Surface-acoustic-wave- (SAW) based devices emerge as promising technology in magnetic field sensing by integrating a magnetostrictive layer with the giant ΔE/ΔG effect. However, almost all SAW magnetic field sensors require a bias field to obtain high sensitivity. In addition, the true nature of magnetoacoustic coupling still presents a major challenge in understanding and designing this kind of device. Here, a dynamic magnetoelastic model for the ΔE/ΔG effect is established in consideration of the important role of the dipole-dipole interaction. The model is also implemented in finite-element-method software to calculate the resonance-frequency responses of multiple fabricated sensors with different ψ angles between the acoustic wave vector and the induced uniaxial magnetic anisotropy. The measured results are in excellent agreement with the simulated ones. A strong resonance-frequency sensitivity ($S_{\mathrm{RF}}$) of 630.4 kHz/Oe is achieved at zero bias field for the device with an optimized ψ angle. Furthermore, the $S_{\mathrm{RF}}$ measurements along different directions verify its vector-sensing capability.

Metasurfaces providing resonances arising from quasibound states in the continuum have been proposed as sensors in the THz band due to the existence of strong resonances characterized by high-quality factors. Controlling geometrical parameters, the quality factor can be adjusted and, theoretically, designed at will. However, losses in materials critically bound the metasurface performance and limit the quality factor of the resonances. For this reason, all-dielectric metasurfaces have been proposed as an alternative to metal-dielectric structures to reduce losses and achieve extreme functionalities. When implemented by low-loss materials, these structures are usually considered lossless and proposed as ultrasensitive sensors in the THz band. In this paper, we examine the effect of losses in all-dielectric metasurfaces considering realistic materials and study the limitations in the quality factor. In addition, we compare the performance of these structures as sensors with a nanostructure supporting extraordinary optical transmission. Our results show that material loss, even in low-loss materials, severely limits the sensing performance in all-dielectric metasurfaces, and that their performance can be surpassed by structures supporting extraordinary optical transmission.

We propose a method to perform the quantum state tomography (QST) of an $n$-partite qudit state embedded in single photons based on the Hong-Ou-Mandel interference between the target photon and ancillary probe light. Our method requires only passive beam splitters in the target modes by shifting all of the active devices used in conventional QST methods to the probe preparation system. This feature is useful when the active optical devices for QST have large optical losses. Moreover, even when the probe is prepared by a laser light having a mode mismatch with the target photon, the method offers an accurate estimation of the target state. As a proof-of-principle, we perform the experimental demonstration using a polarization qubit. Regardless of the photon statistics of the probe light, the estimated results of state reconstruction are as accurate as those verified by the conventional QST.

Understanding thermal transport by phonons near nanoscale hotspots is critical for many engineering applications, especially for nanoscale thermal measurements and nanoelectronics. Previous studies usually adopted the gray phonon Boltzmann transport equation to consider ballistic phonon transport or the multitemperature model to consider selective phonon excitation. However, nonequilibrium phonon transport near a nanoscale hotspot cannot be fully captured by either method. In this work, we employ the more rigorous nongray phonon Boltzmann transport equation to investigate phonon transport near nanoscale hotspots. We first consider hotspots in a one-dimensional system with two phonon modes to extract the underlying physics. Thermal transport is found to be less efficient near these hotspots, leading to significantly smaller effective thermal conductivity. The mechanism behind this is that different phonon modes have different temperatures and are therefore not in equilibrium. Nonequilibrium is caused by both the selective electron-phonon interaction and ballistic phonon transport. For relatively large hotspots, nonequilibrium is primarily contributed to by the selective excitation effect. When the hotspot size further reduces, the contribution from ballistic phonon transport becomes increasingly more important. Furthermore, we quantitatively study the thermal transport of laser-heated hotspots in Raman experiments and Joule heating in fin field-effect transistors (FETs) using the nongray phonon Boltzmann transport equation. We find that the measured thermal conductivity of single-layer graphene can be significantly underestimated if the nonequilibrium effect is ignored. Additionally, the peak temperature rise in a silicon fin FET is much larger than that calculated by the heat-diffusion equation.

Many low-electron-density Drude (LEDD) materials such as transparent conductive oxides or nitrides have recently attracted interest as alternative plasmonic materials and extremely nonlinear thermal and optical materials. These materials attract research interest because they are CMOS compatible; hence, they can enable new functionalities within existing devices. However, the rapidly growing number of experimental studies has so far not been supported by a systematic theory of the electronic, thermal, and optical response of these materials. Here, we use the techniques previously derived in the context of noble metals to go beyond a simple electromagnetic modeling of LEDD materials and provide an electron-dynamics model for their electronic and thermal response. We find that the low electron density makes momentum conservation in electron-phonon interactions more important, more complex, and more sensitive to the temperatures compared with noble metals; moreover, we find that electron-electron interactions are becoming more effective due to the weaker screening. Most importantly, we show that the low electron density makes the electron heat capacity much smaller than in noble metals, such that the electrons in LEDD materials tend to heat up much more and cool down faster compared to noble metals. While here we focus on indium tin oxide (ITO), our analytical results can easily be applied to any LEDD materials.

Breaking the so-called diffraction limit on the resolution of optical devices and achieving subwavelength focusing requires tailoring the evanescent spectrum of wave fields. There are several possible approaches, all of which have limitations, such as the generation of strong additional scattering, limited focusing power, issues at the implementation step, and the need for a drain at the focal point. This paper presents a feasible strategy based on the concepts of the perfect lens and power flow-conformal metasurfaces. Desired fields for subwavelength focusing are integrated using double-negative media and then the surface profile of a focusing reflector is designed to be tangential to the desired power flow, so that the metasurface can be modeled as a local impedance boundary, and can be easily implemented using passive and lossless elements. Full-wave simulations demonstrate that an example reactive metasurface is able to break the diffraction limit and provide near-field focusing with subwavelength hotspot size. We expect that the outcome will find applications in antennas, beam-shaping devices, nonradiative wireless power transfer systems, microscopy, and lithography.

The strain-mediated method is considered to be a promising solution for implementing energy-efficient magnetization rotation. However, current methods strictly require a voltage clock. Here, a multiferroic nanomagnet composed of bicomponent magnetic materials (Terfenol-D:$\mathrm{Ni}$ = 1:2) is developed to study strain-mediated magnetization switching behavior. With micromagnetic simulations, what is demonstrated is that the strict requirements for a precise applied voltage period can be overcome in such a bicomponent nanomagnet, where the threshold of the square-wave voltage pulse width required for complete and repeated magnetization reversal is only 0.42 ns if the amplitude of the voltage is 1 V. Besides deterministic magnetization switching, further study shows that the unique strain-mediated stochastic magnetization reversal behavior of the designed device can be used to mimic the biological neuron. With the application of the derived neural device parameters, a three-layer artificial neural network is further constructed to recognize a handwritten dataset, based on which, an accuracy of more than 98% can be achieved. Overall, these results open an intriguing way toward straintronic memory and neuromorphic systems.

Cavity photons and ferromagnetic spin excitations can exchange information coherently in hybrid architectures, at speeds set by their mutual coupling strength. Speed enhancement is usually achieved by optimizing the geometry of the electromagnetic cavity. Here we show that the geometry of the ferromagnet also plays a role, by setting the fundamental frequency of the magnonic resonator. Using focused ion-beam patterning, we vary the aspect ratio of different Permalloy samples reaching operation frequencies above 10 GHz while working at low external magnetic fields. Additionally, we perform broadband ferromagnetic resonance measurements and cavity experiments that demonstrate that the light-matter coupling strength can be estimated using either open transmission lines or resonant cavities, yielding very good agreement. Finally, we describe a simple theoretical framework based on electromagnetic and micromagnetic simulations that successfully accounts for the experimental results. This approach can be used to design hybrid quantum systems exploiting magnetostatic mode excited in ferromagnets of arbitrary size and shape and to tune their operation conditions.

Localizing an intruder submerged in a strongly scattering medium, such as a dense granular suspension, is a practical challenge. Here we extract the coherent ultrasonic echo from a steel ball submerged in a dense glass-bead packing saturated by water, by using a standard single-element ultrasonic transducer thanks to a configuration averaging process. Different configurations of the granular packing are created by the nonaffine motion of the beads with a mixing blade, akin to the Brownian motion, in the vicinity of the intruder. We investigate the efficiency of this process to reduce the so-called material noise from multiply scattered ultrasound, as a function of the configuration number, the shear rate and the blade-intruder distance. Nonaffine motions of the beads in the shear zone induced by the blade are then analyzed on the basis of a split-bottom rotating shear cell. This method helps to develop not only ultrasonic imaging tools of buried objects in turbid marine sediments, but also the local rheology based on a ball falling monitored by ultrasonic tracking.

The emergence of localized structures originating from instabilities is of particular interest to scientists interested in exploring the rich nonlinear dynamics associated with complex dynamical systems. In the context of ultrafast lasers, the localized structures can be broadly divided into two categories: coherent structures of the dissipative soliton (DS) and incoherent structures such as the noiselike pulse (NLP). These two different types of structures have unique properties and applications and their physical origins are markedly different. Having both in a single fiber laser is an intriguing but challenging problem. Here we demonstrate the formation of the DS and NLP in a bidirectional ultrafast fiber laser and study their buildup dynamics from $Q$-switched instabilities. The clockwise light is a DS while the counterclockwise light is a NLP. The synchronized buildup dynamics of a DS or NLP from $Q$-switched instabilities provides vital evidence of the $Q$-switched instabilities as a universal indicator for the generation of localized structures in ultrafast lasers. The evenly spaced $Q$-switched spikes before the onset of a DS (NLP) exhibit intensity fluctuations, which is strikingly different than in the mode-locking regime in unidirectional oscillators, hence providing decisive evidence of the interaction between the counterpropagating light fields in the bidirectional lasers. The spectra cross-correlation and the histogram of the spectral intensity confirm the concurrent formation of a DS and NLP. Numerical simulations confirmed experimental observations. These findings help deepen understanding of the behavior of bidirectional ultrafast lasers and also demonstrate their potential as multifunctional ultrafast laser sources.

A complete simulation scheme is established to model the terahertz (THz) properties of vanadium dioxide (${\mathrm{VO}}_2$) from ab initio to computational electromagnetic method. For the monoclinic insulating phase of ${\mathrm{VO}}_2$, dielectric properties are computed by Hubbard-corrected density functional perturbation theory. Meanwhile, the electron-phonon interactions are studied to model the electronic transport properties of the tetragonal metallic phase of ${\mathrm{VO}}_2$. In order to build a bridge from the microscopic effects of first-principles calculations to the macroscopic THz properties, the Drude model and effective medium theory are applied to model the dispersion properties of the material. To validate the proposed model, a ${\mathrm{VO}}_2$ film grown on a sapphire substrate is fabricated and measured by a terahertz time-domain spectroscopy system. The measurement results have excellent agreements with the proposed simulations. Further, a ${\mathrm{VO}}_2$-based THz metamaterial modulator is simulated by using the proposed model, in great agreement with the reported measurement result. Our ab initio computational scheme improves the simulation accuracy of design, which provides a solid THz modeling approach to the ${\mathrm{VO}}_2$ and other insulator-metal transition metal oxides.

Recently, real-space representations, revealing in depth the duality of topological descriptions in real and momentum space, have been applied to show multidimensional crystalline topology. Here we propose nontrivial phononic crystals by constructing different Wannier configurations based on real-space representations. We experimentally achieve the symmetry-protected interface topology in multiple dimensions containing anomalous interface states and higher-order corner states. Nontrivial acoustic pseudospin polarizations are critical to the interface states and have been directly observed in experiment. In a consistent frame, the higher-order corner states arising from the fractional charge anomaly are further observed. Our study provides a scheme based on Wannier configurations for designing topological metamaterials, which is fruitful for characterizing and manipulating multidimensional topological states and can be generalized to other classical systems.

Spin-wave transport across multidimensional networks of magnonic waveguides represents a crucial and necessary aspect of prospective densely packed three-dimensional spin-wave architectures. Here, we report the results of investigations of spin-wave propagation through magnonic waveguides extending along and bending across two and three dimensions. We consider three designs of in-plane two-dimensional bends, namely with right-angled, diagonal, and curved geometries. Our numerical and experimental results show that such bends facilitate the conversion of spin-wave types, with the output modal number depending on the spin-wave frequency. At the same time, variation of the width of lateral magnonic bends enables the spin-wave wavelength to be modified. When propagating across three-dimensional waveguide bends in the form of out-of-plane junctions, the spin-wave wavelength can similarly be tuned by adjusting the stripe’s thickness. Our results show that magnonic waveguides can serve not only as passive conduits but also as active elements in modifying the properties of the transmitted spin wave in three-dimensional magnonic networks.

Fault-tolerant quantum information processing with flawed qubits and gates requires highly efficient, quantum nondemolition qubit readout. In superconducting circuits, qubit readout using coherent light with fidelity above 99% has been achieved by using quantum limited parametric amplifiers such as the Josephson parametric converter (JPC). However, further improvement of such measurement faces many challenges, including the vacuum fluctuation of the coherent light that limits measurement fidelity. In this work we demonstrate dispersive qubit readout with a two-mode squeezed light interferometer formed by two JPCs. The first JPC generates two-mode squeezed vacuum at its output, which is coherently recombined by the second JPC after one branch is phase shifted and displaced by its interaction with the qubit-cavity system on that arm of the interferometer. We observe a 31% improvement in the power signal-to-noise ratio (SNR) of projective readout after replacing vacuum fluctuation in the readout cavity with two-mode squeezed vacuum containing on average one photon. This demonstrates a more efficient way of improving SNR than by increasing readout power. Furthermore, we investigate the quantum properties of the two-mode squeezed light in the system through weak measurement. Surprisingly, we find that tuning the interferometer to be less projective increases the measurement efficiency relative to the optimum setting for projective measurement. These enhancements may enable remote entanglement with lower efficiency components in a system with qubits in both arms of the interferometer.

Understanding the interaction of a kilojoule picosecond laser pulse with long-scale-length preplasma or homogeneous near-critical-density (NCD) plasma is crucial for guiding experiments at national short-pulse laser facilities. Using full three-dimensional particle-in-cell simulations, we demonstrate that in this regime, cross-filament stochastic acceleration is an important mechanism that contributes to the production of superponderomotive high-flux electron beams. Since the laser power significantly exceeds the threshold of the relativistic self-focusing, multiple filaments are generated and can propagate independently over a long distance. Electrons jump across the filaments during the acceleration and their motion becomes stochastic. We find that the effective temperature of electrons increases with the total interaction time following a scaling like $T_{\mathrm{eff}}\propto {\tau }_i^{0.65}$. By irradiating a submillimeter-thick NCD target, the space charge of electrons with energy above 2.5 MeV reaches tens of microcoulombs. Such high-flux electrons with superponderomotive energies significantly facilitate applications in high-energy-density science, nuclear science, secondary sources, and diagnostic techniques.

For decades, researchers have been exploring pattern-forming instabilities in free surface thin films in the hope of developing alternative lithographic techniques for applications only requiring resolution limits in the submicron range. Previous studies have shown how the pitch and shape of elements in an array can be varied by adjusting the magnitude of surface forces and growth time prior to solidification in situ. Since the formations emerge naturally from an initial flat molten film, the final arrays exhibit ultrasmooth interfaces and are therefore ideally suited to beam-shaping applications such as thin-film micro-optics. Progress in this field has stalled, however, due to the very nature of the formation process. Even when great care is taken to ensure that initial films are defect-free, final arrays still exhibit unacceptable variability in pitch, shape, and height due to ubiquitous sources of noise responsible for instability and growth. In this work, we focus on a thermocapillary instability in slender molten films exposed to a very large thermal gradient. We begin with a discussion and demonstration of why this instability inextricably leads to highly disordered arrays even if initialized by a film with very small amplitude surface roughness. We then demonstrate how spatially periodic modulation of the thermal field, implemented in three different ways, can induce synchronous growth of highly uniform periodic arrays despite noisy initial conditions. Results based on linear and weakly nonlinear stability analysis, Bloch wave analysis, and direct numerical simulation of the interface equation reveal how resonant wavelength excitations occurring between the modulation and instability driving fields are responsible for such rapid and coherent growth. An additional benefit is that the modulation field can be selected to yield an array pitch much smaller than in unmodulated systems.

Corner states (CSs) in higher-order topological insulators (HOTIs) have recently been of great interest in both crystals and quasicrystals. In contrast to electronic systems, HOTIs have not been found in photonic quasicrystals (PQCs). Here, we systemically study the higher-order topology in the two-dimensional Thue-Morse photonic quasicrystals (TM PQCs). Not only the topological phase transition and the nontrivial CSs with fractional charge induced by multipole moments, but also another type of CSs are found due to the complex structure of TM PQCs near corners. The different origins of these CSs are also analyzed based on the tight-binding model. Our work opens the door to explore richer HOT physics beyond photonic crystals and the robustness of CSs in PQC shows the potential for applications.