微波大气击穿阈值的理论研究

本文通过对使用有效场强(或均方根场强)得到的微波大气击穿阈值表达式进行讨论, 指出其推导中所做的假设及这些假设应用到微波大气击穿过程中存在的问题. 然后分别使用解析理论和数值模拟对微波大气击穿过程中的有效电子温度变化过程和击穿阈值进行研究, 并将其与直流电场进行比较. 分析发现在高气压下, 电子能量转移频率高, 有效电子温度随电场大幅振荡, 由于电离频率随有效电子温度的增长率大于电子能量损失随有效电子温度的增长率, 因此在高气压时, 微波大气击穿阈值低于使用有效场强的击穿阈值. 通过大量分析, 给出了理论推导和数值模拟得到的微波大气击穿阈值拟合表达式.     

Long-time asymptotics in the one-dimensional trapping problem with large bias

The survival probability of a particle which moves according to a biased random walk in a one-dimensional lattice containing randomly distributed deep traps is studied at large times. Exact asymptotic expansions are deduced for fields exceeding a certain threshold, using the method of images. In order to cover the whole range of fields, we also derive the behavior of the survival probability below this threshold, using the eigenvalue expansion method. The connection with the continuous diffusion model is discussed.     

Approximation Analytical Solution for a Current-Carrying Ion Sheath and its Chaos Control

The fourth-order quantum interference of squeezed fields generated by two independent optical parametric oscillators(OPOs) is discussed based on the semi-classical theory. It is shown that at the oscillation threshold of OPOs the squeezing of two output fields is the highest but the fringe visibility of fourth-order interference is the lowest(33.3%). When the pump power is away from the threshold, the squeezing decreases while the correlation between them increases.     

Helical instability in variband semiconductors

In this paper the characteristics of the helical instability in a semiconductor plate in which the width of the forbidden band varies linearly over the thickness are studied theoretically. The dependences of the threshold product of the external fields and the wave vector which arise at the threshold of instability of the disturbances on the gradient of the width of the forbidden band and the orientations of the external electric and magnetic fields are calculated. It is shown that nonuniformity of the band structure can result in a lower excitation threshold of instability than in the case of a homogeneous-band semiconductor.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 54–58, January, 1991.     

Quantum correlations of pulses of optical parametric oscillator synchronously pumped above threshold

The quantum analysis of radiation from a degenerate optical parametric oscillator synchronously pumped above its oscillation threshold is presented. It is shown that pulses of signal and pump fields at the output of the oscillator have the following properties: quantum fluctuations of the fields are independent in each individual pulse, but correlated in pulses of the pulse train with a temporal step multiple of the pulse period. The number of essentially correlated pulses is on the order of the oscillator cavity finesse. Cross-correlations between the pump and signal pulses are established above the oscillation threshold. These correlations lead to a significant quantum effect in the integral characteristics of the fields. A theoretical analysis revealed that the spectrum of field fluctuations measured using a balanced homodyne detection technique of phase quadratures of the fields with a pulsed local oscillator reveals quantum noise suppression in the vicinity of frequencies that are multiples of the pulse repetition rate.     

The influence of transverse magnetic field on Gunn effect threshold

The effect of magnetic field has been studied on the Gunn effect threshold electric field for short planar Gunn devices. For low magnetic fields, experimental results are in good agreement with the theoretical results but a large increase in threshold electric field has been observed for higher magnetic fields.     

Threshold intensity and coefficient of raman scattering amplification in a high-<Emphasis Type="Italic">Q</Emphasis> bilayer microresonator during the formation of internal and external submicron photonic jets: A photonic nanojet in the near field

Using quantum and semiclassical approaches, the energy excitation threshold for induced Raman scattering is estimated and a relationship between the excitation threshold and the concentration of optically active molecules in a bilayer microresonator is established. Estimates are made during the formation of specially configured optical fields: internal and external photonic nanojets. Based on the amount of stored energy per mode and the value of the threshold intensity, an additional generalized selection rule for whispering gallery modes is suggested. It is shown that the bilayer microresonator can focus incident radiation (laser pumping) into a submicron focal volume at a low threshold intensity.     

Effect of destabilizing fields on hydrodynamic instabilities in nematics

The effect of destabilizing fields on the roll instability (RI) threshold for shear flow and on the homogeneous instability (HI) threshold for plane Poiseuille flow of nematic HBAB (μ _s>0) is studied on the basis of the continuum theory of nematics for flow cells of infinite lateral width. It turns out that the critical shear rate and wave vector at RI threshold decrease with increasing destabilizing field but do not approach zero at the Freedericksz transition. However calculations show that beyond the Freedericksz threshold HI may be favourable over a range of destabilizing field with shear in the stabilizing role. For plane Poiseuille flow a similar analysis points to the existence of a HI threshold in the presence of destabilizing field beyond the Freedericksz threshold again with shear acting as a stabilizing field. These results are compared with theoretical results obtained previously for MBBA.     

Defect motion at finite temperature on a hydrogen-bonded chain

We have previously considered the energetics and dynamics of ionic and orientational defects in the proton displacement pattern on a hydrogen-bonded chain at zero temperature. Here we examine thermal effects on ionic defect motion in such a chain. The chain is represented by a classical lattice hamiltonian and a modified tight-binding electronic hamiltonian. Temperature is incorporated using the Nose-Hoover method. We find that the threshold field for defect motion is reduced somewhat by temperature, as would be expected. The degree of reduction depends on whether the heavy-ion sublattice is taken to be stationary or mobile, but in either case the threshold fields at 200 K are still comparable to breakdown fields for reasonable values of the dielectric screening. Mobilities are on the order of 0.005 cm²/V s, well below the values initially observed by Eigen and de Maeyer, but consistent with more recent observations.     

The influence of substrate geometry on the emission properties of a liquid metal ion source

The effect of needle radius, cone angle and shaft diameter on the threshold voltage and angular intensity — total current relationships for a Ga liquid-metal ion source (LMIS) was investigated. The variation of threshold voltage with needle geometry could be described in terms of the Taylor theory of liquid cone formation by electrostatic fields. The beam energy spread was mainly a function of total source current and was not a sensitive function of emitter geometry. Source angular intensity at a constant total current increased linearly with threshold voltage when the latter was altered due to source geometry.     

Influence of Fano interference and incoherent processes on optical bistability in a four-level quantum dot nanostructure

Role of Fano interference and incoherent pumping field on optical bistability in a four-level designed In Ga N/Ga N quantum dot nanostructure embedded in a unidirectional ring cavity are analyzed. It is found that intensity threshold of optical bistability can be manipulated by Fano interference. It is shown that incoherent pumping fields make the threshold of optical bistability behave differently by Fano interference. Moreover, in the presence of Fano interference the medium becomes phase-dependent. Therefore, the relative phase of applied fields can affect the behaviors of optical bistability and intensity threshold can be controlled easily.     

Surface anchoring energy and the Freedericksz transitions in ferronematics

This paper analyzes the influence of anchoring conditions on the two-field-induced Freedericksz transition in ferronematics. Using the Euler-Lagrange analytical method we find a correlation between the threshold intensities of the magnetic and the laser fields, the anchoring strength, the material and device parameters. The threshold fields in our case are smaller than those obtained in the rigid anchoring limit. We show that for finite anchoring energy, the saturation transition is possible, and find the correlation between the saturation fields, the anchoring strength, the material and cell parameters. Our results can be useful in designing magneto-optical devices using ferronematics.     

On the role of relaxation processes in the acoustic mechanism of supramolecular structure formation in nematic liquid crystals

We have for the first time developed and tested a model constructed on the basis of nonequilibrium hydrodynamics and postulating that the structural relaxation processes of a nematic liquid crystal (NLC) in wave fields are important for the formation of supramolecular structures in the form of a system of linear domains in a planar mesophase layer. Distortions in the macrostructure of an NLC layer in the field of longitudinal waves were observed in the frequency range of 0.9–18.9 MHz. The values of the spatial period of domains at the threshold of the effect and the threshold amplitudes of the vibrational speed were determined for 10–300-μm-thick layers in wave fields with different degrees of uniformity for the temperature range where the mesophase exists. The simulation results are compared to experimental data.     

Controlling the optical bistability via quantum interference in a four-level N-type atomic system

We investigate the optical bistability (OB) and optical multi-stability (OM) in a four-level N-type atomic system. The effect of spontaneously generated coherence (SGC) on OB and OM is then discussed. It is found that SGC makes the medium phase dependent, so the optical bistability and multi-stability threshold can be controlled via relative phase between applied fields. We realize that the frequency detuning of probe and coupling fields with the corresponding atomic transition plays an important role in creation OB and OM. Moreover, the effect of laser coupling fields and an incoherent pumping field on reduction of OB and OM threshold is then discussed.     

压缩态光场的四阶量子干涉

采用半经典理论讨论了同一抽运源抽运的两个独立光学参量振荡腔(OPO)所产生的压缩态光场的四阶量子干涉效应．结果表明：在OPO振荡阈值处,输出光场压缩度最高但四阶量子干涉最弱,条纹可见度为33.3%,远离阈值时干涉越来越强. 关键词：     

Direct detection of nonlinear ferromagnetic resonance in thin films by the magneto-optical Kerr effect

The longitudinal magneto-optical Kerr effect is used to obtain a calibrated measure of the dynamic magnetization response over the ferromagnetic resonance (FMR) profile for in-plane magnetized Permalloy films excited with high power in-plane transverse microwave fields at 1.25 to 3.75 GHz and in-plane precession angles up to about 20 degrees. The data provide a profound demonstration of the Suhl threshold effect for parametric spin wave generation for angles above about 14 degrees, the magnetization precession lock-up just above threshold, and the complicated response over the full FMR profile at very high powers.     

Laser-induced recombination studies with the adiabatically expanded electron beam of the Heidelberg TSR

The enhancement of spontaneous radiative recombination of C⁶⁺ ions with free electrons in a resonant laser field has been investigated at the Heidelberg cooler ring TSR for different transverse electron temperatures realized by adiabatically expanding the magnetically guided electron beam. The recombination spectra close to the ionization threshold strongly deviate from calculations for free independent electrons, showing important contributions at energies below this threshold. Shape and relative size of these contributions change significantly as the transverse temperature is varied. These changes are not consistent with the expected behaviour of sub-threshold contributions due to electric stray fields.     

Investigation of the Disordered Antiferromagnetic Ising Model with the Effects of Random Magnetic Fields

Computer simulation of phase transitions is made by the Monte Carlo method using a three-dimensional disordered antiferromagnetic Ising model in the external magnetic field. It is found that in the case where the spin concentration in a system is lower than a threshold one, the effects of random magnetic fields destroy the second-order phase transition and lead to the first-order phase transition into a new phase state of the system characterized by a ground spin-glassy state and metastable energy states at finite temperatures. The dependence of the threshold concentration on the external magnetic field is revealed.