Unraveling real eigenfrequencies in non-Hermitian $\mathcal{P}\mathcal{T}$-symmetric Hamiltonians has opened avenues in quantum physics, photonics, and most recently, phononics. However, the existing literature squarely focuses on exploiting such systems in the context of scattering profiles (i.e., transmission and reflection) at the boundaries of a modulated waveguide, rather than the rich dynamics of the non-Hermitian medium itself. In this work, we investigate the wave propagation behavior of a one-dimensional non-Hermitian elastic medium with a universal complex stiffness modulation that encompasses a static term in addition to real and imaginary harmonic variations in both space and time. Using a plane-wave expansion, we conduct a comprehensive dispersion analysis for a wide set of subscenarios to quantify the onset of complex conjugate eigenfrequencies, and set forth the existence conditions for gaps that emerge along the wave-number space. Upon defining the hierarchy and examining the asymmetry of these wave-number gaps, we show that both the position with respect to the wave-number axis and the imaginary component of the oscillatory frequency largely depend on the modulation type and gap order. Finally, we demonstrate the coalescence of multiple Bloch-wave modes at the emergent exceptional points where significant direction-dependent amplification can be realized by triggering specific harmonics through a process that is detailed herein.

With the increasing demand for miniaturized devices and integrated circuits, ultrasmall-scale device units have attracted increasing attention. However, the short-channel effects severely limit the development of high-performance micro- and nanodevices. Here, we design sub-5-nm dual-gate monolayer $\mathrm{Ga}\mathrm{Se}$ metal-oxide-semiconductor field-effect transistors (MOSFETs) and systemically analyze the transmission spectrum, local density of states, on-state current ($I_{on}$), and subthreshold swing (SS), considering different dielectric layer thicknesses, dielectric constants, and underlap lengths. The results show that, with decreasing equivalent oxide thickness, the SS ($I_{on}$) shows a downward (uptrend) trend. Compared with ${\mathrm{Al}}_2{\mathrm{O}}_3$ and ${\mathrm{Hf}\mathrm{O}}_2$ substrates, the SS and $I_{on}$ can be modified obviously through the dielectric layer thickness for 3-nm $\mathrm{Ga}\mathrm{Se}$ MOSFETs with ${\mathrm{Si}\mathrm{O}}_2$ substrate. The $I_{on}$ can be tuned from 904 to 1766 µA/µm, which is about 2 times higher than the high-performance requirements of the International Technology Roadmap for Semiconductors (ITRS) (900 µA/µm) for 2028. Meanwhile, the SS is upgraded from 134.8 to 62.7 mV/dec, closing the Boltzmann tyranny (60 mV/dec). Therefore, this work provides a route to realize ultrashort-scale MOSFETs with an ultralow subthreshold swing and a high on-state current through engineering the dielectric layer.

Magic angles, namely, relative rotations between two parallel graphene flakes that create bigger periodic superlattices, are recently found to inherit the bilayer structure with exotic properties totally unknown to a single isolated graphite sheet. An electromagnetic analog to these regimes is provided by modeling the two monolayers of the same two-dimensional medium as anisotropic surface conductivities, which, if properly tilted, give substantial enhancement for the fields across the formed cavity. Such results may apply the paradigm of twistronics to nanophotonic design involving stacked optical materials, for serving a wide range of applications: from electromagnetic sensing and signal processing to radiative heat transfer and light harvesting.

The measurement of ac susceptibility $\chi (\omega )$ down to $0.1\mathrm{mHz}$ of test masses is vital for TianQin, a Chinese space-based gravitational-wave detection mission, since the magnetic induced disturbing force is one of the main contributors to residual acceleration noise of test masses. Here we report a method based on a torsion pendulum with dual magnetic field modulation to measure $\chi (\omega )$ of centimeter-scale test mass from $50\mu \mathrm{Hz}$ to $80$ Hz. The measurement resolution can reach a level of ${10}^{-8}$, meeting the requirements of TianQin. Meanwhile, this method also provides an important ground-validation approach for the down-converted effects [Phys. Rev. Lett. 120, 061101 (2018)] from ac magnetic fields, potentially reducing the risk of failure in space.

The generation and control of surface acoustic waves (SAWs) in a magnetic material are objects of an intense research effort focused on magnetoelastic properties, with fruitful ramifications in spin-wave-based quantum logic and magnonics. We implement a transient grating setup to optically generate SAWs also seeding coherent spin waves via magnetoelastic coupling in ferromagnetic media. In this work we report on SAW-driven ferromagnetic resonance (FMR) experiments performed on polycrystalline Ni thin films in combination with time-resolved Faraday polarimetry, which allows extraction of the value of the effective magnetization and of the Gilbert damping. The results are in full agreement with measurements on the very same samples from standard FMR. Higher-order effects due to parametric modulation of the magnetization dynamics, such as down-conversion, up-conversion, and frequency mixing, are observed, testifying the high sensitivity of this technique.

We numerically demonstrate a general method to achieve acoustic high absorption in water with a wide range of incident angles and a broad frequency bandwidth. Different from previous studies considering local resonances with a narrow bandwidth or porous materials with a gradient design, we show that flat multilayer solid composites can be engineered to be perfectly impedance matched with water over a wide range of angles and a broad spectrum. By adding a proper loss factor to the composites, sound waves can be gradually dissipated and high absorption can be realized. Such a method is robust and adaptive to a broad range of material parameters. Our work could promise broad underwater applications.

The study of helicity in the context of light-matter interactions is an increasing area of research. However, some fundamental aspects of the helicity content of light fields inside scatterers have been overlooked. In this work, we demonstrate that the helicity of light fields inside lossless spherical cavities cannot be either conserved or sign flipped as a result of a scattering event. The underlying reason is that the internal electric and magnetic Mie coefficients cannot simultaneously oscillate with equal amplitude and equal or opposite phase. Our analytical demonstration is fulfilled regardless of the refractive index, sphere size, and multipolar order. In addition, we show that the helicity of light fields inside lossy spheres can be conserved. This fact is in striking contrast to the behavior of the scattered field, whose helicity content cannot be conserved precisely when the sphere has losses. Finally, we show that the helicity content of internal fields can be flipped for materials with gain.

The optical anisotropy of $r$-plane $\mathrm{Al}$${}_x$$\mathrm{Ga}$${}_{1-x}$$\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ quantum wells (QWs) is investigated theoretically and experimentally. Theoretical calculations predict that as the $\mathrm{Al}$ composition is increased from 0 to 1, the in-plane polarization switches from $\mathbf{E}\perp$ [11$\overline{2}$0] to $\mathbf{E}\perp$ [1$\overline{1}$01] at an $\mathrm{Al}$ composition of approximately 0.2. The polarization properties are nearly independent of the well width, particularly when the well width is thicker than approximately $1.5\mathrm{nm}$. Photoluminescence spectroscopy of the fabricated $r$-plane $\mathrm{Al}$${}_x$$\mathrm{Ga}$${}_{1-x}$$\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ QWs experimentally confirms the theoretical prediction. The $\mathbf{E}\perp$ [1$\overline{1}$01] polarization enables the emitted light to propagate along the [11$\overline{2}$0] direction. Using this property, photopumped lasing at approximately 250-nm wavelength at 11 K is demonstrated from an $r$-plane $\mathrm{Al}$${}_x$$\mathrm{Ga}$${}_{1-x}$$\mathrm{N}$/$\mathrm{Al}\mathrm{N}$ QW with the cleaved (11$\overline{2}$0) $a$ planes as cavity mirrors.

Neutron beams are unique tools to probe samples in many fields, such as biology medicine and fusion research. Access to such sources is traditionally limited to large particle accelerators. However, laser-driven pulsed neutron sources have attracted attention in recent years due to their compactness and cost savings. Although, in practical applications, the major obstacle is terribly low neutron flux due to the isotropic scattering. Here we propose a roadmap for laser-driven neutron sources based on “kinematic focusing,” which is ideal for producing highly collimated neutron beams and leads to the neutron-flux enhancement by more than 1 order of magnitude. Such a good collimation is conducive to the detection of samples located several meters from neutron sources, and it becomes feasible to place sensitive detectors adjacent to the neutron sources without the necessity of heavy shielding. A theoretical model for kinematic focusing and supporting two-dimensional particle-in-cell simulations are presented. It is shown that a well-collimated neutron beam, with the maximal divergence angle of about ${30}^{\circ }$, is generated by bombardment of a hydrogen target with ${\mathrm{Li}}^{3+}$ beams, which come from a metallic lithium foil irradiated by a short laser pulse at an intensity of $5.5\times {10}^{20}\mathrm{W}{\mathrm{cm}}^{-2}$. This proposed method can be achieved by present-day petawatt laser facilities.

Lack of nonreciprocity—in particular, nonreciprocity of phase accumulation—is one of the major drawbacks of microwave solid-state acoustic devices, which has prevented the development of acoustic isolators and circulators. Here we report the observation of the phase nonreciprocity of hybridized surface acoustic waves (SAWs) and spin waves in a magnetoelastic heterostructure. Our system consists of a $\text{Fe-Ga-B}/{\mathrm{Al}}_2{\mathrm{O}}_3/\text{Fe-Ga-B}$ multilayer on top of a ${\mathrm{LiNbO}}_3$ crystal. Maximum values of the observed nonreciprocal phase accumulation easily exceed π radians over a broad range of field conditions, which is necessary for the development of an effective circulator. In addition, under the application of bias magnetic field, the structure demonstrates tunable giant nonreciprocity of propagation losses with isolation as high as 48 dB, necessary for the development of isolators. Theoretical calculations provide an insight into the observed phenomena and demonstrate a pathway for further improvement of nonreciprocal SAW devices based on magnetoelastic coupling.

Microwave technology plays a central role in current wireless communications, including mobile communication and local area networks. The microwave range shows relevant advantages with respect to other frequencies in open-air transmission, such as low absorption losses and low-energy consumption, and in addition, it is the natural working frequency in superconducting quantum technologies. Entanglement distribution between separate parties is at the core of secure quantum communications. Therefore, understanding its limitations in realistic open-air settings, especially in the rather unexplored microwave regime, is crucial for transforming microwave quantum communications into a mainstream technology. Here, we investigate the feasibility of an open-air entanglement distribution scheme with microwave two-mode squeezed states. First, we study the reach of direct entanglement transmission in open air, obtaining a maximum distance of approximately 500 m with parameters feasible for state-of-the-art experiments. Subsequently, we adapt entanglement distillation and entanglement swapping protocols to microwave technology in order to reduce the environment-induced entanglement degradation. The employed entanglement distillation helps to increase quantum correlations in the short-distance low-squeezing regime by up to $46\mathrm{\%}$, and the reach of entanglement increases by $14\mathrm{\%}$ with entanglement swapping. Importantly, we compute the fidelity of a continuous-variable quantum teleportation protocol using open-air-distributed entanglement as a resource. Finally, we adapt this machinery to explore the limitations of quantum communication between satellites, where the impact of thermal noise is substantially reduced and diffraction losses are dominant.

Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetization and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping.

The convolution operation is widely used in signal and image processing and represents the most computationally intensive step in convolutional neural networks. We introduce a scheme to achieve arbitrary convolution kernels in the synthetic frequency dimension with a simple setup consisting of a ring resonator incorporating a phase and an amplitude modulator. This scheme can be used to perform multidimensional convolutions. We provide an analytic approach that determines the required modulation profile for any convolution kernel. Our work points to a direction of using optical computing to remove the computational bottleneck in traditional electronic circuits and may be useful in improving machine-learning hardware in artificial-intelligence applications.

The development and performance of quantum technologies heavily relies on the properties of the quantum states, which often require careful optimization of the driving conditions of all underlying components. In quantum key distribution (QKD), optical injection locking (OIL) of pulsed lasers has recently been shown as a promising technique to realize high-speed quantum transmitters with efficient system design. However, due to the complex underlying laser dynamics, tuning such laser system is both a challenging and time-consuming task. Here, we experimentally demonstrate an OIL-based QKD transmitter that can be automatically tuned to its optimum operating state by employing a genetic algorithm. Starting with minimal knowledge of the laser operating parameters, the phase coherence and the quantum bit error rate of the system are optimized autonomously to a level matching the state of the art.

A convenient and versatile scheme to generate continuously adjustable cylindrical vector (CV) and vortex beams by multiplexing and cascading passive vortex half-wave plates (VHPs) is proposed. (−24)–24 order CV or vortex beams are generated only based on VHPs with m = 1, 3, and 8. The coaxial and small-angle interference are introduced to detect orders. For coaxial interference of $l_1$-order and $l_2$-order vortex beams, the intensity distribution is spirally fan shaped. The partition number is N = |$l_2$− $l_1$|, and the spiral direction can distinguish the sign of topological charge with higher value. Fork-shaped fringes appear in the center for small-angle interference between vortex beams with incident angle ${\beta }_1$ and ${\beta }_2$. The forking number between two fringes meets N = |$l_2$− $l_1$|, and the forks face upward when ($l_2$− $l_1$)(${\beta }_2$− ${\beta }_1$) < 0, and face downward when ($l_2$− $l_1$)(${\beta }_2$− ${\beta }_1$) > 0. Particularly, when one of the beams is a Gaussian beam, such as $l_1$ = 0, the value and sign of the topological charge $l_2$ can be directly detected. And for the interference between CV and Gaussian plane beams, the partition number N is positively correlated with the polarization order P, with N = |P| or N = 2|P| for coaxial or small-angle interference, respectively, and corresponding sign can be identified by the polarization azimuth distributions measured by Stokes parameters. It is experimentally demonstrated that the proposed scheme can effectively generate continuously adjustable CV and vortex beams by programming limited VHPs, greatly expanding order numbers with low cost, low energy consumption, and high utilization, and the coaxial and small-angle interference method can conveniently detect the corresponding orders without borrowing redundant devices.

Inspired by transformation optics, negative permeability materials enable the transfer of static magnetic fields to a distant location far away from the current source. This is useful for situations in which it is difficult to place a current source near the region where magnetic fields are required. However, existing artificial negative permeability materials are three dimensional (3D) and bulky. In this study, we theoretically design and experimentally demonstrate a metasurface with static negative permeability that displaces volumetric 3D magnetic fields to a distant region. The design overcomes the existing limitation because the dimensions of the structure are reduced to two, and the control of the magnetic field has an increased degree of freedom. The proposed method can be further extended to tools and strategies to precisely displace and/or manipulate dc or quasistatic magnetic fields in regions of interest for various applications such as medical care and measurement precision.