The intrinsic sensitivity of whispering-gallery-mode resonators aimed at measuring refractive index can be extremely high, although their practical performance is compromised by temperature fluctuations that masquerade as refractive-index changes. We present a triple-mode approach that delivers simultaneous and independent sensing of temperature and refractive-index changes in the same resonator. The frequency difference between two orthogonally polarized modes is used to sense temperature which is then actively stabilized to $\sim 1\mu \mathrm{K}$ over 15 minutes. We then detect a frequency difference between two modes of different wavelengths to obtain a refractive-index measurement that is free of temperature fluctuations. This triple-mode technique delivers a state-of-the-art detection limit of $8\times 10^{-9}$ refractive-index units, despite the resonator size being 100 times larger than that typically used for sensitive refractometric sensing.

Time-domain current measurements are widely used to characterize semiconductor material properties, such as carrier mobility, doping concentration, carrier lifetime, and the static dielectric constant. It is therefore critical that these measurements be theoretically understood if they are to be successfully applied to assess the properties of materials and devices. In this paper, we derive generalized relations for describing current-density transients in planar semiconductor devices at uniform temperature. By spatially averaging the charge densities inside the semiconductor, we are able to provide a rigorous, straightforward, and experimentally relevant way to interpret these measurements. The formalism details several subtle aspects of current transients, including how the electrode charge relates to applied bias and internal space charge, how the displacement current can alter the apparent free-carrier current, and how to understand the integral of a charge-extraction transient. We also demonstrate how the formalism can be employed to derive the current transients arising from simple physical models, like those used to describe charge extraction by linearly increasing voltage (CELIV) and time-of-flight experiments. In doing so, we find that there is a nonintuitive factor-of-2 reduction in the apparent free-carrier concentration that can be easily missed, for example, in the application of charge-extraction models. Finally, to validate our theory and better understand the different current contributions, we perform a full time-domain drift-diffusion simulation of a CELIV trace and compare the results to our formalism. As expected, our analytic equations match precisely with the numerical solutions to the drift-diffusion, Poisson, and continuity equations. Thus, overall, our formalism provides a straightforward and general way to think about how the internal space-charge distribution, the electrode charge, and the externally applied bias translate into a measured current transient in a planar semiconductor device.

The electronic structural and phonon properties of ${\mathrm{Na}}_{2-\alpha }{\mathit{M}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ ($M=\mathrm{Li}$,K, $\alpha =0.0$,0.5,1.0,1.5,2.0) are investigated by first-principles density-functional theory and phonon dynamics. The thermodynamic properties of ${\mathrm{CO}}_2$ absorption and desorption in these materials are also analyzed. With increasing doping level $\alpha$, the binding energies of ${\mathrm{Na}}_{2-\alpha }{\mathrm{Li}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ are increased while the binding energies of ${\mathrm{Na}}_{2-\alpha }{\mathrm{K}}_{\alpha }{\mathrm{ZrO}}_3$ are decreased to destabilize the structures. The calculated band structures and density of states also show that, at the same doping level, the doping sites play a significant role in the electronic properties. The phonon dispersion results show that few soft modes are found in several doped configurations, which indicates that these structures are less stable than other configurations with different doping levels. From the calculated relationships among the chemical-potential change, the ${\mathrm{CO}}_2$ pressure, and the temperature of the ${\mathrm{CO}}_2$ capture reactions by ${\mathrm{Na}}_{2-\alpha }{\mathit{M}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$, and from thermogravimetric-analysis experimental measurements, the Li- and K-doped mixtures ${\mathrm{Na}}_{2-\alpha }{\mathit{M}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ have lower turnover temperatures ($T_t$) and higher ${\mathrm{CO}}_2$ capture capacities, compared to pure ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$. The Li-doped systems have a larger $T_t$ decrease than the K-doped systems. When increasing the Li-doping level $\alpha$, the $T_t$ of the corresponding mixture ${\mathrm{Na}}_{2-\alpha }{\mathrm{Li}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$ decreases further to a low-temperature range. However, in the case of K-doped systems ${\mathrm{Na}}_{2-\alpha }{\mathrm{K}}_{\alpha }\mathrm{Zr}{\mathrm{O}}_3$, although doping K into ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$ initially shifts its $T_t$ to lower temperatures, further increases of the K-doping level $\alpha$ causes $T_t$ to increase. Therefore, doping Li into ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$ has a larger influence on its ${\mathrm{CO}}_2$ capture performance than the K-doped ${\mathrm{Na}}_2\mathrm{Zr}{\mathrm{O}}_3$. Compared with pure solids ${\mathit{M}}_2\mathrm{Zr}{\mathrm{O}}_3$, after doping with other elements, these doped systems’ ${\mathrm{CO}}_2$ capture performances are improved.

We report on tuning the THz emission of a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (BSCCO) intrinsic Josephson-junction stack by a focused laser beam which is scanned across the stack. The emission power $P_e$ increases by up to 75% upon laser irradiation for a bath temperature near 22 K. The laser-induced changes in the voltage $V_{\mathrm{dc}}$ across the stack and in the emission power are measured simultaneously. The maximum of the laser-induced changes in emission power $\mathrm{\Delta }{P}_e$ is achieved by irradiating the stack on the location where the local temperature is about the critical temperature $T_c$. However, $\mathrm{\Delta }{P}_e$ is found to be proportional to the laser-induced global voltage change $\mathrm{\Delta }{V}_{\mathrm{dc}}$, irrespective of the laser position. This unexpected global response is likely to be related to a change in the average stack temperature and is consistent with the change in $P_e$ when increasing the bath temperature by about 0.2 K. This tuning method can be employed in the application of BSCCO THz sources.

We fabricate ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) direct-current nano- superconducting quantum-interference devices (nano-SQUIDs) based on grain-boundary Josephson junctions by focused-ion-beam patterning. Characterization of electric transport and noise properties at 4.2 K in a magnetically shielded environment yields a very small inductance $L$ of a few pH for an optimized device geometry. This, in turn, results in very low values of flux noise $<50\mathrm{n}{\mathrm{\Phi }}_0/{\mathrm{Hz}}^{1/2}$ in the thermal white-noise limit, which yields spin sensitivities of a few ${\mu }_B/{\mathrm{Hz}}^{1/2}$ (${\mathrm{\Phi }}_0$ is the magnetic flux quantum, and ${\mu }_B$ is the Bohr magneton). We observe frequency-dependent excess noise up to 7 MHz, which can be eliminated only partially by bias reversal readout. This behavior indicates the presence of fluctuators of unknown origin, possibly related to defect-induced spins in the ${\mathrm{SrTiO}}_3$ substrate. We demonstrate the potential of using YBCO nano-SQUIDs for the investigation of small spin systems, by placing a 39-nm-diameter Fe nanowire encapsulated in a carbon nanotube on top of a nonoptimized YBCO nano-SQUID and by measuring the magnetization reversal of the Fe nanowire via the change of magnetic flux coupled to the nano-SQUID. The measured flux signals upon magnetization reversal of the Fe nanowire are in very good agreement with estimated values, and the determined switching fields indicate magnetization reversal of the nanowire via curling mode.

Time-resolved measurements of carrier recombination are reported for a midwave infrared $\mathrm{InAs}/\mathrm{In}{\mathrm{As}}_{0.66}{\mathrm{Sb}}_{0.34}$ type-II superlattice (T2SL) as a function of pump intensity and sample temperature. By including the T2SL doping level in the analysis, the Shockley-Read-Hall (SRH), radiative, and Auger recombination components of the carrier lifetime are uniquely distinguished at each temperature. SRH is the limiting recombination mechanism for excess carrier densities less than the doping level (the low-injection regime) and temperatures less than 175 K. A SRH defect energy of 95 meV, either below the T2SL conduction-band edge or above the T2SL valence-band edge, is identified. Auger recombination limits the carrier lifetimes for excess carrier densities greater than the doping level (the high-injection regime) for all temperatures tested. Additionally, at temperatures greater than 225 K, Auger recombination also limits the low-injection carrier lifetime due to the onset of the intrinsic temperature range and large intrinsic carrier densities. Radiative recombination is found to not have a significant contribution to the total lifetime for all temperatures and injection regimes, with the data implying a photon recycling factor of 15. Using the measured lifetime data, diffusion currents are calculated and compared to calculated ${\mathrm{H}\mathrm{g}}_{1-x}{\mathrm{C}\mathrm{d}}_x\mathrm{T}\mathrm{e}$ dark current, indicating that the T2SL can have a lower dark current with mitigation of the SRH defect states. These results illustrate the potential for $\mathrm{InAs}/\mathrm{In}{\mathrm{As}}_{1-x}{\mathrm{Sb}}_x$ T2SLs as absorbers in infrared photodetectors.

A precise measurement of dephasing over a range of time scales is critical for improving quantum gates beyond the error correction threshold. We present a metrological tool based on randomized benchmarking capable of greatly increasing the precision of Ramsey and spin-echo sequences by the repeated but incoherent addition of phase noise. We find our superconducting-quantum-interference-device-based qubit is not limited by $1/f$ flux noise at short time scales but instead observe a telegraph noise mechanism that is not amenable to study with standard measurement techniques.

We investigate the atomic and electronic properties of O interstitial defects in amorphous In-Ga-Zn-O ($a$-IGZO) through density-functional calculations. We find that an O interstitial forms a dimer with a host O atom in its neutral state, but the O—O dimer bond is easily broken upon electron capture. When bond-breaking relaxations take place, the antibonding defect level of the dimer is significantly lowered from the conduction band toward the valence-band edge, making it energetically more favorable for the dimer to capture two electrons. With the hybrid functional for the exchange-correlation energy, the agreement of the energy barrier for two-electron capture with an experiment is greatly improved. The implication of the results is that O interstitials act as electron traps for the Fermi level close to the conduction-band edge; thus, excess O atoms can be the origin of positive shifts of threshold voltage observed under positive-bias stress in $a$-IGZO thin-film transistors. On the other hand, under light-illumination or negative-bias stress, which lower the Fermi level to the valence-band edge, the original dimer configuration is recovered by capturing hole carriers without any energy barrier, and the stability of current-voltage characteristics is restored.

We propose and demonstrate a physical mechanism for producing liquid microjets by taking an optoacoustic approach that can convert light to sound through a carbon-nanotube-coated lens, where light from a pulsed laser is converted to high momentum sound wave. The carbon-nanotube lens can focus high-amplitude sound waves to a microspot of $<100\mu \mathrm{m}$ near the air-water interface from the water side, leading to microbubbles in water and subsequent microjets into the air. Laser-flash shadowgraphy visualizes two consecutive jets closely correlated with bubble dynamics. Because of the acoustic scattering from the interface, negative pressure amplitudes are significantly increased up to 80 MPa, even allowing homogeneous bubble nucleation. As a demonstration, this nozzle-free approach is applied to inject colored liquid into a tissue-mimicking gel as well as print a material on a glass substrate.

Strong optical coupling is a carrier extraction method for hot-carrier solar cells, where carriers are extracted as photons from an absorber material and then reabsorbed by a conventional photovoltaic (PV) cell. The design and working principle of this kind of optically coupled device is investigated using a relaxation-time model with practical operating conditions. This investigation is performed in the so-called “down-conversion” configuration, where the band gap of the absorber material is the same as that in the final photovoltaic cell. In this configuration, carrier impact ionization rates that are faster than thermalization rates by at least 50 times are needed to effectively down-convert the hot carriers’ energy to the electronic band edge. Photon emission through radiative recombination must be enhanced by more than 500 times through photonic engineering in order to reduce Auger loss during carrier extraction. The strong luminescence from the PV cell feeding back to the absorber material will further limit the optical extraction in the near-field coupled structure, reducing the overall conversion efficiency to be lower than the ideal expectation reported previously. The strong similarity between the hot-carrier down-conversion and carrier multiplication also suggests the possible application of such optical extraction for multiple-exciton-generation solar cells, making it potentially a general carrier extraction approach for third-generation solar cells.

Ultrasound-driven oscillating microbubbles are used as active actuators in microfluidic devices to perform manifold tasks such as mixing, sorting, and manipulation of microparticles. A common configuration consists of side bubbles created by trapping air pockets in blind channels perpendicular to the main channel direction. This configuration consists of acoustically excited bubbles with a semicylindrical shape that generate significant streaming flow. Because of the geometry of the channels, such flows are generally considered as quasi-two-dimensional. Similar assumptions are often made in many other microfluidic systems based on flat microchannels. However, in this Letter we show that microparticle trajectories actually present a much richer behavior, with particularly strong out-of-plane dynamics in regions close to the microbubble interface. Using astigmatism particle-tracking velocimetry, we reveal that the apparent planar streamlines are actually projections of a stream surface with a pseudotoroidal shape. We, therefore, show that acoustic streaming cannot generally be assumed as a two-dimensional phenomenon in confined systems. The results have crucial consequences for most of the applications involving acoustic streaming such as particle trapping, sorting, and mixing.

We report the observation of all-optical squeezing in an on-chip monolithically integrated CMOS-compatible platform. Our device consists of a low-loss silicon nitride microring optical parametric oscillator (OPO) with a gigahertz cavity linewidth. We measure 1.7 dB (5 dB corrected for losses) of sub-shot-noise quantum correlations between bright twin beams generated in the microring four-wave-mixing OPO pumped above threshold. This experiment demonstrates a compact, robust, and scalable platform for quantum-optics and quantum-information experiments on chip.

We consider the effect of metal electrodes on the polarizability and nucleation of metal phases responsible for the operations of the emerging solid-state memory. Our analysis is based on the image-charge approach. We find results for point dipoles in static and oscillatory fields as well as an erect cylindrical nucleus near metal surfaces in resistive switching memories. We predict a large increase in polarizability and nucleation rate due to the metal electrode effects.

We show that optical forces acting on carbon nanotubes are substantially enhanced when the optical wavelength is tuned to resonances in the electronic band structure. Using a tunable laser source, we experimentally demonstrate a resonant optical scattering force on single-walled carbon nanotubes and, by tuning the wavelength, exploit this force to achieve chirality enrichment of four chiralities of nanotubes. Our results represent a significant step towards optical manipulation and sorting of carbon nanotubes based on their chiral vector.

The mode shape of bending waves in thin silicon and silicon-carbide membranes is measured as a function of space and time, using a phase-shift interferometer with stroboscopic light. The mode shapes hold information about all the relevant mechanical parameters of the samples, including the spatial distribution of static prestress. We present a simple algorithm to obtain a map of the lateral tensor components of the prestress, with a spatial resolution much better than the wavelength of the bending waves. The method is not limited to measuring the stress of bending waves. It is applicable in almost any situation, where the fields determining the state of the system can be measured as a function of space and time.

Spin-orbit coupling in ferromagnets gives rise to the anomalous Hall effect and the anisotropic magnetoresistance, both of which can be used to create spin-transfer torques in a similar manner as the spin Hall effect. In this paper, we show how these effects can be used to reliably switch perpendicularly magnetized layers and to move domain walls. A drift-diffusion treatment of the anomalous Hall effect and the anisotropic magnetoresistance describes the spin currents that flow in directions perpendicular to the electric field. In systems with two ferromagnetic layers separated by a spacer layer, an in-plane electric field causes spin currents to be injected from one layer into the other, creating spin-transfer torques. Unlike the related spin Hall effect in nonmagnetic materials, the anomalous Hall effect and the anisotropic magnetoresistance allow control of the orientation of the injected spins, and hence torques, by changing the direction of the magnetization in the injecting layer. The torques on one layer show a rich angular dependence as a function of the orientation of the magnetization in the other layer. The control of the torques afforded by changing the orientation of the magnetization in a fixed layer makes it possible to reliably switch a perpendicularly magnetized free layer. Our calculated critical current densities for a representative CoFe/Cu/FePt structure show that the switching can be efficient for appropriate material choices. Similarly, control of the magnetization direction can drive domain-wall motion, as shown for NiFe/Cu/NiFe structures.

The two-dimensional electron gas (2DEG) observed at the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ heterointerface has attracted intense research interest in recent years. The high mobility, electric tunability, and giant persistent photoconductivity suggest its potential for electronic and photonic applications. The lack of a $p$-type counterpart as well as a highly spin-polarized carrier in the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ system, however, restricts its widespread application, since both multiple carriers and high spin polarization are very desirable for electronic devices. Here, we report a system of ${\text{LaAlO}}_3/{\text{EuTiO}}_3$ digital heterostructures that may overcome these limitations. Results from first-principles calculations reveal that the 2DEG in the $n$-type ${\text{LaAlO}}_3/{\text{EuTiO}}_3$ is a normal ferromagnet. The $p$-type two-dimensional hole gas, on the other hand, is a 100% spin-polarized half-metal. For digital heterostructures with alternating $n$-type and $p$-type interfaces, a magnetic-field-driven insulator-to-metal transition, together with spatially separated electrons and holes, can be realized by tuning the intrinsic polar field. At low temperatures, the spin-polarized electron-hole pairs may result in spin-triplet exciton condensation, which provides an experimentally accessible system for achieving the theoretically proposed dissipationless spin transport. Our findings open a path for exploring spintronics at the heterointerface of transition-metal oxides.

In this work, we study the folding and unfolding of flowing viscous jets by imposing an electric field. We demonstrate that a folded viscous jet can be induced to unfold through jet widening in a sufficiently strong electric field. The folded jets unfold above a critical slenderness, which increases as the jet capillary number increases. Our systematic elucidation of the mechanisms behind the controlled folding has important implications on processes such as nozzle designs for industrial applications that rely on the manipulation of high-speed viscous jets, including liquid dispensing, printing, and food processing.

We obtain robust perpendicular magnetic anisotropy in a $\beta \text{−}\mathrm{W}/{\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}/\mathrm{MgO}$ structure without the need of any insertion layer between W and ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$. This is achieved within a broad range of W thicknesses (3.0–9.0 nm), using a simple fabrication technique. We determine the spin Hall angle (0.40) and spin-diffusion length for the bulk $\beta$ form of tungsten with a large spin-orbit coupling. As a result of the giant spin Hall effect in $\beta \text{−}\mathrm{W}$ and careful magnetic annealing, we significantly reduce the critical current density for the spin-transfer-torque-induced magnetic switching in ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$. The elemental $\beta \text{−}\mathrm{W}$ is a superior candidate for magnetic memory and spin-logic applications.