Among the magnetostrictive alloys, the one formed of iron and gallium (called “Galfenol” from its U.S. Office of Naval Research discoverers in the late 1990s) is attractive for its low hysteresis, good tensile stress, good machinability, and its rare-earth-free composition. One of its applications is its association with a piezoelectric material to form an extrinsic multiferroic composite as an alternative to the rare room-temperature intrinsic multiferroics such as ${\mathrm{Bi}\mathrm{Fe}\mathrm{O}}_3$. This study focuses on thin ${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$ films of thickness 5, 10, 20, and 60 nm deposited by sputtering onto glass substrates under a deposition field $H_{\mathrm{dep}}\sim 300$ Oe. Magnetization reversal study reveals a well-defined symmetry with two principal directions independent of the thickness. The magnetic signature of this magnetic anisotropy decreases with increasing ${\mathrm{Fe}}_{81}{\mathrm{Ga}}_{19}$ thickness due to an increase of the nonpreferential polycrystalline arrangement, as revealed by transmission electron microscopy (TEM) observations. Thus, when magnetic field is applied along these specific nontrivial directions, magnetization reversal is mainly coherent for the thinnest sample, as seen from the transverse magnetization cycles. The magnetostriction coefficient reaches 20 ppm for the 5-nm film and decreases for thicker samples, where the polycrystalline part with nonpreferential orientation prevails.

All-optical helicity-dependent switching in ferromagnetic $\mathrm{Co}/\mathrm{Pt}$ multilayers is investigated using magneto-optical microscopy and anomalous Hall effect measurements. A state diagram is built by studying the effect of pulse duration, fluence, and spot size. We use numerical solutions of the three-temperature model to explain that the all-optical helicity-dependent switching mechanism relies on the spin bath reaching temperatures close to the Curie point. Further insights into the reversal process are provided by the experimental demonstration of significant helicity-dependent reversal after a single laser pulse that reveals the involvement of direct angular momentum transfer. Moreover, based on the observation that longer pulse durations and larger spot sizes lead to enhanced reversal efficiency, we identify experimental conditions that lead to saturated magnetization reversal after just a few tens of laser pulses.

The application of imaging techniques based on ensembles of nitrogen-vacancy ($\mathrm{N}$-$V$) sensors in diamond to characterize electrical devices has been proposed, but the compatibility of $\mathrm{N}$-$V$ sensing with operational gated devices remains largely unexplored. Here we report the fabrication of graphene field-effect transistors directly on the diamond surface and their characterization by $\mathrm{N}$-$V$ microscopy. The current density within the gated graphene is reconstructed from $\mathrm{N}$-$V$-magnetometry measurements under both mostly $p$- and $n$-type doping, but the exact doping level is found to be affected by the measurements. Additionally, we observe a surprisingly large modulation of the electric field at the diamond surface under an applied gate potential, seen in $\mathrm{N}$-$V$-photoluminescence and $\mathrm{N}$-$V$-electrometry measurements, suggesting a complex electrostatic response of the oxide-graphene-diamond structure. Possible solutions to mitigate these effects are discussed.

We investigate the real-time closed-loop control of the compensation point in a $\mathrm{K}\text{−}\mathrm{Rb}{\text{−}}^{21}\mathrm{Ne}$ comagnetometer operated in the spin-exchange relaxation-free regime. By locking the electron resonance, the alkali-metal electrons are free from the fluctuations of the longitudinal ambient magnetic field and nuclear magnetization, which could improve the systematic stability, enlarge the linear measuring range, and suppress the cross-talk error of the comagnetometer. The demonstration of closed-loop control of the magnetic field in the single nuclear species comagnetometer is of great significance for rotation sensing such as in gyroscopes and other high-precision metrology applications of the comagnetometer.

Raman spectroscopy is one of the most valuable experimental techniques for quality assessment and structural characterization of sample materials. As such, it has been applied to understand the mechanism of the staging of anions and cations into graphite-based electrodes in a variety of energy storage devices such as $\mathrm{Al}$ batteries, dual-ion cells, and $\mathrm{Li}$-ion batteries. However, the correlation between the Raman peaks and intercalation stages is still unclear in most of these systems. This is due to the fact that the modeling of electron-phonon coupling in highly doped graphite systems is beyond the standard Born-Oppenheimer approximation. Here, we simulate the Raman peaks for ${\mathrm{Al}\mathrm{Cl}}_4^{-}$-intercalated graphite in $\mathrm{Al}$ batteries by using a nonadiabatic coupling theory. Specifically, we successfully correlate the Raman peaks of the $G$ phonon in ${\mathrm{Al}\mathrm{Cl}}_4^{-}$-doped graphite with experiment for intercalation stages 1, 2, and 4, while stage 3 appears to be absent. Stages 1 and 2 have not been observed in experimental XRD patterns. We therefore believe that the ${\mathrm{Al}\mathrm{Cl}}_4^{-}$-graphite intercalation compound has a core-shell structure with a maximum stage of 4 or 3 in the core and 2 or 1 in the shell. In addition, the observed intense narrow Raman bands for the $\mathrm{Al}$-ion battery cathode are due to the high level of graphite doping and are explained in terms of low electron-phonon decay rates.

$\mathrm{Au}$-hyperdoped $\mathrm{Si}$ produced by ion implantation and pulsed laser melting exhibits sub-band-gap absorption in the near infrared, a property that is interesting for $\mathrm{Si}$ photonics. However, the sub-band-gap absorption has previously been shown to be thermally metastable. In this work, we study the atomistic processes that occur during the thermal relaxation of $\mathrm{Au}$-hyperdoped $\mathrm{Si}$. We show that the first step in thermal relaxation is the release of substitutional $\mathrm{Au}$ from lattice sites. This process is characterized by an activation energy of around 1.6 eV, a value similar to that associated with $\mathrm{Au}$ diffusion in $\mathrm{Si}$, suggesting that both processes could be rate limited by the exchange of substitutional and interstitial $\mathrm{Au}$ atoms. As the system further relaxes, $\mathrm{Au}$ is found to locally diffuse and become trapped at nearby lattice defects, notably vacancies and vacancy complexes. In fact, density-functional theory results suggest that the formation of $\mathrm{Au}$ dimers is energetically favourable after the $\mathrm{Au}$ becomes locally trapped. The dimers could subsequently evolve into trimers, etc., as other diffusing $\mathrm{Au}$ atoms become trapped at the dimer. At low $\mathrm{Au}$ concentrations, this clustering process does not form visible precipitation after annealing at $750{}^{\circ }\mathrm{C}$ for 3 min. In contrast, spherical $\mathrm{Au}$ precipitates are found in samples with higher $\mathrm{Au}$ concentrations ($>0.14\mathrm{at.}\mathrm{\%}$), where the $\mathrm{Au}$ atoms and the associated lattice defect distributions are laterally inhomogeneous.

Passive targeted drug delivery to solid tumors is driven by the permeation of drug carriers through porous vasculature. Due to a dominant transport mechanism for the entry and migration of the drug carriers being diffusion, the passive drug delivery is relatively slow and ineffective in delivering large carriers to the desired location. Here, we propose a method for delivering liposomes into the interstitium at orders of magnitude faster than the diffusion. Using microfluidic model tumor microenvironment, we show that by exchanging the solutes of the interstitial fluid, the liposomes can respond to the change in the chemistry of the surrounding fluid, thereby penetrating deep into the confined pore space at an accelerated transport rate. In addition, by further exchanging the environment with a hypotonic solution, the delivered liposomes can expel their inner content continuously via periodic osmotic bursting, allowing controlled release of encapsulated molecules in hard-to-reach spaces. Our study suggests an active delivery strategy to enhance the permeation of therapeutic molecules into the interstitium.

Efficient generation of single photons on demand at a high repetition rate is a key to the practical realization of quantum-communication networks and optical quantum computations. Color centers in diamond and related wide-band-gap semiconductors are considered to be the most promising candidates for building such single-photon sources due to their outstanding emission properties at room temperature. However, efficient electrical excitation of color centers in most materials remains a challenge due to the inability to create a high density of free carriers. We predict a superinjection effect in diamond p-i-n diodes. By employing a comprehensive theoretical approach, we numerically demonstrate that one can overcome the doping problem in diamond and inject four orders of magnitude more electrons into the i region of the diamond p-i-n diode than the doping of the n region allows. This high density of free electrons can be efficiently used to boost the single-photon electroluminescence process and enhance the brightness of the diamond single-photon source by more than three orders of magnitude. Moreover, we show that such a high single-photon emission rate can be achieved at exceptionally low injection current densities of only $0.001\mathrm{A/m}{\mathrm{m}}^2$, which creates the backbone for the development of low-power and cost-efficient diamond quantum optoelectronic devices for quantum information technologies.

Metasurfaces, which are planar surfaces or interfaces constructed by placing subwavelength scatterers into a two-dimensional array pattern, have attracted much attention in recent years. However, it has been shown that transmission-type metasurfaces without vias are hard to simultaneously achieve high transmission and 360° phase control when the number of cascaded layers is less than three. An ultrathin Huygens’ metasurface composed of only double-layer transmitarray elements without vias is proposed to achieve both high transmission (−2.5-dB criteria) and 360° phase control. Benefiting from the antisymmetric design of the double-layer transmitarray element, the proposed Huygens’ metasurface excites both electric and magnetic dipole resonances with mutual interference, allowing the realization of high transmission and 360° phase control at the same time, which cannot be fulfilled by symmetric unit structures. Based on the ultrathin Huygens’ metasurface, a high-gain transmitarray antenna is designed, fabricated, and measured, which can achieve high aperture efficiency up to 61.04% with a very small thickness of only 0.033 wavelengths at a microwave frequency of 13 GHz.

We report the experimental realization of real-time and broadband acoustic cloaking and holography by manipulating wave fields through their boundary conditions. The method of immersive boundary conditions (IBCs) enables us to virtually replace part of a physical experiment with a virtual computational environment. By introducing a source surface enclosing the virtual environment and calculating the source strengths in real time, the wave field in the physical experiment can be controlled such that incoming waves are suppressed and transmissions through the arbitrary virtual domain are radiated in a desired manner. A one-dimensional experiment shows that scattering objects can be made undetectable without prior knowledge about the incident wave field. Moreover, virtual objects or media can be emulated where they are not present—even objects or media exhibiting properties that do not occur in nature. Our results provide useful insights into how to implement IBCs for dynamic cloaking and holography, suggesting that higher-dimensional experiments are feasible.

We present an ultrahigh-$Q$, solid-silica microrod resonator operated under ambient conditions that supports laser-fractional-frequency stabilization to the thermal-noise limit of $3\times {10}^{-13}$ and a linewidth of 62 Hz. We characterize the technical-noise mechanisms for laser stabilization, which contribute significantly less than thermal noise. With fiber photonics, we generate optical and microwave reference signals provided by the microrod modes and the free-spectral range, respectively. Our results suggest the future physical considerations for a miniature, low noise, and robust optical-frequency source.

The metrology of laser-induced damage usually finds a single transition from 0% to 100% damage probability when progressively increasing the laser energy in experiments. We observe that picosecond pulses at 2-µm wavelength focused inside silicon provide a response that strongly deviates from this. Supported by nonlinear propagation simulations and energy flow analyses, we reveal an increased light delocalization for near critical power conditions. This leads to a nonmonotonic evolution of the peak delivered fluence as a function of the incoming pulse of the energy, a situation more complex than the clamping of the intensity until now observed in ultrafast regimes. Compared to femtosecond lasers, our measurements show that picosecond sources lead to reduced thresholds for three-dimensional (3D) writing inside silicon that is highly desirable. However, strong interplays between nonlinear effects persist and should not be ignored for the performance of future technological developments. We illustrate this aspect by carefully retrieving from the study the conditions for a demonstration of 3D data inscription inside a silicon wafer.

Magnetic skyrmions are topological quasiparticles with great potential for applications in future information storage and processing devices because of their nanoscale size, high stability, and large velocity. Recently, the high-frequency properties of skyrmions have been explored for magnon nanodevices. We systematically study the dynamics of an isolated skyrmion under voltage excitation through the voltage-controlled magnetic anisotropy effect in a circular thin film. A theoretical model considering the demagnetization energy, which has often been neglected or treated superficially in previous skyrmion research but is demonstrated to have importance in determining the skyrmion dynamic state, is developed. With our model, the periodic oscillation of the skyrmion radius can be solved numerically with similar precision compared to micromagnetic simulations, and the characteristic frequency $(f_c)$ of the skyrmion breathing can be determined analytically with greater precision than previous studies. Furthermore, we find that the breathing skyrmion can be analogized as a modulator by investigating its linear modulation functionality under sinusoidal-form voltage excitation. Different from the conventional modulation system with complex CMOS circuits, this skyrmion “modulator” device integrates with both the modulation and carrier wave generation functionality, thus showing greater convenience and efficiency in applications. Our findings can provide useful guidance for both theoretical and experimental skyrmions research as well as the development of skyrmion-based magnonic devices with significant potential applicability in future communication systems.

The transport behaviors of valley kink states with potential applications have become a research hotspot for acoustic waves in phononic crystals, which are usually constructed in hexagonal lattices, such as graphene and kagome lattices. We realize an acoustic valley material of a square lattice by stacking the dimerized chains of resonators in which topological phase transition is implemented by tuning the height ratio of nonequivalent resonators. The topological transports of the acoustic kink waves with reflection immunity are observed along the interface between distinct valley phases. The topological negative refraction of kink states outcoupling into free space is experimentally visualized with a tunable refraction angle. In particular, an abnormal partition transport of the acoustic kink states at the channel intersection is demonstrated in phononic crystals. These findings may have potential applications in directional vocalization, noise reduction, and beam splitting.

Asymmetric absorptivity has recently been demonstrated in phase-gradient metasurfaces. Such an effect varies from an integer m, which is the number of discrete unit cells in a supercell. There is still lack of analytical formulas to reveal the relationship between the absorptivity and the value of m. We enrich the previous studies by considering a subwavelength metallic grating at optical frequencies and present a simple and intuitive semianalytical approach to explain the observed asymmetric absorptivity. Our theory clearly indicates that the phenomenon of m dependent asymmetric absorptivity is very robust and can be observed in any grating with a phase gradient. The mechanism is that the phase gradient ensures an additional process of multiple total internal reflections inside the gratings, leading to more dissipation and an asymmetric diffraction response.

Nematic liquid crystals exhibit a fast optical response when the applied electric field modifies the degree of order but does not change the direction of molecular orientation. The effect requires a relatively high electric field, on the order of ${10}^8\mathrm{V}/\mathrm{m}$ for a field-induced birefringence change of 0.01. To address this detrimental issue, we explore electrically induced modification of the order parameter in a material with a giant dielectric anisotropy of +200. A relatively weak field of $3\times {10}^{7}V/m$ causes a significant change of birefringence of about 0.04. Both switching on and off times are approximately 10 µs. The effect is called a microsecond electrically modified order parameter effect and can be used in electro-optical devices, such as fast electro-optic shutters, phase modulators, and beamsteerers that require microsecond response times.

Recent progress has been made in creating terahertz magnons using ultrathin ferromagnetic layers. Due to their short lifetimes, these can be difficult to measure. Here we detail a calculation that shows that infrared magnons can be detected using standard reflection and attenuated total reflection measurements from a thin-film heterostructure made up of alternating ultrathin magnetic and nonmagnetic layers. We use an entire-cell effective-medium calculation to find the magnetic permeability of the heterostructure and then use electromagnetic boundary conditions to calculate reflectivity as a function of frequency. There are appreciable dips in the reflectivity at infrared magnon resonance frequencies, for realistic material parameters. Moreover, the strong coupling of magnon photons indicates the possible use of 50 GHz–1 THz magnons in integrated signal-processing devices.

We show that an intensity speckle can be directly interpreted as the properties of incident light – amplitude, phase, polarization, and coherency over spatial positions. Revisiting the speckle-correlation scattering matrix method [Lee and Park, Nat. Comm. 7, 13359 (2016)], we successfully extract the intact information of incident light from an intensity speckle snapshot as the form of a coherency matrix. The idea is experimentally verified by introducing the peculiar states of light that exhibit uneven amplitude, phase, polarization, and coherency features. We also find substantial practical advantage of the proposed method compared to the conventional coherency matrix measuring techniques such as Stokes polarimetry. We believe this physical interpretation of an intensity speckle could open a new avenue to study and utilize the speckle phenomenon in vast subfields of wave physics.

We propose an efficient optomechanical mass sensor operating at exceptional points (EPs), non-Hermitian degeneracies where eigenvalues of a system and their corresponding eigenvectors simultaneously coalesce. The benchmark system consists of two optomechanical cavities that are mechanically coupled, where we engineer mechanical gain (loss) by driving the cavity with a blue-detuned (red-detuned) laser. The system features an EP at the gain and loss balance, where any perturbation induces a frequency splitting that scales as the square root of the perturbation strength, resulting in a giant sensitivity-factor enhancement compared with conventional optomechanical sensors. For nonidentical mechanical resonators, quadratic optomechanical coupling is used to tune the frequency mismatch to get closer to the EP, extending the efficiency of our sensing scheme to mismatched resonators. This work paves the way toward new levels of sensitivity for optomechanical sensors, which could find applications in many other fields, including nanoparticle detection, precision measurement, and quantum metrology.