External Dc Electric Field Research Articles

The hydrogen atom in an external DC electric field: the discretization approach

We propose a finite element approach to treat the problem of the hydrogen atom in a static electric field. We show the efficiency of a L2 basis set, i.e. B-spline functions. Particular attention is placed on the problem of the normalization in the context of parabolic coordinates. The method is applied to the study of hydrogen photoionization from the fundamental (1s) and excited (n = 2,3,4) states in a weak external DC electric field. Oscillator strengths have been calculated in the region of the field-free atomic threshold. The pertinence of the finite element approach is also discussed for the study of Stark energies of hydrogen. The results are compared with other accurate calculations when available.

Molecular orientation in superfluid liquid helium droplets: high resolution infrared spectroscopy as a probe of solvent–solute interactions

Reported here are three aspects of molecular stereochemistry associated with polar molecules in an exotic solvent, namely, superfluid liquid helium. Infrared spectroscopy is used to study the nature of the solvent–solute and solute–solute interactions in this unique environment. First, we use an external dc electric field to orient the helium solvated polar molecules in the laboratory frame of reference and we characterize this orientation using pendular state spectroscopy. Second, we present results that demonstrate non-equilibrium self-assembly of polar molecules into linear chain structures, due to the mutual orientation imposed by the associated dipole–dipole interactions. Finally, the spectroscopic results provide information on the anisotropic interactions experienced by the solvated molecules, which result in significant molecular orientation in the droplet frame of reference. Studies of this type provide important new insights into the nature of the interactions between the molecules and the superfluid solvent.

Theory of the nonlinear susceptibility of relaxor ferroelectrics

The theory of the nonlinear dielectric response to an external dc electric field is developed in the random-field-theory framework. The equations describing the dependence of the order parameter and the dielectric susceptibility (both linear and nonlinear) on the temperature, dc electric field, frequency, parameters of the host lattice and random-field sources are obtained. The numerical solution of these equations for several random-field-source concentrations and other parameters has shown that in the dipole-glass phase the dc electric field always decreases the dielectric response, while in the mixed ferroelectric-glass phase the dc field can either decrease or increase this response. Approximation of the numerical results for the nonlinear part of the susceptibility leads to with ( is the dimensionless dc-field value). It was shown that, for all of the cases considered, remains finite and has a maximum ( is the dipole-glass freezing temperature). The absence of critical divergency of the nonlinear susceptibility both in theory and experiment proves that, unlike conventional spin glass, the dipole-glass state in relaxors is a metastable state with long (up to infinite) relaxation times. A comparison of the theoretical results obtained with available experimental data for PMN and PMN-10PT is carried out. The calculated temperature and dc-field dependences of the nonlinear susceptibility are in agreement with observed data.

Helically modulated electroclinical effects in twist grain boundary liquid crystals

We report the first observation of an electroclinical effect at the TGB\(\)-TGB\(\) transition induced by an external DC electric field applied perpendicular to the pitch direction. Upon increasing the field, the smectic layers rather than the director field tilt over relative to the helical axis, allowing to detect the effect by X-ray scattering from well aligned samples. The observations are qualitatively interpreted in the frame of a mean field phenomenological model of a helically modulated electroclinical effect.

Observation of high gain in a liquid-crystal panel with photoconducting polymeric layers

A novel, to our knowledge, liquid-crystal panel suitable for real-time holographic purposes has been prepared. A nematic liquid-crystal layer sandwiched between photoconducting polymeric layers, when exposed to a sinusoidal light-intensity pattern, shows efficient formation of refractive-index gratings. The unique feature of the presented panel is its ability to switch energy from beam to beam in a manner similar to the charge-diffusion-controlled photorefractive effect. In a two-wave-mixing experiment multiple orders of diffraction are present, and a very high two-beam coupling-gain ratio (2.5) and a net exponential gain coefficient of ? = 931 cm(-1) have been measured. This gain was achieved in samples biased by a dc external electric field and tilted with respect to the beam-incidence bisector at 45 degrees . The time constants for grating formation and erasure in the studied system are functions of the applied voltage and can be made as short as a few milliseconds under favorable conditions. The mechanism of beam coupling is linked with an electric-field-driven reorientation of the nematic director as a result of a spatially modulated space-charge field created by light in a photoconducting poly(3-octyl)thiophene polymeric layer.

Nonlinear dielectric response of relaxor ferroelectrics

The calculations of nonlinear dielectric susceptibility on external dc electric field is performed in the random field theory framework. The equations which describe the dependence of order parameter and dielectric susceptibility on temperature, dc electric field, frequency, parameters of host lattice and random field sources are obtained. The numerical solutions of these equations for several random field sources concentrations and parameters have shown that nonlinear dielectric susceptibility at low frequencies smaller than linear one in dipole glass state, meanwhile in mixed ferroglass phase nonlinear part can be both negative and positive. No critical divergency of nonlinear susceptibility at freezing temperature was obtained in accordance with experiment. This proves that contrary to conventional spin glasses dipole glasses in relaxors are methastable states with long (up to infinity) relaxation times. The comparison of calculated and observed nonlinear susceptibility is carried out.

A generation model of small-scale geomagnetic field-aligned plasma inhomogeneities in the ionosphere

Abstract The dissipative instability of acoustic-gravity waves in the ionosphere is investigated. This instability results in formatting the horizontal periodic structure of conductivity disturbances which move with the velocities considerably less than the sound velocity. In the presence of an external DC electric field the conductivity variations in the E-layer lead to an appearance of polarization electric fields propagating into the upper ionosphere and generating the plasma density variations at these altitudes. Estimates of the space-time and amplitude characteristics of the excited ionospheric disturbances and their comparison with the experimental data show that the developed mechanism can be applied to generation model of the earthquake-related small-scale plasma inhomogeneities in the upper ionosphere.

Fast mutually pumped phase conjugation induced by a transient photorefractive surface wave

Mutually pumped phase conjugation with a recording time of few milliseconds is obtained in Bi12TiO20 crystals by use of transient photorefractive beam coupling under a dc external electric field. It is demonstrated that the additional acceleration of the positive-feedback-loop formation is required for successful generation of transient phase-conjugate wave fronts. This acceleration is provided by the high-intensity transient photorefractive surface wave that appears immediately after application of the external electric field as the result of coupling of the incident beam with the reflected fanning beams. To the authors’ knowledge, this is the first experimental observation of a transient photorefractive surface wave.

Two-dimensional Perturbative Analysis of Slant Grating in Photorefractive Crystals in an External Electric Field

A slant grating was recorded with two laser beams incident on photorefractive crystal in an external dc electric field. Two-dimensional perturbative analytical solution for the first two-order space-charge field for steady-state photorefractive slant grating recording has been obtained. It is shown that the inclusion of the longitudinal component of the space-charge field during photorefractive two-wave coupling can predict reduction of the fundamental harmonic and enhancement or reduction of the second-order harmonic for various applied field strength and various grating slant angles. The dependence of the first two-order harmonics on the slant angle and on the applied field strength is examined.

Electric field induced molecular reorientation in stilbazolium salt Langmuir-Blodgett monolayers

Electric field induced molecular reorientation in stilbazolium salt Langmuir-Blodgett monolayers have been investigated by using rotation-angle second harmonic generation technique. The stilbazolium salt molecules were preferentially oriented along the dipping direction and this anisotropic feature was enhanced by increasing the dipping speed. An external DC electric field (6.7 × 10★ V/m) forced the molecules to reorient along the field direction, and caused dissociation of aggregate species, as demonstrated by the steady state and time resolved fluorescence measurements.

Self-focusing in photorefractive BaTiO3 crystal under external DC electric field

Experimental observations of 1D and 2D self-focusing in a BaTiO3 crystal under externally DC applied electric field are reported. Stationary self-focusing behaviour can be obtained with and without additional uniform illumination.

Characteristics of Electrorheological Responses in an Emulsion System

The characteristics of electrorheological (ER) responses in an emulsion system have been examined for the first time. The emulsion system consists of a chlorinated paraffin oil and a silicone oil; these two liquids have similar dielectric constants, but the former liquid is 1000 times more conductive than the latter liquid. The mismatch in electric properties between the dispersed phase and the dispersing medium can be reversed in an emulsion by inverting the phase distribution. Transient ER responses in the emulsion system have been studied over a range of shear rates following step changes (on/off) of an external dc electric field. The relative response strength characterizing the ER effect increases with the field strength and with the volume fraction of silicone oil, but decreases with increasing shear rate imposed. The paraffin oil-in-silicone oil emulsion exhibits a response much stronger than that observed in the phase-reversed silicone oil-in-paraffin oil emulsion. No negative ER response is detected in the silicone oil-in-paraffin oil emulsion. With an optical microscope, very different microscopic response characteristics are observed for the phase-reversed emulsions under static conditions, and this can be explained according to the leaky dielectric liquid model of Taylor.

Analytical solutions for the dynamic Kerr effect: Linear response of polar and polarizable molecules to a weak ac electric field superimposed on a strong dc bias field

The infinite hierarchy of differential-recurrence relations for ensemble averages of the spherical harmonics pertaining to the noninertial rotational Brownian motion of an ensemble of polar and anisotropically polarizable molecules in a strong external dc electric field is derived by averaging the underlying Langevin equation. This procedure avoids recourse to the Fokker–Planck equation, the solution of which involves complicated mathematical manipulations. By calculating the Laplace transforms of the relaxation functions for the dynamic Kerr effect of symmetric top molecules, two equilibrium correlation functions are established, thus allowing one to express the corresponding birefringence ac responses by using linear response theory. Exact analytic solutions for the spectra of these correlation functions and relaxation times are first calculated for two limiting cases, namely, pure induced dipole moments and pure permanent moments, using the continued fraction method. The general case where both types of moments are taken into account, is then considered using matrix continued fractions. Furthermore, exact analytical expressions for the Kerr effect relaxation time are also derived in terms of integrals (which are evaluated exactly) and compared with the matrix continued fraction result. Plots of the relaxation time are presented for various values of the parameters ξ and σ characterizing the permanent and the induced dipole moments. Features of the relaxation behavior are emphasized in figures showing the real and imaginary parts of the spectra of the birefringence function. Moreover, Cole–Cole diagrams are presented for various values of ξ and σ in order to see how they deviate from the Debye-like spectra.

Visualization of the core-scattering dynamics of Rydberg wave packets

We present classical and quantum studies of the scattering dynamics of Rydberg electron wave packets from the electronic core of alkali atoms. In quantum systems an ideal state for studying such effects is an angularly localized wavepacket in which the primary effect of the scattering is to cause precession. The scattering is enhanced by the application of an external dc electric field. We calculate and animate the field-induced dynamics of both hydrogenic and alkali wavepackets and compare them to classical atomic models. We end that in alkali systems the scattered wave function can be divided into two components: one whose nearly hydrogenic behavior is due to quantum interference near the core, and another which exhibits the orbital precession found in classical models of nonhydrogenic atoms.

Nonlinear electro-optical response. I. Steady state Kerr effect relaxation arising from a weak ac electric field superimposed on a strong dc bias field

The dynamic Kerr effect (excluding inertial effects) of an assembly of both polar and anisotropically polarizable molecules acted on by a strong external dc electric field superimposed on a weak ac electric field is evaluated by starting from the Smoluchowski equation valid in the noninertial limit. The calculation proceeds by expanding the expectation value of the second Legendre polynomial which describes the Kerr effect relaxation, as a power series up to the second order in the small ac field strength, so that frequency components in ω and 2ω exist. This is accomplished using the matrix continued fraction method which allows one to express exactly the solution of the infinite hierarchy of differential-recurrence relations for the first and second order ac responses of ensemble averages of the Legendre polynomials (relaxation functions). In order to illustrate these results, diagrams showing the behavior of the real and imaginary parts of the complex birefringence functions are presented. In particular, the relaxation spectra clearly indicate the differences between the first order response (equivalent to that obtained using linear response theory) and the second order one. We present also the solutions for the particular cases where only either permanent or induced dipole moments are taken into account. The calculation then radically simplifies as the matrix continued fractions now reduce to scalar ones.

Dynamics of photoinduced lens formation in a photorefractive Bi_12TiO_20 crystal under an external dc electric field

We report experimental results on the dynamics of formation of a photoinduced lens produced by a two-dimensional cw He–Ne Gaussian laser beam (633 nm) in a cubic Bi12TiO20 photorefractive crystal under an external dc electric field. The main features of the effect, namely, astigmatism of the dynamic lens formed, the pulse shape of the far-field response after switching on of the focused beam, inverse linear dependence of time of its growth on the beam intensity, and approximate constancy of its peak level, are explained in the framework of the analytical model for a one-dimensional Gaussian beam and numerical calculations for a two-dimensional one.

Nonlinear dielectric relaxation and dynamic Kerr effect in a strong dc electric field suddenly switched on: Exact solutions for the three-dimensional rotational diffusion model.

The infinite hierarchy of differential-recurrence relations for ensemble averages of the spherical harmonics pertaining to the noninertial rotational Brownian motion of an ensemble of polar and anisotropically polarizable molecules in a strong external dc electric field is derived by averaging the underlying Langevin equation. This procedure avoids recourse to the Fokker-Planck equation, the solution of which involves complicated mathematical manipulations. Exact analytic solutions for the spectra of the relaxation functions and relaxation times for nonlinear dielectric relaxation and dynamic Kerr effect of symmetric top molecules are calculated for two limiting cases, namely, pure induced dipole moments and pure permanent moments, using the continued fraction method. The general case where both types of moment are taken into account is then considered by using matrix continued fractions. Exact expressions for the dielectric and Kerr effect relaxation times are also derived as functions of the parameters $\ensuremath{\xi}$ and $\ensuremath{\sigma}$ characterizing the field-off and the induced dipole moments. Plots of these relaxation times are presented for various values of $\ensuremath{\xi}$ and $\ensuremath{\sigma}$. The nonlinear relaxation behavior is emphasized in figures showing how the real and imaginary parts of the spectra of the relaxation functions deviate from the Lorentzian profiles.

Electric field suppression of epileptiform activity in hippocampal slices.

1. The effects of relatively small external DC electric fields on synchronous activity in CA1 and CA3 from transverse and longitudinal type hippocampal slices were studied. 2. To record neuronal activity during significant field changes, differential DC amplification was employed with a reference electrode aligned along an isopotential with the recording electrode. 3. Suppression of epileptiform activity was observed in 31 of 33 slices independent of region studied and type of slice but was highly dependent on field orientation with respect to the apical dendritic-somatic axis. 4. Modulation of neuronal activity in these experiments was readily observed at field strengths < or = 5-10 mV/mm. Suppression was seen with the field oriented (positive to negative potential) from the soma to the apical dentrites. 5. In vivo application of these results may be feasible.

Dye-doped liquid crystal composite for real-time holography

In this paper we report the results of dynamic phase grating formation in an anthraquinone derivative dichroic dye-doped nematic liquid crystal in which an external DC electric field was used to change the planar alignment of liquid crystal molecules into a near homeotropic one. This field was also needed to produce an efficient grating via photoconductivity-induced molecular reorientation in a degenerate two-wave mixing experiment with He - Ne (632.8 nm) laser light serving as a low power excitation source. The characteristic of the measured effect shows that the mechanism of grating formation is evidently a non-photorefractive one. A diffraction efficiency of up to 20% has been measured in the system. Fast hologram recording and erasing times (1 ms) make this system attractive for image processing applications.

Non-steady-state photoelectromotive force in semiconductor photorefractive crystals biased by dc field

A microscopic model of the non-steady-state photoelectromotive force excited by a vibrating interference pattern in a bipolar photoconductor under an external dc electric field is elaborated for the case in which the lifetime of one type of carrier is longer than the dielectric relaxation time. It is shown theoretically that when two types of carriers with different relaxation times are used their presence leads to some new effects in non-steady-state photoelectromotive-force behavior. First, the frequency transfer function presents a two-shelf form (i.e., it has two regions in which the signal is frequency independent) instead of the high-pass band filter form usually seen with the monopolar model. Second, for both frequency shelves under an external dc field one can observe anomalous quadratic growth of the photoelectromotive-force signal amplitude without change of its sign. Unusual spatial-frequency dependencies typical for this model are also discussed. The analysis is used to explain effects observed in GaAs in the visible and the near-IR spectral regions for which interband optical absorption is typical, with both types of carriers being efficiently generated.