In typical organic light-emitting diodes (OLEDs), the device turn-on voltages are above the HOMO-LUMO energy gaps of emitters. This paper reports that various combinations of active materials can be strategically selected to construct OLEDs with device working voltages below the HOMO-LUMO energy gaps of emitters; i.e., emitted photon energies are higher than applied electrical energies. The operation mechanism of this type of OLED is found to be related to ballistic Auger-electron injection at organic heterojunctions. This Auger-electron injection acts as a built-in voltage booster. The Auger-electron process predicts that the turn-on voltage scales linearly with the energy difference between an acceptor’s LUMO and a donor’s HOMO. This prediction is in excellent agreement with experimental data.

We demonstrate a general method to assemble and propel highly maneuverable colloidal carpets which can be steered via remote control in any direction of the plane. These colloidal micropropellers are composed by an ensemble of spinning rotors and can be readily used to entrap, transport, and release biological cargos on command via a hydrodynamic conveyor-belt effect. An efficient control of the cargo transportation combined with remarkable “healing” ability to surpass obstacles demonstrate a great potential towards development of multifunctional smart devices at the microscale.

We report on highly tunable radio-frequency (rf) characteristics of a low-loss and compact three-dimensional (3D) metamaterial made of superconducting thin-film spiral resonators. The rf transmission spectrum of a single element of the metamaterial shows a fundamental resonance peak at $\sim 24.95\mathrm{MHz}$ that shifts to a 25% smaller frequency and becomes degenerate when a 3D array of such elements is created. The metamaterial shows an in situ tunable narrow frequency band in which the real part of the effective permeability is negative over a wide range of temperature. This narrow frequency band gradually possesses near-zero and positive values for the real part of permeability as the superconducting critical temperature is approached. The studied 3D metamaterial can be used for increasing power-transfer efficiency and tunability of electrically small rf antennas.

We demonstrate ultrabroadband and ultrafast midinfrared (MIR) pump-probe spectroscopy by employing the chirped-pulse up-conversion technique with four-wave difference-frequency generation in a gas. The full spectra of the MIR probe pulses, which spread from 200 to $5000{\mathrm{cm}}^{-1}$, are recorded on a single-shot basis. We apply this MIR pump-probe spectroscopy to measure the ultrafast carrier dynamics of an intrinsic germanium crystal wafer. Ultrafast reflectivity change induced by the excitation of carriers is observed in the region from far to near infrared. The behavior of the reflectivity-change signals at each delay time is explained by the Drude model.

We present an oscillating micromirror with mechanical quality factors $Q$ up to $1.2\times 10^6$ at cryogenic temperature and optical losses lower than 20 ppm. The device is specifically designed to ease the detection of ponderomotive squeezing (or, more generally, to produce a cavity quantum optomechanical system) at frequencies of about 100 kHz. The design allows one to keep under control both the structural loss in the optical coating and the mechanical energy leakage through the support. The comparison between devices with different shapes shows that the residual mechanical loss at 4.2 K is equally contributed by the intrinsic loss of the silicon substrate and of the coating, while at higher temperatures the dominant loss mechanism is thermoelasticity in the substrate. As the modal response of the device is tailored for its use in optical cavities, these features make the device very promising for quantum-optics experiments.

We study experimentally how wettability impacts fluid-fluid-displacement patterns in granular media. We inject a low-viscosity fluid (air) into a thin bed of glass beads initially saturated with a more-viscous fluid (a water-glycerol mixture). Chemical treatment of glass surfaces allows us to control the wetting properties of the medium and modify the contact angle $\theta$ from 5° (drainage) to 120° (imbibition). We demonstrate that wettability exerts a powerful influence on the invasion morphology of unfavorable mobility displacements: increasing $\theta$ stabilizes fluid invasion into the granular pack at all capillary numbers. In particular, we report the striking observation of a stable radial displacement at low capillary numbers, whose origin lies on the cooperative nature of fluid invasion at the pore scale.

We demonstrate that a two-photon absorption process occurs in a ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$-based solar cell in which a ${\mathrm{GaAs}}_{0.31}{\mathrm{P}}_{0.68}{\mathrm{N}}_{0.01}$ layer is implemented in the heterostructure and the $E_{-}$ energy band is isolated using an AlP blocking layer. The observed transition energies in external quantum-efficiency spectra correspond to the $E_{-}$ and $E_{+}$ energy bands of the ${\mathrm{GaAs}}_{0.31}{\mathrm{P}}_{0.68}{\mathrm{N}}_{0.01}$ alloy and agree with the photoreflectance results. With photon energy smaller than the transition between the valence band and the $E_{-}$ band, the absorption of IR light increases the quantum efficiency by over 10% at room temperature and significantly more at lower temperatures due to the suppression in the thermionic emission. The effects of two-photon absorption on the $I\text{−}V$ characteristics and output power of the fabricated ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$ solar cell are studied by current transport simulations. We show how doping of the intermediate band and current transport between the intermediate band and surrounding regions affect the operation of ${\mathrm{GaAs}}_{1-x-y}{\mathrm{P}}_y{\mathrm{N}}_x$ solar cells. Simulations under 1 Sun illumination result in an enhancement of 6% in the power density compared to a corresponding reference single-junction cell.

We study microfluidic self-digitization in Hele-Shaw cells using pancake droplets anchored to surface-tension traps. We show that above a critical flow rate, large anchored droplets break up to form two daughter droplets, one of which remains in the anchor. Below the critical flow velocity for breakup, the shape of the anchored drop is given by an elastica equation that depends on the capillary number of the outer fluid. As the velocity crosses the critical value, the equation stops admitting a solution that satisfies the boundary conditions; the drop breaks up in spite of the neck still having finite width. A similar breaking event also takes place between the holes of an array of anchors, which we use to produce a 2D array of stationary drops in situ.

Bulk semiconductors and insulators typically have continuous valence and conduction bands. Here, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., ${\mathrm{Cs}}_2{\mathrm{NaInBr}}_6$, ${\mathrm{Cs}}_2{\mathrm{NaBiCl}}_6$, and ${\mathrm{Tl}}_2{\mathrm{NaBiCl}}_6$. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use ${\mathrm{Cs}}_2{\mathrm{NaInBr}}_6$ as an example to show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. These characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.

The generation of random numbers via quantum processes is an efficient and reliable method to obtain true indeterministic random numbers that are of vital importance to cryptographic communication and large-scale computer modeling. However, in realistic scenarios, the raw output of a quantum random-number generator is inevitably tainted by classical technical noise. The integrity of the device can be compromised if this noise is tampered with or even controlled by some malicious party. To safeguard against this, we propose and experimentally demonstrate an approach that produces side-information-independent randomness that is quantified by min-entropy conditioned on this classical noise. We present a method for maximizing the conditional min entropy of the number sequence generated from a given quantum-to-classical-noise ratio. The detected photocurrent in our experiment is shown to have a real-time random-number generation rate of $14(\mathrm{Mb}\mathrm{i}\mathrm{t}/\mathrm{s})/\mathrm{MHz}$. The spectral response of the detection system shows the potential to deliver more than $70\mathrm{Gbit}/\mathrm{s}$ of random numbers in our experimental setup.

The creation of efficient thermoelectric devices remains a technological challenge. Using nanoscale-engineered devices offers some potential advantages over bulk materials; however, they also present new problems. The microscopic Hamiltonian of a device which optimizes the efficiency and power output for a particular load and temperature profile is not necessarily optimum for another temperature difference and external load. Furthermore, one cannot necessarily manufacture a particular Hamiltonian. In this paper, we calculate the nonlinear thermoelectric transport through a gate-modulated one-dimensional disordered semiconducting nanowire connected to two large leads. The disorder is chosen to be Lorentzian, which allows exact results for transmission through the wire for all strengths of disorder. By tuning the gate voltage acting on the nanowire, we show that the thermodynamic efficiency can be made large enough to be industrially competitive. The gate voltage allows one to maximize the efficiency and power output for particular temperature differences between the leads as well as different external loads.

We study arrays of parallel doped semiconductor nanowires in a temperature range where the electrons propagate through the nanowires by phonon-assisted hops between localized states. By solving the random-resistor-network problem, we compute the thermopower $S$, the electrical conductance $G$, and the electronic thermal conductance $K^e$ of the device. We investigate how those quantities depend on the position—which can be tuned with a back gate—of the nanowire impurity band with respect to the equilibrium electrochemical potential. We show that large power factors can be reached near the band edges, when $S$ self-averages to large values while $G$ is small but scales with the number of wires. Calculating the amount of heat exchanged locally between the electrons inside the nanowires and the phonons of the environment, we show that phonons are mainly absorbed near one electrode and emitted near the other when a charge current is driven through the nanowires near their band edges. This phenomenon could be exploited for a field control of the heat exchange between the phonons and the electrons at submicron scales in electronic circuits. It could be also used for cooling hot spots.

Inelastic electron tunneling provides a low-energy pathway for the excitation of surface plasmons and light emission. We theoretically investigate tunnel junctions based on metals and graphene. We show that graphene is potentially a highly efficient material for tunneling excitation of plasmons because of its narrow plasmon linewidths, strong emission, and large tunability in the midinfrared wavelength regime. Compared to gold and silver, the enhancement can be up to 10 times for similar wavelengths and up to 5 orders at their respective plasmon operating wavelengths. Tunneling excitation of graphene plasmons promises an efficient technology for on-chip electrical generation and manipulation of plasmons for graphene-based optoelectronics and nanophotonic integrated circuits.

Low-temperature research laboratories with typical liquid-helium consumption of the order of tens of liters per day have greatly benefited from the recent development of small-scale liquefiers. In general, these liquefiers are based on Gifford-McMahon or pulse-tube closed-cycle refrigerators with a nominal cooling power ranging from 1 to 1.5 W at 4.2 K. The liquefaction rate for these cryocooler-based liquefiers depends on the pressure at which the helium is liquefied, although the final user conditions of the produced liquid helium are always atmospheric pressure and boiling temperature (e.g., 4.2 K at 100 kPa). Here, we show a systematic study on this effect, in which an enhancement in excess of 70% in liquefaction rate is found experimentally for pressures near and above the critical point of helium (220 kPa). We propose that the underlying mechanism for the liquefaction enhancement is based on the increase in cryocooler cooling power with temperature and the decrease of the helium enthalpy with pressure.

Previous studies show that air pollution and wind erosion, which damage a leaf’s epicuticular wax layer, can change leaf surface properties from hydrophobic to hydrophilic. However, the dynamic response of a damaged leaf to a raindrop impact has not been investigated and could clarify the direct influence of changes in wettability on early leaf abscission. In this article, we investigate how leaves with different surface properties respond to falling raindrops, viewing this as a unique system of coupled elasticity and drop dynamics. An elastic beam with tunable surface wettability properties is used as a simple leaf model. We find that wettable beams experience much higher torque and bending energy than nonwettable beams. This is because a drop sticks to a wettable beam, while a drop falls off a nonwettable beam. An analytical model using momentum balance and simple cantilever beam theory quantifies the bending energy and torque experienced by wettable and nonwettable beams. The results elucidate the potential damage caused by raindrops impacting a leaf as a function of its surface wettability and are correlated with environmental factors contributing to premature changes of leaf surface properties.

Atomic-resolution optical imaging that overcomes the diffraction limit of light is a challenging task. However, atomic-resolution imaging of the optical field has not yet been achieved. We investigate the atomic-resolution imaging of the optical near field on a surface. With this method, the surface photovoltage of the silicon probe tip induced by the optical near field on the surface is measured. We achieve atomic-resolution imaging of the optical near field on a sapphire $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3(0001)$ surface. We observe the strong tip-sample-distance dependence of the surface photovoltage. This success is a promising development in the exploration of atomic-scale physical and chemical interactions between light and atoms or molecules and offers deeper insight into the various photonic processes and functions on surfaces.

We show how the deposition of laser energy induces propulsion and strong deformation of an absorbing liquid body. Combining high speed with stroboscopic imaging, we observe that a millimeter-sized dyed water drop hit by a millijoule nanosecond laser pulse propels forward at several meters per second and deforms until it eventually fragments. The drop motion results from the recoil momentum imparted at the drop surface by water vaporization. We measure the propulsion speed and the time-deformation law of the drop, complemented by boundary-integral simulations. The drop propulsion and shaping are explained in terms of the laser-pulse energy, the drop size, and the liquid properties. These findings are, for instance, crucial for the generation of extreme ultraviolet light in nanolithography machines.

We study the self-heating of a large stack of ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ intrinsic Josephson junctions, of a configuration designed for terahertz generation. We find good qualitative agreement between direct thermoluminescent measurements of the device surface temperature and low-temperature scanning laser microscopy images. In particular, the two techniques both reveal a mode of thermal instability through the asymmetric nucleation of a small hot spot near a corner or edge of the sample. This behavior conforms with a theoretical stability analysis, and the radius of the hot spot is in excellent agreement with theoretical predictions, as is its growth with increasing bias current and bath temperature. Narrow hot spots may offer a possible means of enhancing the terahertz emission power from this type of device.

The optimized soliton mode locking of the erbium-doped fiber laser (EDFL) and its pulse-shortening dynamic with the graphene nanosheet is demonstrated by precisely detuning the weakly negative group-delay dispersion (GDD) and maintaining strong self-phase-modulation (SPM), to obtain the shortest pulse width of 449 fs with a spectral linewidth of 6.02 nm. The pulse evolution with the mode-locking mechanism changing from the self-amplitude-modulation of the saturable absorber, to the soliton compression caused by the GDD and SPM is experimentally and numerically investigated in detail. Under high pumping powers, the enlarged up-chirp inside the EDFL cavity can induce a significant Kelly-sideband shift of up to 0.5 nm. The passively-mode-locked EDFL pulse width is controllable by detuning the GDD and SPM parameters, so that the pulse width can be compressed from 642 to 449 fs while reducing the negative GDD from $-0.354$ to $-0.154{\mathrm{ps}}^2$. The compression ratio can be also improved by strengthening the SPM at this stage.