Lattice-Boltzmann numerical simulations are conducted to explore the pore-scale flow behavior inside modeled porous media over the Darcy regime. We use static (fixed) and dynamic (rotating) particles to form the porous media. The pore flow behavior (tortuosity) is found to be constant in the static medium within the Darcy range. However, the study reveals distinctively different flow structures in the dynamic case depending on the macroscopic Darcy flow rate and the level of internal energy imposed to the system (via the angular velocity of particles). With small Darcy flow rates, tortuous flow develops with vortices occupying a large portion of the pore space but contributing little to the net flow. The formation of the vortices is linked to spatial fluctuations of local pore fluid pressure. As the Darcy flow rate (and, hence, the global fluid pressure gradient across the medium) increases, the effect of local pressure fluctuations diminishes, and the flow becomes more channelized. Despite the large variations of the pore-scale flow characteristics in the dynamic porous media, the macroscopic flow satisfies Darcy’s law with an invariant permeability. The applicability of Darcy’s law is proven for an internally disturbed flow through porous media. The results raise questions concerning the generality of the models describing the Darcy flow as being channelized with constant (structure-dependent) tortuosity and how the internal sources of energy imposed to the porous media flow are considered.

In this paper, the luminescence properties of self-trapped excitons (STEs) of undoped and ${\mathrm{Eu}}^{2+}$-doped perovskite-type materials $\mathrm{CsMg}{X}_3$ ($X=\mathrm{Cl}$, Br, I) are presented. The three compounds crystallize isostructurally in a hexagonal crystal system that exhibits an intrinsic pseudo-one-dimensionality. This feature has a highly stabilizing effect on the localization of excitons. The similarities to the properties of STEs in alkali halides are drawn that are justified by the band-structure and density-of-states calculations. The luminescence spectra of all three halides are characterized and interpreted despite their high complexity with many emissive transitions. It is illustrated that both STEs and impurity-localized self-trapped excitons (IL STEs) are responsible for the features in the spectra. The impurity localization of the STEs is proven by doping the hosts with ${\mathrm{Sr}}^{2+}$ ions instead of ${\mathrm{Eu}}^{2+}$ ions. The decay times in the microsecond range indicate that emission predominantly occurs from a triplet state of the STEs with a prominent afterglow component for the IL STEs that ideally suits a trapping model along the one-dimensional chains of the halides. Moreover, by thermal activation, the excitons tend to annihilate at the ${\mathrm{Eu}}^{2+}$ traps, thereby inducing an energy transfer to the ${\mathrm{Eu}}^{2+}$ ions. Because of this action, an extreme increase of the intensity of the ${\mathrm{Eu}}^{2+}$-based $4f^{6}5d\text{−}4f^7$ emission at room temperature is observed, which might be a general explanation for unusual temperature-dependent emission intensities of ${\mathrm{Eu}}^{2+}$ ions. In general, an understanding of the basic optical properties of the STEs may give some insights into the mechanism of currently used x-ray storage phosphors as well as scintillators.

We demonstrate a scheme for amplifying nanomechanical motion based on strong parametric interactions between two independent flexural modes of a single nanobeam. A static electric field polarizes a dielectric nanobeam and additional radio-frequency voltage excites the motion of the beam. When the excitation frequency equals the difference of the two resonance frequencies, we observe a mode-splitting feature in the strong-coupling regime. If the excitation frequency overlaps the sum of the two resonance frequencies, the optical signal from the thermal motion of the nanomechanical resonator is amplified by more than 30 dB. We demonstrate that coupling between the odd- and even-numbered oscillatory mode can be realized and a coupled-mode theory is developed from a simple capacitor model to explain our observations. We conclude that the results are thoroughly explained by parametric interactions between two independent nanomechanical modes in thermal motion. This observation of parametric control can be employed in nanomechanical force detection and molecular sensing applications.

Most quantum error-correcting codes assume that the decoherence of each physical qubit is independent of the decoherence of any other physical qubit. We can test the validity of this assumption in an experimental setup where a microwave feedline couples to multiple qubits by examining correlations between the qubits. We investigate the correlations between fluxonium qubits located in a single waveguide. Despite being in a wide-bandwidth electromagnetic environment, the qubits have measured relaxation times in excess of $100\mu \mathrm{s}$. We use cascaded Josephson parametric amplifiers to measure the quantum jumps of two fluxonium qubits simultaneously. No correlations are observed between the relaxation times of the two fluxonium qubits, which indicates that the sources of relaxation are local to each qubit in our setup. Our correlation analysis can be generalized to different types of qubits and our architecture can easily be scaled to monitor larger numbers of qubits.

Many superconducting technologies such as rapid single-flux quantum computing and superconducting quantum-interference devices rely on the modulation of nonlinear dynamics in Josephson junctions for functionality. More recently, however, superconducting devices have been developed based on the switching and thermal heating of nanowires for use in fields such as single-photon detection and digital logic. In this paper, we use resistive shunting to control the nonlinear heating of a superconducting nanowire and compare the resulting dynamics to those observed in Josephson junctions. We show that interaction of the hotspot-impedance with an external shunt produces high-frequency relaxation oscillations with similar behavior to that observed in Josephson junctions due to their ability to be modulated by a weak periodic signal. In particular, we use a microwave drive to pull and mix the oscillation frequency, resulting in phase-locked features that resemble the Shapiro steps observed in the ac Josephson effect. Microwave nanowire devices based on these conclusions have promising applications in fields such as parametric amplification and frequency mixing.

A rigorous analytical approach is proposed based on the Bergman spectral representation and the Milton spectral representation of the effective permittivity for a composite system in order to design the anisotropic broadband $ϵ$-near-zero metamaterials. According to the Bergman and Milton spectral representations, the inverse problem for the anisotropic broadband $ϵ$-near-zero metamaterials is analytically established and solved by introducing the exactly proved identities. Meanwhile, the introduced identities theoretically reveal the identity of the Bergman and Milton spectral representations in mathematics and physics. With the precise solutions to the inverse problem, the conditions for designing the anisotropic broadband $ϵ$-near-zero metamaterials are clarified and demonstrated by the specific anisotropic broadband $ϵ$-near-zero meta-atoms of the lamination and the Hashin-Shtrikman coated-sphere microstructures through a theoretical analysis and a computational full-wave simulation.

Single self-assembled $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots are a promising solid-state quantum technology, with which vacuum Rabi splitting, single-photon-level nonlinearities, and bright, pure, and indistinguishable single-photon generation have been demonstrated. For such achievements, nanofabrication is used to create structures in which the quantum dot preferentially interacts with strongly confined optical modes. An open question is the extent to which such nanofabrication may also have an adverse influence, through the creation of traps and surface states that could induce blinking, spectral diffusion, and dephasing. Here, we use photoluminescence imaging to locate the positions of single $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots with respect to alignment marks with $<5\mathrm{nm}$ uncertainty, allowing us to measure their behavior before and after fabrication. We track the quantum-dot emission linewidth and photon statistics as a function of the distance from an etched surface and find that the linewidth is significantly broadened (up to several gigahertz) for etched surfaces within a couple hundred nanometers of the quantum dot. However, we do not observe an appreciable reduction of the quantum-dot radiative efficiency due to blinking. We also show that atomic-layer deposition can stabilize spectral diffusion of the quantum-dot emission and partially recover its linewidth.

The topologically protected magnetic spin configurations known as Skyrmions offer promising applications due to their stability, mobility, and localization. We emphasize how to leverage the thermally driven dynamics of an ensemble of such particles to perform computing tasks. We propose a device employing a Skyrmion gas to reshuffle a random signal into an uncorrelated copy of itself. This device is demonstrated by modeling the ensemble dynamics in a collective coordinate approach where Skyrmion-Skyrmion and Skyrmion-boundary interactions are accounted for phenomenologically. Our numerical results are used to develop a proof of concept for an energy efficient (approximately in the range of microwatts) device with a low area imprint (roughly on the order of $\mu {\mathrm{m}}^2$). Whereas its immediate application to stochastic computing circuit designs will be made apparent, we argue that its basic functionality, reminiscent of an integrate-and-fire neuron, qualifies it as an alternative bioinspired building block.

The problem of noise mitigation has been extensively studied in the conventional real domain of representation of waves, e.g., for signals in the time domain or spatial images. However, in many practical cases, the desired information is acquired in the Fourier domain representation of the relevant signal, e.g., the frequency domain for temporal waveforms, and the treatment and control of random noise in this domain remains a significant challenge. In this work, we propose a concept for noiseless amplification of the Fourier spectrum of a given periodic waveform, here demonstrated for the relevant case of a frequency comb. The concept involves a coherent redistribution of the original comb energy into fewer frequency lines, without altering the uncorrelated noise background. This translates into an effective amplification of the comb peak intensity over the noise floor in a completely passive fashion, thus avoiding the external noise contributions that are intrinsic to conventional active-gain processes. This result is achieved through suitable linear phase manipulations of the comb signal, in a process that can be regarded as a generalization of the spectral Talbot effect. We experimentally demonstrate recovery and measurement of frequency combs originally buried completely under random uncorrelated noise, without any prior knowledge of the absolute frequencies of the comb lines.

The generation and the manipulation of current-induced spin-orbit torques are of essential interest in spintronics. However, in spite of the vital progress in spin orbitronics, electric control of the spin-torque generation still remains elusive and challenging. We report on electric control of the spin-torque generation using ionic-liquid gating of ultrathin Au. We show that by simply depositing a ${\mathrm{SiO}}_2$ capping layer on an ultrathin-$\mathrm{Au}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ bilayer, the spin-torque generation efficiency is drastically enhanced by a maximum of 7 times. This enhancement is verified to be originated from the rough ultrathin-$\mathrm{Au}/{\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ interface induced by the ${\mathrm{SiO}}_2$ deposition, which results in the enhancement of the interface spin-orbit scattering. We further show that the spin-torque generation efficiency from the ultrathin Au film can be reversibly manipulated by a factor of 2 using the ionic gating with an external electric field within a small range of 1 V. These results pave a way towards the efficient control of the spin-torque generation in spintronic applications.

Radiative dipolar resonance with Lorentzian line-shape induces the otherwise dark quadrupolar resonances resulting in electromagnetically induced transparency (EIT). The two interfering excitation pathways of the dipole are earlier shown to result in a Fano line shape with a high figure of merit suitable for sensing. In metamaterials made of metal nanorods or antennas, the plasmonic EIT (PIT) efficiency depends on the overlap of the dark and bright mode spectra as well as the asymmetry resulting from the separation between the monomer (dipole) and dimer (quadrupole) that governs the coupling strength. Increasing asymmetry in these structures leads to the reduction of the figure of merit due to a broadening of the Fano resonance. We demonstrate a PIT system in which the simultaneous excitation of two dipoles result in double PIT. The corresponding two quadrupoles interact and control the quality factor ($Q$) of the PIT resonance. We show an antiresonancelike symmetric line shape with nonzero asymmetry factors. The PIT resonance vanishes due to quadrupole-quadrupole coupling. A $Q$ factor of more than 100 at 0.977 THz is observed, which is limited by the experimental resolution of 6 GHz. From polarization-dependent studies we show that the broadening of the Lorentzian resonance is due to scattering-induced excitation of orthogonally oriented dipoles in the monomer and dimer bars in the terahertz regime. The high $Q$ factors in the terahertz frequency region demonstrated here are interesting for sensing application.

Surface acoustic waves (SAWs) propagating in a piezoelectric substrate covered with a thin ferromagnetic–heavy-metal bilayer are found to exhibit a substantial degree of nonreciprocity, i.e., the frequencies of these waves are nondegenerate with respect to the inversion of the SAW propagation direction. The simultaneous action of the magnetoelastic interaction in the ferromagnetic layer and the interfacial Dzyaloshinskii-Moriya interaction in the ferromagnetic–heavy-metal interface results in the openings of magnetoelastic band gaps in the SAW spectrum, and the frequency position of these band gaps is different for opposite SAW propagation directions. The band-gap widths and the frequency separation between them can be controlled by a proper selection of the magnetization angle and the thickness of the ferromagnetic layer. Using numerical simulations, we demonstrate that the isolation between SAWs propagating in opposite directions in such a system can exceed the direct SAW propagation losses by more than 1 order of magnitude.

Soft mechanical metamaterials working on the basis of instability have numerous potential applications in the context of “machine materials.” Controlling the onset of instability is usually required when rationally designing such metamaterials. We study the isolated and modulated effects of geometrical design and material distribution on the onset of instability in multimaterial cellular metamaterials. We use multimaterial additive manufacturing to fabricate cellular specimens whose unit cells are divided into void space, a square element, and an intermediate ligament. The ratio of the elastic modulus of the ligament to that of the square element $[(E_L)/(E_S)]$ is changed by using different material types. Computational models are also developed, validated against experimental observations, and used to study a wide range of possible designs. The critical stress can be adjusted independently from the critical strain by changing the material type while keeping $[(E_L)/(E_S)]$ constant. The critical strain shows a power-law relationship with $[(E_L)/(E_S)]$ within the range $[(E_L)/(E_S)]=0.1–10$. The void shape design alters the critical strain by up to threefold, while the combined effects of the void shape and material distribution cause up to a ninefold change in the critical strain. Our findings highlight the strong influence of material distribution on the onset of the instability and buckling mode.

This article presents a class of space-time-varying media with giant linear nonreciprocity, zero space-time local reflections, and zero photonic band gap. This is achieved via equilibrium in the electric and magnetic properties of unidirectionally space-time-modulated media. The enhanced nonreciprocity is accompanied by a larger sonic regime interval which provides extra design freedom for achieving strong nonreciprocity by a weak pumping strength. We show that the width of photonic band gaps in general periodic space-time permittivity- and permeability-modulated media is proportional to the absolute difference between the electric and magnetic pumping strengths. We derive a rigorous analytical solution for investigation of wave propagation and scattering from general periodic space-time permittivity- and permeability-modulated media. In contrast with weak photonic transitions, from the excited mode to its two adjacent modes, in conventional space-time permittivity-modulated media, in an equilibrated space-time-varying medium, strong photonic transitions occur from the excited mode to its four adjacent modes. We study the enhanced nonreciprocity and zero band gap in equilibrated space-time-modulated media by analysis of their dispersion diagrams. In contrast to conventional space-time permittivity-modulated media, equilibrated space-time media exhibit different phase and group velocities for forward and backward harmonics. Furthermore, the numerical simulation scheme of general space-time permittivity- and permeability-modulated media is presented, which is based on the finite-difference time-domain technique. Our analytical and numerical results provide insights into general space-time refractive-index-modulated media, paving the way toward optimal isolators, nonreciprocal integrated systems, and subharmonic frequency generators.

We report the nonlinear acoustic streaming effect and the fast manipulation of microparticles by microelectromechanical Lamb-wave resonators in a microliter droplet. The device, consisting of four Lamb-wave resonators on a silicon die, generates cylindrical traveling waves in a liquid and efficiently drives nine horizontal vortices within a $1\text{−}\mu \mathrm{l}$ droplet; the performance of the device coincides with the numerical model prediction. Experimentally, the particles are enriched at the stagnation center of the main vortex on the free surface of the droplet in open space without microfluidic channels. In addition, the trajectories of the particles in the droplet can be controlled by the excitation power.

A directional superconducting parametric amplifier in the GHz frequency range is designed and analyzed, suitable for low-power read-out of microwave kinetic inductance detectors employed in astrophysics and when combined with a nonreciprocal device at its input also for circuit quantum electrodynamics. It consists of a one-wavelength-long nondegenerate Josephson parametric reflection amplifier circuit. The device has two Josephson-junction oscillators, connected via a tailored impedance to an on-chip passive circuit which directs the in- to the output port. The amplifier provides a gain of 20 dB over a bandwidth of 220 MHz on the signal as well as on the idler portion of the amplified input and the total photon shot noise referred to the input corresponds to maximally approximately $1.3$ photons per second per Hertz of bandwidth. We predict a factor of 4 increase in dynamic range compared to conventional Josephson parametric amplifiers.

Structured electromagnetic waves carrying nonvanishing orbital angular momentum (OAM) have recently opened up alternative frontiers in the field of wave physics, holding great promise for a wide range of potential applications. By leveraging geometric phases originating from spin-to-orbital interactions, spin-dependent wave phenomena can be created, leading to a more versatile realm of dispersionless wave-front manipulation. However, the currently available transmissive vortex-beam generators suffer from a narrow bandwidth, require an optically thick device profile, or are limited by a low efficiency, severely restricting their integration into systems and/or widespread usage for practical applications. We present the design methodology and a physical analysis and complete experimental characterization of a class of millimeter-wave Pancharatnam-Berry transmit-arrays with a thickness of about ${\lambda }_0/3$, which enables highly efficient generation and separation of spin-controlled vortex beams over a broad bandwidth, achieving an unprecedented peak efficiency of 88% for a single vortex beam and 71% for dual vortex beams. The proposed transmit-array, which is capable of providing two-dimensional OAM multiplexing and demultiplexing without normal-mode background interference, overcomes all previous roadblocks and paves the way for high-efficiency electromagnetic vortex-beam generation as well as other wave-front–shaping devices from microwave frequencies to optical wavelengths.

The photoluminescence (PL) of semiconductors can be used to determine their absorption coefficient ($\alpha$) using Planck’s generalized law. The standard method, suitable only for self-supported thick samples, like wafers, is extended to multilayer thin films by means of the transfer-matrix method to include the effect of the substrate and optional front layers. $\alpha$ values measured on various thin-film solar-cell absorbers by both PL and photothermal deflection spectroscopy (PDS) show good agreement. PL measurements are extremely sensitive to the semiconductor absorption and allow us to advantageously circumvent parasitic absorption from the substrate; thus, $\alpha$ can be accurately determined down to very low values, allowing us to investigate deep band tails with a higher dynamic range than in any other method, including spectrophotometry and PDS.

Hydrodynamic simulations of the subterahertz and terahertz response in a ratchet field-effect structure with several gate fingers reveal a significant responsivity enhancement compared to similar nonratchet structures. In high-mobility structures, the response exhibits a plasmonic peak with a half width determined by impurity and phonon scattering and by the viscosity of the electronic fluid. The device with a feature size of 100 nm exhibits a reasonable response up to 8 THz. At high frequencies, high mobilities, and short feature sizes (on the order of 50 nm), the viscosity of the 2D electronic fluid becomes a dominant attenuation mechanism. The finite ratchet structures also exhibit plasmonic peaks at lower plasmonic frequencies corresponding to the wavelengths determined by the overall lengths of the structures. These results are important for the modeling, design, and optimization of highly sensitive subterahertz and terahertz ratchet plasmonic devices.

We propose a hybrid quantum model combining cavity QED and optomechanics, which allows the occurrence of an equilibrium superradiant quantum phase transition (QPT) triggered by a single photon. This single-photon-triggered QPT exists in the cases of both ignoring and including the so-called $A^2$ term; i.e., it is immune to the no-go theorem. It originally comes from the photon-dependent quantum criticality featured by the proposed hybrid quantum model. Moreover, a reversed superradiant QPT is induced by the competition between the introduced $A^2$ term and the optomechanical interaction. This work offers an approach to manipulate QPT with a single photon, which should inspire the exploration of single-photon quantum-criticality physics and the engineering of new single-photon quantum devices.