Temporal modulation of components of electromagnetic systems provides an exceptional opportunity to engineer the response of those systems in a desired fashion, both in the time and frequency domains. For engineering time-modulated systems, one needs to thoroughly study the basic concepts and understand the salient characteristics of temporal modulation. In this paper, we carefully study physical models of basic bulk circuit elements—capacitors, inductors, and resistors—as frequency dispersive and time-varying components and study their effects in the case of periodical time modulations. We develop a solid theory for understanding these elements, and apply it to two important applications: wireless power transfer and antennas. For the first application, we show that, by periodically modulating the mutual inductance between the transmitter and receiver, the fundamental limits of classical wireless power transfer systems can be overcome. Regarding the second application, we consider a time-varying source for electrically small dipole antennas and show how time modulation can enhance the antenna performance. The developed theory of electromagnetic systems engineered by temporal modulation is applicable from radio frequencies to optical wavelengths.

Creating perpendicular fields by spin-orbit torques (SOTs) is an effective way to realize field-free and highly efficient SOT switching for magnetic films with perpendicular magnetic anisotropy (PMA). Here, (1) using a magnetic trilayer composed of a layer with PMA, a spacer, and a layer with in-plane magnetic anisotropy; and (2) by injecting spin currents with the same polarization into the two magnetic layers, we can generate the desired effective perpendicular field through SOT and further utilize this effective field to assist in field-free SOT switching. The SOT efficiency to produce the perpendicular field reaches $2.1\mathrm{Oe}/{10}^{10}{\mathrm{Am}}^{-2}$. This study can be instructive for designing energy-efficient SOT magnetic random-access memories.

The electromagnetic scattering from a network of high-permittivity dielectric thin wires with the network’s size smaller than or almost equal to the operating wavelength is investigated. A simple lumped-element theory for the polarization current intensities induced in the wires is developed. The network elements are capacitances and inductances between the wires. An analytical expression for the induced polarization currents is obtained. The connection between the spectral properties of the loop inductance matrix and the network’s resonances is established. The number of the allowed current modes and resonances is deduced from the topology of the circuit’s digraph. The radiative coupling is also included, and the radiative frequency shifts and the quality factors are derived. The introduced concept and methods may find applications both at radio frequencies and in nanophotonics.

We present the fabrication of 570-$\mu \mathrm{m}$-thick millimeter-sized soda-lime-silicate float-glass blocks with a 1-$\mu \mathrm{m}$-thick $\mathrm{Al}\mathrm{N}$ thin-film piezoelectric transducer sandwiched between thin metallic electrodes and deposited on the top surface. The electromechanical properties are characterized by electrical-impedance measurements in the frequency range from 0.1 to 10 MHz with a peak-to-peak voltage of 0.5 V applied to the electrodes. We measure the electrical-impedance spectra of 35 devices, all of width 2 mm, but with nine different lengths ranging from 2 to 6 mm and with two to seven copies of each individual geometry. Each impedance spectrum exhibits many resonance peaks, and we carefully measure the five most prominent ones in each spectrum. We compare the resulting 173 experimental resonance frequencies with simulation results from a finite-element-method model that we develop. When we use the material parameters from the manufacturer, we obtain an average relative deviation of the 173 simulated resonance frequencies from the experimental values of $(-4.6\pm 0.1)\mathrm{\%}$. When we optimize the values of the Young’s modulus and Poisson’s ratio of the float glass in the simulation, this relative deviation decreases to $(-0.5\pm 0.1)\mathrm{\%}$. Our results suggest a method for an accurate in situ determination of the acoustic parameters at ultrasound frequencies of any elastic solid onto which a thin-film transducer can be attached.

The use of low-dimensional objects in the field of cavity optomechanics is limited by their low scattering cross section compared with the size of the optical cavity mode. Fiber-based Fabry-Perot microcavities can feature tiny mode cross sections and still maintain a high finesse, boosting the light-matter interaction and thus enabling the sensitive detection of the displacement of minute objects. Here we present such an ultrasensitive microcavity setup with the highest finesse reported so far in loaded fiber cavities, $\mathcal{F}=195000$. We are able to position-tune the static optomechanical coupling to a silicon nitride membrane stripe, reaching frequency pull parameters of up to $\mid G/2\pi \mid =1\mathrm{GHz}{\mathrm{nm}}^{-1}$. We also demonstrate radiation pressure backaction in the regime of an ultrahigh finesse up to $\mathcal{F}=165000$.

We propose, design, and experimentally test a non-Hermitian acoustic superlattice that acts as a tunable precise filter. The superlattice is composed of two concatenated sublattices. The first sublattice is Hermitian, while the other can be adjusted to be Hermitian or non-Hermitian. The existence of non-Hermiticity, in terms of an induced loss in the second sublattice, results in the generation of absorption resonances that appear in the reflected spectrum. This provides us with a powerful knob to absorb or reflect several frequencies at will with high accuracy. The number of filtered frequencies can be controlled by designing the resonances in the first sublattice. Our proposed tunable acoustic filter can be extended to higher-frequency ranges, such as ultrasound, and other areas, such as photonics.

We present a current-monitoring system based on optical magnetometry, which is able to discriminate true current variations from environmental effects at room temperature. The system consists of a dedicated thermally stable magnetic-field-confining coil and an array of four optically pumped magnetometers arranged in a two-dimensional gradiometer configuration. These magnetometers monitor magnetic field variations inside the coil, which correlate with the variations of the driving current of the coil. The system uses a digital signal-processing unit to extract and record in real time the magnetic field values measured by the magnetometers, which allows real-time monitoring of the current. The coil of the system, which is made out of printed circuit boards, can easily be changed to adapt the current-to-field conversion. Thus, we can expand the applicability of this system to a wide range of currents. By using this system to actively feedback control a current source, we stabilize a current of 20 mA on a level better than $5\times {10}^{-9}$ for an integration time of 70 min.

Using the geometry of the spin field-effect transistor, we investigate the spin transport of in-plane spins in an array of narrow channels formed in a two-dimensional electron gas confined in an $(\mathrm{In},\mathrm{Ga})\mathrm{As}$ quantum well. We demonstrate a clear enhancement of the spin-diffusion length with decreasing channel width, yielding up to 10 µm for the narrowest channels. We discuss the observed enhancement in terms of the suppression of the Dyakonov-Perel mechanism of spin relaxation, which has been invoked in previous studies in the case of diffusion of out-of-plane oriented spins. We also show that the spin signal is significantly higher when injecting into an array of narrow channels, compared with injecting into a single narrow channel.

We propose an injection-locked pulsed laser with an electro-optic phase modulator in a Ti:sapphire laser cavity to realize both broadband optical spectra and long pulse durations. Further, we experimentally and theoretically demonstrate that the output microsecond-long pulse consists of a train of short pulses in the time domain and a comblike broadband spectrum with fast chirping in the positive and negative directions in the frequency domain. Finally, we discuss the potential of the proposed system for efficient Doppler cooling of positronium, a short-lived exotic atom.

The imaging of dynamical processes at interfaces and on the nanoscale is of great importance throughout science and technology. While light-optical imaging techniques often cannot provide the necessary spatial resolution, electron-optical techniques damage the specimen and cause dose-induced artifacts. Here, optical near-field electron microscopy (ONEM) is proposed, an imaging technique that combines noninvasive probing with light, with a high-spatial-resolution readout via electron optics. Close to the specimen, the optical near fields are converted into a spatially varying electron flux using a planar photocathode. The electron flux is imaged using low-energy electron microscopy, enabling label-free nanometric resolution without the need to scan a probe across the sample. The specimen is never exposed to damaging electrons.

We study information-theoretical security for space links between a satellite and a ground station. Quantum key distribution (QKD) is a well-established method for information-theoretical secure communication, giving the eavesdropper unlimited access to the channel and technological resources limited by only the laws of quantum physics. But QKD for space links is extremely challenging, the achieved key rates are extremely low, and daytime operating impossible. However, eavesdropping on a channel in free space without being noticed seems complicated, given the constraints imposed by orbital mechanics. If we also exclude the eavesdropper’s presence in a given area around the emitter and receiver, we can guarantee that he has access to only a fraction of the optical signal. In this setting, quantum keyless private (direct) communication based on the wiretap channel model is a valid alternative to provide information-theoretical security. Like for QKD, we assume that the legitimate users are limited by state-of-the-art technology, while the potential eavesdropper is only limited by physical laws: either by specifying her detection strategy (Helstrom detector) or by bounding her knowledge, assuming the most powerful strategy through the Holevo information. Nevertheless, we demonstrate information-theoretical secure communication rates (positive keyless private capacity) over a classical-quantum wiretap channel using on-off keying of coherent states. We present numerical results for a setting equivalent to the recent experiments with the Micius satellite and compare them to the fundamental limit for the secret key rate of QKD. We obtain much higher rates compared with QKD with an exclusion area of less than 13 m for low earth orbit satellites. Moreover, we show that the wiretap channel quantum keyless privacy is much less sensitive to noise and signal dynamics and daytime operation is possible.

The ability to recognize dynamical processes in physical events and to abstract physical concepts or even to reveal physical laws, lies at the core of human cognitive development. Modern machine learning methods have been given high hopes for realizing such ability, but are still in their infancy. Here we present our progress in this capability. The main purposes of this paper are to use neural networks for classifying the dynamical skyrmion phases of videos and to demonstrate that neural networks can learn physical concepts from the dynamical process. To this end, we employ multiple neural networks to recognize the static phases (image format) and dynamical phases (video format) of a particle-based skyrmion model. Our results show that neural networks, without any prior knowledge, can not only correctly classify these phases, but also predict the phase boundaries that agree with those obtained by simulation. We further propose a parameter visualization scheme to interpret what neural networks have learned. We show that neural networks can learn two order parameters from videos of dynamical phases and predict the critical values of two order parameters. Finally, we demonstrate that only two order parameters are needed to identify videos of skyrmion dynamical phases. Hence, this parameter visualization scheme can be used to determine how many order parameters are needed to fully recognize the input phases. Our work sheds light on the future use of neural networks in discovering meaningful physical concepts and revealing unknown yet physical laws from videos.

Optical frequency combs (OFCs) and microwave frequency combs (MFCs) are of intense interest in fields ranging from metrology to multifrequency light sources. MFCs in particular have attracted interest for applications in microwave photonics. Timing jitter is a key limiting factor of the quality of OFCs and MFCs, and despite the central importance of timing jitter, a clear generic account of its effect on OFCs and MFCs that provides a connection between the two is lacking. We consider the power spectral density (PSD) of the OFCs and MFCs produced by active and hybrid active-passive mode-locked lasers in which the active mode locking is effected by a high-spectral-purity oscillator, but with timing noise originating in both the active locking and the mode-locked oscillators. Based on an analytically tractable theoretical treatment, we account quantitatively for the characteristic comb line shape consisting of a narrow central peak lying atop a broad pedestal. The theory provides a quantitative comparison between characteristic OFC and MFC line shapes and identifies the origin of the difference.

The mimicking of chiral effects at the nanoscale usually requires complex nanostructured systems fabricated by using highly sophisticated techniques. Here, we demonstrate that low-cost diffractive metasurfaces, fabricated by nanosphere lithography, can offer a remarkably rich chiral broadband response in the near-infrared range. We measure the spin-dependent response as a function of the incident angle and wavelength. The resulting extinction maps reveal switching of the resonances governed by extrinsic chirality. We further employ numerical modeling to gain an insight into the electromagnetic behavior over the unit cell. The simulation results agree with experiments, and we further investigate the possibility of tailoring chirality by matching the refractive index below and above the metasurface. Finally, we investigate chiral emission of the ensemble of electric dipoles embedded in the refractive-index-matching layer. This approach opens alternative perspectives for chiral surface-lattice resonances and chiral emission in low-cost materials.

Vortex beams carrying orbital angular momentum enrich the wave-matter interaction physics and show great potential applications, such as optical or acoustic tweezers and long-range signal communication. However, passive vortex generators commonly suffer from drawbacks of low efficiency and bulky configurations. In this work, by harnessing nonlocal coupling between subunits, we present a thin layer of metagratings to change efficiently the topological charge of a vortex beam in a waveguide. Our numerical and experimental results show that thin and planar metagratings can convert a plane-wave mode into first-, second-, and third-order vortex beams with nearly perfect efficiency (more than 99%). Our work breaks through the barrier to obtain highly efficient acoustic vortex beams and may offer opportunities for the development of devices for acoustic communication and manipulation.

We demonstrate that the injection of the ballistic electrons into the two-dimensional electron plasma in lateral $n^{+}$-$i$-$n$-$n^{+}$ graphene field-effect transistors (GFETs) might lead to a substantial Coulomb drag of the quasiequilibrium electrons due the violation of the Galilean and Lorentz invariance in the systems with a linear electron dispersion. This effect can result in S-shaped current-voltage ($I$-$V$) characteristics. The resulting negative differential conductivity enables the hysteresis effects and current filamentation that can be used for the implementation of voltage-switching devices. Due to a strong nonlinearity of the $I$-$V$ characteristics, the GFETs can be used for an effective frequency multiplication and detection of terahertz radiation.