Parametric amplification—injecting energy into waves via periodic modulation of system parameters—is typically restricted to specific multiples of the modulation frequency. However, broadband parametric amplification can be achieved in active metamaterials which allow local parameters to be modulated both in space and in time. Inspired by the concept of luminal metamaterials in optics, we describe a mechanism for one-way amplification of sound waves across an entire frequency band using spacetime-periodic modulation of local stiffnesses in the form of a traveling wave. When the speed of the modulation wave approaches that of the speed of sound in the metamaterial—a regime called the sonic limit—nearly all modes in the forward-propagating acoustic band are amplified, whereas no amplification occurs in the reverse-propagating band. To eliminate divergences that are inherent to the sonic limit in continuum materials, we use an exact Floquet-Bloch approach to compute the dynamic excitation bands of discrete periodic systems. We find wide ranges of parameters for which the amplification is nearly uniform across the lowest-frequency band, enabling amplification of wave packets while preserving their speed, shape, and spectral content. Our mechanism provides a route to designing acoustic metamaterials which can propagate wave pulses without losses or distortion across a wide range of frequencies.

We investigate the impact of threading dislocations with an edge component ($a$ or $a+c$ type) on carrier recombination and diffusion in $\mathrm{Ga}\mathrm{N}$(0001) layers close to the surface as well as in the bulk. To this end, we utilize cathodoluminescence imaging of the top surface of a $\mathrm{Ga}\mathrm{N}$(0001) layer with a deeply buried (In,$\mathrm{Ga}$)N quantum well. Varying the acceleration voltage of the primary electrons and comparing the signal from the layer and the quantum well enables us to probe carrier recombination at depths ranging from the close vicinity of the surface to the position of the quantum well. Our experiments are accompanied by fully three-dimensional Monte Carlo simulations of carrier drift, diffusion, and recombination in the presence of the surface, the quantum well, and the dislocation, taking into account the dislocation strain field and the resulting piezoelectric field at the dislocation outcrop. Near the surface, this field establishes an exciton dead zone around the dislocation, the extent of which is not related to the carrier diffusion length. However, reliable values of the carrier diffusion length can be extracted from the dipolelike energy shift observed in hyperspectral cathodoluminescence maps recorded around the dislocation outcrop at low acceleration voltages. For high acceleration voltages, allowing us to probe a depth where carrier recombination is unaffected by surface effects, we observe a much stronger contrast than expected from the piezoelectric field alone. This finding provides unambiguous experimental evidence for the strong nonradiative activity of edge threading dislocations in bulk $\mathrm{Ga}\mathrm{N}$ and hence also in buried heterostructures.

We determine the diffusion length of excess carriers in $\mathrm{Ga}\mathrm{N}$ by spatially resolved cathodoluminescence spectroscopy utilizing a single quantum well as a carrier collector or carrier sink. Monochromatic intensity profiles across the quantum well are recorded for temperatures between 10 and 300 K. A classical diffusion model accounts for the profiles acquired between 120 and 300 K, while for temperatures lower than 120 K, a quantum capture process has to be taken into account in addition. Combining the diffusion length extracted from these profiles and the effective carrier lifetime measured by time-resolved photoluminescence experiments, we deduce the carrier diffusivity as a function of temperature. The experimental values are found to be close to theoretical ones for the ambipolar diffusivity of free carriers limited only by intrinsic phonon scattering. This agreement is shown to be fortuitous. The high diffusivity at low temperatures instead originates from an increasing participation of excitons in the diffusion process.

The determination of the carrier diffusion length of semiconductors such as $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ by cathodoluminescence imaging requires accurate knowledge about the spatial distribution of generated carriers. To obtain the lateral distribution of generated carriers for sample temperatures between 10 and 300 K, we utilize cathodoluminescence intensity profiles measured across single quantum wells embedded in thick $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ layers. Thin ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{N}$ and ($\mathrm{Al}$,$\mathrm{Ga}$)$\mathrm{As}$ barriers respectively prevent carriers diffusing in the $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ layers to reach the well, which would broaden the profiles. The experimental cathodoluminescence profiles are found to be systematically wider than the energy loss distributions calculated by means of the Monte Carlo program casino, with the width monotonically increasing with decreasing temperature. This effect is observed for both $\mathrm{Ga}\mathrm{N}$ and $\mathrm{Ga}\mathrm{As}$ and becomes more pronounced for higher acceleration voltages. We discuss this phenomenon in terms of both the electron-phonon interaction controlling the energy relaxation of hot carriers and the shape of the initial carrier distribution. Finally, we present a phenomenological approach to simulate the carrier generation volume that can be used for the investigation of the temperature dependence of carrier diffusion.

We consider the motion of a light sail that is accelerated by a powerful laser beam. We derive the equations of motion for two proof-of-concept sail designs with damped internal degrees of freedom. Using linear stability analysis, we show that perturbations of the sail movement in all lateral degrees of freedom can be damped passively: the sail is asymptotically stable. This analysis also shows complicated behavior akin to that associated with exceptional points in PT-symmetric systems in optics and quantum mechanics. We show that the excess heat that is produced by the damping mechanism is likely to be substantially smaller than the expected heating due to the partial absorption of the incident laser beam by the sail.

The height dependence of Raman light scattering on organic molecules deposited onto thick one-dimensional metal-dielectric gratings is investigated theoretically and experimentally. We observe oscillations of the intensity of Raman light scattering as a function of the height of the metastructure’s strips and demonstrate that these oscillations are explicitly associated with a type of Fabry-Perot effect. We show that the intensity of surface-enhanced Raman scattering can be additionally increased by local field enhancement (by an order of magnitude) at resonances of both pump and Stokes as well as anti-Stokes frequency, due to coupling of the surface plasmon polaritons on the top and bottom metal parts of the gratings via the vertical Fabry-Perot resonances over their middle dielectric part. A semianalytical one-mode model to describe the effect qualitatively is proposed. A significant deviation of the dispersion of Fabry-Perot-coupled plasmon polaritons from the surface-plasmon-polariton dispersion just folded into the first Brillouin zone is demonstrated.

Fewer-qubit quantum logic gate, serving as a basic unit for constructing universal multiqubit gates, has been widely applied in quantum computing and quantum information. However, traditional constructions for fewer-qubit gates often utilize a multipulse protocol, which inevitably suffers from serious intrinsic errors during the gate execution. In this paper, we report an optimal model about universal two- and three-qubit cnot gates mediated by excitation to Rydberg states with easily accessible van der Waals interactions. This gate depends on a global optimization to implement amplitude- and phase-modulated pulses via genetic algorithm, which can facilitate the gate operation with fewer optical pulses. Compared to conventional multipulse piecewise schemes, our gate can be realized by simultaneous excitation of atoms to the Rydberg states, saving the time for multipulse switching at different spatial locations. Our numerical simulations show that a single-pulse two- (three-) qubit cnot gate is possibly achieved with a fidelity of 99.23% (90.39%) for two qubits separated by $7.10\mu \mathrm{m}$ when the fluctuation of Rydberg interactions is excluded. Our work is promising for achieving fast and convenient multiqubit quantum computing in the study of neutral-atom quantum technology.

Spin echo can be used to refocus random dynamical phases caused by inhomogeneities in control fields and thereby retain the purity of a spatial distribution of quantum spins. This technique for accurate spin control is an essential ingredient in many applications, such as nuclear magnetic resonance, magnetic resonance imaging, and quantum information processing. Here, we show how all the elements of a spin echo sequence can be performed at high speed by means of transitionless quantum driving. This technique promises accurate control of rapid quantum spin evolution. We apply the scheme to universal nonadiabatic geometric single- and two-qubit gates in a nuclear magnetic resonance setting.

We reveal that a synthetic photonic lattice based on coupled optical loops can be utilized as a feed-forward neural network for processing of optical pulse sequences in time domain. As a proof of concept, we train the optical system to restore an initial shape of the pulse train from the signal distorted due to linear dispersion in a fiber-optic link. We also show efficient training of the optical network with an intrinsic Kerr-type nonlinearity for the realization of target nonlinear transmission functions and inference functionality for the discrimination of different pulse sequences. The theoretical modeling is performed under practical conditions and can guide future experimental realizations.

Droplet evaporation is a versatile process to deposit solute materials on a substrate unless it induces solute accumulation at droplet perimeter because of outward capillary flows. Despite many efforts to generate inward Marangoni flows to suppress outward capillary flows, it is challenging to control Marangoni flows in colloidal droplets that include binary solvents. To understand and manipulate Marangoni flows in colloidal binary droplets, we investigate droplet evaporation and deposition dynamics by changing the mass ratio of binary solvents. We show that island deposits emerge when Marangoni flows overwhelm capillary flows during most evaporation. The formation of island deposits in colloidal binary droplets would be a helpful route to achieve uniform deposits of colloidal particles or nanoparticles.

We propose how to achieve strong phonon blockade (PB) in a hybrid spin-mechanical system in the weak-coupling regime. We demonstrate the implementation of magnetically induced two-phonon interactions between a mechanical cantilever resonator and an embedded nitrogen-vacancy center, which, when combined with parametric amplification of the mechanical motion, produces significantly enlarged anharmonicity in the eigenenergy spectrum. In the weak-driving regime, we show that strong PB appears in the hybrid system along with a large mean phonon number, even in the presence of strong mechanical dissipation. We also show flexible tunability of phonon statistics by controlling the strength of mechanical parametric amplification. Our work opens up prospects for the implementation of an efficient single-phonon source, with potential applications in quantum phononics and phononic quantum networks.

Few-layer metasurfaces, which are planar artificial arrays composed of more than one functional layer, have been showing unprecedented capabilities for the implementation of integrated and miniaturized optical devices with high efficiency and broad working bandwidth. However, the rich design freedoms of few-layer metasurfaces severely challenge their design and optimization. A universal strategy for the design of few-layer metasurfaces with different desired optical functionalities and an arbitrary number of layers, which can lower the design complexity and the time cost for structural optimization, is still eagerly anticipated by the scientific community. Here, we demonstrate an inverse design strategy based on deep-learning technology for the design of few-layer metasurfaces. By combining the matrix theory of multilayer optics, the proposed algorithm can predict the entire scattering matrix of a few-layer metasurface in tens of seconds with an acceptable accuracy and realize the inverse design of few-layer metasurfaces with different desired functionalities. Thus, the proposed inverse design strategy provides an efficient solution for the reduction of the design complexity of few-layer metasurfaces and significantly lowers the time cost for the structural optimization when compared with the numerical simulation methods based on an iterative process of trial and error, which will be of benefit to and expand the related research.

We investigate the electrical transport in amorphous oxide semiconductors by applying the percolation-based random band-edge model proposed by Nenashev et al. [Phys. Rev. B 100, 125202 (2019)] to multicationic, multianionic, and multinary materials. We modify the model by introducing the Hall-scattering factor. This allows us to evaluate, besides the conductivity, the temperature-dependent free charge-carrier concentration and Hall mobility, additionally enabling the inclusion of localized tail states in the model. The extended model allows analysis of pulsed-laser deposited amorphous zinc tin oxide, magnetron-sputtered amorphous zinc oxynitride, and zinc magnesium oxynitride thin films with carrier concentrations ranging from ${10}^{16}$ to ${10}^{19}{\mathrm{cm}}^{-3}$ at room temperature. Excellent agreement of the extended model data with the measured electrical properties is found for the investigated temperature regime of $50$ to $300\mathrm{K}$. In addition, we extract critical parameters of the random band-edge model and discuss them with regard to varying deposition parameters and different anion and cation concentrations. In particular, we find the standard deviation of the mobility edge distribution to be ranging between $20$ and $60\mathrm{meV}$ for the investigated materials.

Autocorrelated noise appears in many solid-state qubit systems and hence needs to be taken into account when developing gate operations for quantum information processing. However, explicitly simulating this kind of noise is often less efficient than approximate methods. Here, we focus on the filter function formalism, which allows the computation of gate fidelities in the presence of autocorrelated classical noise. Hence, this formalism can be combined with optimal control algorithms to design control pulses, which optimally implement quantum gates. To enable the use of gradient-based algorithms with fast convergence, we present analytically derived filter function gradients with respect to control pulse amplitudes, and analyze the computational complexity of our results. When comparing pulse optimization using our derivatives to a gradient-free approach, we find that the gradient-based method is roughly 2 orders of magnitude faster for our test cases. We also provide a modular computational implementation compatible with quantum optimal control packages.

Transition-metal dichalcogenides (TMDs) are promising candidates for a wide variety of ultrascaled electronic, optoelectronic, and quantum-computing applications. The electronic density of states exponentially decaying into the band gap (also known as the band tail) has a strong impact on the performance of TMD applications. In this work, the band tails of various TMD monolayer and multilayer systems is predicted with density-functional-theory-based nonequilibrium Green’s functions when placed on several dielectric substrates such as ${\mathrm{Hf}\mathrm{O}}_2$, ${\mathrm{Al}}_2{\mathrm{O}}_3$, ${\mathrm{Si}\mathrm{O}}_2$, and $h$-BN. Nonlocal scattering of electrons on polar optical phonons, charged impurities, and remote scattering on phonons in the dielectric materials is included in the self-consistent Born approximation. The band tails are found to critically depend on the layer thickness, temperature, doping concentration, and particularly on the chosen dielectric substrate. The underlying physical mechanisms are studied in high detail and an analytical interpolation formula is given to provide a quick reference for Urbach parameters and to guide the design work in ${\mathrm{Mo}\mathrm{S}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{WSe}}_2$.

We propose a practical biaxial signal decoupling method for the longitudinal magnetic-field-modulated spin-exchange-relaxation-free comagnetometer used in the inertial rotation measurement. The method is implemented based on a model, which had been verified by a rotation response experiment. According to our model, by adjusting the magnetic field in the pumping direction to the decoupling point, the modulated comagnetometer can independently output the rotation information of two orthogonal directions. The rotational scale factor also reaches the maximum value, and the rotation sensitivity and long-term stability can be further improved. Our method not only retains the advantages of the modulated comagnetometer that it can output the dual-axis signal simultaneously with only one probe beam, but also realizes biaxial signal decoupling, and achieves better practical performance.

We show using three-dimensional kinetic Monte Carlo simulations that the injection of charge carriers from a metallic electrode into a disordered organic semiconductor is under nominally Ohmic injection conditions strongly impeded by the short-range Coulomb interactions between the charge carriers in the image-force-stabilized interfacial dipole layer. In contrast, master equation and conventional one-dimensional drift-diffusion simulations underestimate these Coulomb interactions due to their mean-field approximation, and are found not to reveal the effect. The simulations predict a reduction of the current density in organic semiconductor devices when the nominal injection barrier is taken very small or even negative, consistent with recent experimental results [Kotadiya et al., Nat. Mater. 17, 329 (2018)]. However, whereas in that work a modification of the energetic disorder near the interface is assumed, we find that the effect is already obtained after including charge-charge interactions beyond a one-dimensional and mean-field approximation.

Noisy intermediate-scale quantum (NISQ) devices are nowadays starting to become available to the final user, hence potentially allowing the quantum speedups predicted by quantum-information theory to be shown. However, before implementing any quantum algorithm, it is crucial to have at least a partial or possibly full knowledge on the type and amount of noise affecting the quantum machine. Here, by generalizing quantum generative adversarial learning from quantum states (QGANs) to quantum operations, superoperators, and channels (here named super QGANs), we propose a very promising framework to characterize noise in a realistic quantum device, even in the case of spatially and temporally correlated noise (memory channels) affecting quantum circuits. The key idea is to learn about the noise by mimicking it in a way that one cannot distinguish between the real (to be sensed) and the fake (generated) one. We find that, when applied to the benchmarking case of Pauli channels, the super-QGAN protocol is able to learn the associated error rates even in the case of spatially and temporally correlated noise. Moreover, we also show how to employ it for quantum metrology applications. We believe our super QGANs pave the way for the development of hybrid quantum-classical machine-learning protocols for a better characterization and control of the current and future unavoidably noisy quantum devices.