We present a numerical study of a microelectromechanical-system-based design of a fiber cavity integrated with an ion-trap system. Each fiber mirror is supported by a microactuator that controls the mirror’s position in three dimensions. The mechanical stability is investigated by a feasibility analysis, which shows that the actuator offers stable support of the fiber. The actuators move the fibers’ positions continuously with a stroke of more than $10\mu \mathrm{m}$, with mechanical resonance frequencies on the order of kilohertz. A calculation of the trapping potential shows that a separation between the ion and the fiber consistent with strong ion-cavity coupling is feasible. Our miniaturized ion-photon interface constitutes a viable approach to integrated hardware for quantum information.

We develop a concept for a traveling-wave parametric amplifier driven by a magnetic flux wave. The circuit consists of a serial array of symmetric dc superconducting quantum-interference devices coupled inductively to a separate superconducting $LC$ transmission line carrying the pump wave. The adjusted phase velocity of the pump flux wave of frequency ${\omega }_p$ ensures amplification of the signal (${\omega }_s$) and idler (${\omega }_i$) waves, with frequencies obeying the relation ${\omega }_s+{\omega }_i={\omega }_p$. The advantage of the proposed flux-driven linear circuit includes a large gain over a wide frequency range and overcoming of the pump depletion problem. Unlike conventional traveling-wave amplifiers, the signal and pump in the proposed circuit are applied to different ports, what can greatly simplify the whole measurement setup. The experimental parameters and characteristics of this amplifier are evaluated and show promise for applications in quantum-information single-photon circuits.

Ambient seismic noise contains valuable information about the structural properties of Earth’s subsurface. The superconducting gravimeter (SG) is the most suitable instrument for temporal gravity, but not for microseisms due to its limited bandwidth. We report an alternative type of vertical superconducting accelerometer with a bandwidth as high as 0.7 Hz. Its sensitivity is comparable with the current SG. The accelerometer consists of a superconducting mass-spring oscillator and a displacement sensor based on a superconducting quantum interference device. The oscillator is constructed by levitating a Meisnner-state proof mass using combined positive-stiffness and negative-stiffness superconducting coils. Its natural frequency can be adjusted by the coil currents over a large range. The enhanced sensitivity of displacement sensing compensates the sensitivity loss imposed by the broadened bandwidth. Experimental tests show that both the gravity variation and the microseism can be extracted from the measurement data. It would be quite meaningful to monitor the temporal gravity and structural evolution using such an accelerometer array in sensitive zones, for example, the crustal fault and volcano zones.

Manipulation of tunneling spin-polarized electrons via a ferroelectric interlayer sandwiched between two ferromagnetic electrodes, dubbed multiferroic tunnel junctions (MFTJs), can be achieved not only by the magnetic alignment of two ferromagnets, but also by the electric polarization of the ferroelectric interlayer; this provides great opportunities for next-generation multistate memory devices. Here, we show that a ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{Mn}\mathrm{O}}_3(\mathrm{LSMO})/{\mathrm{Pb}\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3(\mathrm{PZT})/\mathrm{Co}$ structured MFTJ device can exhibit multilevel resistance states in the presence of gradually reversed ferroelectric domains via tunneling electroresistance and tunneling magnetoresistance, respectively. Nonvolatile ferroelectric control in the MFTJ can be attributed to separate contributions that arise from two independent ferroelectric channels in the PZT interlayer with opposite polarization. Our study shows the dominant role of “mixed” ferroelectric states on achieving accumulative electrical modulation of multilevel resistance states in MFTJs; thus paving the way for multifunctional device applications.

Existing thermal metamaterials are mostly designed to work at room temperature, where thermal conduction is the dominant method of heat transfer. Unfortunately, as the temperature increases, thermal radiation becomes non-negligible, and hence these thermal metamaterials can no longer work. The inability to handle thermal radiation largely limits practical applications at high temperatures, such as thermal protection. To solve this problem, here we propose an effective-medium theory to manipulate thermal radiation with the Rosseland diffusion approximation. This theory helps to design three radiative metamaterials even with anisotropic geometries, including transparency, cloak, and expander. Theoretical analyses are further confirmed by finite-element simulations, which indicate that these radiative metamaterials behave well in both steady and transient states. This work not only offers an effective-medium theory to manipulate thermal radiation but also designs three radiative metamaterials. These results may provide hints for novel thermal management and have potential applications in radiative camouflage, radiative diodes, etc.

The performance of solid-state quantum sensors based on electronic spin defects is often limited by the presence of environmental spin impurities that cause decoherence. A promising approach to improve these quantum sensors is to convert environment spins into useful resources for sensing, in particular, entangled states. However, the sensitivity enhancement that can be achieved from entangled states is limited by experimental constraints, such as control errors, decoherence, and time overheads. Here we experimentally demonstrate the efficient use of an unknown electronic spin defect in the proximity of a nitrogen-vacancy center in diamond to achieve both an entangled quantum sensor and a quantum memory for readout. We show that, whereas entanglement alone does not provide an enhancement in sensitivity, combining both entanglement and repetitive readout achieves an enhancement in performance over the use of a single-spin sensor, and more broadly we discuss regimes where sensitivity could be enhanced. Our results critically highlight the challenges in improving quantum sensors using entangled states of electronic spins, while providing an important benchmark in the quest for entanglement-assisted metrology.

Underwater acoustic plane waves are usually generated using transducer arrays with sophisticated control and synchronizing systems. Here, a subwavelength metasurface is proposed to convert incident cylindrical waves into plane waves by rectifying the local transmission phase through pentamode material design. The metasurface allows a high transmission because of the matched impedance between the pentamode material and water. It is also broadband since only the static material property is used in the design. A metasurface with graded unit cells is fabricated and tested in a two-dimensional water waveguide using transient cylindrical wave excitation. The experiment shows that the metasurface can convert cylindrical waves to plane waves over a broad frequency band of 15–23 kHz with an energy converting ratio as high as 69%. The working frequency band and converting efficiency can be further enhanced with smaller cells and a larger lateral size of the metasurface. This research may lead to new applications for controlling low-frequency waterborne sound, such as acoustic surveying, locating, as well as plane-wave generation for underwater acoustic-testing systems.

High-doping-induced Urbach tails and band-gap narrowing play a significant role in determining the performance of tunneling devices and optoelectronic devices such as tunnel field-effect transistors, Esaki diodes, and light-emitting diodes. In this work, Urbach tails and band-gap-narrowing values are calculated explicitly for $\mathrm{Ga}\mathrm{As}$, $\mathrm{In}\mathrm{As}$, $\mathrm{Ga}\mathrm{Sb}$, and $\mathrm{Ga}\mathrm{N}$ as well as for ultrathin bodies and nanowires of the same materials. Electrons are solved for by the nonequilibrium Green’s function method in multiband atomistic tight binding. Scattering on polar optical phonons and charged impurities is solved for in the self-consistent Born approximation. The corresponding nonlocal scattering self-energies and numerically efficient formulations for them are introduced for ultrathin bodies and nanowires. The predicted Urbach band tails and conduction-band-gap narrowing agree well with the experimental literature for a range of temperatures and doping concentrations. Polynomial fits of the Urbach tail and band-gap narrowing as a function of doping are tabulated for quick reference.

Using a time-resolved optically pumped scanning-optical-microscopy technique, we demonstrate the laser-driven excitation and propagation of spin waves in a 20-nm film of a ferromagnetic metallic alloy Galfenol epitaxially grown on a $\mathrm{Ga}\mathrm{As}$ substrate. In contrast to previous all-optical studies of spin waves, we employ laser-induced thermal changes of magnetocrystalline anisotropy as an excitation mechanism. A tightly focused 70-fs laser pulse excites packets of magnetostatic surface waves with an $e^{-1}$-propagation length of $3.4\mu \mathrm{m}$, which is comparable with that of permalloy. As a result, laser-driven magnetostatic spin waves are clearly detectable at distances in excess of $10\mu \mathrm{m}$, which promotes epitaxial Galfenol films to the limited family of materials suitable for magnonic devices. A pronounced in-plane magnetocrystalline anisotropy of the Galfenol film offers an additional degree of freedom for manipulating the spin waves’ parameters. Reorientation of an in-plane external magnetic field relative to the crystallographic axes of the sample tunes the frequency, amplitude, and propagation length of the excited waves.

State-of-the-art spin-transfer-torque magnetic random access memory (STT MRAM) is based on out-of-plane magnetized magnetic tunnel junctions (pMTJ), which commonly comprise a composite storage layer (CSL) of the form ${\mathrm{Fe}}_{72}{\mathrm{Co}}_8{\mathrm{B}}_{20}$/nonmagnetic spacer (e.g., $\mathrm{Ta}$, $\mathrm{W}$, $\mathrm{Mo}$)/${\mathrm{Fe}}_{72}{\mathrm{Co}}_8{\mathrm{B}}_{20}$. Because STT MRAM has to operate over a wide range of temperature (e.g., −40 °C to 150 °C for automotive applications), it is desirable to minimize the temperature variation of the magnetic and transport properties of pMTJs. In this context, we report an alternative design of a dual-$\mathrm{W}$ CSL of the form “${\mathrm{Fe}}_{72}{\mathrm{Co}}_8{\mathrm{B}}_{20}/\mathrm{W}/\mathrm{ferromagnet}/\mathrm{W}/{\mathrm{Fe}}_{72}{\mathrm{Co}}_8{\mathrm{B}}_{20}$”, which brings additional degrees of freedom, to further improve the performance of the storage layer. In particular, this design reduces the thermal variation of magneto-transport properties of dual-W CSL. A detailed investigation of the thermal variation of magnetic and electrical properties of such dual-$\mathrm{W}$ CSLs for various ferromagnet insertions, as characterized by different exchange stiffnesses, is reported. In particular, cobalt insertion between two $\mathrm{W}$ laminations significantly enhances the effective perpendicular anisotropy ($K_{\mathrm{eff}}$) at room temperature, relative to that of the conventional composite storage layer (${\mathrm{Fe}}_{72}{\mathrm{Co}}_8{\mathrm{B}}_{20}/\mathrm{W}/{\mathrm{Fe}}_{72}{\mathrm{Co}}_8{\mathrm{B}}_{20}$), and reduces the relative thermal variation of $K_{\mathrm{eff}}$ in the range 300–400 K. Moreover, STT MRAM cells with diameters of 80 nm comprising $\mathrm{Co}$ inserted into dual-$\mathrm{W}$ CSL exhibit, on average, 15% and 38% higher thermal stability factors, as well as 26% and 15% higher tunneling magnetoresistance ratios at room temperature and 110 °C, respectively, than those of conventional CSLs. These observations are interpreted in terms of magnetic stiffening of the storage layer achieved thanks to the very high Curie temperature (1388 K) of cobalt.

Over the past decades, geometric phases have brought profound insights in both quantum and classical physics. Although metasurfaces based on the Pancharatnam-Berry phase have made great achievements in polarization optics, chiral holograms, and even quantum entanglement, the construction of other types of geometric phases remains unclear. We propose to mimic a single ballistic Aharonov-Bohm ring using meta-atoms in both spin direction and polarization state changes during spin-material interactions, resulting in an analog of the Aharonov-Anandan phase of fermion particles. By combining the Aharonov-Anandan and Pancharatnam-Berry phases, the symmetric restriction of the geometric phase is finally broken. In this way, desired phase profiles can be easily imposed on arbitrary polarization states without restricting the orthogonal limitations. The proposed geometric phases allow flexible manipulations of the photonic spin-Hall effect and enable the superposition of arbitrary ${\mathrm{spin}}_{1/2}$ charges, thus breaking the conjugate constraints of the output orbital angular momentum modes. The results of this work expand the categories of geometric phases in metasurfaces and may find applications in polarization control and spin-enabled optics.

Laser electron accelerators have paved the way to build compact electron accelerators, but the quality of the electron beam should be improved to enable efficient use in various specialized applications. We demonstrate a proof-of-principle experiment for nanoparticle-assisted laser wakefield acceleration (NA LWFA) to improve the quality of the electron beam. Nanoparticles are generated through laser ablation of an aluminum target and introduced into a helium plasma to trigger the injection of electrons into the nonlinear plasma wake excited by 70 TW femtosecond laser pulses. We observe a significant enhancement of the electron beam energy, energy spread, and divergence compared with the case of self-injection. For instance, the best quality electron bunches present a maximum energy of up to 340 MeV, with a relative energy spread of 4.7% and a vertical divergence of 5.9 mrad. The initial results on NA LWFA are very promising and motivate further theoretical and experimental research into developing nanoparticle-assisted laser wakefield acceleration.

A superconducting nanowire single-photon detector (SNSPD) has played a significant role in numerous applications for visible and near-infrared photon detection. SNSPDs with high system detection efficiency (SDE greater than 90%) would enable remarkable experiments in quantum information processing. Currently, niobium nitride- ($\mathrm{Nb}\mathrm{N}$) based SNSPDs are widely used in practical applications since they can operate in an inexpensive compact closed-cycle cryostat. However, it is a challenge to realize a $\mathrm{Nb}\mathrm{N}$ SNSPD with saturated intrinsic detection efficiency (IDE) while maintaining its high SDE at near-infrared wavelengths. We develop a postprocessing method to enhance the IDE of $\mathrm{Nb}\mathrm{N}$ SNSPDs to saturation without sacrificing SDE through defect engineering by a 20-keV helium ion irradiation. The enhancement of IDE is achieved because of the irradiation-induced reduction of the superconducting energy gap and the electron density of states at the Fermi level as determined by the electrical transport measurements. The change in the optical absorptance of the irradiated SNSPD is insignificant at the resonant wavelength as confirmed by the measured optical reflectance and SDE. By taking advantage of the IDE enhancement, the SDE of an irradiated device is significantly increased from 49% to 92% at 2.2 K for a 1550-nm wavelength.

Light-matter interaction at the nanoscale is the foundation of studies in nanophotonics and nanoelectronics. It can be manipulated by modifying the local electromagnetic field. Therefore, detection of the local electromagnetic field is necessary for fundamental research and applications. Scanning probe microscopy has been used for near-field detection but it might perturb the detected field. Here we demonstrate noninvasive subdiffraction electromagnetic field sensing by using optical far-field superresolution microscopy for quantum-state manipulation and detection. The nitrogen-vacancy centers in diamond are used as multifunctional probes to investigate various optical and electrical properties of nanostructures. Both the spatial resolution and the accuracy of sensing are increased in comparison with the results of traditional diffraction-limited optical microscopy. On the basis of this method, we expect that a multifunctional quantum nanoscopy can be developed for future studies in nanoscience.

Graphene–hexagonal-boron-nitride–$\mathrm{In}\mathrm{Sb}$ near-field structures are designed and optimized to enhance the output power and energy efficiency of the thermophotovoltaic systems working in the temperature range of common industrial waste heat, from 400 to 800 K, which is also the working temperature range for conventional thermoelectric devices. We show that the highest output electric power can reach $7.6\times {10}^4\mathrm{W}/{\mathrm{m}}^2$ for the system with a graphene–hexagonal-boron-nitride heterostructure as the emitter and a graphene-covered $\mathrm{In}\mathrm{Sb}$ $p$-$n$ junction as the absorber, while the highest energy efficiency is achieved by the system with the heterostructure as the emitter and an uncovered $\mathrm{In}\mathrm{Sb}$ $p$-$n$ junction as the absorber (reaching to $34\mathrm{\%}$ of the Carnot efficiency). These results show that the performances of near-field thermophotovoltaic systems can be comparable with or even superior to state-of-the-art thermoelectric devices. The underlying physics for the significant enhancement of the thermophotovoltaic performance is understood as due to the resonant coupling between the emitter and the $p$-$n$ junction, where the surface plasmons in graphene and surface-phonon polaritons in boron nitride play crucial roles. Our study provides a stepping stone toward future high-performance thermophotovoltaic systems.

Quantum plasmonics experiments have on multiple occasions resulted in the observation of quantum coherence of discrete plasmons, which exhibit remarkable preservation of quantum interference visibility, a seemingly surprising feature for systems mixing light and matter with high Ohmic losses during propagation. However, most experiments to date used essentially weakly confined plasmons, which experience limited light-matter hybridization, thus limiting the potential for decoherence. In this paper, we investigate experimentally the robustness of coherence preservation in a plasmonic system: our setup is based on a hole-array chip supporting plasmons near the surface plasma frequency, where plasmonic dispersion and confinement are much stronger than in previous experiments, making the plasmons much more susceptible for decoherence processes. We, however, report preservation of quantum coherence even in these extreme conditions. We generate polarization-entangled pairs of photons using type-I spontaneous parametric down-conversion and transmit one of the photons through a plasmonic hole array that is numerically designed to convert incident single photons into highly dispersive single surface-plasmon polaritons. Our results show that the quality of photon entanglement after the plasmonic channel is unperturbed by the introduction of a highly dispersive plasmonic element. Our findings provide a lower bound of 100 fs for the pure dephasing time for dispersive plasmons in gold, and show that even in a highly dispersive regime surface plasmons preserve quantum mechanical correlations, making possible harnessing of the power of extreme light confinement for integrated quantum photonics.

A large magnetoresistance effect controlled by an electric field rather than a magnetic field or electric current is a preferable routine for designing low-power-consumption magnetoresistance-based spintronic devices. We propose an electric-field-controlled antiferromagnetic (AFM) tunnel junction with the structure of the piezoelectric substrate/${\mathrm{Mn}}_3\mathrm{Pt}/{\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3/\mathrm{Pt}$ operating by the magnetic phase transition (MPT) of antiferromagnet ${\mathrm{Mn}}_3\mathrm{Pt}$ through its magneto-volume effect. The transport properties of the proposed AFM tunnel junction are investigated by employing first-principles calculations. Our results show that a magnetoresistance over hundreds of percentages is achievable when ${\mathrm{Mn}}_3\mathrm{Pt}$ undergoes MPT from a collinear AFM state to a noncollinear AFM state. Band structure analysis based on density functional calculations shows that the large tunnel magnetoresistance can be attributed to the joint effect of significantly different Fermi surfaces of ${\mathrm{Mn}}_3\mathrm{Pt}$ at two AFM phases and the band symmetry filtering effect of the ${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_3$ tunnel barrier. In addition, other than the single-crystalline tunnel barrier, we also discuss the robustness of the proposed magnetoresistance effect by considering an amorphous ${\mathrm{Al}\mathrm{O}}_{\mathrm{x}}$ barrier. Our results may open a way for effective electrical writing and reading of the AFM state and its application in energy efficient magnetic memory devices.

Transparent conducting oxides (TCOs) with the functionality of transparency plus conductivity lie at the center of many technological applications. Commonly used TCOs are $n$-type. However, the performances of $p$-type TCOs lag far behind their $n$-type counterparts and the development of high-performance $p$-type TCOs remains an arduous challenge. The electron correlation due to on-site Coulomb interaction in the correlated metal oxides can not only promote modification of the valence band, which forms the basis for $p$-type conduction, but also shift the screened plasma energy to below the visible region for enhancing optical transparency. In this paper, the electron correlation is utilized as a key material design parameter to improve the performance of $p$-type TCOs. This approach is experimentally verified by decreasing electron correlation from ${\mathrm{Cr}}_2{\mathrm{O}}_3$ to ${\mathrm{V}}_2{\mathrm{O}}_3$, which, despite its high hole concentration ($>{10}^{22}{\mathrm{cm}}^{-3}$), has low screened plasma energy (approximately 0.97 eV) and significantly boosts the performance through good balance between the electrical conductivity and optical transparency. These results indicate that the strategy of electron correlation engineering paves an effective way for exploring high-performance $p$-type TCOs in strongly correlated oxides.

Kinetic Monte Carlo (KMC) simulations are used to study transport of guest molecules in a two-phase medium in which the minority phase forms closed regions. This type of model system resembles compositions of mixed-matrix membranes (MMMs) made of a matrix and imbedded filler particles with different permeabilities. Based on an ideal filler-matrix composite morphology as defined in [H. Vinh-Thang, S. Kaliaguine, Chem. Rev. 113, 4980, 2013], the effects of the filler-particle volume fraction, particle size, shape (aspect ratio), and the spatial particle distribution on gas transport through MMMs are addressed. The results obtained for nonoverlapping and randomly placed spherical filler particles are found to be in good agreement with the analytical models available in the literature and have proven their good accuracy also for thin membranes where finite-size effects are expected to play an appreciable role. Furthermore, the prominent influence of the aspect ratio for nonspherical particles on the effective permeability is shown and scenarios are discussed where the alignment of the asymmetric filler particles give rise to anisotropic transport properties potentially favorable for the performance of MMMs. The KMC approach developed guides the optimal spatial arrangement and orientation of the filler particles for the different strategies to increase the membrane permeability and separation selectivity by transport enhancement or inhibition.

A magnetically coupled dielectric elastomer actuator (MCDEA) is an emerging two-degree-of-freedom DEA system that demonstrates rich dynamic behaviors and promising potential applications in robotics, energy harvesting, and smart structures. However, the three-dimensional geometry, nonlinearity, and viscoelasticity in the system lead to complex nonlinear dynamic behavior that is challenging to predict. In this work, we develop a numerical model that can accurately characterize its dynamic responses. This model, along with experimental results, demonstrates the complex dynamic phenomena of this MCDEA system, including superharmonic, primary-harmonic, and subharmonic resonances, bifurcations, and multiple modes of oscillation. Dynamic control strategies including phase tuning and frequency control are proposed in this work, allowing control of the amplitude and the appearance of a specific resonance and the occurrence of emerging dynamic behaviors, such as beat phenomena. These findings have numerous additional applications in areas including active vibrational control, energy harvesting, and programmable soft motors for on-demand locomotion.