We report the generation of an optical-frequency comb in the midinfrared region with 1-GHz comb-line spacing and no offset with respect to absolute-zero frequency. This comb is tunable from 2.5 to $4.2\mu \mathrm{m}$ and covers a critical spectral region for important environmental and industrial applications, such as molecular spectroscopy of trace gases. We obtain such a comb using a highly efficient frequency conversion of a near-infrared frequency comb. The latter is based on a compact diode-pumped semiconductor saturable absorber mirror–mode-locked ytterbium-doped calcium-aluminum gadolynate (Yb:CALGO) laser operating at $1\mu \mathrm{m}$. The frequency-conversion process is based on optical parametric amplification (OPA) in a periodically poled lithium niobate (PPLN) chip containing buried waveguides fabricated by reverse proton exchange. The laser with a repetition rate of 1 GHz is the only active element of the system. It provides the pump pulses for the OPA process as well as seed photons in the range of $1.4–1.8\mu \mathrm{m}$ via supercontinuum generation in a silicon-nitride (${\mathrm{Si}}_3{\mathrm{N}}_4$) waveguide. Both the PPLN and ${\mathrm{Si}}_3{\mathrm{N}}_4$ waveguides represent particularly suitable platforms for low-energy nonlinear interactions; they allow for mid-IR comb powers per comb line at the microwatt level and signal amplification levels up to 35 dB, with 2 orders of magnitude less pulse energy than reported in OPA systems using bulk devices. Based on numerical simulations, we explain how high amplification can be achieved at low energy using the interplay between mode confinement and a favorable group-velocity mismatch configuration where the mid-IR pulse moves at the same velocity as the pump.

The active layers of most organic photovoltaic (OPV) devices are constructed from a blend of two organic compounds. The two materials spontaneously segregate into pure-component phases during device fabrication, creating a bicontinuous network of conduction pathways that are selective for electron or hole charge carriers. The morphological distribution of these materials within the active layer has long been known to influence charge transport and resulting device performance. In addition to the two pure-component phases present in these devices, a third, mixed-composition phase exists at the interface between the two pure phases. The exact effects of this mixed-composition phase on OPV device performance are not well understood, although it is argued that the presence of a mixed phase is necessary for optimal device operation. In this paper, we probe the effects of having a mixed-composition interfacial phase on the performance and charge-transport characteristics of OPV devices through a series of drift-diffusion model simulations. We start with set of model morphologies with only pure-component phases and then introduce an interfacial mixed phase in a controllable fashion. Our simulations show that a modest amount of mixing initially improves device efficiency by reducing the tortuosity of the device’s conduction pathways and easing morphological traps. However, an excessive amount of mixing can actually degrade high-conductivity pathways, reducing photovoltaic performance. The point at which mixing switches from being beneficial to detrimental to OPV performance depends on the average domain size of a device’s morphology. Devices with smaller feature sizes are more susceptible to the debilitating effects of overmixing, so that the presence of a mixed phase may either raise power-conversion efficiency by as much as 100% or lower it by as much as 50%, depending on the average domain size and the extent of mixing. These trends suggest that variations in the amount of mixed-composition phase with different processing conditions is one of the key factors that makes optimizing bulk heterojunction OPV devices difficult.

We apply density-functional theory, in the local-density approximation, to a quasi-one-dimensional electron gas in order to quantify the effect of Coulomb and correlation effects in modulating and, therefore, patterning, the charge-density distribution. Our calculations are presented specifically for surface-gate-defined quasi-one-dimensional quantum wires in a GaAs-(AlGa)As heterostructure, but we expect our results to apply more generally for other low-dimensional semiconductor systems. We show that at high densities with strong confinement, screening of electrons in the direction transverse to the wire is efficient and density modulations are not visible. In the low-density, weak-confinement regime, the exchange-correlation potential induces small density modulations as the electrons are depleted from the wire. At the weakest confinements and lowest densities, the electron density splits into two rows, thereby forming a pair of quantum wires that lies beneath the surface gates. An additional double-well external potential forms at very low density which enhances this row-splitting phenomenon. We produce phase diagrams that show a transition between the presence of a single quantum wire in a split-gate structure and two quantum wires. We suggest that this phenomenon can be used to pattern and modulate the electron density in low-dimensional structures with particular application to systems where a proximity effect from a surface gate is valuable.

This paper investigates how faithfully an electrospray mass spectrometer reports the order of monomers of a single biopolymer in the context of two sequencing strategies. We develop a simplified one-dimensional theoretical model of the dynamics of Brownian particles in the Taylor cone of an electropray source, where free monomers drift towards the apex in an elongational force gradient. The likelihood that neighboring particles will invert their order decreases near the apex because the strength of the force gradient increases. Neighboring monomers on a stretched biopolymer should be cleaved by photofragmentation within about 3 nm of the apex if they are to enter the mass spectrometer in sequence with 95% probability under typical experimental conditions. Alternatively, if the monomers are cleaved processively at milliseconds-long intervals by an enzyme, their sequence will be faithfully reported with 95% confidence if the enzyme is within about 117 nm of the apex.

Three-qubit quantum gates are key ingredients for quantum error correction and quantum-information processing. We generate quantum-control procedures to design three types of three-qubit gates, namely Toffoli, controlled-not-not, and Fredkin gates. The design procedures are applicable to a system comprising three nearest-neighbor-coupled superconducting artificial atoms. For each three-qubit gate, the numerical simulation of the proposed scheme achieves 99.9% fidelity, which is an accepted threshold fidelity for fault-tolerant quantum computing. We test our procedure in the presence of decoherence-induced noise and show its robustness against random external noise generated by the control electronics. The three-qubit gates are designed via the machine-learning algorithm called subspace-selective self-adaptive differential evolution.

Magnetization control of ferromagnetic materials only by circularly polarized light has received increasing attention both as a fundamental probe of the interactions of light and magnetism but also for future high-density magnetic recording technologies. Here we show that for granular FePt films designed for ultrahigh-density magnetic recording, the optical magnetic switching by circularly polarized light is an accumulative effect from multiple optical pulses. The measured results can be reproduced by a simple statistical model where the probability of switching a grain depends on the helicity of the optical pulses. We further show the deterministic switching of high-anisotropy materials by the combination of circularly polarized light and modest external magnetic fields, thus, revealing a pathway towards technological implementation.

We demonstrate that a far-field transistor made up of a phase-change material base with thermal hysteresis can efficiently be used as a thermal device capable of heating and cooling. Based on the principle of energy conservation for the heat currents by radiation, conduction, and convection, it is shown that the base temperature undergoes significant jumps as the transistor amplification factor is optimized. When the collector and emitter of the transistor operate at 350 and 300 K, respectively, a temperature jump of $+18\mathrm{K}$ ($-5\mathrm{K}$) is obtained during the heating (cooling) of a ${\mathrm{VO}}_2$ base excited with $208\mathrm{W}{\mathrm{m}}^{-2}$ ($63\mathrm{W}{\mathrm{m}}^{-2}$). These significant jumps are mainly driven by the photon heat current and could open new perspectives on thermal machines.

We consider properties of a fully superconducting variant of the superconducting quantum-interference proximity transistor, a magnetic-flux sensor. We study the density of states in a finite-size superconducting metal wire in the diffusive limit, and how it depends on the phase gradient of the order parameter. We describe the dependence on the junction length and the interface transparency, and we discuss properties relevant for using the structure in magnetic-flux-detection applications.

Despite the potential advantages of information storage in antiferromagnetically coupled materials, it remains unclear whether one can control the magnetic-moment orientation efficiently because of the canceled magnetic moment. Here, we report spin-orbit-torque-induced magnetization switching of ferrimagnetic ${\mathrm{Co}}_{1-x}{\mathrm{Tb}}_x$ films with perpendicular magnetic anisotropy. Current-induced switching is demonstrated in all of the studied film compositions, including those near the magnetization compensation point. The spin-orbit-torque-induced effective field is further quantified in the domain-wall motion regime. A divergent behavior that scales with the inverse of magnetic moment is confirmed close to the compensation point, which is consistent with angular momentum conservation. Moreover, we also quantify the Dzyaloshinskii-Moriya interaction energy in the $\mathrm{Ta}/{\mathrm{Co}}_{1-x}{\mathrm{Tb}}_x$ system and we find that the energy density increases as a function of the Tb concentration. The demonstrated spin-orbit-torque switching, in combination with the fast magnetic dynamics and minimal net magnetization of ferrimagnetic alloys, promises spintronic devices that are faster and with higher density than traditional ferromagnetic systems.

We experimentally and theoretically investigate injection locking of quantum-dot microlasers in the regime of cavity quantum electrodynamics. We observe frequency locking and phase locking where cavity-enhanced spontaneous emission enables simultaneous stable oscillation at the master frequency and at the solitary frequency of the slave microlaser. Measurements of the second-order autocorrelation function prove this simultaneous presence of both master and slavelike emission, where the former has coherent character with $g^{(2)}(0)=1$, while the latter has thermal character with $g^{(2)}(0)=2$. Semiclassical rate equations explain this peculiar behavior by cavity-enhanced spontaneous emission and a low number of photons in the laser mode.

The electromagnetic properties of an extremely thin metallic checkerboard drastically change from resonant reflection (transmission) to resonant transmission (reflection) when the local electrical conductivity at the interconnection points of the checkerboard is switched. To date, such critical transitions of metasurfaces have been applied only when they have fourfold rotational symmetry, and their application to polarization control, which requires anisotropy, has been unexplored. To overcome this applicability limitation and open up alternative pathways for dynamic deep-subwavelength polarization control by utilizing critical transitions of checkerboardlike metasurfaces, we introduce a universal class of anisotropic Babinet-invertible metasurfaces enabling transmission-reflection switching for each orthogonally polarized wave. As an application of anisotropic Babinet-invertible metasurfaces, we experimentally realize a reconfigurable terahertz polarizer whose transmitting axis can be dynamically rotated by 90°.

Measuring the charge of a nanoparticle is of great importance in many fields including optics, astronomy, biochemistry, atmospheric science, environmental engineering, and dusty plasma. Here, we propose to use a high-$Q$ whispering-gallery-mode (WGM) optical microresonator to detect the surface and bulk charge of a dielectric nanoparticle. Because of the modification of nanoparticle conductivity induced by the surplus electrons, both the coupling strength between the nanoparticle and the WGM and the dissipation changes compared with the case of a neutral nanoparticle. The charge density can be inferred from the transmission spectrum of the WGM microresonator. By monitoring the mode splitting, the linewidth broadening or the resonance dip value of the transmission spectrum, surface (bulk) electron density as low as $0.007{\mathrm{nm}}^{-2}$ ($0.001{\mathrm{nm}}^{-3}$) can be detected for nanoparticles with negative (positive) electron affinity. The high sensitivity is attributed to the ultranarrow resonance linewidth and small mode volume of the microresonator.

This paper demonstrates a method of acoustical self-feedback to suppress spontaneous gas oscillations such as those observed in combustors of gas-turbine engines. Whereas a conventional feedback system consists of electromechanical devices, the present method achieves acoustical self-feedback with a hollow tube that connects two positions of the oscillation system. A model oscillator of combustion-driven gas oscillations is designed and built to demonstrate the applicability of the self-feedback concept. Stability analysis through measurements of $Q$ values (quality factor) of oscillations shows that the desired delay time and gain are obtained when the tube length is equal to the odd integer times half the wavelength of the anticipated acoustic oscillations.

Graphene has emerged as a promising platform for THz metasurfaces supporting electrically tunable deep-subwavelength plasmonic excitations. Here, we introduce a broadband graphene metasurface based on the Hilbert curve, a continuous, space-filling fractal. We demonstrate the enhancement of graphene absorption over a broad frequency band (0.5–60 THz) with an average absorption level exceeding 20%. Owing to the continuous nature of the metasurface patterns, both the absorption level and the bandwidth can be controlled electrically by varying the graphene charge-carrier concentration.

The effect of radiative and nonradiative recombination processes on the efficiency-droop phenomenon is assessed for ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ single quantum wells (SQWs). The macroscopic photoluminescence (PL) intensity has an almost linear dependence on the excitation intensity in blue-emitting ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ SQWs, but a sublinear dependence in green-emitting SQWs. To clarify the reason, we perform PL mappings using a scanning near-field optical microscope. For blue SQWs, although the carriers delocalize from the localized states under a high excitation density, the carriers are hardly captured by nonradiative recombination centers (NRCs) because NRCs are surrounded by potential barriers. Meanwhile, for green SQWs, the increase in carrier density enhances carrier diffusion from the strong PL domains to the weak ones, and because a large number of NRCs are distributed in weak domains, the PL intensity becomes saturated. Such carrier-recombination dynamics are supported by time-resolved PL measurements. The present results indicate the importance of suppressing the carrier-activated nonradiative recombination processes to improve the efficiency droop in green light-emitting diodes.

Careful interpretation of time-resolved photoluminescence (TRPL) measurements can substantially improve our understanding of the complex nature of charge-carrier processes in metal-halide perovskites, including, for instance, charge separation, trapping, and surface and bulk recombination. In this work, we demonstrate that TRPL measurements combined with powerful analytical models and additional supporting experiments can reveal insights into the charge-carrier dynamics that go beyond the determination of minority-charge-carrier lifetimes. While taking into account doping and photon recycling in the absorber layer, we investigate surface and bulk recombination (trap-assisted, radiative, and Auger) by means of the shape of photoluminescence transients. The observed long effective lifetime indicates high material purity and good passivation of perovskite surfaces with exceptionally low surface recombination velocities on the order of about $10\mathrm{cm}/\mathrm{s}$. Finally, we show how to predict the potential open-circuit voltage for a device with ideal contacts based on the transient and steady-state photoluminescence data from a perovskite absorber film and including the effect of photon recycling.

We report the development of a high-sensitivity semiconductor charge sensor based on a quantum dot coupled to a single lead designed to minimize the geometric requirements of a charge sensor for scalable quantum-computing architectures. The quantum dot is fabricated in Si:P using atomic precision lithography, and its charge transitions are measured with rf reflectometry. A second quantum dot with two leads placed 42 nm away serves as both a charge for the sensor to measure and as a conventional rf single-electron transistor (rf SET) with which to make a comparison of the charge-detection sensitivity. We demonstrate sensitivity equivalent to an integration time of 550 ns to detect a single charge with a signal-to-noise ratio of 1 compared with an integration time of 55 ns for the rf SET. This level of sensitivity is suitable for fast ($<15\mu \mathrm{s}$) single-spin readout in quantum-information applications, with a significantly reduced geometric footprint compared to the rf SET.

Uranium dioxide (${\mathrm{UO}}_2$) is the most commonly used fuel in light-water nuclear reactors and thermal conductivity controls the removal of heat produced by fission, thereby governing fuel temperature during normal and accident conditions. The use of fuel performance codes by the industry to predict operational behavior is widespread. A primary source of uncertainty in these codes is thermal conductivity, and optimized fuel utilization may be possible if existing empirical models are replaced with models that incorporate explicit thermal-conductivity-degradation mechanisms during fuel burn up. This approach is able to represent the degradation of thermal conductivity due to each individual defect type, rather than the overall burn-up measure typically used, which is not an accurate representation of the chemical or microstructure state of the fuel that actually governs thermal conductivity and other properties. To generate a mechanistic thermal conductivity model, molecular dynamics (MD) simulations of ${\mathrm{UO}}_2$ thermal conductivity including representative uranium and oxygen defects and fission products are carried out. These calculations employ a standard Buckingham-type interatomic potential and a potential that combines the many-body embedded-atom-method potential with Morse-Buckingham pair potentials. Potential parameters for ${\mathrm{UO}}_{2+x}$ and ${\mathrm{ZrO}}_2$ are developed for the latter potential. Physical insights from the resonant phonon-spin-scattering mechanism due to spins on the magnetic uranium ions are introduced into the treatment of the MD results, with the corresponding relaxation time derived from existing experimental data. High defect scattering is predicted for Xe atoms compared to that of La and Zr ions. Uranium defects reduce the thermal conductivity more than oxygen defects. For each defect and fission product, scattering parameters are derived for application in both a Callaway model and the corresponding high-temperature model typically used in fuel-performance codes. The model is validated by comparison to low-temperature experimental measurements on single-crystal hyperstoichiometric ${\mathrm{UO}}_{2+x}$ samples and high-temperature literature data. Ultimately, this work will enable more accurate fuel-performance simulations and will extend to new fuel types and operating conditions, all of which improve the fuel economics of nuclear energy and maintain high fuel reliability and safety.

Spectroscopy is a key technology for both fundamental and applied science. A long-held desire has been the development of a means to continuously acquire broadband spectral data with simultaneous high time and frequency resolution. Frequency-comb technology can open this door: here, we use a spectroscopic technique based on an electro-optic comb to make continuous observations of cesium vapor across a 3.2-GHz spectral bandwidth with a 2-$\mu \mathrm{s}$ time resolution and with 10-MHz frequency sampling. We use a rapidly switched pump laser to burn narrow features into the spectral line and study the response to this step perturbation. This examination allows us to see a number of unexpected effects, including the temporal evolution of the bandwidth, the amplitude, and the frequency of these burnt features. We also report on the previously unobserved effect of radiation reabsorption, which slowly produces a broad pedestal of perturbation around each feature. We present models that can explain these dynamical effects.

Employing a combination of the many-body configuration-interaction method described by an extended Hubbard model and first-principles calculations, we predict the emergence of high oscillator strength at the near-IR region which originates from the Davydov type of splitting in doped graphene quantum dots (GQDs). Incorporation of the strain in GQDs promotes closely spaced bright states that are pertinent to coherent excitation. Controlling the destructive interference of the functionalized-nanographene quantum states, the dark states can be tuned towards the red end, ensuring that the system is a good candidate for a photocell. On the other hand, the coherent states can be tailored to concentrate the light at a very high intensity, resulting in an opportunity for a photonic device.