In this paper, we propose a subwavelength closed surface consisting of radial gradient grooves for achieving localized acoustic rainbow trapping. Similar to conventional rainbow trapping based on a plane structured surface, we demonstrate that rainbow trapping can also be realized by a closed surface in the deep-subwavelength scale. In particular, the trapped spatial positions on the closed surface can be freely tuned by changing the structural parameters. Based on the advantages of the multiband response and subwavelength dimension of this structure, we propose an acoustic barrier consisting of an array of closed surfaces to achieve multiband sound insulation and high ventilation simultaneously. Investigated results indicate that this kind of acoustic barrier possesses the features of subwavelength scale, and multiband and omnidirectional responses for blocking incident acoustic waves, yet has high ventilation. Finally, we discuss the influence of the channel width on the magnitude and bandwidth of the transmission loss spectrum. The experimental results agree with our theoretical prediction that the material losses can broaden the response bandwidth, and form broadband sound insulation. The acoustic barrier consisting of localized rainbow trapping structures may have many applications in infrastructure requiring air-permeability and sound-proofing.

Wide-field magnetometry can be realized by imaging the optically detected magnetic resonance of diamond nitrogen-vacancy ($\mathrm{N}$-$V$) center ensembles. However, $\mathrm{N}$-$V$ ensemble inhomogeneities significantly limit the magnetic field sensitivity of these measurements. We demonstrate a double-double quantum (DDQ) driving technique to facilitate wide-field magnetic imaging of dynamic magnetic fields at a micron scale. DDQ imaging employs four-tone radio-frequency pulses to suppress inhomogeneity-induced variations of the $\mathrm{N}$-$V$ resonance response. As a proof of principle, we use the DDQ technique to image the dc magnetic field produced by individual magnetic nanoparticles tethered by single DNA molecules to a diamond-sensor surface. This demonstrates the efficacy of the diamond $\mathrm{N}$-$V$ ensemble system in high-frame-rate magnetic microscopy, as well as single-molecule biophysics applications.

We report on the magnetic damping properties of polycrystalline $\mathrm{Fe}\text{−}\mathrm{V}$ alloy thin films that are deposited at room temperature. By varying the concentration of $\mathrm{V}$ in the alloy, the saturation magnetization can be adjusted from that of $\mathrm{Fe}$ to near zero. We show that exceptionally low values of the damping parameter can be maintained over the majority of this range, with a minimum damping at approximately 15%–20% $\mathrm{V}$ concentration. Such a minimum is qualitatively reproduced with ab initio calculations of the damping parameter, although at a concentration closer to 10% $\mathrm{V}$. The measured intrinsic damping has a minimum value of (1.53 ± 0.08) × ${10}^{-3}$, which is approximately a factor of 3 higher than our calculated value of 0.48 × ${10}^{-3}$. From first-principles theory, we outline the factors that are mainly responsible for the trend of the damping parameter in these alloys. In particular, the band structure and resulting damping mechanism is shown to change at $\mathrm{V}$ concentrations greater than approximately 35% $\mathrm{V}$ content.

The advent of reliable nanoscale memristive components is promising for next-generation compute-in-memory paradigms; however, the intrinsic variability in these devices has prevented widespread adoption. Here, we show coherent electron wave functions play a pivotal role in the nanoscale transport properties of these emerging nonvolatile memories. By characterizing both filamentary and nonfilamentary memristive devices as disordered Anderson systems, the switching characteristics and intrinsic variability arise directly from the universality of electron transport in disordered media. Our framework suggests that localization phenomena in nanoscale solid-state memristive systems are directly linked to circuit-level performance. We discuss how quantum conductance fluctuations in the active layer set a lower bound on device variability. This finding implies that there is a fundamental quantum limit on the reliability of memristive devices and that electron coherence will play a decisive role in surpassing or maintaining Moore’s law with these systems.

A mechanism of optical intensity modulation is proposed by utilizing the electro-optic coupling in distributed semiconductor heterojunctions of p-type silicon ($\mathrm{Si}$) and n-type indium tin oxide (ITO) in the form of the subwavelength grating in a rib waveguide. The coupled multiple semiconductor heterojunctions of $\mathrm{Si}$-ITO are made to exhibit efficient optical intensity modulation via electrically tunable permittivity of ITO. The subwavelength grating is a nanophotonic element that not only provides a way to couple multiple heterojunctions, but it also gives rise to efficient optical (fiber to chip) coupling at a wavelength of 1550 nm. Lateral coupling of distributed heterojunctions via depleted charge density distributed along vertical and horizontal directions enable the device to show a high extinction ratio of 24 dB. Also, electrical tuning of the coupling efficiency for an 80-µm long device is reported, which exhibits the multifunctional nature of the proposed nanophotonic device. The proposed modulation scheme, with a modulation efficiency of 0.34 V/mm and energy consumption of 36 pJ, may open pathways for energy-efficient compact devices and circuits for large-scale optoelectronic integration. The proposed mechanism of optical modulation takes advantage of distributed semiconductor heterojunctions and enables electrically tunable inherent optical coupling with a nanophotonic element called a subwavelength grating, which further improves the modulation performance compared with a conventional $\mathrm{Si}$-ITO heterojunction.

In this paper we prove that any conformal transformation of a wave can be produced via a suitably arranged cascade of two, or at most four, discrete phase elements satisfying Laplace’s equation. Although this result is of general applicability, in the case of charged-matter waves it implies that such transformations can be exactly obtained by employing only electrostatic or magnetostatic phase elements. Furthermore, we illustrate how a basis for such generating phase elements is given by integer and fractional charge multipoles, proving that these transformations can be used to perform the efficient sorting of multipole-induced quantum states. This provides a fast, compact, and direct method to measure the strength and orientation of dipole systems and of astigmatism. It thus adds a further observable to the four whose spectrum can already be directly measured via spatial separation on the detector, i.e., position, momentum, energy, and orbital angular momentum. The results hold true in optics and for all kinds of charged-particle beams of sufficient coherence.

Acoustic vortices with subwavelength dimensions and tunable topological charge are theoretically and experimentally synthesized at distances far beyond the Rayleigh diffraction length of the source, using self-demodulation. A dual helical acoustic source is used to generate two primary confocal vortex beams at different frequencies and different topological charges. As a consequence of the conservation of angular momentum during nonlinear wave mixing, a self-demodulated vortex beam at the difference frequency emerges, keeping the spatial features of the primary vortex beams and a topological charge that is the difference of their topological charges. We report subdiffractive vortices the characteristic size of which is 18 times smaller than its wavelength at a distance 2.8 times the Rayleigh diffraction length. The generation and focusing of subwavelength vortices paves the way for long-range communication, biomedical, and wave-matter interaction applications.

Stimulated Brillouin scattering (SBS) is well recognized as being useful in beam shaping, quantum information processing, optical fiber sensing, and LIDAR remote sensing. Here we theoretically and experimentally study the dependence of threshold value and gain coefficient of SBS in water on temperature, pressure, and attenuation coefficient. Numerical simulations of the temporal waveform, threshold value, and gain coefficient of SBS as a function of temperature, pressure, and attenuation coefficient, respectively, are performed by employing distributed noise model and coupled wave equations. A temperature-pressure simulator was designed to carry out the experimental measurements to validate the numerical simulations. We show that the threshold value of SBS increases with the increases of pressure and attenuation coefficient and decreases with the increase of temperature, while the gain coefficient of SBS shows the reverse change trend. We find that the temperature and attenuation coefficient have greater effect on threshold value and gain coefficient of SBS than that of pressure. Our results reveal the roles of temperature, pressure, and attenuation coefficient in threshold value and gain coefficient of SBS, which have so far largely been overlooked. This work is essential to future applications of SBS in laser systems, quantum communication, fiber sensing, and LIDAR technology.

Skyrmions are continuous but topologically nontrivial field configurations that behave like particles. They play an important role in fields ranging from condensed matter to nuclear physics. Realizing and manipulating different kinds of motions of skyrmions are still great challenges. Here, three-dimensional skyrmions, in the form of double-twist tori terminated on two topological point defects called “torons,” are produced in chiral nematic liquid crystals with varied helical pitches. The spontaneous formation of skyrmions and high-degree multiskyrmion configurations called “skyrmion bags” are characterized by polarizing microscopy. The skyrmions show anomalous diffusion in the equilibrium state and perform collective directional motion driven by fluid flows, with a velocity controllable by electric fields and surface anchoring. They can be utilized as vehicles for microcargo transport. We also show that skyrmions can collide and form clusters with tunable shape, anisotropy, and fractal dimension.

The study of nonreciprocal wave propagation is of great interest for both fundamental research and engineering applications. Here we demonstrate theoretically and experimentally a bidirectional, nonreciprocal, and high-quality diode that can rectify elastic waves in both forward and backward directions in an elastic metamaterial designed to exhibit enhanced nonlinearity of resonances. This diode can preserve or vary frequency, rectify low-frequency long wave with small system size, offer high-quality insulation, can be modulated by amplitude, and break reciprocity of both the total energy and fundamental wave. We report three mechanisms to break reciprocity: the amplitude-dependent band gap combining interface reflection, chaotic response combining linear band gap, amplitude-dependent attenuation rate in damping diode. The bidirectional diode paves ways for mutually controlling information and energy transport between two sources, which can be used as wave insulators.

We report on the strong coupling between a metallic ferromagnetic ${\mathrm{Fe}}_{75}{\mathrm{Co}}_{25}$ thin-film patterned element and a range of superconducting $\mathrm{Nb}$ half-wavelength co-planar waveguide (CPW) resonators. By varying the volume of the ferromagnet we demonstrate that the coupling rate scales linearly with the square root of the number of spins and achieve a coupling rate over 700 MHz, approaching the ultrastrong coupling regime. Experiments varying the center conductor width while maintaining constant magnetic volume verify that decreasing the center conductor width increases coupling and cooperativity. Our results show that the frequency dependence of the coupling rate is linear with the fundamental and higher-order odd harmonics of the CPW, but with differing efficiencies. The results show promise for scaling planar superconducting resonator-magnetic hybrid systems to smaller dimensions.

For neutral-atom qubits, two-qubit gates are typically realized via the Rydberg-blockade effect, which hints at the special status of Rydberg levels compared with regular qubit register states. However, the experimental and theoretical studies that have been done to reveal how the ground-Rydberg coherence of the control-qubit atom affects the fidelity of a two-qubit gate are not yet adequate, especially when all experimental two-qubit gates feature a single atom that stays in the Rydberg state for a certain time. Here, we perform a careful investigation of decoherence in ground-Rydberg transitions in a single atom with various pulse sequences. In particular, based on the commonly used controlled-$Z$ ($C_Z$) gate sequence proposed by D. Jaksch et al., Phys. Rev. Lett. 85, 2208 (2000), we observe that a control-qubit atom subject to a two-qubit entangling process with the $H$-$C_Z$ controlled-not gate scheme experiences a process that is essentially similar to ground-Rydberg Ramsey interference. Then, we build a straightforward theoretical model to link the decoherence time ${\tau }_{\mathrm{gr}}$ of the control qubit (extracted from Ramsey fringes of the ground-Rydberg transition) to this $C_Z$ pulse sequence, and also analyze the typical origins of decoherence effects. Finally, we discuss the loss of fidelity of the $C_Z$ gate due to the limits imposed by the ground-Rydberg coherence properties, and the prospect of improving fidelity with refined gate protocols.

The physical properties, including the electrocaloric effect (ECE), of polycrystalline ferroelectrics are highly dependent on grain size in the materials. Using a phase-field model based on the time-dependent Ginzburg-Landau equation, the electrocaloric properties of ferroelectric ceramics with different grain sizes are investigated. The ferroelectric hysteresis loops under different temperatures are calculated and the adiabatic temperature changes (ATCs) are obtained through the indirect method based on the Maxwell relation. Among the investigated models with different grain sizes, the model with larger grain size possesses a larger value of ATC as well as a higher temperature at which the maximum ATC exhibits. In contrast, for the model with smaller grain size, the value of ATC declines and the temperature related to the maximum ATC shifts to a lower temperature. The significant influence of grain sizes on the ECE of polycrystalline ferroelectrics is due to the different type of domain patterns formed inside the grains. Compared to the vortex domains formed in the model with smaller grain size, the 90° domains and single domains shown in larger grain size contribute to a more pronounced change of temperature-dependent polarization, which results in the larger ECE. In addition, the shifting of corresponding temperature to a lower one in smaller grain sizes is due to the size-effect-induced decrease of transition temperature. This work suggests a degree of freedom to understand the grain-size-dependent ECE properties of polycrystalline ferroelectrics from the perspective of domain structure and its evolution with temperature changing, which can be further studied to improve the performance of solid-state cooling devices.

The realization of practical spintronic devices relies on the ability to create and detect pure spin currents. In graphene-based spin valves, this is usually achieved by the injection of spin-polarized electrons from ferromagnetic contacts via a tunnel barrier, with ${\mathrm{Al}}_2{\mathrm{O}}_3$ and $\mathrm{Mg}\mathrm{O}$ used most widely as barrier materials. However, the requirement to make these barriers sufficiently thin often leads to pinholes and low contact resistances, which, in turn, results in low spin-injection efficiencies, typically 5% at room temperature, due to the so-called resistance mismatch problem. Here, we demonstrate an alternative approach to fabricate ultrathin tunnel-barrier contacts to graphene. We show that laser-assisted chemical functionalization of graphene with $s{p}^3$-bonded phenyl groups effectively provides a seed layer for the growth of ultrathin ${\mathrm{Al}}_2{\mathrm{O}}_3$ films, ensuring smooth high-quality tunnel barriers and an enhanced spin-injection efficiency. Importantly, the effect of functionalization on spin transport in the graphene channel itself is relatively weak, so that the enhanced spin injection dominates and leads to an order of magnitude increase in spin signals. Furthermore, the spatial control of functionalization using a focused laser beam and lithographic techniques can, in principle, be used to limit functionalization to contact areas only, further reducing the effect on the graphene channel. Our results open a route towards circumventing the resistance mismatch problem in graphene-based spintronic devices based on easily available and highly stable ${\mathrm{Al}}_2{\mathrm{O}}_3$ and facilitate a step forward in the development of their practical applications.

Experiments have shown that graphene origami (GORI) can be formed by compressing a folio graphene ribbon. In this study, the GORI is introduced to a rotary nanomotor built from a double-walled carbon nanotube. By placing several cantilevered GORI around the long inner tube (rotor) of the nanomotor with their tips toward the rotor, the nonbonding interaction between the tips and the rotor becomes strong when their distance is less than 1 nm. Rotationally symmetrical layout of the GORI tends to induce nonzero unidirectional torque on the rotor. Thereout, the rotor is accelerated to rotate until a stable rotational frequency (SRF) is reached on the condition that the stators (short outer tubes) applying equal but opposite torque on the rotor. The molecular-dynamics simulation approach is adopted to verify the procedure of the proposed rotary nanomotor. Conclusions are drawn for fabrication of the present rotary nanomotor.

Controlling directional emission of nanophotonic radiation sources is fundamental to tailor radiation-matter interaction and to conceive highly efficient nanophotonic devices for on-chip wireless communication and information processing. Nanoantennas coupled to quantum emitters have proven to be very efficient radiation routers, while electrical control of unidirectional emission has been achieved through inelastic tunneling of electrons. Here we prove that the radiation emitted from the interaction of a high-energy electron with a graphene-nanoparticle composite has beams in directions that can be made to continuously span the full azimuthal circle even through small variations of the graphene Fermi energy. Emission directionality stems from the interference between the double-cone-shaped electron transition radiation and the nanoparticle dipolar diffraction radiation. Tunability is enabled since the composite hybrid plasmonic resonances and the graphene plasmon polariton phase drive the nanoparticle dipole moment, thus providing an effective electrical reorientation of the nanoantenna. The flexibility of our method provides a way to exploit graphene plasmon physics to conceive nanosources with ultrafast reconfigurability.

Photoluminescence measurements on high-quality $(\mathrm{In},\mathrm{Ga})\mathrm{N}$ quantum wells reveal that carriers diffuse laterally to long distances at room temperature, up to tens of microns. This behavior, which shows a pronounced dependence on the excitation density, contrasts with the common expectation of a short diffusion length. The data is well explained by a diffusion model taking into account the full carrier recombination dynamics, obtained from time-resolved measurements. These observations have important implications for understanding the high efficiency of III-nitride emitters, but also for proper interpretation of photoluminescence experiments and for the design of efficient small-scale devices.

We examine the role of a silicon-based amorphous insulating substrate in the thermal relaxation in thin $\mathrm{Nb}\mathrm{N}$, ${\mathrm{In}\mathrm{O}}_x$, and $\mathrm{Au}$/$\mathrm{Ni}$ films at temperatures above 5 K. The samples studied consist of metal bridges on an amorphous insulating layer lying on or suspended above a crystalline substrate. Noise thermometry is used to measure the electron temperature $T_e$ of the films as a function of Joule power per unit area $P_{2\mathrm{D}}$. In all samples, we observe a $P_{2\mathrm{D}}\propto T_e^n$ dependence, with exponent $n\simeq 2$, which is inconsistent with both electron-phonon coupling and Kapitza thermal resistance. In suspended samples, the functional dependence of $P_{2\mathrm{D}}(T_e)$ on the length of the amorphous insulating layer is consistent with the linear temperature dependence of the thermal conductivity, which is related to lattice excitations (diffusons) for a phonon mean free path shorter than the dominant phonon wavelength. Our findings are important for understanding the operation of devices embedded in amorphous dielectrics.