We report experimental studies on acoustic scatterers consisting of a pair of coupled decorated membrane resonators that exhibit near extreme contrast in reflection asymmetry and enhanced Willis coupling coefficient with an amplitude of 2.5 and a polarizability cross term of $1.1\times {10}^{-8}{\mathrm{ms}}^2$. The design is based on the theoretical analysis using the Green’s function formulism we developed earlier. The line shapes of the real and the imaginary parts of the Willis coefficient clearly show that the Kramers-Kronig relations are satisfied.

Whether an electrified jet breaks up while the jet maintains a straight line (varicose), or alternatively, the jet trajectory becomes chaotic (whipping), depends on the competition of the interfacial forces. Starting from the competition of normal stresses on the jet surface, we derive scaling laws in different electro-hydrodynamic operating regimes as a function of fluid properties and the flow rate. The onset of whipping occurs when this scaling function reaches a threshold, which is independent of the electric field strength. However, experimental evidence indicates that this onset condition applies only when the viscosity and electrical conductivity of a liquid are small enough. As a result, we further introduce a general parameter to incorporate viscous effects into the scaling law. A unified threshold value for this parameter is found through a substantial number of experiments for liquids having a wide range of properties and under a wide variety of operational conditions of flow rate.

The characteristic impedance of a microwave transmission line is typically constrained to a value $Z_0=50\mathrm{\Omega }$, in part because of the low impedance of free space and the limited range of permittivity and permeability realizable with conventional materials. Here we suggest the possibility of constructing high-impedance transmission lines by exploiting the plasmonic response of edge states associated with the quantum Hall effect in gated devices. We analyze various implementations of quantum Hall transmission lines based on distributed networks and lumped-element circuits, including a detailed account of parasitic capacitance and Coulomb drag effects, which can modify device performance. We additionally conceive of a metamaterial structure comprising arrays of quantum Hall droplets and analyze its unusual properties. The realization of such structures holds promise for efficiently wiring-up quantum circuits on chip, as well as engineering strong coupling between semiconductor qubits and microwave photons.

We report for the first time on the implications of the dielectric nature of transparent electrodes, not only their low conductivity, but also on charge injection and collection. Transparent electrodes are common in organic-based optoelectronic devices because of the small diffusion length of charged carriers that requires complete coverage by the electrode. The commonly reported drawback of transparent electrodes is their low conductivity, which leads to unwanted power dissipation. However, there is no reference to the influence of the dielectric nature on the contact properties. The effect of the dielectric nature is introduced by modifying the expression for the image potential and the resulting barrier lowering at the transparent contact. The model is verified by fabricating and measuring simple unipolar devices, which show excellent agreement with the proposed modification. Without a modification, in order to have a good fitting, there should have been unrealistic values for some parameters, for example, a dielectric constant of 23 for the organic layer. The modified expression is then inspected in numerical simulations to study its nonlinear influence on the device. We find that using our model with typical device parameters, the predicted current density of the device is around half of the value calculated for a metallic electrode. Similar results are obtained by reducing the value of the mobility while keeping the injection condition for a metallic electrode. This explains why this property has been overlooked and is an indication of the extreme care needed when fitting experimental results.

We achieve reflection-type bound states in the continuum (BICs) in the compound grating waveguide structure by tuning the excitation of the guided modes. Assisted by the quasi-BICs with ultrahigh Q factors, the Goos-Hänchen (GH) shift can be greatly enhanced to larger than or equal to four orders of wavelength. In addition, different from the large GH shift based on the Brewster dip or transmission-type resonance, the maximum GH shift assisted by the reflection-type quasi-BIC is located at the reflectance peak with unity reflectance, which can be more easily detected in the experiment. Based on the quasi-BIC-assisted giant GH shift, we propose an ultrasensitive temperature sensor with a minimum resolution in the order of ${10}^{-4}$ °C. Our work provides a route, under the current experimental conditions, to design ultrasensitive sensors, light information storage devices, wavelength division de/multiplexers, optical switches, and polarization beam splitters based on this giant GH shift together with high reflectance.

In this letter, we describe the capillary sorting of particles by size based on dip coating. A substrate withdrawn from a liquid bath entrains a coating whose thickness depends on the withdrawal speed and the liquid properties. If the coating material contains particles, they will only be entrained when the viscous force pulling them with the substrate overcomes the opposing capillary force at the deformable meniscus. This force threshold occurs at different liquid thicknesses for particles of different sizes. Here, we show that this difference can be used to separate small particles from a mixed suspension through capillary filtration. In a bidisperse suspension, we observe three distinct filtration regimes. At low capillary numbers, $\mathrm{Ca}$, no particles are entrained in the liquid coating. At high $\mathrm{Ca}$, all particle sizes are entrained. For a range of capillary numbers between these two extremes, only the smallest particles are entrained while the larger ones remain in the reservoir. We explain how this technique can be applied to polydisperse suspension. We also provide an estimate of the range of capillary number to separate particles of given sizes. The combination of this technique with the scalability and robustness of dip coating makes it a promising candidate for high-throughput separation or purification of industrial and biomedical suspensions.

Due to the complex polarizations of waves in elastic media, achieving broadband wave engineering in elastic metamaterials (EMMs) proves to be much more difficult than in the acoustic or electromagnetic counterparts. By designing a nonresonance-based singly polarized solid (SPS) with deep-subwavelength scale microstructures, we experimentally demonstrate that elastic wave polarization can be tailored in a broad frequency range. With an inverse design on the EMM’s elasticity tensor, interesting wave behavior in a “fluidlike solid” or “shear-wave solid” can be achieved. By measuring the longitudinal as well as shear wave’s propagation through the fabricated SPS layers with less than 1/20λ thickness, nearly total transmission or reflection for the targeted wave polarizations can be observed at different frequencies. Our work opens an alternative avenue for broadband elastic metamaterials design based on wave polarization engineering with various potential applications in the fields of structural monitoring, elastic wave communications, and ultrasonic elastography.

We implement a technique for measuring the singlet-triplet energy splitting responsible for spin-to-charge conversion in semiconductor quantum dots. This method, which requires fast, single-shot charge measurement, reliably extracts both large and small splittings. We perform this technique on an undoped, accumulation-mode $\mathrm{Si}$/$\mathrm{Si}\text{−}\mathrm{Ge}$ triple-quantum dot and find that the measured splitting varies smoothly as a function of confinement gate biases. Not only does this demonstration prove the value of having an in situ excited-state measurement technique as part of a standard tune-up procedure, but it also suggests that in typical $\mathrm{Si}$/$\mathrm{Si}\text{−}\mathrm{Ge}$ quantum-dot devices, spin blockade can be limited by lateral orbital excitation energies rather than valley splitting.

Steering of magnetic nano- and microhelices by a rotating magnetic field is considered a promising technique for the controlled navigation of tiny objects through viscous fluidic environments. It has been recently demonstrated that simple geometrically achiral planar structures can also be steered efficiently. Such planar propellers are interesting for practical reasons, as they can be mass-fabricated using standard micro- and nanolithography techniques. While planar magnetic structures are prone to in-plane magnetization, under the effect of an in-plane rotating magnetic field, they exhibit, at most, propulsion due to spontaneous symmetry breaking; i.e., they can propel either parallel or antiparallel to the rotation axis of the field depending on their initial orientation. Here, we demonstrate that actuation by a conically rotating magnetic field (i.e., superposition of an in-plane rotating field and constant field orthogonal to it) can yield efficient unidirectional propulsion of planar and magnetized in-plane structures. In particular, we find that a highly symmetrical V-shaped structure magnetized along its symmetry axis, which exhibits no net propulsion in the in-plane rotating field, exhibits unidirectional in-sync propulsion with a constant (frequency-independent) velocity when actuated by the conically rotating field.

Metasurfaces have attracted a lot of attention in recent years due to the alternative methods they provide for the efficient wavefront control and engineering of resonant transmission. We discuss an approach that allows the effective control of the appearance of sharp Fano resonances in metasurfaces associated with the bound states in the continuum. We demonstrate that by breaking the symmetry transversely, in the direction perpendicular to a metasurface with a complex unit cell, we can control the number, frequency, and type of high-$Q$ resonances originating from bound states in the continuum. As an example, we demonstrate experimentally the metasurfaces with magnetic dipole and toroidal dipole responses governed by the physics of multipolar bound states.

In neutron-interferometric experiments using polarized neutrons, coherent spin-rotation control is indispensable. In this article, we present a method for Larmor spin rotation around an axis parallel to the outer guide field using topology-optimized three-dimensional (3D) printed magnets. The use of 3D-printed magnets instead of magnetic coils avoids unwanted inductances and offers the advantage of no heat dissipation, which prevents potential loss in interferometric contrast (fringe visibility) due to temperature gradients in the interferometer. We use topology optimization to arrive at a design of the magnet geometry that is optimized for homogeneity of the magnetic action over the neutron-beam profile and adjustability by varying the distance between the 3D-printed magnets. We verify the performance in polarimetric and interferometric neutron experiments.

Phonons at gigahertz frequencies interact with electrons, photons, and atomic systems in solids, and therefore, have extensive applications in signal processing, sensing, and quantum technologies. Surface acoustic wave (SAW) resonators that confine surface phonons can play a crucial role in such integrated phononic systems due to small mode size, low dissipation, and efficient electrical transduction. To date, it has been challenging to achieve a high quality (Q) factor and small phonon mode size for SAW resonators at gigahertz frequencies. We present a methodology to design compact high-Q SAW resonators on lithium niobate operating at gigahertz frequencies. We experimentally verify designs and demonstrate Q factors in excess of $2\times {10}^4$ at room temperature ($6\times {10}^4$ at 4 Kelvin) and mode size as low as 1.87 ${\lambda }^2$. This is achieved by phononic band structure engineering, which provides high confinement with low mechanical loss. The frequency Q products (fQ) of our SAW resonators are greater than ${10}^{13}$. These high-fQ and small mode size SAW resonators could enable applications in quantum phononics and integrated hybrid systems with phonons, photons, and solid-state qubits.

It has recently been highlighted that nonreciprocal surface waves can be stopped in suitably designed waveguides, leading to the formation of hotspots wherein the electromagnetic fields are massively enhanced. Here, we prove that this phenomenon has a topological origin and is related to a breakdown of the bulk-edge correspondence in electromagnetic continua with no spatial cutoff. Our theoretical analysis shows that nonreciprocal electromagnetic continua can be used to realize energy sinks with ultrasingular fields that absorb all the energy generated in their surroundings. Moreover, we prove that similar energy sinks may be formed in fully reciprocal optical platforms with a parity-time-duality ($\mathcal{P}\mathcal{T}\mathcal{D}$) symmetry.

By density-functional-theory calculations, we predict a stable two-dimensional (2D) ferromagnetic semiconductor, ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Se}}_6$, where the Curie temperature $T_C$ can be dramatically increased beyond room temperature by applying a few-percent strain. In addition, the anomalous Hall conductivity in 2D ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Se}}_6$ and ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ is predicted to be comparable to that in the ferromagnetic metals $\mathrm{Fe}$ and $\mathrm{Ni}$, and is an order of magnitude larger than that in the diluted magnetic semiconductor $\mathrm{Ga}$($\mathrm{Mn}$,$\mathrm{As}$). Based on superexchange interactions, the increased $T_C$ in 2D ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Se}}_6$ caused by strain can be understood by the decreased energy difference between $3d$ orbitals of $\mathrm{Cr}$ and $4p$ orbitals of $\mathrm{Se}$. Our findings highlight the microscopic mechanism to obtain room-temperature ferromagnetic semiconductors by strain.

Using point-source atom interferometry (PSI), we characterize the sensitivity of a multiaxis gyroscope based on free-space Raman interrogation of a single source of cold atoms in a glass vacuum cell. The instrument simultaneously measures the acceleration in the direction of the Raman-laser beams and the projection of the rotation vector onto the plane perpendicular to that direction. The sensitivities for the magnitude and direction of the rotation-vector measurement are ${0.033}^{\circ }/\mathrm{s}$ and ${0.27}^{\circ }$ with an averaging time of 1 s, respectively. The fractional acceleration sensitivity $\delta g/g$ is $1.6\times {10}^{-5}/\surd \mathrm{Hz}$. The sensitivity can be increased by increasing the Raman interrogation time, allowing the cold-atom cloud to expand further, correcting the fluctuations in the initial cloud shape, and reducing sources of technical noise. PSI resolves a rotation vector in a plane by measuring a phase gradient. This two-dimensional rotation sensitivity may be specifically important for applications such as tracking the precession of a rotation vector and gyrocompassing.

An electroactive biofilm is a porous layer of bacteria covering an electrode, which plays an important role in bioelectrochemical systems, such as in the microbial fuel cell. We derive a dynamic model of ion transport, biochemical reactions, and electron transport inside such a biofilm. After validating the model against data, we evaluate model output to obtain an understanding of the transport of ions and electrons through a current-producing biofilm. For a system fed with a typical wastewater stream containing organic molecules and producing 5 A ${\mathrm{m}}^{-2}$, our model predicts that transport of the organic molecules is not a limiting factor. However, the pH deep within the biofilm drops significantly, which can inhibit current production of such biofilms. Our results suggest that the electronic conductivity of the biofilm does not limit charge transport significantly, even for a biofilm as thick as 100 $\mu \mathrm{m}$. Our study provides an example of how physics-based modeling helps to understand complex coupled processes in bioelectrochemical systems.

Photon recycling is required for a solar cell to achieve an open-circuit voltage $(V_{\mathrm{OC}})$ and power conversion efficiency (PCE) approaching the Shockley-Queisser theoretical limit. The achievable performance gains from photon recycling in metal halide perovskite solar cells remain uncertain due to high variability in material quality and the nonradiative recombination rate. We quantify the enhancement due to photon recycling for state-of-the-art perovskite ${\mathrm{Cs}}_{0.05}({\mathrm{MA}}_{0.17}{\mathrm{FA}}_{0.83}{)}_{0.95}\mathrm{Pb}({\mathrm{I}}_{0.83}{\mathrm{Br}}_{0.17}{)}_3$ (triple-cation) films and corresponding solar cells. We show that, at the maximum power point (MPP), the absolute PCE can increase up to 2.0% in the radiative limit, primarily due to a 77 mV increase in $(V_{\mathrm{MPP}})$. For this photoactive layer, even with finite nonradiative recombination, benefits from photon recycling can be achieved when nonradiative lifetimes and external light-emitting diode (LED) electroluminescence efficiencies, $Q_e^{\mathrm{LED}}$, exceed 2 µs and 10%, respectively. This analysis quantifies the significance of photon recycling in boosting the real-world performance of perovskite solar cells toward theoretical limits.

We report three-dimensional (3D)-printed acoustic holographic lenses for the formation of ultrasonic fields of complex spatial distribution inside the skull. Using holographic lenses, we experimentally, numerically, and theoretically produce acoustic beams whose spatial distribution matches target structures of the central nervous system. In particular, we produce three types of targets of increasing complexity. First, a set of points are selected at the center of both right and left human hippocampi. Experiments using a skull phantom and 3D-printed acoustic holographic lenses show that the corresponding bifocal lens simultaneously focuses acoustic energy at the target foci, with good agreement between theory and simulations. Second, an arbitrary curve is set as the target inside the skull phantom. Using time-reversal methods, the holographic beam bends following the target path, in a similar way as self-bending beams do in free space. Finally, the right human hippocampus is selected as a target volume. The focus of the corresponding holographic lens overlaps with the target volume in excellent agreement between theory in free media, and experiments and simulations including the skull phantom. The precise control of focused ultrasound into the central nervous system is mainly limited due to the strong phase aberrations produced by refraction and attenuation of the skull. Using the present method, the ultrasonic beam can be focused not only at a single point but overlapping one or various target structures simultaneously using low-cost 3D-printed acoustic holographic lens. The results open alternative paths to spread incoming biomedical ultrasound applications, including blood-brain-barrier opening or neuromodulation.

We study the optical properties of tetravalent-vanadium impurities in $4H$ silicon carbide. Light emission from two crystalline sites is observed at wavelengths of 1.28 and $1.33\mu \mathrm{m}$, with optical lifetimes of 163 and 43 ns, respectively, which remains stable up to 50 and 20 K, respectively. Moreover, spectrally broad photoluminescence is observed up to room temperature. Group-theory and ab initio density-functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. Specifically, our numerical simulations indicate that the site assignment is reversed with respect to previous assumptions. Our calculations show that vanadium in silicon carbide has highly favorable properties for the generation of single photons in the telecommunication wavelength regime. Combined with the available electronic and nuclear degrees of freedom, vanadium presents all the ingredients required for a highly efficient spin-photon interface.

We use ultrasound directed self-assembly (DSA) to create two-dimensional patterns of high-aspect-ratio particles with user-specified orientation. We theoretically derive a method to determine the operating parameters of any arrangement of ultrasound transducers required to align high-aspect-ratio particles in any user-specified orientation. The method finds the ultrasound wave field that maximizes the curvature of the acoustic radiation potential orthogonal to the user-specified particle orientation and in user-specified locations. We simulate the theoretical solution resulting from this method and experimentally validate it using carbon microfibers in water and quantify the position and orientation error. This method enables controlling the location and orientation of high-aspect-ratio particles, including simultaneously orienting multiple high-aspect-ratio particles in different directions. This work finds application in the biomedical field and in using ultrasound DSA as a processing or manufacturing method for engineered materials.