The coalescence of liquid droplets with a liquid stream has profound importance in various emerging applications, such as biochemical assays. Acoustic force-based droplet manipulation, which offers unique advantages, is consequently gaining attention. However, the physics of acoustics-driven coalescence of liquid droplets with a liquid stream is not well understood. Here, we unravel the mechanism of coalescence of aqueous droplets flowing in an immiscible (oil) phase with a coflowing aqueous stream, when the system is exposed to acoustic radiation force due to bulk acoustic waves. Our study reveals that the acoustic coalescence phenomenon is governed by the interplay between two important timescales, acoustic migration timescale $({\tau }_{\mathrm{ac}})$ and advection timescale $({\tau }_{\mathrm{adv}})$, that underpin the phenomenon. We find that the phenomenon is also governed by the acoustic capillary number $(C_{\mathrm{ac}})$ and relative widths of the coflowing oil and aqueous streams (i.e., $W_{\mathrm{aq}}$ and $W_{\mathrm{oil}}$). Our results show that, if $C_{\mathrm{ac}}<0.9$ and $W_{\mathrm{aq}}>W_{\mathrm{oil}}$ are satisfied to ensure the stability of the streams and positioning of the acoustic node in the aqueous phase, respectively, continuous coalescence is observed for $({\tau }_{\mathrm{adv}}/{\tau }_{\mathrm{ac}})\ge 0.85$. We exploit the phenomenon for the extraction of droplet contents (beads and cells) into an aqueous stream.

Magnetic skyrmions are chiral spin textures that hold great promise as nanoscale information carriers. Since their first observation at room temperature, progress has been made in their current-induced manipulation, with fast motion reported in stray-field-coupled multilayers. However, the complex spin textures with hybrid chiralities and large power dissipation in these multilayers limit their practical implementation and the fundamental understanding of their dynamics. Here, we report on the current-driven motion of Néel skyrmions with diameters in the 100-nm range in an ultrathin $\mathrm{Pt}$/$\mathrm{Co}$/$\mathrm{Mg}\mathrm{O}$ trilayer. We find that these skyrmions can be driven at a speed of $100\mathrm{m}{\mathrm{s}}^{-1}$ and exhibit a drive-dependent skyrmion Hall effect, which is accounted for by the effect of pinning. Our experiments are well substantiated by an analytical model of the skyrmion dynamics as well as by micromagnetic simulations including material inhomogeneities. This good agreement is enabled by the simple skyrmion spin structure in our system and a thorough characterization of its static and dynamical properties.

We investigate spin-wave resonance modes and their damping in insulating thin films of bismuth-substituted yttrium iron garnet by performing femtosecond magneto-optical pump-probe experiments. For large magnetic fields in the range below the magnetization saturation, we find that the damping of high-order standing spin-wave (SSW) modes is about 40 times lower than that for the fundamental one. The observed phenomenon can be explained by considering different features of magnetic anisotropy and exchange fields that, respectively, define the precession frequency for fundamental and high-order SSWs. These results provide further insight into SSWs in iron garnets and may be exploited in many new photomagnonic devices.

Highly accurate microelectromechanical-systems inertial sensors have a wide range of potential applications, including inertial navigation and seismometry. Conventional approaches to the implementation of inertial sensors rely on transducers that convert the external acceleration into changes in displacement of a proof mass or shifts in resonant frequencies. Recently, it has been demonstrated that inertial forces can also be measured through the monitoring of the spatial energy distribution between two coupled microresonators. To extend this approach, we show that a weak dynamic coupling can be established through periodic modulation of the stiffness for a mechanically coupled microresonator system integrated as part of an accelerometer, enhancing the scale factor and resolution of the accelerometer. The resulting capability of parametric modulation also enables the tuning of the operating point of the accelerometer through the modulation frequency. Utilizing this technique, we show that the scale factor of the accelerometer can be enhanced by a factor of 188 and a factor-of-25 improvement in sensor resolution is demonstrated. Dynamic tuning of the sensor scale factor and inherent noise filtering are also demonstrated.

A two-dimensional Stampfli-triangle photonic crystal (2D S-T PC), which arranges the basic structural unit of a Stampfli-type photonic quasicrystal (PQC) in a triangular lattice, is proposed. The relationship between the structural parameters and the topological trivial or nontrivial state generated in the 2D S-T PC and the relationship between the structural parameters and the frequency of the one-way propagation are analyzed. One-way propagation based on a U-shaped waveguide composed of 2D S-T PCs, with topological trivial and nontrivial states sharing a common band gap, is realized. The topological edge state, which satisfies $C_6$ symmetry, is realized by changing just the diameters of the cylinders in the 2D S-T PC. This provides an idea for the generation of a topological edge state based on a 2D PQC and increases the diversity and adjustability of the method to realize the topological edge state.

A major challenge in spin-based electronics is reducing power consumption for magnetization switching of ferromagnets, which is implemented by injecting a large spin-polarized current. The alternative approach is to control the magnetic anisotropy (MA) of the ferromagnet by an electric field. However, the voltage-induced MA is too weak to deterministically switch the magnetization without an assisting magnetic field and a strategy towards this goal remains elusive. Here, we demonstrate a scheme of orbital-controlled magnetization switching (OCMS). A sharp change in the MA is induced when the Fermi level is moved between energy bands with different orbital symmetries. Using a ferromagnetic oxide interface, we show that OCMS can be used to achieve a deterministic and magnetic-field-free 90°-magnetization switching solely by applying a small electric field of 0.05 V ${\mathrm{nm}}^{-1}$ with a negligibly small current density of about ${10}^{-2}$ A ${\mathrm{cm}}^{-2}$. Our results highlight the huge potential of band engineering in ferromagnetic materials for efficient magnetization control.

Treatment of laboratory-grown diamond by electron irradiation and annealing has enabled quantum sensors based on negatively charged nitrogen-vacancy ($\mathrm{N}$-$V^{-}$) centers to demonstrate record sensitivities. Here we investigate the irradiation and annealing process applied to 28 diamond samples using an ambient-temperature, all-optical approach. As the presence of the neutrally charged nitrogen-vacancy ($\mathrm{N}$-$V^0$) center is deleterious to sensor performance, this photoluminescence decomposition analysis is first used to determine the concentration ratio of $\mathrm{N}$-$V^{-}$ to $\mathrm{N}$-$V^0$ in diamond samples from the measured photoluminescence spectrum. The analysis hinges on (i) isolating each $\mathrm{N}$-$V$ charge state’s emission spectrum and (ii) measuring the $\mathrm{N}$-$V^{-}$ to $\mathrm{N}$-$V^0$ emission ratio, which is found to be $2.5\pm 0.5$ under low-intensity 532-nm illumination. Using the photoluminescence-decomposition-analysis method, we measure the effects of irradiation and annealing on conversion of substitutional nitrogen to $\mathrm{N}$-$V$ centers. Combining these measurements with a phenomenological model for diamond irradiation and annealing, we extract an estimated monovacancy creation rate of $0.52\pm 0.26{\mathrm{cm}}^{-1}$ for 1-MeV electron irradiation and an estimated monovacancy diffusion coefficient of 1.8 ${\mathrm{nm}}^2/$s at $850{}^{\circ }\mathrm{C}$. Finally, we find that irradiation doses of ${10}^{18}{e}^{-}$/${\mathrm{cm}}^2$ or more deteriorate the $\mathrm{N}$-$V^{-}$ decoherence time $T_2$, whereas $T_1$ is unaffected up to the the maximum investigated dose of $5\times {10}^{18}{e}^{-}$/${\mathrm{cm}}^2$.

Consolidated granular materials and materials with damage at the microstructural level exhibit anomalous elastic behavior even when excited by low-amplitude elastic waves. Their response is given by a combination of slow- and fast-dynamics effects which, as their definition implies, act on very different time scales. In particular, conditioning (a transition to an elastic state dependent on the strain amplitude) and relaxation (full recovery of the elastic properties when the strain is removed) have been observed in different materials and under different dynamic excitations. An experimental parametric analysis of the phenomenon, aiming to establish correlations between the effects on different elastic physical properties (wave velocity and attenuation coefficient) and between the evolution of conditioning and relaxation is proposed here, with the goal of better characterizing slow dynamics and allowing one to go beyond the phenomenological description of elastic hysteresis currently available. At the same time, by studying different materials, we wish to highlight the possibility of using slow dynamics as an additional tool for materials characterization.

Magnetoelectric (ME) heterostructures are widely studied to realize functional applications, such as magnetometers, ME random access memory (MERAM), ME antennas, energy harvesters, and voltage microwave devices. A good understanding of the nonlinearity of ME heterostructures can lead to potentially improved performance. Here, we present an investigation into the voltage-driven nonlinear phenomena of a ME heterostructure near its electromechanical resonance. The Stoner-Wohlfarth model and Duffing equation are used to study the ΔE effect in amorphous Metglas alloy and the nonlinear behavior of a ME heterostructure, respectively. Then, the dependence of the nonlinearity on bias field, driving voltage, mechanical quality factor, and the frequency sweeping direction are systematically studied and verified. Experimental results show that spring-hardening and -softening behavior is separately obtained at bias fields of 25 Oe and 50 Oe, respectively. In addition, hysteresis is observed when sweeping the frequency forward and then backward at a driving voltage of 5 V; this agrees well with qualitative analysis. This work provides a route to induce, control, and possibly exploit the nonlinear behavior of ME devices, such as magnetic-field energy harvesters and ME sensors and antennas.

The growing field of quantum computing is based on the concept of a q-bit, which is a delicate superposition of 0 and 1, requiring cryogenic temperatures for its physical realization along with challenging coherent coupling techniques for entangling them. By contrast, a probabilistic bit or a $p$-bit is a robust classical entity that fluctuates between 0 and 1 and can be implemented at room temperature using present-day technology. Here, we show that a probabilistic coprocessor built out of room-temperature $p$-bits can be used to accelerate simulations of a special class of quantum many-body systems that are sign-problem–free or “stoquastic,” leveraging the well-known Suzuki-Trotter decomposition that maps a $d$-dimensional quantum many-body Hamiltonian to a $d+1$-dimensional classical Hamiltonian. This mapping allows an efficient emulation of a quantum system by classical computers and is commonly used in software to perform quantum Monte Carlo (QMC) algorithms. By contrast, we show that a compact, embedded magnetic tunnel junction (MTJ)-based coprocessor can serve as a highly efficient hardware accelerator for such QMC algorithms, providing an improvement in speed of several orders of magnitude compared to optimized CPU implementations. Using realistic device-level spice simulations, we demonstrate that the correct quantum correlations can be obtained using a classical $p$-circuit built with existing technology and operating at room temperature. The proposed coprocessor can serve as a tool to study stoquastic quantum many-body systems, overcoming challenges associated with physical quantum annealers.

In wireless communications, electromagnetic (EM) waves carrying orbital angular momentum (OAM) can obviously improve data transmission efficiency due to their particular characteristic of multiple orthogonal modes. However, the divergent peculiarity of vortex waves limits seriously the propagation distance of communication. In this paper, a medium performing as a collimating lens for vortex waves is designed with a transformation optics concept and manufactured by additive printing technology. The design concept is presented and the lens is both numerically simulated and experimentally measured at microwave frequencies. The measured near-field amplitude distribution indicates that the divergence angle and the diameter of the doughnut-shaped vortex wave passing through the collimating lens are greatly reduced over a broad frequency range. The far-field antenna gain patterns show that the energy density of the vortex wave is significantly improved, while the phase profile and mode purity show that the topology charge of the vortex wave remains unchanged after passing through the collimating lens. The proposed all-dielectric collimating lens allows us to increase the realized gain of vortex waves and may extend the propagation distance, laying the foundation for future vortex wave communications.

Indistinguishable single photons are key ingredients for a plethora of quantum-information-processing applications, ranging from quantum communications to photonic quantum computing. A mainstream platform to produce indistinguishable single photons over a wide spectral range is based on biphoton generation through spontaneous parametric down-conversion in nonlinear crystals. The purity of the biphotons produced is, however, limited by their spectral correlations. Here we present a design recipe, based on a machine-learning framework, for the engineering of biphoton joint spectral amplitudes over a wide spectral range. By customizing the poling profile of the ${\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}\mathrm{O}}_4$ crystal, we show, numerically, that spectral purities of 99.22%, 99.99%, and 99.82%, respectively, can be achieved in the 1310-, 1550-, and 1600-nm bands after applying a moderate 8-nm filter. The machine-learning framework thus enables the generation of near-indistinguishable single photons over the entire telecommunication band without resorting to the ${\mathrm{K}\mathrm{Ti}\mathrm{O}\mathrm{P}\mathrm{O}}_4$ crystal’s group-velocity-matching wavelength window near 1582 nm.

Hardware-implemented reservoir computing (RC) has been gaining considerable interest in recent years, in particular for classification and nonlinear-prediction tasks. Such RC systems often perform analog computation and, therefore, may be more sensitive to noise than digital systems; noise has been found to often degrade the computational performance. In contrast, here we demonstrate that noise can also play a constructive role in hardware-based RC. Using a hybrid delay-based RC system with an analog part (nonlinearity) and a digital part, we show that the replication of chaotic attractor dynamics is overall improved when the reservoir is trained with an input signal modified by additive Gaussian noise. To quantify the performance of the attractor replication, we suggest two different methods based on recurrence plots and power spectra.

We investigate the accessibility and projected low-noise performance of single-soliton Kerr frequency combs in silicon nitride microresonators enabled by operating at cryogenic temperatures as low as 7 K. The resulting 2-orders-of-magnitude reduction in the thermorefractive coefficient relative to room temperature enables direct access to single bright Kerr soliton states through adiabatic frequency tuning of the pump laser while remaining in thermal equilibrium. Our experimental results, supported by theoretical modeling, show that single solitons are easily accessible at temperatures below 60 K for the microresonator device under study. We further demonstrate that the cryogenic temperature primarily impacts the thermorefractive coefficient. Other parameters critical to the generation of solitons, such as quality factor, dispersion, and effective nonlinearity, are unaltered. Finally, we discuss the potential improvement in thermorefractive noise resulting from cryogenic operation. The results of this study open up new directions in advancing chip-scale frequency-comb optical clocks and metrology at cryogenic temperatures.

Many published biomedical and chemical experimental results involving ultrasound suffer inconsistencies. One of the main reasons is due to the nonuniform field produced by conventional piston-type ultrasonic transducers; severe intensity fluctuation occurs in the near-field region where most experiments are conducted. We show both theoretically and experimentally that if the transducer surface vibration can be controlled to have an amplitude distribution in the form of $J_0[{(\rho /R_0)}^{\gamma }{U}_1]$, where ρ is the radial coordinate, $R_0$ is the radius of the acoustic source, γ ≥ 1, and $U_1$ is the first zero of the zeroth-order Bessel function of the first kind $J_0(r)$, then the pressure amplitude space variation in the near-field can be eliminated to produce a three-dimensional (3D) cylindrical region with uniform pressure distribution near the transducer surface. The width of the cylindrical region increases while its height decreases with γ, which gives the flexibility of manipulating the shape of the uniform pressure field region. The presented design is beneficial for many biomedical and chemical experiments, as well as industrial applications that require uniform pressure sonication.

The effects of a local external magnetic field on microwave scattering from a laser-generated plasma are investigated by modeling and experiment, leading to the prediction and observation of magnetically induced depolarization of scattered microwaves. The sample volume is localized by the small laser-generated plasma and depolarization is shown to depend on both the magnitude and direction of the magnetic field; this indicates the potential for standoff detection and measurement of vector magnetic fields in gases.

We study theoretically how loss impacts the amplification and squeezing performance of a generic quantum traveling-wave parametric amplifier. Unlike previous studies, we analyze how having different levels of loss at signal and idler frequencies can dramatically alter properties compared to the case of frequency-independent loss. We find that loss asymmetries increase the amplifier’s added noise in comparison to the symmetric loss case. More surprisingly, even small levels of loss asymmetry can completely destroy any quantum squeezing of symmetric collective-output quadratures, while nonetheless leaving the output state strongly entangled.

We experimentally demonstrate a photon-pair source, based on continuous-wave pumped spontaneous four-wave mixing in a silicon microring resonator, which efficiently generates photon pairs with high coincidence counts, high brightness, and high coincidence-to-accidental ratio at multiple resonance wavelengths matching a standard International Telecommunication Union (ITU) frequency grid. The signal-idler joint spectral intensity is measured with a very high resolution of $2\mathrm{pm}$ over multiple wavelengths by an optical spectrum analyzer through probe-swept stimulated four-wave mixing, allowing an accurate quantification of the spectral correlation. Strong spectral correlation and uniform degree of quantum correlation are observed from the source, and the measured amount of spectral modes agrees well with that obtained by unheralded second-order correlation measurements.

Solitons are revealed to undergo sudden structural collapse upon nonlinear propagation and robust recurrence to the original state afterwards. Such fascinating dynamics, termed soliton explosions, are ubiquitous in dissipative systems, but, surprisingly, are, so far, found exclusively in parameter-invariant stationary solitons. Many nonlinear systems support dynamic solutions, such as breathing dissipative solitons (DSs), the energy and duration of which experience periodic evolutions. It is of fundamental importance to unveil whether breathing DSs also exhibit explosive dynamics. Here, we report on experimental observation of breathing DS explosions in mode-locked fiber lasers. A bifurcation diagram clearly shows how different laser regimes, including breathing DS explosions, breathing DSs, and continuous-wave mode locking, are switched by varying the pump power. While soliton explosions are found above the pump power for generating stable solitons, breathing DS explosions occur under the pump power to support stable breathing DSs. Nonlinearity-mediated giant Q-switching is critical to stimulate breathing DS explosions. Moreover, rogue waves are observed during breathing DS explosions. A correlation analysis shows that the spectral correlation evolves during breathing DS explosions from a low value at the explosion phase to a high one at the revival phase. Our results can contribute to the design of ultrafast lasers and are conducive to understanding the complex dynamics of nonlinear systems.

Sensitive detection of phase-locked terahertz (THz) pulses is routinely achieved using electro-optic sampling (EOS), a technique that harnesses second-order optical nonlinearities in a crystalline semiconductor. The thickness and electro-optic coefficient of such a crystal set the detection sensitivity in EOS, while its linear optical properties often determine the accessible detection bandwidth through the phase-matching conditions. Here, we demonstrate how a periodically patterned structure on the incident surface of an EOS crystal can be used to overcome bandwidth limitations by enabling noncollinear propagation between the THz waves and a near-infrared (NIR) gating pulse. The concept is demonstrated with phase gratings etched on a 1-mm-thick gallium phosphide ($\mathrm{Ga}\mathrm{P}$) semiconductor. The use of a diffracted NIR gating pulse to retrieve the THz waveform results in a sensitivity enhancement of a factor of approximately 2 with regard to the spectral maximum at 2 THz and exceeding an order of magnitude at 5 THz. This extends the spectral detection bandwidth up to 5 THz, which is twice as large as that achievable in a similar configuration with collinear THz and NIR pulses, facilitating the broadband detection of weak signals in THz spectroscopy.