We investigate the lasing dynamics of optically-pumped submicrowatt-threshold Raman silicon lasers that employ a high-quality (high-Q) nanocavity design. The measurements reveal that free carriers generated by two-photon absorption induce dynamic effects during the initial lasing process, even at the very low threshold power of 0.12 µW. At higher excitation powers, the Raman laser signal exhibits a significant reduction within a few hundred nanoseconds after the initial rise, followed by clear oscillations. We find that the temporal behavior of the laser signal strongly depends on the excitation wavelength. However, the Raman laser signal converges to a stable continuous operation within a few microseconds after the initial rise for any excitation power employed in this work. Numerical simulations indicate that the oscillations reflect the dynamic shift of the resonant wavelength due to the thermo-optic effect and the carrier-plasma effect, which are induced by free carriers generated via two-photon absorption. These results are useful for understanding the correct design of future devices that employ Raman silicon nanocavity lasers.

Transient photovoltage and photocurrent responses of organic solar cells provide valuable information regarding device performance and charge-carrier dynamics. However, the techniques used for analyzing these data have only been shown to be valid in a limited set of circumstances. Here we make use of a quasi-three-dimensional model to simulate these measurements in order to quantitatively evaluate the validity of the parameters extracted. With the experimentalist in mind, we produce a set of criteria that can be easily tested and demonstrate a good degree of generality for different sample parameters. This approach allows us to explore the use of “large perturbation” conditions in order to increase the signal-to-noise ratio and speed of the measurement. We propose the introduction of an “effective fixed capacitance,” which is shown to significantly reduce the uncertainty associated with the deviation from the small-perturbation regime.

High-mobility field-effect transistors (FETs) can serve as resonant detectors of terahertz radiation due to excitation of plasmons in the channel. The modeling of these devices previously relied either on approximate techniques or on complex time-consuming full-wave simulations. In this paper, we obtain an exact solution for driven electrical oscillations in a plasmonic FET with realistic contact geometry. The solution obtained highlights the importance of evanescent plasma waves excited near the contacts, which qualitatively modify the detector responsivity spectra. We also derive the boundary condition on the ac floating electrodes that generalizes the open-circuit (Dyakonov-Shur) and short-circuit (fixed-voltage) conditions and reduces to them in limting cases. In both limits, the FET photovoltage has resonant fringes; however, the absolute value of the voltage is greater in the open-circuit case.

We present a device to continuously focus microparticles in a liquid-filled open channel subjected to lateral vibration at frequencies of the order of 10 Hz. The vibration generates a capillary wave at the liquid-air interface. This capillary wave leads to a net motion of microparticles over multiple cycles, causing collection under the displacement nodes of the capillary wave. These accumulated particles are observed as a concentrated stream in the presence of a continuous flow along the open channel, which means that the channel can be designed such that the focused particle stream exits through one outlet, while the filtrate is removed via interspaced outlets on each side of the particle stream. A numerical model is proposed, which superimposes the periodic flow field due to the capillary wave and the inlet-induced transverse flow field between the inlet and the outlet. The model is utilized to predict the smallest distance from the inlet at which the focused stream of particles is obtained, termed here the collection length. In addition, experiments are performed for different channel and inlet sizes, vibration actuation amplitudes, flow rates, and particle sizes. By considering the design factors extracted from the modeling data, we demonstrate that the resulting device is capable of continuous particle collection down to $1\mu \mathrm{m}$ diameter, at flow rates of up to 1.2 ml/min.

Synthetic antiferromagnets with high Curie temperature (approximately 290 K) are realized among all-perovskite ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ $(\mathrm{LSMO})/\mathrm{Ca}{\mathrm{Ru}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ (CRTO) superlattices, which show antiferromagnetic (AF) interlayer exchange coupling (IEC) rather than traditional interfacial exchange coupling that is usually observed under the Curie temperature of the spacer. The system shows layer-resolved magnetic switching, resulting in sharp steplike hysteresis loops with magnetization plateaus depending on the repetition number of the stacking bilayers. This bilinear IEC can be easily changed to biquadratic IEC by change of orthorhombic $\mathrm{Nd}\mathrm{Ga}{\mathrm{O}}_3(001)$ substrate to cubic $(\mathrm{La}\mathrm{Al}{\mathrm{O}}_3{)}_{0.3}(\mathrm{Sr}{\mathrm{Al}}_{0.5}{\mathrm{Ta}}_{0.5}{\mathrm{O}}_3{)}_{0.7}(001)$ substrate. The strength of AF IEC increases with decrease of temperature, and the modeling results suggest that both magnetic LSMO and the CRTO space layer play a role in this behavior. Remarkably, the LSMO/CRTO system shows controllability of AF-IEC behavior under a moderate magnetic field of hundreds of oersteds. These observations may have potential applications in future oxide spintronic devices.

Nonlinear thermal transport can arise naturally in materials with strongly temperature-dependent thermal conductivities; however, this is exceedingly rare and weak. If a general strategy could be devised to yield nonlinear thermal transport, it would provide an avenue to controlling heat flow and realizing nonlinear thermal devices. Phase-change materials, which can exist in two states with distinct thermal conductivities, provide a unique opportunity to realize nonlinear thermal transport. In this work, we develop an analytical framework upon which we propose a material architecture for actualizing one type of nonlinear thermal transport, thermal rectification, where heat flux is biased in one direction. Our findings show that a heterojunction of two tunable phase-change materials can demonstrate strong thermal rectification. The magnitude of thermal rectification is analyzed as a function of the phase-change properties of each material, and we determine the fundamental heat-flux relations for each direction in these heterojunctions and criteria to separate the various phase situations that can occur. Finally, the analytical framework is applied to a junction comprising two phase-change materials, polyethylene and vanadium dioxide, and demonstrate a maximal theoretical rectification factor of approximately 140%. This analysis provides an important analytical tool in helping researchers design thermal circuits or advanced thermal energy storage media.

Nondegenerate two-photon absorption (ND TPA) is studied in a nanophotonic silicon waveguide in a configuration such that the dispersion of the nonlinear absorption and refraction cannot be ignored. It is shown that a signal wave around 1330 nm can be strongly absorbed by ND TPA with a pulse of 6-W peak power at 1850 nm. Experiments are performed with pulses of about 200 fs in a dispersion engineered waveguide in which the pump and signal pulses propagate at a similar group velocity. The experimental results are very well reproduced by numerical simulations of two coupled generalized nonlinear Schrödinger equations (GNLSEs). We show that the nonlinear dynamics can be well described by a single GNLSE despite the wavelength separation between the pump and the signal waves. We also demonstrate that in silicon nanophotonic waveguides, the dispersion of the nonlinear absorption is much larger than the dispersion of the Kerr effect. This could have an impact in the design of all-optical functions based on ND TPA, as well as on the study of supercontinuum and frequency-comb generation in integrated semiconductor-on-insulator platforms.

Both experiments and simulations show that the polarization state and propagation path of the Bloch surface waves sustained on a dielectric multilayer can be manipulated with the grooves inscribed on this multilayer. These grooves can be easily producible, accessible, and controllable. Various nanodevices for the Bloch surface waves, such as the launcher, beam splitter, reflector, polarization rotator, and even the photonic single-pole double-throw switch, are all experimentally realized with the properly designed grooves, which are consistent with the numerical simulations. The proposed devices will be basic elements for the two-dimensional photonic system and will find numerous applications, including integrated photonics, molecular sensing, imaging, and micromanipulation.

The alloy $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ finds extensive application in spintronics, and in particular the perpendicular magnetic anisotropy characteristic of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Mg}\mathrm{O}$ systems is of great interest. While some efforts have been made to examine the effect of composition on magnetic properties of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ materials, the magnetic-property–composition relationship for the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Mg}\mathrm{O}$ system is still not fully understood. Therefore, it is fundamentally and practically important to understand the $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}$ composition dependence of the magnetic properties of $\mathrm{Co}\text{−}\mathrm{Fe}\text{−}\mathrm{B}\text{−}\mathrm{Mg}\mathrm{O}$ systems. This work focuses on the data-storing free layer of double-barrier magnetic tunnel junctions with perpendicular magnetic anisotropy (PMTJs), which include $({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}{)}_{80}{\mathrm{B}}_{20}$ ultrathin films sandwiched between two $\mathrm{Mg}\mathrm{O}$ layers and a W insert layer. We study magnetic properties of various $({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}{)}_{80}{\mathrm{B}}_{20}$ compositions at different annealing conditions and find x ∼ 35% to have the highest anisotropy energy to ensure high thermal stability, while maintaining the lowest Gilbert damping parameter which is essential to achieve a low critical switching current. This composition also shows the highest thermal stability at elevated temperatures. In addition, we perform first-principle calculations to explain the anomalous composition trends of the magnetization and Gilbert damping parameter. Moreover, we find that the conventional Slater-Pauling curve is not applicable, and it is necessary to consider the magnetization’s dependence on the magnetic anisotropy which in turn depends on $({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}{)}_{80}{\mathrm{B}}_{20}$ composition and the oxide interface. Our results provide a perspective for a better understanding of metal-oxide systems with desirable properties for DBL PMTJ applications.

Achieving exact balance between spatially separated gain and loss is generally regarded as a necessary condition for parity-time- (PT-)symmetric optical systems. We introduce generalized PT- (GPT-)symmetric optical structures, which have an asymmetric and unbalanced gain-loss profile, while exhibiting similar scattering properties and phase transitions as their traditional PT-symmetric counterparts. In particular, we show that the concept of GPT symmetry may help to reduce the threshold gain in achieving newly discovered PT-enabled applications, such as the coherent perfect absorber laser and exceptional points. The concept proposed herein will facilitate the practice of PT-symmetric optical and photonic devices by offering greater design flexibility to tailor gain-loss profiles and their thresholds.

We study spin-orbit-torque-driven ferromagnetic resonance (FMR) in ferromagnetic (FM) bilayers, consisting of $\mathrm{Co}$ and permalloy ($\mathrm{Py}$) sandwiched between $\mathrm{Pt}$ and $\mathrm{Mg}\mathrm{O}$ layers. We find that the FM layer in contact with the $\mathrm{Pt}$ layer dominantly determines the spin Hall angle, which is consistent with the spin-transparency model. By contrast, the FMR linewidths are considerably influenced not only by the spin-pumping effect across the $\mathrm{Pt}/\mathrm{FM}$ interface but also by the spin relaxation such as two-magnon scattering at the $\mathrm{FM}/\mathrm{Mg}\mathrm{O}$ interface. The $\mathrm{Co}/\mathrm{Mg}\mathrm{O}$ interface leads to notably increased FMR linewidths, while the $\mathrm{Py}/\mathrm{Mg}\mathrm{O}$ interface has less effect. The different contributions of each interface to the spin Hall angle and dissipation parameter suggest that the stack configuration of $\mathrm{Pt}/\mathrm{Co}/\mathrm{Py}/\mathrm{Mg}\mathrm{O}$ requires less writing energy than $\mathrm{Pt}/\mathrm{Py}/\mathrm{Co}/\mathrm{Mg}\mathrm{O}$ in spin-orbit-torque-driven magnetic switching. Our approach offers a practical method to optimize material parameters by engineering either interfaces in contact with the heavy metal or the oxide layer.

Inserting graphene into van der Waals (vdW) heterostructures (HTSs) may result in unprecedented opportunities to design high-performance nanodevices, although the underlying physical mechanisms are unclear. Here, we present theoretical studies on electronic structures, carrier concentrations, and electronic transport of the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ ($X=\mathrm{S},\mathrm{Se}$) vdW HTSs. The studies show that the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ sandwiched systems have the desired characteristics of highly thermal stability to at least 500 K, staggered band alignments, and intact Dirac cone. In particular, the transport currents of the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ HTS are two orders of magnitude higher than that of the $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{W}{X}_2$ HTS, which indicates that the insertion of graphene obviously enhances the transport properties. Moreover, the external electric field can induce p-type doping of graphene and remarkably enhance the carrier concentration from approximately ${10}^{11}$ to approximately ${10}^{13}{\mathrm{cm}}^{-2}$. The results reveal the physical nature of $\mathrm{Mo}{\mathrm{S}}_2/\mathrm{graphene}/\mathrm{W}{X}_2$ vdW HTSs, which are helpful in providing a route to design two-dimensional semiconductor-based high-performance optoelectronic devices.

We report a mode-locked laser design using $\mathrm{Yb}$-doped fiber lasers for low-noise frequency-comb generation. The frequency comb covers the spectral range from 700 to 1400 nm. Although this range is more practical for many measurements than that produced by the more commonly used Er-fiber lasers, it has been addressed in only a handful of reports, mainly due to the difficulty of generating a fully coherent supercontinuum at $1\mu \mathrm{m}$. We overcome this difficulty by a tailored design of the mode-locking dynamics that succeeds in generating energetic 33-fs-long pulses without even using higher-order-dispersion compensation, while ensuring that the laser operates with net zero cavity dispersion for low-noise supercontinuum generation. After locking to a $\mathrm{Cs}$ atomic clock, this frequency comb is used for absolute-frequency measurements of a $\mathrm{Nd}:\mathrm{YAG}\text{−}{\mathrm{I}}_2$ laser to verify its accuracy by comparison with results from the International Committee for Weights and Measures. After this verification, it is further used to measure the absolute frequency of a 543-nm two-mode stabilized He-Ne laser, which is routinely used for length measurements in our institute, thus verifying its practical utility in metrology applications. The entire setup is built with readily available components for easy duplication by other researchers.

$\mathrm{Ce}$-activated $A^{2+}{B}^{4+}{\mathrm{O}}_3$ perovskites represent a class of compounds currently under active exploration for their potential as scintillators. Depending on the chemistry and synthesis conditions, perovskites can crystallize in multiple crystal structures, and a $\mathrm{Ce}$ substitutional dopant in an $AB{\mathrm{O}}_3$ perovskite can adopt different charge states (i.e., ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Ce}}^{4+}$) as well as different substitutional sites (namely, the 12-fold-coordinated $A$ site or the octahedrally coordinated $B$ site). Here, we use first-principles density-functional-theory- and hybrid-functional-based computations to study relative trends in the structure, energetics, and electronic structure of bulk $AB{\mathrm{O}}_3$ perovskites, where $A=\mathrm{Ca},\mathrm{Sr}$, or $\mathrm{Ba}$ and $B=\mathrm{Hf}$ or $\mathrm{Zr}$. Subsequently, we consider the relative energetics of preferential solution sites for $\mathrm{Ce}$ as a function of charge states, chemical potential, and defect configurations. Our results reveal that while ${\mathrm{Ce}}^{3+}$ or ${\mathrm{Ce}}^{4+}$ defects can be thermodynamically stable, depending on the choice of the substitutional site and synthesis conditions (i.e., prevailing chemical potential), only ${\mathrm{Ce}}^{3+}$ dopant at the $A$ site leads to an electronic structure that can exhibit scintillation. Our comparative analysis shows that while the positions of the $5d^1$ and $4f$ levels of ${\mathrm{Ce}}^{3+}$ as a dopant at the $A$ site are favorably placed in the band structure, these levels are consistently higher for the ${\mathrm{Ce}}^{4+}$ charge state and are unlikely to manifest any luminescence. The findings of this study are also discussed in relation to previously reported results and display excellent agreement with past experimental observations. In general, it is demonstrated that control of the $\mathrm{Ce}$ charge state and local chemical environment can be used—in addition to band-gap and band-edge engineering—to manipulate the relative position of scintillating states with respect to the valence-band maximum and conduction-band minimum. While this study specifically focuses on perovskites, the results (in particular, the relative alignment of the positions of the $5d^1$ and $4f$ levels of $\mathrm{Ce}$ dopant as a function of the activator’s charge state) are expected to be general and thus transferable to other chemistries.

Driven in part by the extreme requirements of gravitational-wave detection, recent research about interferometric phase estimation has focused on the use of nonclassical states of light in order to beat the shot-noise limit, allowing further improvements in sensitivity once the detected optical power has been maximized. Still, for a large class of interferometric applications, the efficient use of these states remains prohibitively complicated. Seeking simple improvements to instruments illuminated by classical laser light, we revisit the passive or unmodulated Mach-Zehnder interferometer by structuring the estimation problem in the form of a linear model for which an unbiased estimator attaining the Cramér-Rao bound is easily computed. This paper compares the performance of such linear and efficient estimators for several detection schemes used with two-output and four-output (in-phase and quadrature) interferometers. We find that the independent monitoring of all available output ports leads to an overall sensitivity that is, in most cases, better than that of balanced detection and single-output detection. In addition, it allows the cancellation of the technical amplitude noise contribution at all operating phases without resorting to modulation techniques. This seldom used but simple detection scheme should therefore be considered when designing instruments for operation close to and perhaps below the shot-noise limit.

This paper proposes a 2D single-point-fed metamaterial-like transmission-line- (TL-)grid antenna design that achieves extremely high aperture efficiency levels. Suitable unloaded transmission-line grids exhibit Dirac dispersion cones at the $\mathrm{\Gamma }$ point when accidental degeneracy of the three eigenmodes of interest is achieved, similar to the case of photonic crystals. The proposed antenna design employs a particular $\mathrm{\Gamma }$-point eigenmode of such unloaded transmission-line grids that is shown to be symmetric with respect to the unit cell and is invariant to the direction of wave propagation. The design is scalable to millimeter-wave and terahertz frequencies as it is simply a grid of interconnected meandered transmission lines. The TL-grid structure is center fed from the back using a coaxial cable and is operated as a standing-wave antenna. The entire structure is also of extremely low profile and printed using microstrip technology. The proposed antenna is shown to be versatile in the sense that it achieves this high aperture efficiency for various array sizes, providing customized levels of gain. The proposed antenna generates a broadside pencil beam with extremely high aperture efficiency, low sidelobe levels along the principle planes, low cross-polarization levels and an almost circularly symmetric beamwidth. The generated beam is maintained over a reasonable bandwidth and does not squint or split with frequency. The design is fabricated and experimentally tested confirming the high level of aperture efficiency achieved and proving the successful excitation of the symmetric eigenmode at the $\mathrm{\Gamma }$ point.

High-intensity laser irradiation can effectively couple to practically all kinds of materials, causing modification of their physical properties due to laser-induced phase transformations. This gives rise to a broad field of laser micro- and nanofabrication with its various technological applications. We demonstrate that, in a 40-nm-thick film of ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ alloy, a short laser pulse is capable of (re)writing the ferromagnetism observed at room temperature (RT). The energy of the pulse generating a ferromagnetic region has to be sufficiently high to induce melting of the ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ layer, while the ferromagnetic state can be erased by various kinds of lower-intensity thermal treatment. This cycling of RT ferromagnetism can be explained in terms of the chemical order (B2)-disorder (A2) phase transition in the ${\mathrm{Fe}}_{60}{\mathrm{Al}}_{40}$ crystal lattice, which is affected by laser-induced melting and rapid resolidification. Our finding has implications for the development of a magnetic memory technology that would use the reversibility of the modulus of the magnetization vector instead of its direction. This promises to circumvent the problem of the superparamagnetic limit for magnetic data storage density.

Two-dimensional (2D) semiconductors, e.g., $\mathrm{Mo}{\mathrm{S}}_2$ and phosphorene, are promising candidates for the channel materials of next-generation field-effect transistors (FETs). Although 2D $\mathrm{Mo}{\mathrm{S}}_2$ FETs with the gate length $L_g$ scaled down to 1 nm have been fabricated with a quite small threshold swing, they suffer from a rather low ON current and are unsuitable for a high-performance device. Herein, we simulate sub-5-nm monolayer (ML) phosphorene MOSFETs using ab initio quantum-transport simulations. We predict that the ON current, delay time, and power dissipation indicator of the sub-5-nm double-gated ML phosphorene MOSFETs with proper underlap structure can fulfill the requirements of the international technology roadmap for semiconductors for both high-performance (along both the armchair and zigzag directions) and low-power (along the zigzag direction) devices in 2028 until $L_g$ is scaled down to 2 nm. Therefore, phosphorene is more suitable for ultrascaled FETs than 2D $\mathrm{Mo}{\mathrm{S}}_2$ in the post-silicon era as far as the ON current is concerned.

Tritium diffusion in the nuclear fusion reactor breeder material ${\mathrm{Li}}_2\mathrm{Ti}{\mathrm{O}}_3$ is a fundamentally important process for tritium-release kinetics. The energy barrier of tritium diffusion in ${\mathrm{Li}}_2\mathrm{Ti}{\mathrm{O}}_3$ has been reported with a considerable uncertainty in previous experiments. Here, we perform systematic density-functional-theory (DFT) studies for the diffusion processes of positively charged tritium, which is the preferential charge state of the tritium interstitial in a single ${\mathrm{Li}}_2\mathrm{Ti}{\mathrm{O}}_3$ crystal. By calculating various local-diffusion minimum-energy paths, we find that the diffusion of tritium is strongly anisotropic along different crystalline directions. The most favorable diffusion paths appear within a ${\mathrm{Li}}_6$ atomic single layer of ${\mathrm{Li}}_2\mathrm{Ti}{\mathrm{O}}_3$ and the corresponding DFT diffusion barrier is 0.334 eV, while the diffusion barrier for the most favorable diffusion paths crossing a ${\mathrm{Li}}_2{\mathrm{Ti}}_4$ atomic layer is 1.006 eV.

Free-space communication links are severely affected by atmospheric turbulence, which causes degradation in the transmitted signal. One of the most common solutions to overcome this is to exploit diversity. In this approach, information is sent in parallel over different paths using two or more transmitters that are spatially separated, with each beam therefore experiencing different atmospheric turbulence, lowering the probability of a receive error. In this work we propose and experimentally demonstrate a alternative method of diversity based on spatial modes of light, which we call modal diversity. We remove the need for a physical separation of the transmitters by exploiting the fact that spatial modes of light experience different perturbations, even when traveling along the same path. For this proof-of-principle, we select identical radius modes from the Hermite-Gaussian and Laguerre-Gaussian basis sets and demonstrate an improvement in bit error rate by up to 54% without increasing the total transmit power or receive aperture radius.