Full-waveform inversion (FWI) can produce previously unobtainable levels of accuracy and is revolutionizing the field of wave imaging. The basic principle is that a numerically produced data set is matched to the measured waveforms, enabling a high-resolution image to be produced since the model being inverted fully captures the physical behavior without approximation. This is achieved by gradually updating the numerical model using optimization algorithms. Currently, most FWI methods aim to recover material properties of a medium containing penetrable scatterers; however, there are many applications that, instead, require the boundary shapes of impenetrable objects to be reconstructed. Conventional velocity-style FWI will be trapped in local minima, with such problems being due to the extremely sharp contrast at the boundary. We propose a FWI procedure to directly recover the geometrical parameters of impenetrable obstacles via shape optimizations. The geometry is reconstructed by iteratively deforming the boundary of the target, following the negative direction of the geometrical boundary gradient. The boundary gradient is calculated from the shape derivatives of mass and stiffness matrices of a finite-element (FE) representation, when distorting the elements attached at the boundary. In addition, multiple-scattering events, which are more likely to occur between impenetrable obstacles, can be utilized automatically to provide substantial information for the inversion. Numerical and experimental results are shown to demonstrate the accuracy of the procedure for an example taken from the field of nondestructive evaluation, giving sizing within fractions of a wavelength for the tested cases; this step change in accuracy could be critical in sizing defects, enabling significantly more reliable decisions to be made about whether it is safe to continue using a component. Mathematical derivations and physical reasons for the success of our approach are illustrated.

The orbital angular momentum (OAM) of light has attracted a great amount of interest in recent times, due to the wide variety of applications it has made possible. Generally, the OAM spectrum is produced by coaxial superposition of several vortex beams, which might increase the complexity of the system. Here, we propose and experimentally demonstrate a simple way to produce a state of light with a controllable OAM spectrum using a binary array of pinholes. More specifically, we show that a spiral structure can convert a plane wave into a beam with a wide OAM spectrum, which can be easily tuned to pure or multiple OAM modes by adapting the structure of the pinhole plate. Furthermore, we show that a simple pinhole plate can produce structured beams with particular OAM states, such as photonic gears (superposition of OAM modes with opposite topological charges ±ℓ) and OAM combs (an optical mode formed by a series of discrete and equally spaced OAM modes, akin to an optical frequency comb). It is worth noting that we demonstrate the OAM comb experimentally. This study provides an avenue for the flexible generation of OAM spectra and the simplicity of the setup could make this approach convenient for many applications, such as optical communications and quantum information.

We consider an approach to fault tolerant quantum computing based on a simple error-detecting code operating as the substrate for a conventional surface code. We anticipate that the approach will be efficient when the noise in the hardware is skewed towards one channel (e.g., phase), or alternatively when the hardware is modular in such a fashion that the local error-detect operations have higher fidelity. We employ a customized decoder where information about the likely location of errors, obtained from the error-detection code, is translated into an advanced variant of the minimum-weight perfect-matching algorithm. A threshold gate-level error rate of 1.42% is found for the concatenated code given highly asymmetric noise. This is superior to the standard surface code and remains so as we introduce a significant component of depolarizing noise; specifically, until the latter is 70% the strength of the former. Moreover, given the asymmetric noise case, the threshold rises to 6.24% if we additionally assume that local operations have 20 times higher fidelity than long-range gates. Thus, for systems that are both modular and prone to asymmetric noise our code structure can be very advantageous.

Nitrogen-vacancy centers (NVCs) in diamond show promise for quantum computing, communication, and sensing. However, the best current method for entangling two NVCs requires that each one is in a separate cryostat, which is not scalable. We show that single NVCs can be laser written 6–15-µm deep inside of a diamond with spin coherence times that are an order of magnitude longer than previous laser-written NVCs and at least as long as naturally occurring NVCs. This depth is suitable for integration with solid immersion lenses or optical cavities and we present depth-dependent ${\mathrm{T}}_2$ measurements. 200 000 of these NVCs would fit into one diamond.

The switching of spin-transfer-torque magnetic random-access memory (STT MRAM) in the simple macrospin model is determined by the amplitude and pulse duration of the applied current, and it requires a current that is higher than a critical current, ${\mathit{I}}_{\mathit{c}}$. However, this critical current misses one fundamental physical issue for the commercialization of STT MRAM; the so-called nonswitching probability ($P_{\mathrm{NS}}$) or write soft-error rate (WSER), which is influenced by the stochastic nature of the switching process at finite temperature. Herein, we propose a limited stochastic switching (LSS) current, which is another definition for the critical current with the $P_{\mathrm{NS}}$ incorporated. The definition of the LSS current and the analytical expressions are obtained by solving the Fokker-Planck equation with a given specific $P_{\mathrm{NS}}$ value. Most importantly, by using the LSS current and optimizing it together with the related pulse-duration time, we find the optimum combination of current amplitude and pulse duration, which may reduce the energy consumption of the STT MRAM by up to 75%.

Antiferromagnetic materials as active components in spintronic devices promise insensitivity against external magnetic fields, the absence of own magnetic stray fields, and ultrafast dynamics at the picosecond time scale. Materials with certain crystal symmetry show an intrinsic Néel-order spin-orbit torque that can efficiently switch the magnetic order of an antiferromagnet. The tetragonal variant of $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$ has been shown to be electrically switchable by this intrinsic spin-orbit effect and its use in memory cells with memristive properties has recently been demonstrated for high-quality films grown using molecular-beam epitaxy. Here, we demonstrate that the magnetic order of magnetron-sputtered $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$ films can also be manipulated by electrical current pulses. The switching efficiency and relaxation as a function of the temperature, the current density, and the pulse width can be described by a thermal-activation model. Our findings demonstrate that $\mathrm{Cu}\mathrm{Mn}\mathrm{As}$ can be fabricated with an industry-compatible deposition technique, which will accelerate the development cycle of devices based on this remarkable material.

A negative electron affinity photocathode based on $\mathrm{Ga}\mathrm{As}$ nanopillar-array (NPA) Mie-type resonators is demonstrated and significant quantum efficiency enhancement is observed. Nanophotonic resonance-assisted photoelectron emission into vacuum is investigated, indicating an enhanced density of optical states due to increased light concentration and increased electron emission area. For visible wavelengths, the Mie resonances in $\mathrm{Ga}\mathrm{As}$ NPA reduce light reflectivity to less than 6% compared to a typical value >35% and result in maximum quantum efficiency 3.5 times greater than a $\mathrm{Ga}\mathrm{As}$ wafer photocathode without the NPA structure. Comprehensive simulations and experimental studies are presented.

Heat-recovery (HERC) solar cell—a concept of solar cells utilizing the heat for improving the power conversion efficiency—is presented. HERC solar cell, characterized by an absorber hotter than electrodes, recovers heat as electricity to have high conversion efficiency exceeding the detailed balance limit when carrier-energy filtering is employed. Being different from hot-carrier solar cells, HERC solar cell does not require fast carrier extraction within the thermalization time, which largely improves its feasibility (feasible even with Si). An increment in the conversion efficiency originates from thermoelectricity produced by the temperature difference. Requirements for materials of the filtering layers are also given based on a nonideal device simulation with the thermoelectric properties.

The emerging field of ionics-controlled magnetism is highly promising towards switching magnetism on and off magnetism at the touch of a button and efficient low-power-consumption spintronic devices. It can be achieved through the doping of ions, such as ${\mathrm{O}}^{2-}$ and ${\mathrm{Li}}^{+}$, into reactive magnetic materials. The advantages of a solid-state electrolyte far exceed those of a conventional electrolyte. Here, we utilize the all-solid-state ${\mathrm{Li}}^{+}$ redox capacitor structure to modulate the magnetism of amorphous ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ thin films. The change in magnetism of ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ due to lithiation-delithiation is investigated by measuring its magnetic hysteresis loop along the out-of-plane (in-plane) direction under the charge (discharge) state. Reversible saturation magnetization of 27% in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ thin films is achieved due to conversion reactions with ${\mathrm{Li}}^{+}$. Further lithiation causes the out-of-plane axis to become harder, and thus, leads to an increase in the magnetic anisotropy along the in-plane direction. This provides a means to control the magnetic properties of metallic magnetic thin films by solid-state ionics.

Acoustic resonators play a key role in the development of subwavelength-sized technologies capable of interacting with airborne audible sound, from its emission and absorption to its manipulation and processing. Specifically, artificial acoustic media made from an ensemble of subwavelength resonators, namely, acoustic metamaterials and metasurfaces, have enabled sound manipulation possibilities well beyond what is typically achievable using natural materials. Yet, the transition of such concepts from physics-driven explorations to practical applications has been drastically hindered by the major difficulty in controlling the resonance frequency, absorption level, and bandwidth of these resonators, making acoustic metamaterials often too narrowband, or sensitive to disorder and absorption losses. Here, we demonstrate the relevance of active electroacoustic resonators to address such limitations. We propose a feedback control scheme for loudspeakers (used as acoustic scatterers), which involves passband current control based on real-time sensing and processing of the pressure signal by field-programmable gate-array technologies. We demonstrate externally reconfigurable subwavelength acoustic resonators with independently tunable levels of absorption (including near-zero and near-one), bandwidth, and resonance frequency. We believe that this work demonstrates a viable route to overcome the current limitations of metamaterials and enable their practical applications.

We demonstrate an optical frequency standard based on the $5S_{1/2}\to 5D_{5/2}$ two-photon transition of rubidium. The transition is interrogated in a Doppler-free arrangement by two lasers at 780 and 776 nm, with the sum frequency of the two lasers locked to the two-photon transition. We measure the fractional frequency stability of the frequency standard to be $1.5\times {10}^{-13}$ at 1 s, reaching $4.9\times {10}^{-14}$ at approximately $100\mathrm{s}$, by comparison with an optical frequency comb. Performance limitations are presented and a clear pathway to an order-of-magnitude improvement is discussed. This platform has the potential to deliver a compact, robust standard for commercial and industrial applications.

A memtranstor that directly correlates charge (q) and magnetic flux (φ) via the nonlinear magnetoelectric effects is considered as the fourth memelement in addition to the memristor, memcapacitor, and meminductor. Just like the well-known memristor, the memtranstor also holds a great potential for developing advanced electronic devices such as nonvolatile memory and artificial synapses. Complete Boolean logic functions can be implemented in a single memtranstor made of the ${\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}/[\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3{]}_{0.7}[\mathrm{Pb}\mathrm{Ti}\mathrm{O}{]}_{0.3}/{\mathrm{Fe}}_{0.81}{\mathrm{Ga}}_{0.19}$ multiferroic heterostructure. There are four variables in a memtranstor: the initial state, the input electric potentials of two terminals, and the phase angle. By assigning different values to the four variables, 16 complete Boolean logic functions are demonstrated in experiments using a single memtranstor. The sign of the magnetoelectric voltage of the device is considered as logic output. The concurrence of nonvolatile memory and complete Boolean logic functions makes the memtranstor a promising candidate for future computing systems beyond von Neumann architecture.

Nonreciprocal propagation of acoustic waves is often achieved by magnetoelastic coupling with spin waves (SWs) and is limited to a narrow magnetoelastic gap (approximately 10–100 MHz) near the crossing point of the acoustic and SW dispersions. We propose a different method to achieve a giant frequency band of the surface acoustic wave (SAW) nonreciprocity in an artificial structure, where a nonmagnetic acoustic crystal is magnetoelastically coupled to a synthetic antiferromagnet—a pair of thin ferromagnetic layers the static magnetizations of which are held opposite by Ruderman-Kittel-Kasuya-Yoshida interaction. Strongly nonreciprocal and approximately linear dispersion of SWs in this system makes it possible to match dispersions of SAWs and SWs in a wide frequency band for one direction of the wave propagation. Example calculations performed for the ${\mathrm{Li}\mathrm{Nb}\mathrm{O}}_3/\mathrm{Co}/\mathrm{Ru}/\mathrm{Co}$ structure confirm the giant nonreciprocity of the SAW propagation in a frequency band exceeding 6 GHz.

Understanding the dynamics of the magnetic skyrmion, a particlelike topologically stable spin texture, and its response dynamics to external fields are indispensable for applications in spintronic devices. Here, Lorentz transmission electron microscopy is used to investigate the spin chirality of magnetic skyrmion bubbles (SKBs) in the centrosymmetric magnet $({\mathrm{Mn}}_{1-x}{\mathrm{Ni}}_x{)}_{65}{\mathrm{Ga}}_{35}$ at room temperature. The reversal of SKBs excited by the in-plane magnetic field is revealed. Moreover, the collective behavior of interacting spin chirality can be manipulated by reversing the directions of the magnetic fields on a wedge-shaped thin plate. The dynamic behavior of bubbles at different positions on the thin plate is explored through micromagnetic simulation, which indicates a nonuniform and nontrivial dynamic magnetization on the surfaces and center of the thin plate during spin chirality reversal. The results suggest that the controllable symmetry breaking of the SKBs, arising from thickness variation, provides the ability to manipulate the collective behavior of the spin chirality with small external fields, leading to a promising application in nonvolatile spintronic devices for magnetic skyrmions.

We demonstrate acoustic superscattering from a subwavelength scatterer consisting of multiple deep-subwavelength resonators. We show that superscattering is enabled by strong coupling between these resonators and the discrete rotational symmetry of the scatterer results in angle-dependent superscattering. From the cylindrical-wave expansion of an incident plane wave, we observe that radially arranged resonators can effectively couple to these expanded cylindrical waves. In addition, we provide an analytical model characterizing the coupled resonators, providing a critical understanding of superscattering. As an experimental demonstration, we present a superscatterer with six acoustic resonators that shows angle-dependent superscattering.

We consider theoretically exciton-polaritons in a strip of honeycomb lattice with zigzag edges and it is shown that the interplay among the spin-orbit coupling, Zeeman splitting, and an on-site detuning between sublattices can give rise to a band structure where all the edge states of the system split in energy. Within an energy interval, one of the spin-polarized edge states resides with the gapless bulk that has opposite spin. Being surrounded by opposite spin, and the absence of the backward-propagating edge state, ensures both reflection-free and feedback-suppressed one-way flow of polaritons with one particular spin in the system. The edge states in this system are more localized than those in the standard topological-polariton systems and are fully spin polarized. This paves the way for feedback-free spin-selective polariton channels for transferring information in polariton networks.

Simultaneous spatiotemporal confinement of energetic electron pulses to femtosecond and nanometer scales is a topic of great interest in the scientific community, given the potential impact of such developments across a wide spectrum of scientific and industrial applications. For example, in ultrafast electron scattering, nanoscale probes would enable accurate maps of structural dynamics in materials with nanoscale heterogeneity, thereby leading to an understanding of the role of boundaries and defects on macroscopic properties. On the other hand, advances in this field are mostly limited by the brightness and size of the electron source. We present the design, fabrication, and optical characterization of bullseye plasmonic lenses for next-generation ultrafast electron sources. Using electromagnetic simulations, we examine how the interplay between light-plasmon coupling, plasmon propagation, dispersion, and resonance governs the properties of the photoemitted electron pulse. We also illustrate how the pulse duration and strength can be tuned by geometric design and predict that sub-10-fs pulses with nanoscale diameter can be achieved. We then fabricate lenses in gold films and characterize their plasmonic properties using cathodoluminescence spectromicroscopy, demonstrating suitable plasmonic behavior for ultrafast nanoscale photoemission.

Fast magnetic fluctuations due to thermal torques have useful technological functionality ranging from cryptography to probabilistic computing. The characteristic time of fluctuations in typical uniaxial anisotropy magnets studied so far is bounded from below by the well-known energy-relaxation mechanism. This time scales as ${\alpha }^{-1}$, where $\alpha$ parameterizes the strength of dissipative processes. Here, we theoretically analyze the fluctuating dynamics in easy-plane and antiferromagnetically coupled nanomagnets. In such magnets, we find the dynamics are strongly influenced by fluctuating intrinsic fields, which give rise to an additional dephasing-type mechanism for washing out correlations. In particular, we establish two time scales for characterizing fluctuations: (i) the average time for a nanomagnet to reverse, which for the experimentally relevant regime of low damping is governed primarily by dephasing and becomes independent of $\alpha$; (ii) the time scale for memory loss of a single nanomagnet, which scales as ${\alpha }^{-1/3}$ and is governed by a combination of energy dissipation and dephasing mechanism. For typical experimentally accessible values of intrinsic fields, the resultant thermal-fluctuation rate is increased by multiple orders of magnitude when compared with the bound set solely by the energy-relaxation mechanism in uniaxial magnets. This could lead to higher operating speeds of emerging devices exploiting magnetic fluctuations.

High-power energetic nanosecond lasers, such as the Laser Mégajoule, require optical smoothing techniques to make their focal spots as homogeneous as possible. This strongly reduces parametric instabilities that occur during laser-plasma interaction, such as stimulated Brillouin scattering. Smoothing by spectral dispersion is one of the most-used techniques. It requires spectral broadening through phase modulation. However, because of propagation impairments, part of the phase modulation is converted into unwanted intensity modulation that induces laser nonlinearities and optical damage. This is called “FM-to-AM conversion.” Taking advantage of the architecture of the Laser Mégajoule, for which each “beam” (quadruplet) is split into four sub-beams, we show that the spectral broadening can be distributed on the sub-beams, reducing FM-to-AM conversion from 30% to 10% while maintaining smoothing performance for mitigation of stimulated Brillouin scattering.

We demonstrate the tunable quantum beat of single photons through the co-development of core nonlinear nanophotonic technologies for frequency-domain manipulation of quantum states in a common physical platform. Spontaneous four-wave mixing in a nonlinear resonator is used to produce nondegenerate, quantum-correlated photon pairs. One photon from each pair is then frequency shifted, without degradation of photon statistics, using four-wave-mixing Bragg scattering in a second nonlinear resonator. Fine tuning of the applied frequency shift enables tunable quantum interference of the two photons as they are impinged on a beam splitter, with an oscillating signature that depends on their frequency difference. Our work showcases the potential of nonlinear nanophotonic devices as a valuable resource for photonic quantum-information science.