contact-dominated noise as the gate voltage is increased. The results can be interpreted in terms of a Hooge relationship associated with the channel noise, a transition region, and a saturated high-gate-voltage regime whose characteristics are determined by a voltage-independent conductance and noise source associated with the metallurgical contact and the interlayer resistance. Both the channel Hooge coefficient and the channel/access noise amplitude decrease with increasing channel thickness over the range of 3–15 atomic layers, with the former remaining approximately constant and the latter increasing over a range of 20–40 atomic layers. The analysis can be extended to devices based on other TMDCs.

We experimentally demonstrate long-distance continuous-variable quantum key distribution over a 50-km standard optical fiber based on continuous-variable Einstein-Podolsky-Rosen entangled states. The entanglement survives despite being distributed over a high-loss optical fiber channel. At a channel excess noise level of 0.01 shot-noise units, we achieve an asymptotic secret key rate of 0.03 bit per sample, which is superior to the optimized coherent-state protocol. The superiority is even more evident at a high-channel excess noise level of 0.1 shot-noise units. Our work paves the way toward practical applications of continuous-variable quantum key distribution under high amounts of excess channel noise.

Dominant recombination paths in $\mathrm{Al}\mathrm{N}$ and ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$-related structures are investigated using cathodoluminescence (CL) mapping measurements and photoluminescence (PL) spectroscopy. The dark spot contrasts originating from nonradiative recombination at threading dislocations (TDs), which are observed in CL intensity maps, drastically decrease upon elevating the temperature. This is because carriers can reach TDs at low temperatures (9–60 K), but are captured by point defects (PDs) even in the vicinity of TDs near RT. Calculations based on the experimental results indicate that in the current $\mathrm{Al}\mathrm{N}$ and $\mathrm{Al}$-rich ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ crystals, TDs scarcely affect the internal quantum efficiency (IQE) at RT as long as the TD density is less than $2.6\times {10}^{10}{\mathrm{cm}}^{-2}$. Because a TD density less than $2.6\times {10}^{10}{\mathrm{cm}}^{-2}$ has already been achieved even for heteroepitaxially grown $\mathrm{Al}\mathrm{N}$ films on sapphire substrates, it is evident that the most effective method to further improve the IQE of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$-related materials is to reduce PDs not TDs. Moreover, we clarify the existence of two types of PD states, which mainly degrade the emission efficiency, using temperature-dependent PL measurements. Combining the CL and PL results allows the activation energies of these PDs and TDs to be evaluated. Furthermore, we highlight the probability that PDs, which predominantly act as nonradiative recombination centers at room temperature, are complexes formed by $\mathrm{Al}$ vacancies and oxygen impurities that enhance the deep-level emissions at 3.2 and 3.5–3.7 eV near room temperature. Such a large impact of PDs on the efficiency degradation may be attributed to the high density of $\mathrm{Al}$-vacancy–related PDs in $\mathrm{Al}\mathrm{N}$ and $\mathrm{Al}$-rich ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ compared with that of $\mathrm{Ga}$-vacancy–related PDs in $\mathrm{Ga}\mathrm{N}$ due to the small formation energy.

In conventional materials, Cherenkov radiation (CR) due to a moving charged particle is associated with a broad frequency range and velocity threshold. Here, we show that using a periodic grating structure, which possesses photonic bound states in the continuum (BICs) and quasi-BICs, unusual CR can be generated in a very narrow frequency band at a particle velocity below the common threshold. This effect arises from an interesting light-amplification process in the BIC structure, which can dramatically improve the efficiencies of evanescent-to-propagating wave conversion and CR generation. Our results present a mechanism for realizing Cherenkov lasing at low electron velocity and may find applications in compact free-electron lasers.

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.