A turbine can convert translational kinetic energy of convective flows into rotation and is one of the most important machines to harvest energy from nature. Here, we propose a theoretical prototype of a microturbine that is powered purely by an external diffusive heat or mass flux. The turbine rotates perpendicular to the external thermal or chemical gradient due to fore-and-aft asymmetric thermophoresis or diffusiophoresis. Thus, its performance does not need alignment. The prototype turbine is validated by means of computer simulations. The results show that the angular velocity of the turbine sensitively depends on its structure and interactions with the solvent. This perpendicular-axis phoretic turbine provides large flexibility and the possibility to exploit existing thermal or chemical energies at small scales.

An atmospheric pressure plasma jet (APPJ) is used to deposit highly transparent and conductive impurity doped zinc oxide thin films. Although studies have shown that plasma reactivity can be affected due to quenching by ${\mathrm{O}}_2$ from the ambient air in the downstream, the correlation between the quenching effect and thin film properties has not yet been demonstrated. We propose a nozzle design with an additional feature for reducing the quench. We perform comprehensive numerical simulations, considering gas-phase reaction kinetics and turbulent flow, to show that the ${\mathrm{O}}_2$ quenching is indeed significantly reduced. We prepare $\mathrm{Ga}$-doped $\mathrm{Zn}\mathrm{O}$ (GZO) films using the modified APPJ system and confirm that both resistivity and deposition rate are improved. Moreover, we obtain a low resistivity of 3.3 × ${10}^{-4}$ Ω cm in open air, which is comparable to GZO films prepared by the state-of-the-art sputtering tools. The physical insight and concept may be applied to other subjects of plasma reactions in atmospheric ambience where surrounding oxygen may yield significant quenching and reduce overall performance, for instance, in medical and surface treatments.

Single-pixel imaging is suitable for low light-level scenarios because a bucket detector is used to maximally collect the light from an object. However, one of the challenges is its slow imaging speed, mainly due to the slow light-modulation technique. We here demonstrate 1.4-MHz video imaging based on computational ghost imaging with a red-green-blue light-emitting diode array having a full-range frame rate up to 100 MHz. With this method, the motion of a high-speed propeller is observed. Moreover, by use of single-photon detectors to increase the detection efficiency, this method is developed for ultrahigh-speed imaging under a low light level.

A hybrid semiconductor-superconductor nanowire on the top of a magnetic film in the stripe phase experiences a magnetic texture from the underlying fringing fields. The Zeeman interaction with the highly inhomogeneous magnetic textures generates a large synthetic spin-orbit coupling. We show that this platform can support the formation of Majorana bound states (MBS) localized at the ends of the nanowire. The transition to the topological superconducting phase not only depends on the nanowire parameters and stripe size but also on the relative orientation of the stripes with respect to the nanowire axis. Topological phase transitions with the corresponding emergence or destruction of MBS can be induced by reorienting the stripes or shifting their position, which can be achieved by passing a charge current through the magnetic film or by applying electrically controlled strain to it. The proposed platform removes the need for external magnetic fields and offers a noninvasive electrical tuning of MBS with the perturbation (current or strain) acting only on the magnetic film.

The generation, control, and detection of spin currents in solid-state devices are critical for Joule-heating minimization, spin-based computation, and electrical-energy generation from thermal gradients. Although incorporation of spin functionality into technologically important architectures is still in its infancy, advantages over all-electric devices are increasingly becoming clear. Here, we utilize the spin Seebeck effect (SSE) in $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ devices to detect light from 390 to 2200 nm. We find that the device responsivity is remarkably flat across this technologically important wavelength range, closely following the $\mathrm{Pt}$ absorption coefficient. As expected from a SSE-generation mechanism, we observe that the photovoltage and $\mathrm{Pt}$ heating dynamics are in strong agreement. To precisely determine the optically created thermal gradient produced from a pointlike heat source, we introduce a field-modulation method for measuring the SSE. Our results show that broadband optical detection can be performed with devices based solely on spin-current generation and detection.

Imaging below the diffraction limit is always of public interest because of the restricted resolution of conventional imaging systems. To beat the limit, evanescent harmonics decaying in space must participate in the imaging process. We introduce the method of spatial spectrum sampling, a far-field superresolution imaging method for microwave and terahertz regimes. Strong dispersion and momentum conservation allow the spoof surface plasmon polaritons (SSP) structure to become a sensitive probe for spatial harmonics. This allows the spatial information of the targets, including both propagating and evanescent components, to be extracted by tuning and recording SSP in the far field. Then, the image with subwavelength resolution is constructed by the inversed Fourier transform of the sampled spatial spectrum. Using the modified subwavelength metallic grating as the probe, a far-field resolution of 0.17λ is numerically and experimentally verified, and two-dimensional imaging ability is also fully discussed. The imaging ability and flexibility can be further improved by optimizing the SSP structures. We are confident that our working mechanism will have great potentials in superresolution imaging applications in the microwave and terahertz frequency ranges.

We report on the use of direct frequency comb spectroscopy to accurately measure the concentrations of ${}^{12}{\mathrm{C}}^{16}{\mathrm{O}}_2$ and ${}^{13}{\mathrm{C}}^{16}{\mathrm{O}}_2$ isotopologs in a gas sample. We demonstrate an accuracy of 0.5% and 12% for concentration measurements of ${}^{12}{\mathrm{C}}^{16}{\mathrm{O}}_2$ and ${}^{13}{\mathrm{C}}^{16}{\mathrm{O}}_2$, respectively, with the measured isotopic ratio in excellent agreement with that expected from their natural abundances. The precision of the concentration measurements is also high, at 0.03% and 1.24% for ${}^{12}{\mathrm{C}}^{16}{\mathrm{O}}_2$ and ${}^{13}{\mathrm{C}}^{16}{\mathrm{O}}_2$, respectively. The measurement technique is verified to be highly linear for concentrations ranging over 3 orders of magnitude. Direct frequency comb spectroscopy can be applied to numerous molecular species, and is therefore a promising technique for measurements in environmental monitoring and biomedical sciences.

The enhancement of thermal conductivity via surface electromagnetic waves (SEWs) supported in nanostructures has recently attracted attention as a remedy for issues raised due to the reduction of thermal conductivity in nanoscale confinement. Among them, multilayered structures on a substrate are prevalent in nanosized systems, such as electronic nanodevices, meaning that analysis on those structures is indispensable. In this work, three basic multilayered structures are selected and the analytical expressions for SEWs supported in each structure are derived. This analytical approach enables us to figure out which factors are crucial for enhancing SEW thermal conductivity using multilayers. It is also found that the solution can be extended to various materials and provides guidelines on which configurations are desirable for increasing the thermal conductivity. Furthermore, the analytical solutions reduce the calculation time significantly such that the optimal configuration, which can yield a SEW thermal conductivity of $1.27\mathrm{W}/\mathrm{m}\mathrm{K}$, corresponding to 90% of the thermal conductivity of bulk glass, is found using a genetic algorithm. This study thus provides a method for efficiently managing thermal issues in nanosized devices.

There is a growing and drastically underutilized abundance of energy stored on earth at small temperature difference, most familiarly in the form of low-grade waste heat. For the purpose of making use of this stockpile of energy, the direct conversion of heat to electricity is demonstrated using first-order phase transformations in lead-free ${\mathrm{Ba}\mathrm{Ti}\mathrm{O}}_3$ ferroelectrics. The thermodynamics of solid-state energy conversion using first-order phase transformations is investigated using a free-energy approach wherein the importance of the well-known Clausius-Clapeyron relation in opening up the mixed-phase region is discussed. A simple two-capacitor circuit is introduced to experimentally demonstrate the direct conversion of heat to electricity. By fluctuating the temperature by just $\pm 5{}^{\circ }\mathrm{C}$, current spikes of roughly 1 $\mu \mathrm{A}$ are sent back and forth across an electric load with no external battery attached. Finally, a supercritical ferroelectric Carnot cycle that would produce an energy density of 1.15 ${\mathrm{J}/\mathrm{cm}}^3$ with a thermal efficiency of $15\mathrm{\%}$ is introduced, simulated, and compared with state-of-the-art pyroelectric energy-conversion methods.

A transducer capable of converting quantum information stored as microwaves into telecom-wavelength signals is a critical piece of future quantum technology as it promises to enable the networking of quantum processors. Cavity optomechanical devices that are simultaneously coupled to microwave fields and optical resonances are being pursued in this regard. Yet even in the classical regime, developing optical modulators based on cavity optomechanics could provide alternatives to current technology. Here we demonstrate a magnetically mediated wavelength-conversion technique based on mixing high-frequency tones with an optomechanical torsional resonator. This process can act either as an optical phase modulator or an amplitude modulator depending on the experimental configuration, and the carrier modulation is always coherent with the input tone. Such coherence allows classical information transduction and transmission via the technique of phase-shift keying. We demonstrate that we can encode up to eight bins of information, corresponding to three bits, simultaneously and demonstrate the transmission of a 52 500-pixel image over 6 km of optical fiber with just 0.67% error. Furthermore, we show that magneto-optomechanical transduction can be described in a fully quantum manner, implying that this is a possible approach to signal transduction in the quantum regime.

Applying nonharmonic driving-field waveforms to nanoparticles for magnetic hyperthermia has beneficial effects on their ability to generate heating power. The response of superparamagnetic nanoparticles described as double-well systems with randomly distributed easy axes is studied by means of a rate-equation approach in the typical range of frequencies adopted in therapeutic applications. Magnetic hysteresis loops obtained at various frequencies and vertex fields show that sawtooth and square field waveforms substantially enhance the specific loss power (SLP) with respect to a harmonic excitation of the same frequency and amplitude. This improvement is related to the presence of fast, quasiadiabatic transformations where the occupancy numbers in the two potential wells are virtually frozen, resulting in largely off-equilibrium magnetization states and in hysteresis loops with a greater area. The square driving-field waveform is particularly promising for magnetic hyperthermia, the SLP being not only most effectively enhanced but also independent of particle size over a large interval of diameters. The special features of the hysteresis loops generated by the square driving-field waveform are studied in detail; a peculiar effect of loop instability is described, and a condition for having stable hysteresis loops is defined. The considered driving-field waveforms can be made to comply with the physiological restrictions imposed by magnetic hypertermia treatments in vivo.

A micropump is the heart of any microfluidic device that finds applications in several lab-on-chip devices. Passive micropumps are highly desirable for this purpose due to their ease of integration, low energy requirements, and simplistic design and operation. The design of a plant leaf serves as natural inspiration for the development of an evaporation-assisted passive micropump. The presence of a branching-channel-like venation pattern ensures water distribution to the spongy mesophyll cells, increasing the surface area for evaporation. However, because of its multiscale design and the complexity of the venation pattern, emulating a leaf’s design is challenging. Apart from the lack of understanding of design parameters that affect fluid flow, manufacturing limitations impede the development of such bioinspired micropumps. Inspired by the multiscale design of the leaf, in this work we propose a passive micropump mimicking the structure of a leaf. Using evaporation and capillary pressure as the pumping mechanism, our leaf-mimicking micropump consists of a microporous membrane integrated with a branched, fractal channel network resembling a leaf’s venation pattern. Our proposed fabrication method is simple, scalable, and inexpensive and uses readily available materials. We demonstrate a significant increase in the fluid flow rate due to the addition of this branched-channel network. We support our experimental observations using an analytical model, wherein we discuss the design parameters that affect the pumping rate. Correspondingly, the performance of these micropumps can be optimized on the basis of intrinsic and extrinsic factors as per the desired applications.

A passive parity-time (PT) symmetric medium provides a feasible scheme to experimentally investigate non-Hermitian systems. We experimentally demonstrate an acoustic asymmetric diffraction grating based on a designed passive PT-symmetric medium. The proposed passive PT-symmetric medium is composed of additional cavities and leaked holes. By skillfully combining these two unit cells, the composite unit cell can simultaneously modulate the real and imaginary parts of the refractive index. When the modulation amplitudes of the real and imaginary parts are equal, asymmetric diffraction is observed in both simulated and experimental results at an exceptional point. Compared to the previous design method of a passive PT-symmetric medium, the proposed scheme could prevent the influence of interferential diffraction beams and enhance the diffraction ability with more compact modulation. In addition, the designed metamaterial structure provides a practical way to construct passive PT-symmetric acoustic potential and opens alternative possibilities for further investigation of acoustic wave control in non-Hermitian systems.

The reference-frame-independent quantum key distribution (RFI QKD) protocol can reduce the requirements on the alignment of reference frames in practical systems. However, compared with the Bennett-Brassard (BB84) QKD protocol, the main drawback of RFI QKD is that Alice needs to prepare six encoding states in three mutually unbiased bases ($X$, $Y$, and $Z$), and Bob also needs to measure the quantum state in three such bases. Here, we show that the RFI QKD protocol can be secured in a case where Alice sends fewer states. In particular, we find that transmitting three states (two eigenstates of the $Z$ basis and one of the eigenstates of the $X$ basis) is sufficient to obtain comparable secret-key rates and distances covered, even when security against coherent attacks with statistical fluctuations of the finite-key size is considered. Finally, a proof-of-principle experiment based on time-bin encoding is demonstrated to show the feasibility of our scheme, and with the three-state protocol, the experimental complexity is effectively alleviated; in particular, a high-speed phase modulator is no longer required by Alice.

Transition-metal dichalcogenides (TMDs) are promising nanoscale materials with a wide range of applications. Chemical doping is a powerful tool for tailoring the physical and chemical properties of TMDs for targeted functionalities. As one of the most important TMDs, ${\mathrm{W}\mathrm{Se}}_2$ has great potential for applications in field effect transistor and complementary metal oxide semiconductor technologies for its bipolar dopability. However, precise control over the type and density of free carriers remains challenging. First-principles calculations are performed to study intrinsic defects and transition-metal (TM) dopants in ${\mathrm{W}\mathrm{Se}}_2$. Our results show that TM doping can effectively control the Fermi level in ${\mathrm{W}\mathrm{Se}}_2$ with no significant compensation by intrinsic defects. $\mathrm{Nb}$ and $\mathrm{Ta}$ are effective p-type dopants capable of generating a high free hole density in ${\mathrm{W}\mathrm{Se}}_2$. While n-type doping is possible by $\mathrm{Re}$ (under the $\mathrm{Se}$-rich condition) and $\mathrm{Cu}$ (under the $\mathrm{W}$-rich condition), the doping efficiency is reduced due to the lower attainable dopant concentration and higher ionization energies. The chemical trend in the attainable concentration of various substitutional $\mathrm{TM}$ dopants in ${\mathrm{W}\mathrm{Se}}_2$ is largely determined by the competition between the dopant incorporation in ${\mathrm{W}\mathrm{Se}}_2$ and the formation of the secondary phase ${\mathrm{TM}\mathrm{Se}}_2$. Such a competition is strongly affected by the different crystal environments of the $\mathrm{TM}$ ion in ${\mathrm{TMSe}}_2$ and ${\mathrm{W}\mathrm{Se}}_2$, which determine the crystal field splitting and electron filling of $\mathrm{TM}$ d levels and consequently the formation energy of the $\mathrm{TM}$ dopant.

High spatiotemporal resolution study of magnetic dynamic transitions in $({\mathrm{Mn}}_{0.5}{\mathrm{Ni}}_{0.5}{)}_{65}{\mathrm{Ga}}_{35}$ (hereafter referred to as $\mathrm{Mn}\text{−}\mathrm{Ni}\text{−}\mathrm{Ga})$ is carried out using Lorentz ultrafast transmission electron microscopy (UTEM). The Lorentz UTEM images with a subpicosecond temporal and sub-50-nm spatial resolution reveal remarkable features of magnetic transient states after a femtosecond pulsed laser excitation and three successive dynamic processes involving four distinct magnetic states are evidently observed in $\mathrm{Mn}\text{−}\mathrm{Ni}\text{−}\mathrm{Ga}$ crystals. Moreover, a full reversible dynamic cycle for the photoinduced ferroparamagnetic transition in a time span of up to tens of microseconds is also examined by using nanosecond-laser-generated photoelectrons. The temporal changes of electron, spin, and lattice in the present system are qualitatively analyzed based on a three-temperature approximation. Our Lorentz UTEM observations highlight the feasibility of direct imaging ultrafast demagnetization and relevant dynamic natures in complex spin textures with a nanometer-picosecond spatiotemporal resolution.

We present a direct-tomography protocol that is capable of characterizing the transverse spatial profile of both the polarization and the complex amplitude of fully polarized vector light beams in a single-shot measurement. This protocol entails a sequence of steps: a coherent mode transformation, a weak polarization perturbation, and a polarization-resolved imaging process. The final readout is directly proportional to the complex-amplitude profile of the two polarization components of the vector beam. We experimentally demonstrate our direct measurement protocol on a variety of commonly used vector beams, including vector-vortex beams and full Poincaré beams. Our method provides the capability of acquiring all the information needed to characterize a fully polarized vector beam in a single-shot measurement. Such a real-time complete-tomography protocol has the potential to create new opportunities in emerging applications of vector beams as well as the fundamental study of complex physical systems with multiple degrees of freedom.

An interrogation protocol based on coherent population trapping in an $N$-level scheme atomic system leads to dark resonances involving three different photons. An ensemble of several hundred radio-frequency-trapped ${\mathrm{Ca}}^{+}$ ions is probed by three lasers simultaneously locked onto the same optical frequency comb, resulting in high-contrast spectral lines referenced to an atomic transition in the terahertz domain. We discuss the cause of uncertainties and limitations of this method and show that reaching subkilohertz resolution is experimentally possible by this interrogation protocol.

High-quality $\mathrm{Ga}\mathrm{As}$/$(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ heterostructures with quantum wells grown by molecular-beam-epitaxy technology are experimentally studied by means of optical spectroscopy of exciton states. The exciton resonances observed in the reflectance spectra are analyzed in the framework of phenomenological and microscopic models. The exciton energies, the radiative ($\hslash {\mathrm{\Gamma }}_0$) and nonradiative ($\hslash \mathrm{\Gamma }$) broadening, and the phases of resonant reflection are obtained from the modeling for each exciton resonance. These parameters are used for careful analysis of the parameters and quality of the structures. Particular attention is paid to the exciton energies and phases, which are used to determine the quantum-well and barrier-layer thicknesses with high accuracy.

In linear acoustics, the reciprocal behavior of waves traveling in periodic materials can be manipulated by imposing external or configurational biases to the system. However, the nonreciprocity observed in linear systems is energetically expensive. We demonstrate that strongly nonlinear, asymmetric lattices can be designed to exhibit strong nonreciprocity that is passively adaptive and tunable. An alternative class of waveguides consisting of two coupled nonlinear lattices, one stiffer than the other, allows wave propagation preferentially in one direction at certain energy ranges. This “giant” nonreciprocal behavior is obtained passively by tuning the propagation zones of these lattices in the frequency-energy domain. We present numerical simulations corroborated by experiments to show an instance of this alternative class of nonlinear waveguides. Specifically, at low input energy, wave packets generated by an applied impulse at the lateral boundaries of the waveguide are blocked at the interface of the two lattices. However, at intermediate energy ranges, wave packets initiated at the free boundary of the softer lattice propagate through the waveguide, whereas wave packets initiated at the free boundary of the stiffer lattice are blocked at the interface. The nonreciprocal acoustics persist until at the critical level of input energy, above which waves propagate in both directions within the waveguide. The range of energy over which nonreciprocal wave transmission occurs is passively tunable by appropriately manipulating the nonlinear propagation zones of the lattices in the frequency-energy domain. The nonreciprocity concept is applicable to materials and systems capable of exhibiting strongly nonlinear behavior and can find broad applications in cases where passive targeted (directed) energy transfer in space and/or frequency is a desired outcome. For example, nonlinear nonreciprocal metamaterials can be used in passive acoustic isolation designs with the capacity for unidirectional sound transmission, thus eliminating their “acoustic signature”; in ultrasonics, to yield better wave focusing at preferential frequencies, and thus improved signal-to-noise ratios; in shock isolation systems, for example, by rapid nonreciprocal low-to-high nonlinear energy transfers, yielding fast structural response attenuation; or in networks of coupled oscillators enabling passive, irreversible energy transmission in preferential directions. Clearly, such capabilities for passive nonreciprocity are not attainable in linear systems.