The terahertz band has been historically hindered by the lack of efficient generators and detectors, but a series of recent breakthroughs have helped to effectively close the “terahertz gap.” A rapid development of terahertz technology has been possible thanks to the translation of revolutionary concepts from other regions of the electromagnetic spectrum. Among them, metamaterials stand out for their unprecedented ability to control wave propagation and manipulate electromagnetic response of matter. They have become a workhorse in the development of terahertz devices such as lenses, polarizers, etc., with fascinating features. In particular, epsilon-near-zero (ENZ) metamaterials have attracted much attention in the past several years due to their unusual properties such as squeezing, tunneling, and supercoupling where a wave traveling inside an electrically small channel filled with an ENZ medium can be tunneled through it, reducing reflections and coupling most of its energy. Here, we design and experimentally demonstrate an ENZ graded-index (GRIN) metamaterial lens operating at terahertz with a power enhancement of 16.2 dB, using an array of narrow hollow rectangular waveguides working near their cutoff frequencies. This is a demonstration of an ENZ GRIN device at terahertz and can open the path towards other realizations of similar devices enabling full quasioptical processing of terahertz signals.

We demonstrate the application of a fiber-coupled quantum dot (QD) in a tip as a scanning probe for electric-field imaging. We map the out-of-plane component of the electric field induced by a pair of electrodes by the measurement of the quantum-confined Stark effect induced on a QD spectral line. Our results are in agreement with finite-element simulations of the experiment. Furthermore, we present results from analytic calculations and simulations which are relevant to any electric-field sensor embedded in a dielectric tip. In particular, we highlight the impact of the tip geometry on both the resolution and sensitivity.

A hysteresis loop with three polarization states is obtained in the case of a symmetric epitaxial ferroelectric-interlayer-ferroelectric structure with bottom and top ${\mathrm{SrRuO}}_3$ electrodes. The ferroelectric layers are of $\mathrm{Pb}{\mathrm{Zr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$, while the interlayer is ${\mathrm{CoFe}}_2{\mathrm{O}}_4$. It is shown that the three polarization states can be separately accessed, suggesting that this type of structure can be used as building element for a three-state nonvolatile ferroelectric random-access memory (FERAM). The presence of the three-state memory effect is explained through a simple phenomenological model based on Landau-Ginzburg-Devonshire theory. The findings of this study can pave the way to multistate all-oxide FERAM devices, resulting in a 50% increase in the storage density compared to actual nonvolatile memories.

Ferroelectric oxides are attractive materials for constructing efficient solar cells. Nevertheless, a wide band gap of nearly 3.0 eV in these ferroelectric oxides would result in poor overall sunlight absorption and, hence, low energy conversion efficiency. Here, by systematic first-principles density-functional calculations, we demonstrate that double-perovskite semiconductors ${\text{ScFe}}_{1-x}{\mathrm{Cr}}_x{\mathrm{O}}_3$ ($1/6\le x\le 5/6$) with a narrow band gap of approximately 1.8 eV would simultaneously exhibit large ferroelectric polarization ($100\mu \mathrm{C}/{\mathrm{cm}}^2$) and ferrimagnetic magnetization ($170\text{emu}/{\mathrm{cm}}^3$). Within a Schottky-based model for a typical sandwich solar-cell structure, a power-conversion efficiency of 9.0% can be reached by neglecting all other sources of photovoltaicity in ferroelectric materials. This value is larger than the largest value of 8.1% observed in ferroelectric oxides. Furthermore, these double perovskites are found to be single-spin semiconductors, and the obtained photocurrent is fully spin polarized over almost the entire Sun spectrum. These fascinating advantages would make ${\mathrm{ScFe}}_x{\mathrm{Cr}}_{1-x}{\mathrm{O}}_3$ ($1/6\le x\le 5/6$) semiconductors promising candidates for highly efficient solar cells and spin photovoltaic devices.

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, but currently most metadevices work either in pure reflection or pure transmission mode, leaving half of the EM space completely unexplored. Here, we propose an alternative type of metasurface, composed of specifically designed meta-atoms with polarization-dependent transmission and reflection properties, to efficiently manipulate EM waves in the full space. As a proof of concept, three microwave metadevices are designed, fabricated, and experimentally characterized. The first two metadevices can bend or focus EM waves at different sides (i.e., transmission and reflection sides) of the metasurfaces, depending on the incident polarization, while the third one changes from a wave bender for the reflected wave to a focusing lens for the transmitted wave as the excitation polarization is rotated, with all of these functionalities exhibiting very high efficiencies (in the range of 85%–91%) and total thickness $\sim \lambda /8$. Our findings significantly expand the capabilities of metasurfaces in controlling EM waves, and can stimulate high-performance multifunctional metadevices facing more challenging and diversified application demands.

Relaxor ferroelectric single crystals have been documented to possess a skin effect in the near-surface region, which may play an important role in emerging micro-nano-scale piezoelectric devices that are surface dominated. Here, a metastable ferroelectric tetragonal phase is induced by an electric field applied above the Curie temperature (i.e., poling at high temperature) in rhombohedral-structured $\mathrm{Pb}({\mathrm{In}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_3\text{−}\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_3\text{−}{\mathrm{PbTiO}}_3$ ternary relaxor ferroelectric crystals. Most interesting, this metastable tetragonal structure unexpectedly transforms into a monoclinic $B$-type ($M_B$) phase, as revealed by reciprocal-space mesh scans via high-resolution x-ray diffraction. The domain configurations of the $M_B$ phase obtained by poling above the Curie temperature are similar to that obtained by poling at room temperature; however, the bulk piezoelectricity is extremely weak relative to that induced by the latter process. This controversy between microstructure and macroproperty is understood by a “surface-interior” heterogeneous structure, which provides insight into the existence of skin effects in the relaxor ferroelectric single crystals.

Attractive mechanical, optical, and electronic properties of semiconducting ZnO nanowires make them prime candidates for a variety on energy-harvesting technologies, including photovoltaics and piezoelectric nanogenerators. In order to enhance the efficiency and versatility of such devices, it is paramount to elucidate the connections between the different property realms, i.e., to establish how mechanical distortions can affect the electronic and optical response of the nanowires, depending on their size, shape, and morphology. For example, it was recently demonstrated that band-gap downshifts of up to $-0.1\mathrm{eV}$ can be induced in monolithic ZnO nanowires by an application of tensile strain [see Wei et al., Nano Lett. 12, 4595 (2012)]. Here, we conduct mesoscale-level, finite-element-method-based modeling of the coupled elastic and electronic properties of both already-synthesized monolithic ZnO nanowires and yet-to-be-fabricated Zn-ZnO core-shell structures with diameters ranging from 100 to 800 nm. Our investigation suggests that, after an optimization of the size, shape, and mutual crystallographic orientations of the core and shell regions, core-shell nanowires can exhibit downward band-gap shifts of up to $-0.3\mathrm{eV}$ (i.e., approximately 10% of the stress-free ZnO band-gap value) under tensile distortions, which can greatly expand the utility of such nanostructures for optoelectronic applications.

The effect of energy-selective extraction on a hot carrier’s population is addressed in this study. Using an evaporative cooling model inspired by the field of cold atoms, we derive an analytical expression supported by numerical calculations to account for the removal of particles from the distribution and subsequent energy redistribution among the remaining carriers. Depending on the filtering dimensionality and energy level of extraction, the distribution can be either heated up or cooled down, resulting in a modification of the current-voltage characteristic associated to the structure. The negative differential resistance peak indicating the selective extraction is shown to be markedly reduced when evaporation is considered, which may lead to an overestimation of the tunneling current in previous models. These results provide insights into the interpretation of experimental results on energy-selective contacts, as well as a straightforward method to unequivocally demonstrate the energy filtering of hot carriers in a structure operating under continuous illumination.

We study the optical transmission of a waveguide that is side coupled to a high-$Q$ circular microresonator. The coupling is critical if the intrinsic resonator losses equal the coupling losses to the waveguide. When this happens, the transmittance of the waveguide displays resonance dips with maximal depth as the frequency is swept through the resonators’ resonances. We show that multiple configurations, parametrized by the minimal distance between the resonator and the waveguide, can lead to critical coupling. Indeed, for a sufficiently large resonator radius, the flow of power between the waveguide and the resonator can change sign several times within a single pass. This leads to an oscillatory coupling parameter as a function of the separation distance. As a result, multiple geometrical configurations can lead to critical coupling, even if the waveguide lies in the equatorial plane of the resonator. These results are explained using coupled-mode theory and full-wave numerical simulations. In the vicinity of secondary or higher-order critical coupling, the depth of the transmittance dip is very sensitive to the environment. We discuss how this effect can be exploited for sensing purpose. Alternatively, by actively controlling the environment in the secondary critical configuration, the waveguide-resonator system can be driven as an optical switch.

The $\mathrm{InAs}/{\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ superlattice system distinctly differs from two well-studied superlattice systems $\mathrm{GaAs}/\mathrm{AlAs}$ and $\mathrm{InAs}/\mathrm{GaSb}$ in terms of electronic band alignment, common elements at the interface, and phonon spectrum overlapping of the constituents. This fact leads to the unique electronic and vibrational properties of the $\mathrm{InAs}/{\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ system when compared to the other two systems. In this work, we report a polarized Raman study of the vibrational properties of the $\mathrm{InAs}/{\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ superlattices (SLs) as well as selected ${\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ alloys, all grown on GaSb substrates by either MBE or metalorganic chemical vapor deposition (MOCVD) from both the growth surface and cleaved edge. In the SL, from the (001) backscattering geometry, an InAs-like longitudinal optical (LO) mode is observed as the primary feature, and its intensity is found to increase with increasing Sb composition. From the (110) cleaved-edge backscattering geometry, an InAs-like transverse optical (TO) mode is observed as the main feature in two cross-polarization configurations, but an additional InAs-like “forbidden” LO mode is observed in two parallel-polarization configurations. The ${\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ alloys lattice matched to the substrate ($x_{\mathrm{Sb}}\sim 0.09$) grown by MBE are also found to exhibit the forbidden LO mode, implying the existence of some unexpected [001] modulation. However, the strained samples ($x_{\mathrm{Sb}}\sim 0.35$) grown by MOCVD are found to behave like a disordered alloy. The primary conclusions are (1) the InAs-like LO or TO mode can be either a confined or quasiconfined mode in the InAs layers of the SL or extended mode of the whole structure depending on the Sb composition. (2) $\mathrm{InAs}/{\mathrm{InAs}}_{1-x}{\mathrm{Sb}}_x$ and $\mathrm{InAs}/\mathrm{GaSb}$ SLs exhibit significantly different behaviors in the cleaved-edge geometry but qualitatively similar in the (001) geometry. (3) The appearance of the forbidden LO-like mode is a universal signature for SLs and bulk systems resulting from the mixing of phonon modes due to structural modulation or symmetry reduction.

Complex systems close to their critical state can exhibit abrupt transitions—avalanches—between their metastable states. It is a challenging task to understand the mechanism of the avalanches and control their behavior. Here, we investigate microwave stimulation of avalanches in the so-called vortex matter of type-II superconductors—a system of interacting Abrikosov vortices close to the critical (Bean) state. Our main finding is that the avalanche incubation strongly depends on the excitation frequency, a completely unexpected behavior observed close to the so-called depinning frequencies. Namely, the triggered vortex avalanches in Pb superconducting films become effectively inhibited approaching the critical temperature or critical magnetic field when the microwave stimulus is close to the vortex depinning frequency. We suggest a simple model explaining the observed counterintuitive behaviors as a manifestation of the strongly nonlinear dependence of the driven vortex core size on the microwave excitation intensity. This paves the way to controlling avalanches in superconductor-based devices through their nonlinear response.

The photoemission and injection properties of two GaAs/(Al,Ga)As electrodes with effective negative electron affinity (NEA) are studied in the parallel-plate capacitorlike vacuum photodiode. Both electrodes are bonded to the glass of the input windows, allowing measuring the quantum yield in the transmission and reflection modes. The photodiode with NEA states of both electrodes is sensitive to the illumination in the 400–900 nm range and produces the photocurrent with no bias applied between electrodes. The energy distribution of emitted electrons is studied as a function of the transverse energy component to the surface in the temperature range of 20–300 K. The presence of the fine structure in the photoemission spectra is associated with the electron-phonon coupling in two-dimensional quantized states in the band-bending region. The two-electrode vacuum photoemission system demonstrates the negative differential conductivity. The cathodoluminescence signal is measured as a function of free-electron injection energy with the threshold appearance less than 0.05 V between electrodes.

We experimentally investigate superconducting coplanar waveguide resonators in external magnetic fields and present two strategies to reduce field-induced dissipation channels and resonance frequency shifts. One of our approaches is to significantly reduce the superconducting ground-plane areas, which leads to reduced magnetic-field focussing and thus to lower effective magnetic fields inside the waveguide cavity. By this measure, the field-induced losses can be reduced by more than one order of magnitude in mT out-of-plane magnetic fields. When these resonators are additionally coupled inductively instead of capacitively to the microwave feed lines, an intrinsic closed superconducting loop is effectively shielding the heart of the resonator from magnetic fields by means of flux conservation. In total, we achieve a reduction of the field-induced resonance frequency shift by up to 2 orders of magnitude. We combine systematic parameter variations on the experimental side with numerical magnetic-field calculations to explain the effects of our approaches and to support our conclusions. The presented results are relevant for all areas, where high-performance superconducting resonators need to be operated in magnetic fields, e.g., for quantum hybrid devices with superconducting circuits or electron spin resonance detectors based on coplanar waveguide cavities.

We study the radiative decay, or fluorescence, of excitons in organic solar cells as a function of its geometrical parameters. Contrary to their nonradiative counterpart, fluorescence losses strongly depend on the environment. By properly tuning the thicknesses of the buffer layers between the active regions of the cell and the electrodes, the exciton lifetime and, hence, the exciton diffusion length can be increased. The importance of this phenomenon depends on the radiative quantum efficiency, which is the fraction of the exciton decay that is intrinsically due to fluorescence. Besides this effect, interferences within the cell control the efficiency of sunlight injection into the active layers. The optimal cell design must rely on a consideration of these two aspects. By properly managing fluorescence losses, one can significantly improve the cell performance. To demonstrate this fact, we use realistic material parameters inspired from literature data and obtain an increase of power-conversion efficiency from 11.3% to 12.7%. Conversely, not to take into account the strong dependence of fluorescence on the environment may lead to a suboptimal cell design and a degradation of cell performance. The presence of radiative losses, however small, significantly changes the optimal set of thicknesses. We illustrate the latter situation with experimental material data.

Here, we show an ultralow-noise regime of operation in a simple quantum memory in warm ${}^{87}\mathrm{Rb}$ atomic vapor. By modeling the quantum dynamics of four-level room-temperature atoms, we achieve fidelities $>90\%$ for single-photon-level polarization qubits, surpassing any classical strategies exploiting the nonunitary memory efficiency. Additionally, we show experimental techniques capable of producing fidelities close to unity. Our results demonstrate the potential of simple, resource-moderate experimental room-temperature quantum devices.

We report on a picosecond-fast optical removal of spin polarization from a self-confined photocarrier system at an undoped $\mathrm{GaAs}/(\mathrm{Al},\mathrm{Ga})\mathrm{As}$ interface possessing superior long-range and high-speed spin-transport properties. We employ a modified resonant-spin-amplification technique with unequal intensities of subsequent pump pulses to experimentally distinguish the evolution of spin populations originating from different excitation laser pulses. We demonstrate that the density of spins, which is injected into the system by means of the optical orientation, can be controlled by reducing the electrostatic confinement of the system using an additional generation of photocarriers. It is also shown that the disturbed confinement recovers within hundreds of picoseconds after which spins can be again photoinjected into the system.

We present a scalable scheme for executing the error-correction cycle of a monolithic surface-code fabric composed of fast-flux-tunable transmon qubits with nearest-neighbor coupling. An eight-qubit unit cell forms the basis for repeating both the quantum hardware and coherent control, enabling spatial multiplexing. This control uses three fixed frequencies for all single-qubit gates and a unique frequency-detuning pattern for each qubit in the cell. By pipelining the interaction and readout steps of ancilla-based $X$- and $Z$-type stabilizer measurements, we can engineer detuning patterns that avoid all second-order transmon-transmon interactions except those exploited in controlled-phase gates, regardless of fabric size. Our scheme is applicable to defect-based and planar logical qubits, including lattice surgery.

Very high values of carrier mobility have been recently reported in newly developed materials for field-effect transistors (FETs) or thin-film transistors (TFTs). However, there is an increasing concern of whether the values are overestimated. In this paper, we investigate how much contact resistance a FET or TFT can tolerate to allow the conventional current-voltage equations, which is derived for no contact resistance. We contend that mobility in transistors with resistive contact can be underestimated with the presence of the injection barrier, whereas mobility in transistors with gated Schottky contact can be overestimated by more than 10 times. The latter phenomenon occurs even in long-channel devices, and it becomes more severe when using low-$k$ dielectrics. This is because the band bending and injection barrier experience a complicated evolution on account of electrostatic doping in the semiconducting layer; thus, they do not follow a capacitance approximation. When the band bending is weak, the accumulation is as weak as that in the subthreshold regime. Accordingly, the carrier concentration nonlinearly increases with the gate field. This mechanism can occur with or without exhibiting the “kink” feature in the transfer curves, which has been suggested as the signature of overestimation. For precision, carrier mobility should be presented against gate voltage and should be examined by other recommended extraction methods.

We investigate spin and charge dynamics of a quantum dot of phosphorus atoms coupled to a radio-frequency single-electron transistor (SET) using full counting statistics. We show how the magnetic field plays a role in determining the bunching or antibunching tunneling statistics of the donor dot and SET system. Using the counting statistics, we show how to determine the lowest magnetic field where spin readout is possible. We then show how such a measurement can be used to investigate and optimize single-electron spin-readout fidelity.

We study ion partitioning and self-charging of nanoporous electrodes with room-temperature ionic liquids using a classical density-functional theory that accounts for molecular-excluded volume effects and electrostatic correlations. Nanopores of zero electrical potential are predicted to favor adsorption of small ions even without specific surface attraction, and the imbalanced distributions of cations and anions inside the pore induces a net surface charge that promotes further enrichment of small ions. The self-amplified ion partitioning is most significant when the nanopore and the ionic species are of comparable dimension.