Focusing light using electrically thin layers is of paramount importance in several applications, from integrated optics to microwave engineering and sensing. Recently, gradient metasurfaces, which are electrically thin arrays of densely located polarizable particles, have been employed to perform different wave-front transformations, including focusing. In comparison to a bulk lens, these designs provide ultrathin geometries, but they suffer from fundamental limitations on their overall efficiency and achievable numerical aperture. Metagratings offer a solution for efficient beam steering at large angles, but it is challenging to utilize them in the small-angle limit. Here, we introduce a hybrid metalens design, which provides dramatic enhancement in lensing performance compared with that of state-of-the-art metasurfaces, combining metagratings and conventional gradient approaches. Our experimental prototype enables microwave focusing with large efficiency $(\eta =0.479)$ and near-unity numerical aperture $(\mathrm{NA}=0.98)$, yielding a sharp focal point at the diffraction limit in the far field $(\mathrm{FWHM}=0.332\lambda )$. We propose a hybrid metalens design with exceptional performance in terms of efficiency and numerical aperture, opening up opportunities for high-throughput optical lithography, high-density data recording, focal plane arrays, radar, and communication systems.

Band engineering near Dirac points is an emerging topic in condensed matter physics and photonics, enabling multifaceted devices of record-high conductivity and zero refractive index in both electronic and photonic structures. Recently, an extended class of Dirac cone, type I, II, or III, has attracted much attention with its controlled directionality of singular wave behaviors near the Dirac points. However, despite the significance of the discovery and applications of each type of Dirac cone, the theoretical framework and unified design structure bridging different types of Dirac cones have not been clearly elucidated. Here, we propose a universal design platform for all types of photonic Dirac cones and reveal the key parameter determining their types and transformations. Starting from the accidentally induced photonic Dirac cone, we show that the inverse design of an optical potential landscape enables the deterministic control of the spectral ordering, which leads to designer construction of each type of Dirac cone. This approach will pave the way for the design of artificial degeneracies with tunable anisotropy for general wave systems.

Piezotronics is an emerging field, which exploits strain to control the transport properties in condensed matters. At present, piezotronics research majorly focuses on insulators with tunable electric dipole by strain. Metals are excluded in this type of application due to the absence of the electric dipole. The recently discovered Berry-curvature dipole can exist in metals, consequently introduces the possibility of the piezoelectric phenomena in them. In this paper, we predict that strain can switch the Berry-curvature dipole, and lead to the nonlinear Hall effect in the two-dimensional (2D) $1H\text{−}{\mathrm{MX}}_2$ ($\mathrm{M}=\mathrm{Nb},\mathrm{Ta}$; $\mathrm{X}=\mathrm{S},\mathrm{Se}$). Based on symmetry analysis and first-principles calculations, we show these 2D monolayer metals have the desired piezoelectriclike property: without strain, the Berry-curvature dipole is eliminated by symmetry, prohibiting the nonlinear Hall effect; while uniaxial strain can effectively reduce the symmetry to introduce sizable Berry-curvature dipole, and it can generate observable Hall voltage under a reasonable experimental condition. Due to the nonlinear and topological properties, the piezoelectriclike property here is quite different from the traditional one based on the electric dipole. Compared with the traditional piezoelectric materials, which can only exist in insulators, we manifest that the 2D metallic $1H\text{−}{\mathrm{MX}}_2$ ($\mathrm{M}=\mathrm{Nb},\mathrm{Ta}$; $\mathrm{X}=\mathrm{S},\mathrm{Se}$) can be applied in the platform for piezoelectriclike devices such as strain sensors, terahertz detections, energy harvesters etc.

Superdirective end-fire radiation in the low-GHz frequency range is demonstrated using magnetically coupled structures of subwavelength metallic helices. Numerical, experimental, and analytical results are presented on superdirective dimers that are almost 3 times smaller than those of previously demonstrated dimers of split-ring resonators (0.09λ compared with 0.25λ) and provide close to theoretical maximum values of directivity without using complex feeding networks. The size, directivity, efficiency, and operational passband width of such structures is optimized.

The electronic properties of anatase titanium dioxide (${\mathrm{Ti}\mathrm{O}}_2$) thin films epitaxially grown on ${\mathrm{La}\mathrm{Al}\mathrm{O}}_3$ substrates are investigated by synchrotron-x-ray spectroscopy [x-ray absorption spectroscopy (XAS), x-ray photoemission spectroscopy (XPS), and angle-resolved photoemission spectroscopy (ARPES)] and infrared spectroscopy. The ${\mathrm{Ti}}^{3+}$ fraction in ${\mathrm{Ti}\mathrm{O}}_{2-x}$ is varied either by changing the oxygen pressure during deposition or by postgrowth annealing in ultrahigh vacuum (UHV). Structural investigation of the ${\mathrm{Ti}\mathrm{O}}_2$ thin films provides evidence of highly uniform crystallographic order in both as-grown and in situ UHV-annealed samples. The increased amount of ${\mathrm{Ti}}^{3+}$ as a consequence of UHV annealing is calibrated by in situ XPS and XAS analysis. The as-grown ${\mathrm{Ti}\mathrm{O}}_2$ samples, with a low $\mathrm{Ti}$${}^{3+}$ concentration, show distinct electronic properties with respect to the annealed films, namely, absorption in the midinfrared (MIR) region correlated with polaron formation, and another peak in the visible range at 1.6 eV correlated with the presence of localized defect states (DSs). With the increasing level of ${\mathrm{Ti}}^{3+}$ induced by the postannealing process, the MIR peak disappears, while the DS peak is redshifted to the near-infrared region at about 1.0 eV. These results indicate the possibility of tailoring the optical absorption of anatase ${\mathrm{Ti}\mathrm{O}}_2$ films from the visible to the near-infrared region.

Room-temperature operation of a spin exclusive-or (xor) gate is demonstrated in lateral spin-valve devices with nondegenerate silicon ($\mathrm{Si}$) channels. The spin xor gate is a fundamental part of the magnetologic gate that enables reconfigurable and nonvolatile nand or or operation in one device. The device for the spin xor gate consists of three iron ($\mathrm{Fe}$)/cobalt ($\mathrm{Co}$)/magnesium oxide ($\mathrm{Mg}\mathrm{O}$) electrodes (i.e., two input electrodes and one output electrode). Spins whose spin angular momentum corresponds to the binary input 1 or 0 are injected into the $\mathrm{Si}$ channel from the input electrodes. The spin-drift effect is controlled by a lateral electric field in the $\mathrm{Si}$ channel to adjust the spin-accumulation voltages under two different parallel configurations, corresponding to (1, 1) and (0, 0), so that they exhibit the same value. As a result, the spin-accumulation voltage detected by the output electrode exhibits three different voltages, represented by an xor gate. The one-dimensional spin-drift–spin-diffusion model clearly explains the xor behavior obtained. Charge-current detection of the spin xor gate is also demonstrated. The detected charge current has a maximum of 1.67 nA, the highest value in spin xor gates reported thus far. Furthermore, gate-voltage modulation of the spin xor gate is also demonstrated, which enables operation of multiple magnetologic gate devices.

Photon scattering imposes a fundamental restriction on optical imaging in turbid media. In this work, we propose a compressive-sensing-based photoacoustic imaging modality that allows single-shot tomography with a single detector in a scattering medium. The nonuniform optical speckle grains created in the diffusive regime, which act as a conventional drawback of optical modality, are used to generate ultrasound locally. The photoacoustic signals from spatial positions can be well extracted from a superimposed signal via introduction of locally modulated time delays. Taking advantage of the compressed measurement assisted by the acoustic mask, we demonstrate a theoretical compressed framework of three-dimensional photoacoustic surface tomography with a broad field of view after one-time optical illumination. This approach can increase the photoacoustic imaging efficiency greatly and reveals the potential for dynamic imaging in optically diffusive media.

Entanglement is a fundamental resource for quantum information science. However, bipartite entanglement is destroyed when one particle is observed via projective (sharp) measurements, as is typically the case in most experiments. Here, we show experimentally that, if instead of sharp measurements, one performs many sequential unsharp measurements on one particle that are suitably chosen depending on the previous outcomes, then entanglement is preserved and it is possible to reveal quantum correlations through measurements on the second particle at any step of the sequence. Specifically, we observe that pairs of photons entangled in polarization maintain their entanglement when one particle undergoes three sequential measurements and that each of these can be used to violate a Clauser-Horne-Shimony-Holt inequality. This proof-of-principle experiment demonstrates the possibility of repeatedly harnessing two crucial resources, entanglement and Bell nonlocality, that, in most quantum protocols, are destroyed after a single measurement. The protocol we use, which in principle works for an unbounded sequence of measurements, can be useful for randomness extraction.

High-quality micro- and nanomechanical resonators are widely used in sensing, communications, and timing, and have future applications in quantum technologies and fundamental studies of quantum physics. Crystalline thin films are particularly attractive for such resonators due to their prospects for high quality, high intrinsic stress, high yield strength, and low dissipation. However, when such films are grown on a silicon substrate, interfacial defects arising from lattice mismatch with the substrate have been postulated to introduce additional dissipation. Here, we develop a back-side etching process for single-crystal silicon carbide microresonators that allows us to quantitatively verify this prediction. By engineering the geometry of the resonators and removing the defective interfacial layer, we achieve quality factors exceeding a million in silicon carbide trampoline resonators at room temperature, a factor of five higher than those achieved without removal of the interfacial defect layer. We predict that similar devices fabricated from ultrahigh-purity silicon carbide, leveraging its high yield strength, could enable room-temperature quality factors as high as $6\times {10}^9$.

Using the nonequilibrium Green’s function combined with density-functional theory, we systematically study the transport properties of the zigzag graphene nanoribbon (ZGNR). When an external transverse electric field is applied, a ZGNR can exhibit half-metallic characteristics with its conductive channel localized at the edge positions. Accordingly, an in-plane dual-gated spin-valve device is proposed. Thanks to its unique design, the proposed device overcomes the bottleneck of current leakage and avoids contact issues. Remarkably, a 100% spin injection efficiency and a giant tunnel magnetoresistance of up to ${10}^7$ are realized, which represent much better performance than that of traditional magnetic tunnel junctions. In addition, we also explore the effect of architecture reconstruction and impurity doping on spin-polarized transport. It is found that, in general, the tunneling process is hindered if disorder occurs at the conductive edge. By contrast, if we put the deformation or impurities away from the conductive edge, the transport properties may be barely degraded or even improved, depending on the type of disorder. It should be possible to apply these ubiquitous underlying principles to other materials, which could inspire the design of alternative spintronic devices with high performance.

The measurement of a quantum system is often performed by encoding its state in a single observable of a light field. The measurement efficiency of this observable can be reduced by loss or excess noise on the way to the detector. Even a quantum-limited detector that simultaneously measures a second noncommuting observable would double the output noise, therefore limiting the efficiency to $50\mathrm{\%}$. At microwave frequencies, an ideal measurement efficiency can be achieved by noiselessly amplifying the information-carrying quadrature of the light field but this has remained an experimental challenge. Indeed, while state-of-the-art Josephson-junction-based parametric amplifiers can perform an ideal single-quadrature measurement, they require lossy ferrite circulators in the signal path, drastically decreasing the overall efficiency. In this paper, we present a nonreciprocal parametric amplifier that combines single-quadrature measurement and directionality without the use of strong external magnetic fields. We extract a measurement efficiency of ${62}_{-9}^{+17}\mathrm{\%}$ that exceeds the quantum limit and that is not limited by fundamental factors. The amplifier can be readily integrated with superconducting devices, creating a path for ideal measurements of quantum bits and mechanical oscillators.

While conventional nuclear magnetic resonance (NMR) spectroscopy is widely used for chemical analysis of macroscopic sample volumes, quantum NMR sensors based on nitrogen-vacancy (N-V) centers in bulk diamond have brought NMR spectroscopy to the nanoscale, demonstrating single-nucleus sensitivity and chemical structure determination. However, the detection volume for existing nanoscale N-V NMR devices extends only nanometers above a bulk diamond chip, which prevents measurements in thicker structures, such as cells. Here, we introduce N-V NMR devices based on nanodiamonds and use them to detect and distinguish multiple nuclear species in a sample volume of about ${20}^3\mathrm{n}{\mathrm{m}}^3$. We describe how analyte concentrations can be measured in situ using nuclear spins on the nanodiamond surface for calibration. The nanodiamond host provides a small size (∼30 nm diameter), low toxicity, and opportunities for surface functionalization, which makes these devices promising for intracellular applications. These results mark an important step toward feasible and reliable nanoscale NMR measurements inside living cells.

Artificial magnetism at optical frequencies can be realized in metamaterials composed of periodic arrays of subwavelength elements, also called “meta-atoms.” Optically induced magnetic moments can be arranged in both unstaggered structures, naturally associated with ferromagnetic (FM) order, or staggered structures, linked correspondingly to antiferromagnetic (AFM) order. Here we demonstrate that such magnetic dipole orders of the lattices of meta-atoms can appear in low-symmetry Mie-resonant metasurfaces where each asymmetric dielectric (nonmagnetic) meta-atom supports a localized trapped mode. We reveal that these all-dielectric resonant metasurfaces possess not only strong optical magnetic response but also they demonstrate a significant polarization rotation of the propagating electromagnetic waves at both FM and AFM resonances. We confirm these findings experimentally by measuring directly the spectral characteristics of different modes excited in all-dielectric metasurfaces, and mapping near-field patterns of the electromagnetic fields at the microwave frequencies.

Recently, a phase-selective laser-induced breakdown spectroscopy (PS-LIBS) technique has been proposed and developed to diagnose nanoparticles. However, only a few kinds of nanoparticles can generate sufficient strong PS-LIBS signals for single-shot two-dimensional measurement. In this work, we report on the development and validation of a resonance-enhanced PS-LIBS for instantaneous two-dimensional imaging of nanoparticle formation and transport in a turbulent reactive flow. By matching the laser pulse with the absorption line of titanium atoms in the laser-induced nanoplasma, we find that the resonance-enhanced titanium atomic emissions are 1 order of magnitude stronger than the nonresonant spectra ranging from 498 to 502 nm. Based on the spectral characterization, a quantitative relation between the spectra and the particle volume fraction is theoretically established and validated by experiments. Combining the resonance-enhanced PS-LIBS and planar laser-induced fluorescence (PLIF) techniques, instantaneous two-dimensional distributions of the nanoparticle and the hydroxyl radical (OH radical) can be measured in situ in a turbulent reactive flow. The simultaneous measurement demonstrates a strong correlation between the nanoparticle formation and the reactive flow pattern in the turbulent region. The correlation further reveals that the precursor consumption is dominated by the turbulence effect, indicating the precursor chemistry-turbulence coupling.

In this work, we propose a theoretical and experimental investigation of the interaction of guided pure shear-horizontal (SH) wave within a uniaxial multilayered ${\mathrm{Tb}\mathrm{Co}}_2$/$\mathrm{Fe}$$\mathrm{Co}$ thin film deposited on ST-$\mathrm{X}{90}^{\circ }$-cut quartz in a delay-line configuration. We evaluate theoretically the evolution of phase velocity as a function of magnetic field and experimentally the variation of $S_{21}$ transmission coefficient (amplitude and phase). An equivalent piezomagnetic model based on pure magnetoelastic coupling is used (developed to allow us) to calculate the elastic stiffness constants of the multilayer as a function of the bias magnetic field. The model is also implemented for the calculation of acoustic waves’ dispersion curves. We show that the evolution of the phase velocity with respect to the bias magnetic field is dominated by the $C_{66}$ elastic stiffness constant as expected for the case of shear-horizontal surface acoustic wave. In the fabricated device, both fundamental and third-harmonic shear mode are excited at 410 MHz and 1.2 GHz, respectively. For both modes, the theoretical and experimental results are in agreement. At 1.2 GHz, the guiding of the acoustic wave in the ferromagnetic thin film enhances the sensitivity to the bias magnetic field with a maximum phase-velocity shift close to 2.5% and an attenuation reaching 500 dB/cm, for a sensitivity as high as 250 ppm/Oe, which is better than what has been reported in the literature so far. We also report that, from a specific ratio between the thin-film thickness and the acoustic wavelength, the bias magnetic field can induce a breaking of the acoustic wave polarization, leading to an acoustic mode conversion.

The output randomness from a random number generator can be certified by observing the violation of quantum contextuality inequalities based on the Kochen-Specker theorem. Contextuality can be tested in a single quantum system, which significantly simplifies the experimental requirements to observe the violation comparing to the ones based on nonlocality tests. However, it is not yet resolved as to how to ensure compatibilities for sequential measurements that is required in contextuality tests. Here, we employ a modified Klyachko-Can-Binicioğlu-Shumovsky contextuality inequality, which can ease the strict compatibility requirement on measurements. On a trapped single ${}^{138}{\mathrm{Ba}}^{+}$ ion system, we experimentally demonstrate violation of the contextuality inequality and realize quantum random number expansion by closing detection loopholes. We perform $1.29\times {10}^8$ trials of experiments and extract a randomness of $5.28\times {10}^5$ bits with a speed of $270\mathrm{bits}{\mathrm{s}}^{-1}$. Our demonstration paves the way for practical high-speed spot-checking quantum random number expansion and other secure information processing applications.

Acousto-optical devices, such as modulators, filters, or deflectors, implement a simple and effective way of light modulation and signal-processing techniques. However, their operation wavelengths are restricted to the visible and near-infrared frequency regions due to a quadratic decrease of the efficiency of acousto-optical interactions with increasing wavelength. At the same time, almost all materials with a high value of the acousto-optic figure of merit are nontransparent at wavelengths longer than 5 µm, while the transparent materials possess a significantly lower acousto-optic figure of merit. Here, we propose and demonstrate by calculations how these limitations can be overcome using specially designed planar semiconductor structures that support electromagnetic modes strongly coupled to the incident light in the Otto configuration. Such an approach can be used for an efficient acousto-optical device operating in the mid-infrared range of 8–14 µm. An acoustic wave excited by a piezoelectric transducer in a semiconductor prism is utilized to modulate the coupling coefficient of the incident light to the guided mode of the semiconductor structure, which results in up to 100% modulation of the transmitted light at a spatial scale less than the ultrasound wavelength. It allows the utilization of acoustic waves with a short decay distance, and therefore, it provides a unique possibility to achieve an efficient acousto-optical modulation at frequencies about 1 GHz, which are unreachable for traditional acousto-optics.

We demonstrate a largely enhanced nonreciprocal propagation of spin waves (SWs) magnetoelastically excited in $\mathrm{Ni}$/$\mathrm{Si}$ bilayer films. The nonreciprocity is attributed to the shear component of the strain tensor, which can be enlarged by increasing the thickness of the $\mathrm{Si}$ layer. By evaporating a 400-nm-thick $\mathrm{Si}$ layer on a $\mathrm{Ni}$ film, we enhanced the nonreciprocal ratio by up to 1200%. This finding paves the way for rectification of SWs in magnonic devices.