We use transient absorption spectroscopy to measure carrier lifetimes in the multiband semiconductor ${\mathrm{GaP}}_y{\mathrm{As}}_{1-x-y}{\mathrm{N}}_x$. These measurements probe the electron populations in the conduction band, intermediate band, and valence band as a function of time after an excitation pulse. Following photoexcitation of ${\mathrm{GaP}}_{0.32}{\mathrm{As}}_{0.67}{\mathrm{N}}_{0.01}$, we find that the electron population in the conduction band decays exponentially with a time constant ${\tau }_{\mathrm{CB}}=23\mathrm{ps}$. The electron population in the intermediate band exhibits bimolecular recombination with recombination constant $r=2\times {10}^{-8}{\mathrm{cm}}^3/\mathrm{s}$. In our experiment, an optical pump pulse excites electrons from the valence band to the intermediate and conduction bands, and the change in interband absorption due to absorption saturation and induced absorption is probed with a delayed white-light pulse. We model the optical properties of our samples using the band anticrossing model to extract carrier densities as a function of time. These results not only identify the short minority-carrier lifetime as a key factor affecting the performance of ${\mathrm{GaP}}_y{\mathrm{As}}_{1-x-y}{\mathrm{N}}_x$-based intermediate-band solar cells but also provide guidance on ways to address this issue.

We explore a type of synthetic saturable absorber based on quantum-inspired photonic arrays. We demonstrate that the interplay between optical Kerr nonlinearity, interference effects, and non-Hermiticity through radiation loss leads to a nonlinear optical filtering response with two distinct regimes of small and large optical transmissions. More interestingly, we show that the boundary between these two regimes can be very sharp. The threshold optical intensity that marks this abrupt “phase transition” and its steepness can be engineered by varying the number of the guiding elements. The practical feasibility of these structures as well as their potential applications in laser systems and optical signal processing are also discussed.

Chirality is of great importance in the living world. It helps differentiate biochemical reactions such as those that take place during digestion. It may also help differentiate physical processes such as diffusion. Aiming to study the latter effect, we investigate the diffusion of guest chiral molecules in chiral nematic (cholesteric) liquid-crystal hosts. We discover that the diffusion dramatically depends on the handedness of the guest and host molecules and the chiral differentiation is greatly enhanced by the proper alignment of the liquid-crystal host. The diffusion of a guest chiral molecule in a chiral host with the same handedness is much faster than in a chiral host with opposite handedness. We also observe that the differentiation of chirality depends on the diffusion direction with respect to the twisting direction (helical axis). These results might be important in understanding effects of chirality on physical processes that take place in biological organisms. In addition, this effect could be utilized for enantiomer separation.

We experimentally study the spin-wave coupling in adjacent ferrimagnetic-ferroelectric heterostructures. By using the space-resolved Brillouin light-scattering spectroscopy, we perform a systematic study of the coupling of waves propagating in both a magnetic and a ferroelectric layer. We demonstrate the voltage-controlled transfer of the spin-wave energy between the adjacent bilayers. Our results show, that the coupling efficiency of the proposed laterally coupled multiferroics can be tuned by independent variation of the applied magnetic and electric field, which opens exciting prospects for an emerging field of magnonics.

We construct a microwave detector based on the voltage switching of an underdamped Josephson junction that is positioned at a current antinode of a $\lambda /4$ coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures, we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yields a sensitivity parameter of 0.5, in qualitative agreement with theoretical calculations.

In this paper, we perform a detailed numerical analysis of remagnetization processes in nanocomposite magnetic materials consisting of magnetically hard grains (i.e., grains made of a material with a high magnetocrystalline anisotropy) embedded into a magnetically soft phase. Such materials are widely used for the production of permanent magnets because they combine high remanence with large coercivity. We perform simulations of nanocomposites with Sr-ferrite as the hard phase and Fe or Ni as the soft phase, concentrating our efforts on analyzing the effects of (i) the imperfect intergrain exchange and (ii) the nonspherical shape of hard grains. We demonstrate that—in contrast to common belief—the maximal energy product is achieved not for systems with a perfect intergrain exchange, but for materials where this exchange is substantially weakened. We also show that the main parameters of the hysteresis loop—remanence, coercivity, and the energy product—exhibit nontrivial dependencies on the shape of hard grains and provide detailed explanations for our results. Simulation predictions obtained in this work open new ways for the optimization of materials for permanent magnets.

We describe a simple method of interleaving a classical and quantum signal in a secure communication system at a single wavelength. The system transmits data encrypted via a one-time pad on a classical signal and produces a single-photon reflection of the encrypted signal. This attenuated signal can be used to observe eavesdroppers and produce fresh secret bits. The system can be secured against eavesdroppers, detect simple tampering or classical bit errors, produces more secret bits than it consumes, and does not require any entanglement or complex wavelength division multiplexing, thus, making continuous secure two-way communication via one-time pads practical.

Organic electrochemical transistors (OECTs) are increasingly studied as transducers in sensing applications. While much emphasis has been placed on analyzing and maximizing the OECT signal, noise has been mostly ignored, although it determines the resolution of the sensor. The major contribution to the noise in sensing devices is the $1/f$ noise, dominant at low frequency. In this work, we demonstrate that the $1/f$ noise in OECTs follows a charge-noise model, which reveals that the noise is due to charge fluctuations in proximity or within the bulk of the channel material. We present the noise scaling behavior with gate voltage, channel dimensions, and polymer thickness. Our results suggest the use of large area channels in order to maximize the signal-to-noise ratio (SNR) for biochemical and electrostatic sensing applications. A comparison with the literature shows that the magnitude of the noise in OECTs is similar to that observed in graphene transistors, and only slightly higher than that found in carbon nanotubes and silicon nanowire devices. In a model ion-sensing experiment with OECTs, we estimate crucial parameters such as the characteristic SNR and the corresponding limit of detection.

The near-infrared (NIR) photon pumped optical physics of nitrogen-vacancy (NV) centers in diamond is experimentally studied by considering both the charge-state conversion and stimulated emission. We find that the NIR laser can help to highly increase the charge-state conversion rate and, subsequently, can be applied to improve the performance of charge-state-depletion (CSD) nanoscopy. Using a doughnut-shaped visible laser beam and a Gaussian-shaped NIR laser beam for charge-state manipulation, we develop a low-power two-depletion-laser CSD nanoscopy for NV centers. Without dropping the imaging resolution, the visible depletion-laser intensity is approximately 10 times lower than that in the single-depletion-laser CSD. With high spatial resolution and low laser power, the CSD nanoscopy can be used for nanoscale quantum sensing with NV centers.

Relatively recent experiments on the scintillation response of CsI:Tl have found that there are three main decay times of about 730 ns, $3\mu \mathrm{s}$, and $16\mu \mathrm{s}$, i.e., one more principal decay component than had been previously reported; that the pulse shape depends on gamma-ray energy; and that the proportionality curves of each decay component are different, with the energy-dependent light yield of the $16\text{−}\mu \mathrm{s}$ component appearing to be anticorrelated with that of the $0.73\text{−}\mu \mathrm{s}$ component at room temperature. These observations can be explained by the described model of carrier transport and recombination in a particle track. This model takes into account processes of hot and thermalized carrier diffusion, electric-field transport, trapping, nonlinear quenching, and radiative recombination. With one parameter set, the model reproduces multiple observables of CsI:Tl scintillation response, including the pulse shape with rise and three decay components, its energy dependence, the approximate proportionality, and the main trends in proportionality of different decay components. The model offers insights on the spatial and temporal distributions of carriers and their reactions in the track.

A picosecond acoustic pulse is used to study the photoelastic interaction in single zinc-blende $\mathrm{GaN}/{\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ quantum wells. We use an optical time-resolved pump-probe setup and demonstrate that tuning the photon energy to the quantum well’s lowest electron-hole transition makes the experiment sensitive to the quantum well only. Because of the small width, its temporal and spatial resolution allows us to track the few-picosecond-long transit of the acoustic pulse. We further deploy a model to analyze the unknown photoelastic coupling strength of the quantum well for different photon energies and find good agreement with the experiments.

We show that the thermal dissociation of icosahedral viral capsids can be interpreted in terms of a two-dimensional phase transition. The approach provides a useful framework to get direct insights into the interactions at work between viral components. We devise a mean-field lattice model that relates the melting temperature to subunit attractive energy, effective charge, and chemical potential. Through fluorescence thermal shift assay on a plant viral system, we illustrate how the model gives access to the interaction parameters for empty and loaded viral capsids in various ionic conditions. This work should help construct physical models accounting for the assembly and disassembly mechanisms of viruses, with possible fallout in the development of therapeutic inhibitors.

Magnetic heterostructures that combine large spin-orbit torque efficiency, perpendicular magnetic anisotropy, and low resistivity are key to developing electrically controlled memory and logic devices. Here, we report on vector measurements of the current-induced spin-orbit torques and magnetization switching in perpendicularly magnetized $\mathrm{Pd}/\mathrm{Co}/{\mathrm{AlO}}_x$ layers as a function of Pd thickness. We find sizable dampinglike (DL) and fieldlike (FL) torques, on the order of 1 mT per ${10}^7\mathrm{A}/{\mathrm{cm}}^2$, which have different thicknesses and magnetization angle dependencies. The analysis of the DL torque efficiency per unit current density and the electric field using drift-diffusion theory leads to an effective spin Hall angle and spin-diffusion length of Pd larger than 0.03 and 7 nm, respectively. The FL spin-orbit torque includes a significant interface contribution, is larger than estimated using drift-diffusion parameters, and, furthermore, is strongly enhanced upon rotation of the magnetization from the out-of-plane to the in-plane direction. Finally, taking advantage of the large spin-orbit torques in this system, we demonstrate bipolar magnetization switching of $\mathrm{Pd}/\mathrm{Co}/{\mathrm{AlO}}_x$ layers with a similar current density to that used for $\mathrm{Pt}/\mathrm{Co}$ layers with a comparable perpendicular magnetic anisotropy.

Through the theoretical study of electron spin lifetime in the two-dimensional electron gas (2DEG) confined near the surface of doped Si, we highlight a dominant spin-relaxation mechanism induced by the impurity central-cell potential near an interface via intervalley electron scattering. At low temperatures and with modest doping, this Yafet spin-flip mechanism can become more important than the D’yakonov-Perel’ spin relaxation arising from the structural Rashba or Dresselhaus spin-orbit-coupling field. As the leading-order impurity-induced spin flip happens only between two nonopposing valleys in Si, 2DEG systems in Si MOSFETs or SiGe heterostructures are a natural platform to test and utilize this spin-relaxation mechanism due to the valley splitting near the interface and the tunability by electrical gating or applied stress. Our proposed alternative spin-relaxation mechanism may explain part of the spin-relaxation contribution to Si-based 2DEG systems, and it should have spintronic applications in Si-based devices.

A model of pearlite colony formation in carbon steels is developed, with a parametrization based on ab initio calculations. The model describes the processes of decomposition of austenite and formation of cementite through a metastable intermediate structure, with a crucial role of ferromagnetic order arising under the cooling. The autocatalytic mechanism of pearlite colony formation is analyzed and conditions for its implementation are established. We demonstrate that pearlite with lamellar structure is formed by the autocatalytic mechanism when thermodynamic equilibrium between the initial phase (austenite) and the products of its decomposition (cementite and ferrite) cannot be reached. The transformation diagram is suggested, and various scenarios of decomposition kinetics are investigated by phase-field simulations. By using a model expression for the free energy with first-principles parametrization, we find conditions of the formation of both lamellar and globular structures, in good agreement with experimental data.

The dynamical processes of radiative recombination of photocarriers and reabsorption of emitted photons in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ single crystals are studied using time-resolved two-photon-excitation photoluminescence (PL) microscopy. We find that the PL spectrum and its decay dynamics depend on the excitation-depth profile. As the excitation depth increases, the PL spectrum becomes asymmetric, the peak energy redshifts, and the PL decay time becomes longer. These observations can be well explained by a simple model including photon recycling (photon emission and reabsorption) in thick samples with strong band-to-band transitions and high radiative recombination efficiencies.

Mechanical resonators realized on the nanoscale by now offer applications in mass sensing of biomolecules with extraordinary sensitivity. The general idea is that perfect mechanical mass sensors should be of extremely small size to achieve zepto- or yoctogram sensitivity in weighing single molecules similar to a classical scale. However, the small effective size and long response time for weighing biomolecules with a cantilever restricts their usefulness as a high-throughput method. Commercial mass spectrometry (MS), on the other hand, such as electrospray ionization and matrix-assisted laser desorption and ionization (MALDI) time of flight (TOF) and their charge-amplifying detectors are the gold standards to which nanomechanical resonators have to live up to. These two methods rely on the ionization and acceleration of biomolecules and the following ion detection after a mass selection step, such as TOF. The principle we describe here for ion detection is based on the conversion of kinetic energy of the biomolecules into thermal excitation of chemical vapor deposition diamond nanomembranes via phonons followed by phonon-mediated detection via field emission of thermally emitted electrons. We fabricate ultrathin diamond membranes with large lateral dimensions for MALDI TOF MS of high-mass proteins. These diamond membranes are realized by straightforward etching methods based on semiconductor processing. With a minimal thickness of 100 nm and cross sections of up to $400\times 400\mu {\mathrm{m}}^2$, the membranes offer extreme aspect ratios. Ion detection is demonstrated in MALDI TOF analysis over a broad range from insulin to albumin. The resulting data in detection show much enhanced resolution as compared to existing detectors, which can offer better sensitivity and overall performance in resolving protein masses.

A simple and flexible optical system for generating electromagnetic or vector partially coherent sources or beams is presented. The alternative design controls field amplitude (beam shape), coherence, and polarization using only spatial light modulators. This improvement makes the apparatus simpler to construct and significantly increases the flexibility of vector partially coherent source generators by allowing many different types of sources to be produced without changing the physical setup. The system’s layout and theoretical foundations are thoroughly discussed. The utility and flexibility of the proposed system are demonstrated by producing a vector Schell-model and non-Schell-model source. The experimental results are compared to theoretical predictions to validate the design. Lastly, design aspects, which must be considered when building a vector partially coherent source generator for a specific application, are discussed.

We study the magnetic and electronic properties of Cu-doped ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}/{\mathrm{NdO}}_x$ systems with first-principles calculations in order to understand the roles of Cu in improving the coercivity of Nd-Fe-B permanent magnets. By analyzing the formation energies of several model systems, we find that Cu prefers to be at the interface. We conclude that the Cu addition to Nd-Fe-B magnets is a practical way of not only increasing the anisotropy of Nd atoms at the interface but also of lessening the magnetic coupling between the Nd and Fe atoms. Particularly, substituting Fe at the interface of the main phase with Cu works effectively in terms of improving the magnetic anisotropy in Nd atoms. This may explain the coercivity improvements reported recently.

Memtranstor that correlates charge and magnetic flux via nonlinear magnetoelectric effects has a great potential in developing next-generation nonvolatile devices. In addition to multilevel nonvolatile memory, we demonstrate here that nonvolatile logic gates such as nor and nand can be implemented in a single memtranstor made of the $\mathrm{Ni}/\mathrm{PMN}\text{−}\mathrm{PT}/\mathrm{Ni}$ heterostructure. After applying two sequent voltage pulses ($X_1$, $X_2$) as the logic inputs on the memtranstor, the output magnetoelectric voltage can be positive high (logic 1), positive low (logic 0), or negative (logic 0), depending on the levels of $X_1$ and $X_2$. The underlying physical mechanism is related to the complete or partial reversal of ferroelectric polarization controlled by inputting selective voltage pulses, which determines the magnitude and sign of the magnetoelectric voltage coefficient. The combined functions of both memory and logic could enable the memtranstor as a promising candidate for future computing systems beyond von Neumann architecture.