Formation of an intermediate band in ${\mathrm{GaN}}_x{\mathrm{P}}_{0.4}{\mathrm{As}}_{0.6-x}$ alloys due to the isovalent doping by nitrogen is studied by photoreflectance and absorption spectroscopy. The fundamental energy gap transition ($E_0$) observed for an N-free alloy is replaced by two optical transitions ($E_{-}$ and $E_{+}$) in GaNPAs layers. The $E_{-}$ and $E_{+}$ transitions are explained within the band anticrossing model, where the localized level of nitrogen interacts with the conduction band of the GaPAs host, splitting it into two subbands. The valence band (VB) is mostly unaffected by nitrogen incorporation as confirmed by the same spin-orbit splitting for N-free and N-containing alloys. The energy position of the $E_{-}$ subband and a strong optical absorption between the VB and the $E_{-}$ subband indicates the GaNPAs alloys have an electronic structure suitable for intermediate-band solar cells. Such an electronic structure is not observed for other III–V alloys like GaInAs, GaInAsP, etc., for which the virtual crystal approximation can be applied to describe the evolution of the electronic structure with the alloy content. Results obtained in this work clearly show that GaNPAs with a few percent of nitrogen is an unusual material system, for which the electronic structure properties differ very significantly from properties of well-known III–V alloys, and the application of virtual crystal approximation in this case is inappropriate or very limited.

We present measurements of rubrene single-crystal field-effect transistors with textbooklike transfer characteristics, as one would expect for intrinsically trap-free semiconductor devices. Particularly, the high purity of the crystals and the defect-free interface to the gate dielectric are reflected in an unprecedentedly low subthreshold swing of $65\mathrm{mV}/\mathrm{decade}$, remarkably close to the fundamental limit of $58.5\mathrm{mV}/\mathrm{decade}$. From these measurements, we quantify the residual density of traps by a detailed analysis of the subthreshold regime, including a full numerical simulation. An exceedingly low trap density of $D_{\text{bulk}}=1\times 10^{13}{\mathrm{cm}}^{-3}{\mathrm{eV}}^{-1}$ at an energy of approximately 0.62 eV is found. This result corresponds to one trap per eV in $10^8$ rubrene molecules. The equivalent density of traps located at the interface ($D_{\text{it}}=3\times 10^9{\mathrm{cm}}^{-2}{\mathrm{eV}}^{-1}$) is as low as in the best crystalline Si/Si field-effect transistors. These results highlight the benefit of having van der Waals bonded semiconducting crystals without electronically active states due to broken bonds at the surface.

The most general response of a small dipolar particle to electromagnetic fields is determined not by its electric and magnetic polarizabilities only but also by magnetoelectric coupling coefficients. We are interested in creating particles with engineered magnetoelectric coupling of nonreciprocal nature, including artificial “moving particles.” Recently, it has been shown theoretically that using small nonreciprocal bianisotropic particles, one can realize perfect electromagnetic isolators, nonreciprocal twist polarizers, one-way transparent sheets, and other devices. This paper is a necessary step towards these applications. For two electrically small nonreciprocal scatterers, an analytical electromagnetic model of polarizabilities is developed. Both particles are bianisotropic: the so-called Tellegen omega particle and moving chiral particle. Analytical results are compared to the full-wave numerical simulations. Both models satisfy to main physical restrictions and leave no doubts in the possibility to realize these particles experimentally.

Surface plasmon polaritons (SPPs) are electromagnetic excitations coupled to electron-charge density waves at metal-dielectric interfaces. They have recently been found to enable a range of nanophotonic devices for controlling systems at the quantum level, including single-photon sources, transistors, and ultracompact quantum circuitry. An important quantum feature of SPPs yet to be fully explored is their bosonic nature. In this work, we report direct evidence of the bosonic nature of SPPs in a scattering-based beam splitter. A parametric down-conversion source is used to produce two indistinguishable photons, each of which is converted into a SPP on a metal-stripe waveguide and then made to interact through a semitransparent Bragg mirror. In this plasmonic analog of the Hong-Ou-Mandel experiment, we measure a coincidence dip with a visibility of 72%, a key signature that SPPs are bosons and that quantum interference is clearly involved. Our work opens up possibilities for the study of fundamental quantum effects in plasmonic systems and their related applications.

Liquid-crystal–based biosensors utilize the high sensitivity of liquid-crystal alignment to the presence of amphiphiles adsorbed to one of the liquid-crystal surfaces from water. They offer inexpensive, easy optical detection of biologically relevant molecules such as lipids, proteins, and cells. Present techniques use linear polarizers to analyze the alignment of the liquid crystal. The resulting images contain information not only about the liquid-crystal tilt with respect to the surface normal, the quantity which is controlled by surface adsorption, but also on the uncontrolled in-plane liquid-crystal alignment, thus making the detection largely qualitative. Here we show that detecting the liquid-crystal alignment between circular polarizers, which are only sensitive to the liquid-crystal tilt with respect to the interface normal, makes possible quantitative detection by measuring the transmitted light intensity with a spectrophotometer. Following a new procedure, not only the concentration dependence of the optical path difference but also the film thickness and the effective birefringence can be determined accurately. We also introduce a new “dynamic” mode of sensing, where (instead of the conventional “steady” mode, which detects the concentration dependence of the steady-state texture) we increase the concentration at a constant rate.

We investigate the dynamics of molecular transport across the gas boundary layer formed above a cold substrate used in the organic vapor phase deposition of small organic molecules. The boundary layer properties ultimately determine film thickness uniformity and morphology, and material utilization efficiency. We use laser-induced fluorescence to spatially resolve the temperature and the concentration of organic molecules within the boundary layer. Under conditions typically used in organic vapor phase deposition of chamber pressures less than 5 torr, we find that the boundary layer extends to a remarkable distance of over 10 cm from the cooled substrate surface. Analytical and numerical models of molecular transport processes are developed to understand the transport of organic molecules to the substrate. Our models provide insights into conditions required to optimize film uniformity and material utilization efficiency in the growth of organic electronic devices.

Nanopore DNA sequencing holds great promise for producing long read lengths from small amounts of starting material, however, high error rates are a problem. We perform nonequilibrium electron transport calculations within an effective tight-binding model of the DNA molecule to study the intrinsic structural noise in DNA sequencing via transverse current in nanopores. The structural noise arises from the effect of neighboring bases on the tunneling current. We find that it could be comparable to the environmental noise, which is caused by changes of the position of the molecule with respect to the electrodes in the nanopore. Moreover, while the environmental noise can be reduced by continuous measurement and by improving the measurement setup, the structural noise is intrinsic. With the help of our methodology we optimize the dependence of the structural noise on the measurement parameters, such as the type of the electrodes and the applied bias. We also propose a statistical technique, utilizing not only the currents through the nucleotides but also the correlations in the currents, to improve the fidelity of the sequencing.

Despite the lack of reproducible experimental confirmation, group-V elements have been considered as possible sources of $p$-type doping in ZnO in the form of simple and complex defects. Using ab initio calculations, based on state-of-the-art hybrid exchange-correlation functional, we study a wide range of defects and defect complexes related with N, P, As, and Sb impurities. We show that none of the candidates for $p$-type doping can be considered a good source of holes in the valence band due to deep acceptor levels and low formation energies of compensating donor defects. In addition, we discuss the stability of complexes in different regimes.

We demonstrate and characterize a scalable optical subsystem for detecting ion qubit states in a surface electrode ion trap. An array of lithographically fabricated diffractive lenses located below the plane of the trap images ions at multiple locations, relaying the collected light out of the vacuum chamber through multimode fibers. The lenses are designed with solid angle collection efficiencies of 3.58%; with all losses included, a detection efficiency of 0.388% is measured. We measure a minimal effect of the dielectric optical substrate on the temporal variation of stray electric fields and the motional heating rate of the ion.

Interfaces play an important role in emerging organic electronic applications. In order to optimize and control the performance in organic devices such as organic solar cells, a comprehensive understanding of the contacts is essential. However, despite the vast progress made, a fundamental theory of the physical processes taking place at the contacts is still lacking. In this work, a numerical device model is used to clarify the effect of imperfect contacts in organic bulk heterojunction solar cells. The effect of increased injection barriers, reduced surface recombination, interfacial minority carrier doping, and traps for majority carriers at the electrodes causing reduced efficiencies is simulated. Two distinctly different underlying mechanisms leading to different S-shaped features are found, both leading to an effective shift of the built-in voltage. In the case of an extraction barrier to majority carriers at the contact, such as reduced surface recombination, the S kink is due to an induced diffusion potential. In the case of interfacial doping or traps, the S kink results from band bending caused by the fixed or trapped space charge. We derive analytical expressions describing the effective reduction of the built-in voltage and the (effective) open-circuit voltage providing means to quantify and distinguish the mechanisms. We show how to experimentally differentiate between these effects and provide tools to extract the relevant physical parameters.

The neutral biexciton cascade of single quantum dots is a promising source of entangled photon pairs. The character of the entangled state is determined by the energy difference between the excitonic eigenstates known as fine-structure splitting (FSS). Here we reduce the magnitude of the FSS by simultaneously using two independent tuning mechanisms: in-plane magnetic field and vertical electric field. We observe that there exists a minimum possible FSS in each quantum dot which is independent of these tuning mechanisms. However, with simultaneous application of electric and magnetic fields, we show the FSS can be reduced to its minimum value as the energy of emission is tuned over several meV with a 5-T magnet.

Mechanical resonators shaped like microcantilevers have been demonstrated as a platform for very sensitive detection of chemical and biological analytes. However, their use as an analytical tool requires fundamental understanding of the molecular absorption-induced effects in the static and dynamic sensor response. The effect of absorption-induced surface stress on the microcantilever response is here investigated using palladium hydride formation. It is shown that the dynamic and static responses of the cantilever monitored simultaneously exhibit excellent correlation with the crystalline phases of hydride formation. However, the resonance frequency shifts and quasistatic bending are observed to be independent during the solid solution phase. Importantly, absorption-induced changes in the elastic parameters of the palladium film are found to play a dominant role in the static and dynamic response. The presented results help in discerning between changes in swelling and elastic modulus that control the cantilever response as well as the relationships between these parameters.

A phenomenon that can be exploited for the manipulation of magnetization without the conventional current-generated magnetic fields is magnetoelastic coupling, which might, thus, pave the way for low-power data-storage devices. Here, we report a quantitative analysis of the magnetic uniaxial anisotropy induced by piezoelectric strain in Ni nanostructured squares. By applying strain, the magnetic domains in Ni nanostructured squares can be manipulated by the magnetoelastic effect in the Ni. The strain-induced anisotropy displaces the domain walls in the square leading to changes in the domain sizes. By comparing the experiments with micromagnetic simulations, the resulting uniaxial anisotropy is quantified. We find a good agreement for a magnetostrictive constant of ${\lambda }_s=-26\mathrm{ppm}$, confirming a full strain transfer from the piezoelectric to the Ni nanostructures and the retainment of a bulklike ${\lambda }_s$.

The miniaturization of optical devices is a prerequisite for broadband data-processing technology to compete with cutting-edge nanoelectronic circuits. For these future nano-optical circuits, controlling the spatial and temporal evolution of surface plasmons, i.e., propagating optical near fields at metal-insulator interfaces, is a key feature. Here, we design, optimize, and fabricate a nanoscale directional coupler with one input and two output ports, a device that is an essential element of nano-optical circuits. The directional coupler is based on a two-wire transmission line supporting two plasmonic eigenmodes that can be selectively excited. By manipulating the input polarization of ultrashort pulses and pulse pairs and by characterizing the light emitted from both output ports, we demonstrate open-loop ultrafast spatial and spatiotemporal coherent control of plasmon propagation. Because of the intuitive and optimized design, which exploits a controlled near-field interference mechanism, varying the linear input polarization is enough to switch between both output ports of the nanoscale directional coupler. Since we exploit the interference of a finite spectrum of eigenmodes, our experiments represent a very intuitive classical analogue to quantum control in molecules.

We demonstrate microwave manipulation of the spin states of electrically injected spin-polarized electrons in silicon. Although the silicon channel is bounded by ferromagnetic metal films, we show that moderate microwave power can be applied to the devices without altering the device operation significantly. Resonant microwave irradiation is used to induce spin rotation of spin-polarized electrons as they travel across a silicon channel, and the resultant spin polarization is subsequently detected by a ferromagnetic Schottky barrier spin detector. These results demonstrate the potential for combining advanced electron spin resonance techniques to complement the study of semiconductor spintronic devices beyond standard magnetotransport measurements.

We study the charge transfer in multilayer graphene from first principles. We find that highly oriented (Bernal) and misoriented (turbostratic) multilayers show similar charge distributions despite their different electronic structure. We quantify the charge transfer and doping distribution in turbostratic graphene layers, where the screening is affected by vanishing density of states near the Fermi level. The results are in good agreement with an analytic model accounting for the electrostatic interaction and the band filling and point out the importance of system-specific interactions between the surface and the first layer. We find that graphene is an outstanding material for ultrathin electrodes, as most of the benefits of multilayer graphene can be captured with bilayers.

Inkjet printing deposits droplets with a well-controlled narrow size distribution. This paper aims at improving experimental and numerical methods for the optimization of drop formation. We introduce a method to extract the one-dimensional velocity profile inside a single droplet during drop formation. We use a novel experimental approach to capture two detailed images of the very same droplet with a small time delay. The one-dimensional velocity within the droplet is resolved by accurately determining the volume distribution of the droplet. We compare the obtained velocity profiles to a numerical simulation based on the slender jet approximation of the Navier-Stokes equation and we find very good agreement.

Listening is a complex sound selection process thought to be located in the auditory cortex. A biophysically motivated Hopf model of the mammalian cochlea reveals that pitch, a main characteristic in the perception of sound, is already materialized at the level of the mammalian hearing sensor. Here, we provide evidence that major elements of listening may similarly be implemented at the auditory periphery by means of efferent connections to the cochlea that tune the hearing sensor towards an auditory object of interest. The cochlea model we use in our investigations is advocated by its performance quality, the simplicity by which efferent control can be implemented, and by the closeness of the control results compared to the biological data. We tune the Hopf parameters to target on a sound, using pitch as the guiding feature. How well we achieve our goal is tested on real-world sounds and measured by a specifically developed tuning-error measure. The results provide a first estimate of how much the peripheral hearing system can assist a listener in focusing on an auditory signal and, thus, what is contributed by the auditory cortex.

The refractive index of biological specimens is a source of intrinsic contrast that can be explored without any concerns of photobleaching or harmful effects caused by extra contrast agents. In addition, the refractive index contains rich information related to the metabolism of cells at the cellular and subcellular levels. Here, we report a no-moving-parts approach that provides three-dimensional refractive-index maps of biological samples continuously flowing in a microfluidic channel. Specifically, we use line illumination and off-axis digital holography to record the angular spectra of light scattered from flowing samples at high speed. Applying the scalar diffraction theory, we obtain accurate refractive-index maps of the samples from the measured spectra. Using this method, we demonstrate label-free three-dimensional imaging of live RKO human colon cancer cells and RPMI8226 multiple myeloma cells, and obtain the volume, dry mass, and density of these cells from the measured three-dimensional refractive-index maps. Our results show that the reported method, alone or in combination with the existing flow cytometry techniques, shows promise as a quantitative tool for stain-free characterization of a large number of cells.

We present a method by which first-principles calculations are linked quite naturally to experimental observables, which determine and characterize the state of an electrochemical system. The method is based on the formation energies of ions in solution and utilizes a grand canonical framework. This is particularly well suited to use in conjunction with ab initio calculations, since it allows charged defects to be treated individually, rather than requiring the consideration of full charge-neutral reactions. It allows the identification of a region of water stability, the $p\mathrm{H}$ scale, and the electrode potential. We discuss and demonstrate the possibilities and capabilities of the approach by the extraction of ion-hydration energies from the Nernst equation and the construction of a $\text{potential}/p\mathrm{H}$ diagram for zinc by an alternative route deviating from the standard approach used to obtain Pourbaix diagrams.