Modulational instability of a laser beam in a longitudinal magnetized piezoelectric semiconductor

Using the hydrodynamic model of a homogeneous infinite plasma, the modulational instability of a laser beam with an acoustic and a helicon wave has been investigated in a piezoelectric semiconductor. The threshold electric field amplitude and the growth rate of the unstable mode have been obtained analytically and for n-InSb at 77 K the unstable mode is found to be propagating with a growth rate ≈10⁵ s^-1 when the crystal is irradiated with a 337 μm HCN laser.     

Parametric instability in a two-hole species semiconductor-plasma

The parametric excitation of a low frequency wave has been investigated analytically in a two-hole species semiconductor-plasma in the region of kl ? 1 using the hydrodynamic model of the plasmas in the presence of a high frequency oscillatory electric field (E₀ cos ω₀t applied along the X-axis) and a d.c. magnetic field B₀ normal to the electric field (along the Z-axis), the low frequency wave propagating in the X–Z plane making a very small angle θ with the X-axis. The system supports a purely growing unstable mode. The variation of the growth rate of the unstable mode has been studied over a wide range of system parameters for the specific case of an intrinsic GaAs crystal at 300 K. The oscillatory electric field can be obtained by irradiating the crystal with a 119μm H₂O laser.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.     

Parametric excitation of helicon and acoustic waves in piezoelectric semiconductor-plasma

The parametric excitation of a helicon and an acoustic wave in a piezoelectric semiconductor-plasma in the presence of a strong magnetic field has been investigated using the coupled mode theory. The expressions for the threshold value of the electric field required for the onset of instability and for the growth rate well above the threshold have been obtained. It is observed that an acoustic wave of higher frequency and higher phase velocity than that of the pump wave cannot be excited. The analysis has been applied to the case of n type InSb sample where the threshold value of the electric field is found to be of the order 5.2 × 10³ Vm^?1 and the growth rate at an electric field 5.2 × 10⁴ Vm^?1 is of the order of 8.7 × 10¹⁰ sec^?1.     

DFT study on the influence of electric field on surface‐enhanced Raman scattering from pyridine–metal complex

The influence of a static external electric field on surface‐enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine–Au₂₀ complex with the density functional theory and time‐dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm⁻¹) but suppresses the trigonal ring breathing mode (~1051 cm⁻¹). On the contrary, the mode at 1051 cm⁻¹ is more enhanced than the mode at 1009 cm⁻¹ when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field. Copyright © 2013 John Wiley & Sons, Ltd.     

CHCl3电子吸附速率常数的离子迁移谱

利用电晕放电离子迁移谱, 使用高纯氮气作为载气和迁移气体, 研究了电场强度在200～500 V/cm变化时CHCl₃的解离电子吸附速率常数, 得到样品所对应的电子吸附速率常数为1.26×10^-8～8.24×10^-9 cm³/(molecules s)．利用该装置测量了固定电场下,样品的电子吸附速率常数与样品浓度之间的关系．此外利用所获得的离子迁移谱图得到了不同电场强度下Cl^-与CHCl₃之间的离子分子反应速率常数.     

由纳米管和纳米颗粒构成的TiO2膜的制备与机理研究

采用聚苯乙烯小球修饰Ti片表面,并进行阳极氧化,制备出一种由纳米颗粒和纳米管构成的TiO₂膜．通过数值模拟,分析了氧化表面附近的局部电场分布对TiO₂膜形貌的影响．结果表明,覆盖物增强了局部电场,从而加快了O^2-与Ti的反应速率,有利于TiO₂的生长;与此同时,[TiF₆]^6-的扩散受到阻碍,使得TiO₂的溶解速率减慢．可见,覆盖物打破了TiO₂纳米管形成的平衡条件,导致纳米颗粒的生成．此外,通过X射线衍射和Raman光谱的测试分析发现,所制备的TiO₂为锐钛矿结构.     

A contribution to the light pulse instability of an N2 laser

In this work we found the conditions of pressure and pulse repetition rate under which a N₂ laser operates in a stable or unstable mode. Especially, the preavalanche phase plays a determinative role for the evolution of the discharge. When the voltage rise time across the electrode gap is finite, the electric field sweeps away the electrons from the cathode. The thickness of the electrondeficient region near the cathode at the end of the preavalanche period determines the mode under which the laser system operates.     

磁层顶中的低混杂漂移不稳定性

本文讨论了具有简单结构磁层顶中的低混杂漂移不稳定性,假定在磁层顶中磁场方向是相互平行的,电子与离子的密度处处相等,总压力为一常数,采用1971年Alpers建立的分布函数做为零级分布,计算了下混杂漂移不稳定性的增长率和饱和时相应的反常电阻,计算表明,当磁层顶厚度接近两个质子迴旋半径时,低混杂漂移不稳定性的增长率约为0.26ω_LH(ω_LH为低混杂频率),反常电阻率为10^-5sec,随着磁层顶厚度成倍增加,反常电阻率以指数形式下降。 关键词：     

Electric field effect on low temperature nanoscale oxidation

The influence of electric fields on the low temperature oxidation of individual nanoscale tungsten wires was investigated. In the experiments at room temperature, the nanowires were biased as anode opposite to a macroscopic cathode and H₂O-vapor with a pressure of 10^?7–10¹ mbar was provided as oxygen source. Under the influence of an electric field, a dramatic change of the oxidation behavior is observed with the formation of several 10 nm thick oxide layers for electric fields exceeding a threshold. The chemical composition of the layers formed is determined with laser-assisted atom probe tomography to be slightly understoichiometric WO₃. After an initial period of fast growth, the oxidation rate later rapidly decreases to immeasurable low values. Evaluation of the electric field distribution in the vicinity of the sample by the finite element method reveals that oxide formation only proceeds if a critical field in the range of 0.7–5.0 V/nm, depending on the H₂O-pressure, is present. This critical field is attributed to a field-activated reaction of H₂O at the oxide–vapor interface. Besides for tungsten, field-induced oxidation is also observed for aluminum and p-doped silicon and thus apparently is a widely material independent phenomenon.     

The c-axis oriented AlN solidly mounted resonator operated in thickness shear mode using lateral electric field excitation

The AlN-based solidly mounted resonator operated in thickness shear mode using lateral field excitation is presented both in theory and experiment. The resonator configuration consists of the electrodes parallel to the surface, a highly c-oriented AlN film and an acoustic Bragg reflector. The theoretical analysis of the Christoffel equation predicts that the electric field in any direction normal to the c-axis can excite the shear mode wave along the thickness direction. The electric field characteristics are calculated by finite element modeling in order to design the electrode frame. The testing results of the finished devices show that the thickness shear mode wave can be excited by the lateral electric field in c-axis oriented AlN solidly mounted resonators. The experimental frequency corresponds well to the theoretical one. The resonators operated in thickness shear mode have resonant frequencies near 2 GHz with an average Q _s value of 323 and a K_eff²K_{\mathrm{eff}}^{2} value of 0.83%.     

Thermodynamic and bias field investigation of an antiferroelectric liquid crystal

Dielectric spectroscopy of an antiferroelectric liquid crystal material (S)-4-(1-methylheptyloxycarbonyl)phenyl-4′-(6-pentanoyloxyhex-1-oxy)biphenyl-4-carboxylate 4H6Bi(S) for its helical phases has been carried out under bias electric field in the frequency range 0.1 Hz-10 MHz. Bias field investigation has been carried out in order to explore new relaxation modes. Dielectric characterization of the material, when carried out as a function of DC bias, reveals soft mode around 350 kHz and domain mode at ∼100 Hz in the SmC^? phase as a consequence of suppression of Goldstone mode due to helix unwinding. In the SmC^?_A phase a field induced new relaxation mode has been investigated at 8.0 kV/cm having relaxation frequency from 100 Hz to 600 Hz with increasing bias electric field.     

Rayleigh-Taylor instability of a composite medium with variable viscosity in hydromagnetics

The frictional effect of collisions of ionized with neutral atoms on the Rayleigh-Taylor instability of a composite medium with variable viscosity is considered in the presence of a horizontal magnetic field. It is found that the simultaneous presence of viscosity, magnetic field and collisions has a stabilizing effect and completely stabilizes the wave-number bandk _* wherek _*=(k _x ² V²L/g). The collisions have no effect as such on the stratification, i.e., stable configuration remains stable and unstable configuration remains unstable. However the growth rate, under either of conditions (25), decreases with the increase of collisions.     

Gravitational Instability of a Thermally Conducting Plasma in an Oblique Magnetic Field

The combined influence of the effects of Hall currents, magnetic resistivity and viscosity have been studied on the gravitational instability of a thermally conducting homogeneous unbounded plasma in an oblique magnetic field. The solution has been obtained through the normal mode technique and the dispersion relation has been derived. It is shown that the Jeans' criterion for gravitational instability remains unchanged. Solving numerically the dispersion relation, the dependence of the growth rate of the gravitational unstable mode on the considered physical effects has been obtained for an astrophysical situation. For conditions prevailing in the magnetized collapsing clouds, the numerical calculations for the plot of growth rate against wave number has been obtained for several values of the parameters characterizing Hall currents magnetic resistivity viscosity thermal conductivity. It is found that magnetic resistivity and thermal conductivity have destabilizing influence while viscosity has stabilizing influence on the instability of the plasma of disturbance m(ϱ) = 9 × 10⁻³ kg.     

Field-independence of thermal emission rate in Au-doped silicon

The thermal emission rate of holes, e^t_p, has been measured from dark leakage currents for the Au acceptor level in silicon diodes at different electric fields and found to be field independent for average electric fields below 10⁵V/cm.     

Instability of a spherical drop in the field of an electrical dipole

Analytical calculations show that, as a field in which an initially spherical charged conducting incompressible drop is placed becomes more and more nonuniform, coupling between the drop’s oscillation modes grows and the threshold of stability against the electrical field pressure declines. When an electrostatic parameter characterizing the electric field pressure exceeds a value that is critical for a certain mode to be unstable, the amplitude of this mode exponentially grows in an aperiodic manner and the amplitudes of modes coupled with this mode build up in an oscillatory manner, each mode having its own instability growth rate. In all cases, there exists a threshold value of the dimensionless electric parameter above which all oscillation modes are unstable.     

Effect of a magnetic field on the rate of etching of silicon dioxide in a CF4 + O2 plasma

A near-electrode nonuniform magnetic field crossed with an electric field is found to strongly affect the rate of etching of silicon dioxide on glass substrates in a CF₄ + O₂ plasma when the Larmor frequency (≈10⁹ s⁻¹) is much higher than the frequency of collisions of an electron with surrounding plasma particles (≈10⁶ s⁻¹) and the frequency of the applied rf electric field (≈10⁷ s⁻¹). The confinement of electrons by the magnetic field in the immediate vicinity of the substrate surface to be treated increases the rate of generation of chemically active particles, which increases the etching rate of silicon dioxide.     

Single mode CO2-laser pulses of high power

Single longitudinal mode CO₂-laser pulses of 200 MW power have been produced by seeding an unstable resonator cavity with single mode radiation from a hybrid low pressure — high pressure CO₂ laser system. The results indicate that a ratio of dominant over residual mode intensities of better than 10⁵ were achieved.     

Nonlocal theory of stimulated Raman backscattering in a strongly magnetized plasma

A nonlocal theory of stimulated Raman backscattering (BSRS) parametric instability of a large amplitude electromagnetic (EM) mode in a strongly magnetized plasma e.g., one encountered in a plasma filled backward wave oscillator, is reported. The EM mode is unstable to parametric instability in a magnetized plasma and decays into a Trivelpiece-Gould (TG) mode and a sideband EM mode. The growth rate of instability (Γ) scales proportional to three-fourth power of plasma density. For a typical BWO the growth rate is ∼10⁸ s^-1     

Piezoelectric measurements on BiFeO3 single crystal by Raman scattering

The piezoelectric response of BiFeO₃ at low temperature has been investigated by Raman scattering measurements. The application of an external electric field at T=10 K induces frequency shifts of the lowest frequency mode related to the Bi-O bonds and corresponding to the soft mode of the ferroelectric transition. The piezoelectric effect is responsible for the softening of this mode via the tensile stress leading to the expansion of the crystal. The phonon deformation potential associated with the soft mode has been estimated around −200 cm⁻¹/strain units using the linear piezoelectric coefficient d₃₃=16 pm/V. It found in the range of the ones obtained for typical piezoelectrics.