Longitudinal focusing and cooling of a molecular beam

A neutral polar molecule experiences a force in an inhomogeneous electric field. This electric field can be designed such that a beam of polar molecules is exposed to a harmonic potential in the forward direction. In this potential the longitudinal phase-space distribution of the ensemble of molecules is rotated uniformly. This property is used to longitudinally focus a pulsed beam of ammonia molecules and to produce a beam with a longitudinal velocity spread of 0.76 m/s, corresponding to a temperature of 250 mu K.     

The Influence of Electric and Magnetic Fields on Angular Distribution of Intensity of Light Scattered from a Cold Atomic Ensemble

Optics and Spectroscopy - An angular distribution of the intensity of light scattered from an optically dense cold atomic ensemble placed into homogeneous static electric or magnetic field is...     

Screening due to non-equilibrium carriers in presence of deep repulsive traps

The characteristics of screening length due to non-equilibrium electrons at low lattice temperatures are investigated theoretically, when the lifetime of the carriers is controlled by deep repulsive traps. The calculations have been made covering a wide range of the electric field, starting from a weakly heated carrier ensemble up to the onset of impurity breakdown. Apart from becoming field dependent, the screening length now shows a rather complicated dependence upon the lattice temperature. The numerical results for high-purity covalent semiconductors like Ge and Si show that the characteristics of the screening length are now significantly different from what one obtains for an ensemble of carriers that is in thermodynamic equilibrium with the lattice atoms.     

Effect of dielectric confinement on optical properties of colloidal nanostructures

We review the effects caused by a large difference in the dielectric constants of a semiconductor and its surrounding in colloidal semiconductor nanostructures (NSs) with various shapes, e.g., nanocrystals, nanorods, and nanoplatelets. The difference increases the electron–hole interaction and consequently the exciton binding energy and its oscillator transition strength. On the other hand, this difference reduces the electric field of a photon penetrating the NS (the phenomenon is called the local field effect) and reduces the photon coupling to an exciton. We show that the polarization properties of the individual colloidal NSs as well as of their randomly oriented ensemble are determined both by the anisotropy of the local field effect and by the symmetry of the exciton states participating in optical transitions. The calculations explain the temperature and time dependences of the degree of linear polarization measured in an ensemble of CdSe nanocrystals.     

Calculation of the near fields for the scattering of electromagnetic waves by multiple infinite cylinders at perpendicular incidence

The near field solution for the scattering of a plane monochromatic electromagnetic wave by an ensemble of parallel infinite dielectric cylinders at perpendicular incidence is presented in this paper. The solution is given for the calculation of the electric and magnetic near fields and the Poynting vector. A MATLAB program has been developed to solve the near field formulas which is introduced and validated. The near to far field transition as well as formation and transport of photonic nanojets have been calculated for multiple cylinder scattering.     

Rabi-oscillations without rotating-wave approximation

We consider the interaction of an ensemble of two level atoms with linearly polarized light. This problem is solved in terms of the well-known Bloch equations for various amplitudes of the incident light field without rotating-wave approximation (RWA). Closed-form solutions are obtained for small and very high electric field amplitudes. In the latter case severe deviations from the results obtained with RWA are observed.     

Electron transport in wurtzite InN

Using ensemble Monte Carlo simulation technique, we have calculated the transport properties of InN such as the drift velocity, the drift mobility, the average electron, energy relaxation times and momentum relaxation times at high electric field. The scattering mechanisms included scattering mechanisms are polar optical phonon, ionized impurity, acoustic phonon and intervalley phonon. It is found that the maximum peak velocity only occurs when the electric field is increased to a value above a certain critical field. This critical field is strongly dependent on InN parameters. The steady-state transport parameters are in fair agreement with other recent calculations.     

Energy transport induced by an external alternating field in strongly disordered media.

The delocalization of excitations in an ensemble of two-level systems with a strong disorder due to an external alternating acoustic or electric field is considered. The propagating modes are shown to appear if a ratio of the field amplitude to the frequency is large enough. Two complementary approaches, the static one similar to that of Anderson and the dynamic one related to Landau-Zener, are developed. It is shown that the field-induced relaxation mechanism can have a strong influence on the kinetics. The internal friction is argued to be vastly affected by the relaxation mechanism proposed.     

Influence of Electric and Magnetic Fields on Interference Effects upon Multiple Light Scattering in Cold Atomic Ensembles

The correlation functions of the electromagnetic radiation scattered by an ensemble of atoms cooled to sub-Doppler temperatures and placed in an external static electric or magnetic field have been calculated by the diagram technique. Based on the derived relations, we have studied in detail the effect of coherent backscattering (CBS) of light. We have calculated the enhancement factor for CBS and analyzed its polarization and spectral dependences. We show that external fields affect the nature of multiple light scattering in an atomic ensemble, in particular, the character of interference upon such scattering, by leading to its optical anisotropy and related birefringence and dichroism. This, in turn, affects all of the observed CBS characteristics.     

Autowave Regimes of Thundercloud Electrification

We consider the evolution of the electric field and charge in a one-dimensional electrohydrodynamic (EHD) distributed-parameter system which serves as a simple model of a thundercloud. A diffusion equation for the electric field is proposed which, under reasonable assumptions on the nonlinear dependence of the dissipation current on the electric field (caused by the corona effect around aerosol particles in strong fields), has autowave solutions describing the dynamics of spatially separated electric-charge regions in the thundercloud. The nonlinear set of equations describing the electric-field dynamics in an ensemble of colliding aerosol particles and light ions is reduced to a rather simple basic model which admits solutions in the form of traveling fronts and traveling pulses. We find the asymptotic values of the velocity and thickness of the space-charge front, which are determined by the diffusion and the separation and conductivity currents. Using these values, we estimate the electrostatic-energy growth rate in a thunderstorm cell at the stage of its intensive electrification. A possibility is shown of the formation of a traveling pulse of field and conductivity in such an EHD system with allowance for inductive and noninductive mechanisms of aerosol-particle charging.     

Investigation of velocity overshoot behavior in p-i-n GaAs semiconductor: The contribution of internal electric field

In this work, we present the investigation of the velocity overshoot behavior in p-i-n GaAs semiconductor based on the ensemble Monte Carlo simulations. Our simulations with high-resolution of time and spatial show that the velocity overshoot effect is originated from internal electric field caused by the difference of carrier density. And this effect is dependent not only on the carrier density but also on temperature. Furthermore, we have observed the velocity relaxation effect which was not recognized before because the external electric field is too high comparing to the internal electric field in semiconductor. We point out that the velocity relaxation can be seen as a damping oscillation in which the damping velocity depends strongly on carrier density but not temperature.     

Dielectric properties of pyridine N-oxide aqueous solution under the static electric field and microwave field

Molecular dynamics simulations of the pyridine N-oxide aqueous solution have been performed in the canonical ensemble macroscopic canonical ensemble (NVT) both in the absence and presence of an external electromagnetic field. It extracts the radial distribution function for each concentration solution, dielectric constant and other information on dielectric properties. Analysing the microscopic dielectric information of the aqueous solution under the static electric field (0–3×10⁹ V/m) and microwave frequencies (2.45G, 0–3×10⁹ V/m), and comparing the dielectric information between the different concentrations and different field strengths, we can get the dielectric properties of two kinds of polar aqueous solution under microwave irradiation. Thus, this project can provide the data of the sample to other correlation studies.     

Photon echoes produced by switching electric fields

We demonstrate photon echoes in Eu3+:Y2SiO5 by controlling the inhomogeneous broadening of the Eu3+ 7F0<-->5D0 optical transition. This transition has a linear Stark shift, and we induce inhomogeneous broadening by applying an external electric field gradient. After optical excitation, reversing the polarity of the field rephases the ensemble, resulting in a photon echo. This is the first demonstration of such a photon echo, and its application as a quantum memory is discussed.     

SiC电子输运特性的Monte Carlo数值模拟

利用系综MonteCarlo法研究了2H ,4H和6HSiC的电子输运特性.在模拟中考虑了对其输运过程有着重要影响的声学声子形变势散射、极化光学声子散射、谷间声子散射、电离杂质散射以及中性杂质散射.通过计算,获得了低场下这几种不同SiC多型电子迁移率同温度的关系,并以4H SiC为例,重点分析了中性杂质散射的影响.最后对高场下电子漂移速度的稳态和瞬态变化规律进行了研究.将模拟结果同已有的实验数据进行了比较,发现当阶跃电场强度为10×106V·cm-1时,4H Sic电子横向瞬态速度峰值接近33×107cm·s^-1,6H Sic接近30×107cm·s^-1.     

钎锌矿相GaN电子高场输运特性的Monte Carlo 模拟研究

应用全带多粒子Monte Carlo模拟方法,研究了钎锌矿相GaN 材料电子的高场输运特性. 模拟中利用了基于第一性原理总能量赝势方法计算得到的纤锌矿GaN的能带结构数据. 用Cartier的方法,计算碰撞电离散射率. 计算得到了电子平均漂移速度和电子平均能量与电场的关系曲线. 电离系数的分析表明当电场强度大于1 MV/cm时,才会有明显的碰撞电离发生,量子产额的分析表明当电子的能量大于7 eV时,量子产额随能量增加迅速增大. 研究了在0—4 MV/cm电场强度范围内电子在各导带的分布,低场下电子全部位于 关键词： 碰撞电离 高场输运 能带结构 Monte Carlo模拟     

The effect of external electric fields on molecular liquids and induced translational motion

A computer simulation is reported of the enantiomer S CHBrClF subjected in the liquid state to intense external fields of force : i) uniaxial electric field E_Z; ii) circularly polarised field at optical frequencies. The molecular dynamics are quantified in detail at field - on equilibrium in case i) using a range of auto correlation functions. The foremost result of the investigation is that the simple uniaxial field E_Z makes the sample $\text{translate}$ in a well defined direction. The direct effect of E_Z on an isolated molecule of S-CHBrClF is purely rotational, but intramolecular rotation/translation coupling converts this rotation into coherent centre of mass translation. This gives rise to direct, $\text{laboratory frame}$, cross correlations of the type <v(t)$\text{J}$^T(o)> where $\text{v}$ is the molecular centre of mass translational velocity and $\text{J}$^T the transposed molecular angular momentum vector. (The existence of these invalidates the classical theory of the Kerr effect.) The molecular dynamics of the hypothetical chiral ion S-CHBrClF^? are looked at with a view to corroborating the predictions by Baranova et al. concerning their response to electric field treatment. Despite the inherent instability of such an ensemble of like-charged ions the simulation can be used to produce a range of auto and cross-correlation functions with which to characterise the ionic dynamics. The effect of treating the ionic ensemble with a field E_Z is reported briefly in terms of the non-vanishing ensemble averaged centre of mass velocity <v_Z&>;.The induced translation in an electric field may be demonstrated on most liquids using a simple experimental set up. Its importance in optically active systems is such that it may be used to separate a racemic mixture into its enantiomers, the translational motion induced in the one enantiomer is necessarily, by symmetry, opposite to that induced in the other enantiomer. The observation therefore has technological importance. Other applications are discussed.     

Static and dynamic profile of the electric field within the bulk of fused silica glass during and after thermal poling

We use the electric-field-induced second-harmonic (EFISH) technique to characterize, for the first time to our knowledge, the profile of the electric field induced within the bulk of fused silica glass samples both during and after thermal-poling treatment. During thermal poling, the initially homogeneous electric field decays exponentially from the anodic to the cathodic surface. After the poling treatment, a permanent electric field is induced within the bulk of the sample. It also decays exponentially from a positive value on the anodic surface to a negative value on the cathodic surface. The exponential decay constant depends on the thickness of the sample, the poling time, and the temperature. The temperature evolution of this constant makes it possible to retrieve the activation energy of the rapidly moving ionic species. This ensemble of observations indicates that modifications to models previously proposed are necessary.     

Die Relaxation der Geschwindigkeitsverteilungsfunktion der Elektronen im homogenen elektrischen Feld mit elastischen und anregenden Stößen. II. Das Verhalten des Elektronenensembles bei aperiodischer Feldänderung zwischen zwei stationären Zuständen

Recently the relaxation of the electron component was investigated under field-free conditions after sudden switch-off the electric field and otherwise after sudden changing the electric field to a new value of field strength. To continue these papers we consider now the relaxation process of the electron ensemble in the presence of a monotonous time variable electric field. The investigations are based on a computation of time change in the isotropic part of the velocity distribution function of the electrons and of the macroscopic parameters determined by the distribution function. The start-ing-point is the non-stationary Boltzmann equation with stationary initial states taking into consideration elastic and exciting collisions. Besides the representation of the received numerical results a physical interpretation is obtained for the duration of the whole relaxation process, for its initial stage as well as for the momentary stage of the relaxation by introduction of normalized characteristic time quantities. Further characterizing conditions are found related to the relaxation after quasi-jumplike change of the electric field and in the case of quasistationary field alteration respectively. With the introduced characteristic time quantities statements about the degree of realization of one of this limiting cases are possible for any given monotonous field. The investigations are per-formed in a low ionized non-thermal neon plasma.     

Surface roughness assisted 100 kHz femtosecond laser induced nanostructure formation on silicon surface

Spontaneous nanostructure formations on roughened and smooth silicon surface by the femtosecond laser irradiation with the repetition rate of 100 kHz were systematically studied. In addition to the widely accepted so-called coarse ripple, which has the period analogous to the wavelength of the laser beam and aligns perpendicularly to the electric field of the incident laser beam, the ripple which has the period similar to the wavelength of the incident laser beam but aligns parallel to the electric field of the laser beam was observed on the roughened surface for the lower fluence and the higher number of pulse irradiation. Furthermore, the ensemble of dots formed by the enhancement of the local electric field was found on the roughened surface. This structure is preferentially formed around the scratches aligned perpendicularly to the electric field of the laser beam. These novel nanostructures are considered to be peculiar to the femtosecond laser irradiation and open the possibilities for precise control of the spontaneous nanostructure formation by femtosecond laser irradiation.     

Driven Rydberg atoms reveal quartic level repulsion

The dynamics of Rydberg states of a hydrogen atom subjected simultaneously to uniform static electric field and two microwave fields with commensurate frequencies is considered in the range of small field amplitudes. In the certain range of the parameters of the system the classical secular motion of the electronic ellipse reveals chaotic behavior. Quantum mechanically, when the fine structure of the atom is taken into account, the energy level statistics obey predictions appropriate for the symplectic Gaussian random matrix ensemble.