Inhomogeneous Electrostatic Field Research Articles

Controllable waveform terahertz generation using rippled plasma driven by an inhomogeneous electrostatic field.

We theoretically present the waveform controls of terahertz (THz) radiations generated from homogeneous and rippled plasma within inhomogeneous external electrostatic field. The Particle-in-cell (PIC) simulations is implemented to demonstrate generation and controllability of three types of THz pulses: single frequency THz pulse in homogeneous plasma, broadband THz pulse and dual frequency THz pulse in rippled plasma. The single frequency THz pulse can be tuned via shifting the knob of electron density of homogeneous plasma. Waveform of broadband THz pulse can be regulated into an envelope-like shape by varying amplitude of electron density of rippled plasma. The two center frequencies' interval of dual frequency THz pulse can be controlled by wave numbers of density distribution of rippled plasma. This work provides a potential means to generate the dual frequency THz pulses with two harmonic frequencies (ω+Ωω, Ω=2) or incommensurate frequencies (ω+Ωω, Ω=1.7,1.8, 2.2…).

Wave-Mechanical Electron-Optical Modeling of Field-Emission Electron Sources

Electron source coherence has a very important influence on the imaging capabilities of modern electron microscopes. However, conventional electron source models that are based on geometrical electron optics implicitly assume that the emission from the source surface is fully incoherent, which can complicate the treatment of highly coherent field-emission sources. In an attempt to treat the wave-optical properties of electron sources, models inspired by light optics treatments of (partially) coherent sources, which assume a planar source and free wave propagation, have been developed. In this case the underlying assumptions are problematic, because the source surface of a field emitter can have a radius of curvature on the nanometer scale, and the emitted electrons are accelerated by a strong, inhomogeneous electrostatic field following emission. We introduce a model based on wave-mechanical electron optics that draws on a quantum mechanical description of electron emission and propagation to obtain a physically consistent treatment of the wave-mechanical properties of electron sources. We apply the model to investigate spatial resolution limits in low-energy electron holography and microscopy, where it is shown that aberrations and coherence properties of the electron source are crucial and interrelated. The wave-mechanical electron-optical model can, furthermore, be readily generalized to assess and improve electron source performance in other scenarios and techniques where spatial and temporal coherence, and electron-optical aberrations, are relevant.

Mapping of inhomogeneous quasi-3D electrostatic field in electro-optic materials

This paper describes a new method for direct measurement and evaluation of the inhomogeneous electrostatic vector field with translational symmetry in electro-optic materials exhibiting the Pockels effect. It is based on the evaluation of maximum transmittance of low intensity light passing through a sample under a voltage bias. Here, the sample is located between rotating crossed polarizers, and camera images are obtained at each point to determine the electric field. The evaluation procedure is demonstrated using data acquired on a CdZnTeSe quasi-hemispheric semiconductor gamma-ray detector. In addition to CdTe-related compounds, the method can be used for various other materials showing overline{4}3m symmetry such as GaAs, CdTe, GaP, 3C-SiC, and ZnS. Furthermore, it can be generalized to other crystalline materials showing the Pockels effect. The method can be used to probe the space charge and the electric field in several kinds of electronic components and devices, as well as provide useful data on the role of defects, contact configurations and other surface and bulk inhomogeneities in the material that can affect the distribution of the internal electric field.

High current ion beam formation with strongly inhomogeneous electrostatic field

A new approach to the development of extraction systems capable of forming ion beams with previously inaccessible intensity is proposed. The use of inhomogeneous accelerating field allows to improve the ion beam formation efficiency significantly. The increase of electric field magnitude is achieved by changing the shape of the electrodes only, without increasing the accelerating voltage and decreasing the interelectrode distance. The comparison is made between a new extraction system and a flat traditional one, which is the most common. The use of a new electrode geometry allows to increase the lifetime of the electrodes in sources of intense beams operating in a continuous wave mode. For electron cyclotron resonance ion sources, results demonstrate the possibility to form high-quality ion beams with a current density of more than 1A cm−2.

Ring Waves on the Surface of a Liquid in a Coulomb Field

The realization laws of instability of ring waves on the surface of an electroconductive liquid in a Coulomb field are studied. The relation is derived, which links the frequency of ring capillary gravitational waves on the surface of a conductive liquid in an inhomogeneous electrostatic field of point charge with physical parameters of the problem and with the wave number analog. Similar to the Tonks–Frenkel field parameter, a field parameter is determined, which characterizes the surface stability of a conductive liquid in an inhomogeneous Coulomb field. It is established that the critical conditions of instability of ring waves on the surface of electroconductive liquid in a Coulomb field are less strict than the conditions of Tonks–Frenkel instability.

Instability of a Charged Droplet in an Inhomogeneous Electrostatic Field of a Rod of Finite Thickness

The instability of a charged droplet of an ideal liquid in an inhomogeneous electrostatic field of a rod of finite thickness maintained at a constant electrostatic potential is investigated within the framework of analytic asymptotic calculations. It is shown that the mode amplitudes and the drop oscillation frequencies increase with the rod thickness. The critical conditions of instability of the droplet reduce by several times as compared with the critical conditions of implementation of its instability in the electrostatic field of an infinitely thin filament maintained at a constant electrostatic potential. An analytic dependence between the charge and field parameters, critical for implementation of the instability of a charged droplet in an inhomogeneous electrostatic field and dependent on the rod thickness, is found.

Instability of a charged droplet in an inhomogeneous electrostatic field of a thin rod

The problem of stability of oscillations of a charged droplet in an inhomogeneous electrostatic field of a thin charged rod is investigated in the nonlinear formulation using the asymptotic expansion in two small parameters, namely, the dimensionless equilibrium droplet strain and the ratio of the droplet oscillation amplitude to the droplet radius. It is shown that, when the droplet charge is less than the Rayleigh critical charge, in the inhomogeneous electrostatic field the droplet instability implementation mechanism remains the same as for the charged droplet in the field of a point charge. As the oscillation mode number increases, the critical field parameter reaches saturation tending to the horizontal asymptotics. The longer the rod, the higher the level of the asymptotics. As the rod length increases, the amplitudes of the related droplet oscillations and the increments of the unstable droplet oscillations in the electrostatic field of the rod decrease.

Steady-state negative Wigner functions of nonlinear nanomechanical oscillators

We propose a scheme for preparing nanomechanical oscillators in nonclassical steady states, characterized by a pronounced negative Wigner function. In our optomechanical approach, the mechanical oscillator couples to multiple laser-driven resonances of an optical cavity. By lowering the resonance frequency of the oscillator via an inhomogeneous electrostatic field, we significantly enhance its intrinsic geometric nonlinearity per phonon. This causes the motional sidebands to split into separate spectral lines for each phonon number and transitions between individual phonon Fock states can be selectively addressed. We show that this enables the preparation of the nanomechanical oscillator in a single-phonon Fock state. Our scheme can, for example, be implemented with a carbon nanotube dispersively coupled to the evanescent field of a state of the art whispering gallery mode microcavity.

Stark-selected beam of ground-state OCS molecules characterized by revivals of impulsive alignment

We make use of an inhomogeneous electrostatic dipole field to impart a quantum-state-dependent deflection to a pulsed beam of OCS molecules, and show that those molecules residing in the absolute ground state, X(1)Σ(+), |00(0)0>, J = 0, can be separated out by selecting the most deflected part of the molecular beam. Past the deflector, we irradiate the molecular beam by a linearly polarized pulsed nonresonant laser beam that impulsively aligns the OCS molecules. Their alignment, monitored via velocity-map imaging, is measured as a function of time, and the time dependence of the alignment is used to determine the quantum state composition of the beam. We find significant enhancements of the alignment (<cos(2) θ(2D)> = 0.84) and of state purity (>92%) for a state-selected, deflected beam compared with an undeflected beam.

Dipole moment of a small water cluster. The effect of size, temperature, and electric field

A molecular dynamics simulation of neutral clusters (H2O)n ≤ 21 has been performed in the framework of the flexible polarized model. The formation and evolution of the dipole moment of the cluster have been investigated with a change in the size and temperature of the cluster and an external electric field. It has been shown that at low electric fields corresponding to the experiments on the deflection of clusters in the transverse inhomogeneous electrostatic field (Moro et al., 2006), the induced polarization of the cluster is determined by the orientational polarizability of the “rigid” cluster, rather than by the intracluster reorientation of the molecules. The calculated dependence of the effective polarizability of the cluster in the low field on n qualitatively reproduces the experimental results, but the calculated polarizability is numerically much higher than the experimental value by, e.g., a factor of 4 for n ≈ 20.

Hydrogen gas sensitivity of thin films of tin oxide surface doped by platinum laser plasma of different structure and charge composition

The effect of structure and charge composition of Pt laser plasma in the surface doping of SnO2 thin films on their resistive gas sensitivity towards hydrogen has been studied. The structure and charge composition of plasma varied depending on the value and polarity of the potential (UD) of the inhomogeneous electrostatic field generated between a target and plates of a diaphragm located in a vacuum chamber outside the plasma dispersion zone under the action of Kr-F laser radiation on a platinum target. Doping at UD > 0 results in an increase in gas sensitivity of SnO2 films several times greater than doping by quasineutral plasma (UD = 0). The films doped at UD < 0 with surface concentrations of Pt optimum in terms of the maximum gas sensitivity appear to tolerant towards changes in the composition of the gas phase in a temperature range of 50–600°C.

Controlled free-form fabrication of nanowires by dielectrophoretic dispension of colloids

The vertical growth of conductive gold nanowires in ambient atmosphere through on-demand dielectrophoretically guided deposition of nanoparticle-laden colloids is demonstrated. Accelerated by dielectrophoresis, rapidly evaporating colloidal droplets leave a capillary nozzle (diameter of 500 nm) and are guided through an inhomogeneous electrostatic field. The nanoparticles in the colloidal droplets combine under the action of van der Waals attraction forces on the substrate to form highly flexible vertical nanowires with easily controllable lengths and diameters, here in the range of 150–800 nm. After bending to bridge an electrode pair and after annealing at 300 °C, the sample gold nanowires demonstrate excellent electrical conductivity considering their size.

Formation of an electron beam with a duration shorter than 100 fs during photoemission of electrons by femtosecond laser pulses

Irradiation of a thin metal target by 38-fs laser pulses at a wavelength of 800 nm is shown to generate a beam of photoelectrons that contains a component whose duration is shorter than 100 fs. The ensemble of photoelectrons is formed by photoemission of a gold film about 10 nm thick sputtered on the base of a prism made of fused silica. The laser beam irradiates a dielectric-metal interface and propagates inside the prism at an angle of 45° to a normal to the interface. The photoelectron beam is formed by accelerating photoelectrons in a spatially inhomogeneous electrostatic potential. The ultrashort component of the photoelectron beam is found to be formed under the action of a ponderomotive potential. It is shown that the ultrashort electron component can be separated from the remaining part of the photoelectron beam with the help of an inhomogeneous electrostatic field.

New Oriented-Molecular-Beam Machine for Surface Stereochemistry with X-ray Photoemission Spectroscopy

The stereochemical control of surface reactions is one of the ultimate goals of surface scientists. An oriented-molecular-beam technique based on the Stark effect of a molecule in an inhomogeneous hexapole electrostatic field is a potential tool for achieving this goal. This technique allows us to select a specific rotational quantum state and also an orientation of a reagent molecule. We have designed, built, and tuned a new UHV-compatible oriented-molecular-beam machine for the elucidation of surface chemical reactions and their related surface modifications. This apparatus is equipped with the components for X-ray photoemission spectroscopy (XPS) in order to detect the surface-reaction products. In the dissociative adsorption of NO on a Si{111} surface, we found a steric effect in the reactivity by monitoring the products on the surface with the new machine. The N-end collision is more reactive than the O-end collision at an incident energy of 58 meV. To our knowledge, this is the first measurement of the steric effect appearing in the reaction products on a surface.

New Development of Ultrahigh-Vacuum Oriented-Molecular-Beam Machine and Its Application to Chemical Reactions on Silicon Surface

The stereochemical control of surface reactions is one of the ultimate goals of surface scientists. An oriented-molecular-beam technique based on the Stark effect of a molecule in an inhomogeneous hexapole electrostatic field is a potential tool for achieving such a goal. This technique allows us to select a specific rotational quantum state and also an orientation of a reagent molecule. We have designed, built and tuned up a new UHV-compatible oriented-molecular-beam machine for the elucidation of the reaction dynamics on surfaces and for surface manufacturing application. In the dissociative adsorption of CH3Cl on a Si{100} surface, we found a dynamical steric effect on the initial sticking probability (S0) using the new machine. S0 in Cl-end collision is larger than that in CH3-end collision at an incident energy of 120 meV. To our knowledge, this is the first measurement of the steric effect in the chemisorption of a molecule on a Si surface.

Contributions of the substrate electric field to the molecular adsorbate optical nonlinearities

The nonlinear optical response of an adsorbate, whose structure is altered by the inhomogeneous electrostatic field of the substrate, has been evaluated with a particular emphasis on the sum-frequency generation process. In the limiting case of an homogeneous electrostatic field, besides the contributions associated with the induced dipole moments, we have additional contributions which only exist if the adsorbed molecule has permanent dipole moments. Also, the Franck-Condon factors of the unperturbed molecule weight the internal couplings induced by the electrostatic field. For the more general inhomogeneous electrostatic field case, while the main observations remain valid, the Franck-Condon factors are modified by the molecular structure changes induced by the electrostatic field. In addition, we have a strong redistribution of the vibronic couplings resulting from the analytical $\mathbf{Q}$ dependence of the partial charge distribution which is a signature of the field inhomogeneities.

Electrostatic Method for Determining the Predisposition of Soil to Deflation

The equation of motion of soil particles in an inhomogeneous electrostatic field has been considered and solved numerically. The factors determining the rate at which particles are carried away from the field have been established. It is shown that the processes of the carrying away of particles in the electrostatic field and under the action of the air flow have analogs. The theory is intended to substantiate the rapid method of determination of the predisposition of soils to wind erosion under natural conditions.

Detection on the Intercosmos-24 satellite of VLF and elf waves stimulated in the topside ionosphere by the heating facility Sura

Results are presented of a satellite experiment on the detection of VLF and ELF waves excited by irradiation of the night ionosphere F region by the field of a nonmodulated powerful radio wave. The excited VLF and ELF waves have been detected at topside ionosphere altitudes h = 500–1000 km in the frequency bands 7.5 kHz < f < 15 kHz and 8 Hz < f < 1.5 kHz at distances less than 50–100 km from the centre of the magnetic force tube passing through the heated spot of the ionosphere. The region of wave occurrence has well-defined boundaries. This points to propagation (guiding) of the observed low frequency waves along geomagnetic field lines. Possible generation mechanisms for the low frequency emission observed are discussed. It is shown that longitudinal ELF waves may be excited at altitudes of the topside ionosphere by beams of suprathermal electrons resulting from the action of a high-power radio wave on the ionosphere. These waves have a small group velocity in the direction orthogonal to the magnetic field. Occurrence of ELF oscillations detected onboard of satellite may be caused also by satellite motion in spatially inhomogeneous electrostatic field, connected with artificial irregularities of plasma density and accelerated electron beams. Whistler waves in the VLF band may result from conversion of lower-hybrid oscillations excited in the reflection region of powerful radio wave due to scattering by artificial small scale field-aligned irregularities. The whistlers generated can propagate to the satellite along artificial ducts formed within the disturbed magnetic force tube.

The influence of electrostatic fields on films of liquid helium

Prompted by the recent striking experimental results reported by Babkin and Hakonen that appeared to show that liquid helium-II does not wet magnesium fluoride, we have examined the effects that an inhomogeneous electrostatic field has on thin films of liquid helium at temperatures below 0.5K. Our model includes the influence of gravity, surface tension, the electric field and the van der Waals interaction between the helium and its supporting substrate. We show that, an inhomogeneous charge on the substrate can produce effects that mimic the surface profiles between wetted and non-wetted areas. The calculations also indicate that some special precautions may be necessary when studying films of liquid or solid helium on insulators.

Numerical study of relativistic electron-beam electrostatic pumping by anode aperture

Quality and pumping of an electron beam by means of the inhomogeneous electrostatic field at an anode aperture are investigated using numerical simulation. A specially developed relativistic particle-in-cell code, described here, is employed. The longitudinal velocity spread and the pitch ratio of the beam electrons are derived for different aperture radii and cathode-aperture distances. The presented results could be practically useful for operation of high-power cyclotron resonance masers with a sharpened cathode and an anode aperture. The beam pumping in such devices is typically achieved by more complicated systems (pumping magnet, adiabatically changing magnetic field, etc.). The numerical simulations show a significant pumping effect coming from the anode aperture itself.