The ability to manipulate and select optical modes effectively and accurately is strongly desired to enable single-mode operation. Here, we report an optical system consisting of two directly coupled metal-cladded optical resonators that can realize a single-mode peak through control of the tuning between the resonant wavelengths of the two optical resonators. Additionally, the wavelength of the single-mode peak can be chosen via adjusting the angle of incident light. Furthermore, the linewidth and power of the single-mode output can be controlled completely by the thickness of the resonator guide layer and the angle of incident light. This optical system provides a way to realize optical information processing without the need for complex structures and specific materials.

Charge-to-spin conversion is crucial for the application of emerging spintronic devices. A two-dimensional electron gas (2DEG) at a complex oxide interface usually possesses strong Rashba spin-orbit coupling, and spin-momentum locking offers a great possibility for efficient charge-to-spin conversion through the Rashba-Edelstein effect. Here, we report the fabrication of metallic 2DEGs in γ-${\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{KTaO}}_3$ spinel/perovskite heterostructures and investigate the charge-to-spin conversion for Py/γ-${\mathrm{Al}}_2{\mathrm{O}}_3/{\mathrm{KTaO}}_3$ devices using the technique of spin-torque ferromagnetic resonance. The sizable spin splitting of the band structure results in a large current-induced spin-orbit torque efficiency with values up to around 3.6 at 5 K and about 1.1 at 300 K, which are more than an order of magnitude higher than those of heavy metals (0.07 for $\mathrm{Pt}$ at 300 K). Moreover, both theoretical and experimental results show that the charge-to-spin conversion is strongly dependent on the position of the Fermi level. These results demonstrate that optimizing the band filling of a ${\mathrm{KTaO}}_3$-based 2DEG is crucial for maximizing the conversion efficiency.

Hyperentanglement, defined as the simultaneous entanglement in several independent degrees of freedom (DOFs) of a quantum system, is a fascinating resource in quantum information processing with its outstanding merits. Here we propose heralded hyperentanglement concentration protocols (hyper-ECPs) to concentrate an unknown partially less polarization-spatial hyperentangled Bell state with available linear optics and common single-photon detectors. By introducing time-delay DOFs, the schemes are highly efficient in that the success of the scheme can be accurately heralded by the detection signatures, and postselection techniques or photon-number-resolving detectors, necessary for previous experiments, are not required. Additionally, our linear optical architectures allow certain states, where concentration fails, to be recyclable, and a trick makes the success probabilities of our schemes higher than those of previous linear optical hyper-ECPs.

Iridium ($\mathrm{Ir}$) is a 5d heavy metal with strong spin-orbit coupling (SOC), which introduces various intriguing phenomena when interacting with a ferromagnetic layer. In this study, we utilize $\mathrm{Ir}$ as an insertion layer in $\mathrm{W}/\mathrm{Ir}(t_{\mathrm{Ir}})/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$ heterostructures to tune SOC and study its influence on the spin-orbit torque (SOT), perpendicular magnetic anisotropy (PMA), and Dzyaloshinskii-Moriya interaction (DMI). The SOT efficiency decreases first and then increases with an increase of $\mathrm{Ir}$ insertion layer thickness because of the opposite spin Hall angle between $\mathrm{W}$ and $\mathrm{Ir}$, and the sign changes when the thickness of $\mathrm{Ir}$ is 1.5 nm, which is close to the spin diffusion length of $\mathrm{Ir}$ of 1.35 nm. Importantly, a 0.25-nm-thick $\mathrm{Ir}$ layer enhances largely the effective PMA by 140%–350% under different annealing temperatures compared with that of a sample without insertion. Meanwhile, the DMI constant is also enhanced. The enhancement of PMA and DMI is likely due to the enhancement of interfacial SOC when $\mathrm{Ir}$ diffuses into ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ and leads to the formation of ${\mathrm{Fe}}_{1-x}{\mathrm{Ir}}_x$. These results provide an effective method to tune SOC by inducing a dusting layer with strong SOC to meet different requirements in spintronic devices like small magnetic tunnel junctions.

We present a scheme for measuring a rotational Doppler frequency shift with only a single-shot measurement. Our protocol benefits from the combination of computational temporal ghost imaging and heterodyne detection of the rotational Doppler effect. Specifically, by multiplexing a series of elaborately designed ring-shaped spatial patterns, we map the temporal rotational Doppler signal into the spatial domain and thus can collect the Doppler signal with a camera in a single-shot exposure. Then, by calculating intensity correlations between the resulting elaborate spatial patterns and the image captured by the camera, we reconstruct the temporal Doppler signal. In our experiments, we succeed in demonstrating fairly good performance for measuring the angular velocity of a real spinning object with a rough surface. Our scheme could find potential applications in remote sensing and could also inspire more attempts to develop protocols for temporal detection and processing on the basis of space-time duality.

Optofluidic force induction (OF2i) is an optical nanoparticle characterization scheme which achieves real-time optical counting with single-particle sensitivity and high throughput. In a recent paper [Šimić et al., Phys. Rev. Appl. 18, 024056 (2022)], we have demonstrated the working principle for standardized polystyrene nanoparticles, and have developed a theoretical model to analyze the experimental data. In this paper we give a detailed account of the model ingredients, including the full working equations, provide additional justification for the assumptions underlying OF2i, and discuss directions for further developments and future research.

Direct measurements of far-field thermal infrared emission become increasingly useful because conventional indirect methods are limited or even unfeasible to characterize state-of-the-art thermal devices with engineered spectra, directionalities, and polarizations. The direct collection of the weak far-field emission from nanoscale infrared devices is also challenging because of their tiny footprints and the relatively large background radiation noises from surroundings. Here, we demonstrate a microscopic lock-in Fourier transform infrared spectrometer (FTIR) system that realizes significant improvement in SNR by combining a microscope and a lock-in amplifier with a FTIR. The lock-in FTIR is ultrasensitive, with a specific detectivity ${10}^6$ times higher than commercial ones, to overcome the optical loss and background noise during the emission light collection. Based on an analytical model of the full signal-detection process, we first employ the combination of the global heating and modulated Joule heating to maximize the potential of our system for noise reduction. Our findings show that, compared to previous studies, more than 3 times lower temperatures are sufficient to generate a measurable signal. Under a heating temperature of around 125 °C, we can achieve a SNR of about 23.7, which is far above the true-signal-threshold (SNR of about 3.0). Furthermore, the system can respond fast enough to record spectral-resolved dynamics of microdevices in the frequency domain. The system together with the analytical signal processing can be beneficial for next-generation thermal infrared material and device development, facilitating their applications in lighting, sensing, imaging, and energy harvesting at a small scale.

Electron-mediated spin torque provides a fast and efficient method to manipulate magnetization; however, electron motion inevitably brings about the generation of Joule heat and corresponding power consumption. Magnon-mediated spin torque, without involving moving electrons, could circumvent the energy dissipation issue. In this work, we fabricate a sandwich structure of topological insulator/antiferromagnetic insulator/ferromagnet with perpendicular magnetic anisotropy. We find that the magnon current with spin angular momentum can traverse a 25-nm-thick antiferromagnetic $\mathrm{Ni}\mathrm{O}$ layer and effectively switch the perpendicular magnetization of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ at room temperature with a critical switching current density of 4.1 × ${10}^6\mathrm{A}/{\mathrm{cm}}^2$. The magnon torque efficiency is characterized using spin-torque ferromagnetic resonance measurements to be 0.33 with a magnon diffusion length of 26.6 nm. Our work paves the way for manipulating perpendicular magnetization via magnon torques, facilitating the exploration of magnon-based spintronics with low power consumption.

Certification of quantum systems and their properties has become a field of intensive study. Here, taking advantage of the one-sided device-independent (1SDI) scenario (also known as the quantum steering scenario), we propose a self-testing scheme for all bipartite entangled states using a single family of steering inequalities with the minimal number of two measurements per party. Building on this scheme we then show how to certify all rank-one extremal measurements, including nonprojective $d^2$-outcome measurements, which in turn can be used for certification of the maximal amount of randomness, that is, $2{\log }_2d$ bits. Finally, in the particular case of $d=3$, we propose an extended Bell scenario that transforms a 1SDI scheme to “almost device independent.”

Controlling the spectral response of thermal emitters has become increasingly relevant for a range of energy and sensing applications. Conventional approaches to achieving arbitrary spectrum selectivity in photonic systems have entailed combining multiple resonantly emissive elements together to achieve a range of spectral profiles through numerical optimization, with a universal theoretical framework lacking. Here, we develop a temporal coupled-mode theory for thermal emission from multiple, arbtirarily coupled resonators. We validate our theory against numerical simulations of complex two- and three-dimensional nanophotonic thermal emitters, highlighting the anomalous thermal emission spectra that can emerge when multiple resonators with arbitrary properties couple to each other with varying strengths.

Magnon polaritons (MPs) refer to a light-magnon coupled state, which have the potential to act as information carriers, possibly enabling charge-free computation. However, light-magnon coupling is inherently weak. To achieve sufficiently strong coupling, a large ferromagnet or coupling with a microwave cavity is necessary. Thus, we theoretically propose a 100 nm YIG/1 µm GGG/500 µm YIG structure as a fundamental platform for magnonic and magnon-optical information storage devices to address the aforementioned issues. Furthermore, we discuss the transport properties of the MPs. Owing to the waveguide modes formed by the dielectric constant of the structure, the magnons placed in a nanometer-thin layer are strongly coupled with light. In addition, a longitudinal magnon-magnon coupling between thin and thick magnetic layers yields rich functionalities and a large magnon density to the thick-layer MPs due to the “magnonic antenna effect.” Hence, a large, direction-switchable magnon current in the thin layer is observed. The proposed structure indicates a longer coherence length and suitable figure of merit for the magnonic antenna effect. Therefore, the findings of this study would enable the integration of ferromagnetic micro- and nanostructures for MP-based information devices without any restrictions due to cavities.

Metal photocathodes are an important source of high-brightness electron beams, ubiquitous in the operation of both large-scale accelerators and table-top microscopes. When the surface of a metal is nanoengineered with patterns on the order of the optical wavelength, it can lead to the excitation and confinement of surface-plasmon-polariton waves that drive nonlinear photoemission. In this work, we aim to evaluate gold plasmonic nanogrooves as a concept for producing bright electron beams for accelerators via nonlinear photoemission. We do this by first comparing their optical properties to numerical calculations from first principles to confirm our ability to fabricate these nanoscale structures. Their nonlinear photoemission yield is found by measuring emitted photocurrent as the intensity of their driving laser is varied. Finally, the mean transverse energy of this electron source is found using the solenoid-scan technique. Our data demonstrate the ability of these cathodes to provide a tenfold enhancement in the efficiency of photoemission over flat metals driven with a linear process. We find that these cathodes are robust and capable of reaching sustained average currents over 100 nA at optical intensities larger than $2\mathrm{GW}{\mathrm{cm}}^{-2}$ with no degradation of performance. The emittance of the generated beam is found to be highly asymmetric, a fact we can explain with calculations involving the also asymmetric roughness of the patterned surface. These results demonstrate the use of nanoengineered surfaces as enhanced photocathodes, providing a robust air-stable source of high-average-current electron beams hopefully with great potential for industrial and scientific applications.

Terahertz (THz) waves are extremely sensitive to water content and biological composition (proteins, genes, and metabolites), which are the key regulator of cancer development. Hence, the application of THz spectroscopy and imaging is promising in the field of biomedicine, especially in cancer diagnosis. Currently, the diagnosis of cancer patients (including tumor borders, tumor stage, etc.) mainly rely on optical microscope observation of pathological slices. The preparation of the slices is complicated and time consuming, and the microscopic field of view is small, which cannot effectively guide the precise removal of the tumor during the operation in time. To solve this problem, a label-free real-time high-resolution near-field THz spectroscopy imaging technique was developed. In this study, we successfully apply this technique to the oral tissue. Compared with conventional point-by-point scanning terahertz time-domain spectroscopy imaging, this technique can exhibit a real-time imaging rate (as high as 20 frames/s at 512 × 1024 pixels) and a field of view approximately 5 mm in diameter with a spatial resolution up to λ/10 at 1 THz in near-field condition. The experimental results show that the terahertz spectroscopic imaging in this paper can clearly detect and identify not only biological tissues with different components, but also normal and cancerous tissues, as well as tissues with the same composition but different slice directions. This work lays an experimental foundation for the application of THz spectroscopy imaging to rapid label-free intraoperative diagnosis, and it provides a basis for promoting the wide application of THz technology in the biomedical field.

We investigate the transverse trapping force acting on a small particle on or off the central axis of zero-order finite Bessel beams. Combining the simulated fields with the Gorkov force potential, the transverse trapping behaviors for small objects are analyzed, and the reversal of the trapping behaviors is recovered when varying the paraxiality parameter of the Bessel beam. The results prove the possibility of using axisymmetric Bessel beams to trap both dense and stiff particles as well as light and soft ones. The particles can be trapped at the maximum central pressure in the main lobe and also the maximum pressure of the other transverse locations. A lossy acoustic metastructure with the ability to decouple the phase and amplitude modulations is used to generate the desired sound field, which is used to trap a foam ball as an example. Our research opens a promising avenue to the design and development of simplified acoustic tweezers.

Detecting a weak physical signal immersed in overwhelming noises entails separating the two, a task for which machine learning is naturally suited. In principle, such a signal is generated by a nonlinear dynamical system of intrinsically high dimension for which a mathematical model is not available, rendering unsuitable traditional linear or nonlinear state-estimation methods that require an accurate system model (e.g., extended Kalman filters). We exploit the architectures of reservoir computing and feed-forward neural networks (FNNs) with time-delayed inputs to solve the weak-signal-detection problem. As a prototypical example, we apply the machine-learning schemes to Earth’s magnetic anomaly field-based navigation. In particular, the time series are collected from the interior of the cockpit of a flying aircraft during different maneuvering phases, where the overwhelmingly strong noise background is the result of other components of Earth’s magnetic field and the fields generated by the electronic devices in the cockpit. We demonstrate that, when combined with the traditional Tolles-Lawson model for Earth’s magnetic field, the articulated machine-learning schemes are effective for accurately detecting the weak anomaly field from the noisy time series. The schemes can be applied to detecting weak signals in other domains of science and engineering.

Magnetic hysteresis present in tracer materials for magnetic particle imaging (MPI) either by effect of the driving-field frequency or because of interactions among nanoparticles is shown to play a crucial role when such a technique is exploited to get quantitative information about the tissue section being examined. By means of simulations based on a rate-equation model, the nanoparticles are proven to display magnetic hysteresis at the operating frequencies typical of MPI even if they are superparamagnetic in quasistatic conditions. As a consequence, the system function, a quantity crucial for image reconstruction in MPI, becomes markedly different in nature and shape from the curve produced by superparamagnetic nanoparticles. Not taking into account such an effect has detrimental consequences on quantitative MPI, as evidenced by specific examples of image reconstruction involving both monodisperse and polydisperse nanoparticles. The role played in image reconstruction by magnetic dipolar interaction, no longer negligible when nanoparticles accumulate in a small volume of the tissue, is discussed within the rate-equation model.

Quantum key distribution (QKD) protocols utilize single photons for key exchanging. Heralded single-photon sources (HSPSs) with spontaneous parametric down-conversion (SPDC) is a practical and effective way of producing single photons of high fidelity, bringing extra performance especially at long distances. A pulsed laser is usually used as the pump for HSPS QKD, but it has disadvantages in operational stability and practicality compared to a cw one. On the other hand, a precise model describing the performance of HSPS with a cw pump is still missing. It is inaccurate and unsafe to apply the key rate model of the pulse-pumped HSPS QKD directly to the cw one, and it also overlooks the timing jitter and dead time of the single-photon detectors. In this paper, we study the cw case in detail, including practical imperfections of the experimental instruments. Our model shows that a cw-pumped setup can reach a much longer transmission distance (31 % longer) than the fastest pulsed HSPS QKD system at present, together with other advantages such as higher operational stability and better spectral characteristics.