抽运-检测型非线性磁光旋转铷原子磁力仪的研究

报道了一种抽运-检测型的非线性磁光旋转铷原子磁力仪.其原理是线偏振光通过处于外磁场环境中被极化的原子介质后,由于原子对线偏振光中左、右圆偏成分不同的吸收和色散,导致光的偏振方向会产生与磁场相关的转动.分析了该磁力仪的工作原理,并测试了它对不同磁场大小的响应.测试结果表明,磁力仪测量范围为100—100000 nT,极限灵敏度为0.2 p T/Hz~(1/2),磁场分辨率为0.1 p T.进一步研究了不同磁场下原子系综极化态的横向弛豫时间,讨论了原子磁力仪高磁场采样率的获得方法.本文的原子磁力仪在5000—100000 n T的磁场测量范围内磁场采样率可实现1—1000 Hz范围内可调,能够测量低频的微弱交变磁场.本文的研究内容为大磁场测量范围、高灵敏度、高磁场采样率的原子磁力仪研制提供了重要参考.     

提高激光抽运铯原子磁力仪灵敏度的研究

本文报道了一种基于激光抽运射频共振的铯原子磁力仪. 通过圆偏振光将铯原子抽运到暗态, 实现偏极化. 外磁场存在时, 原子磁矩将以拉莫尔频率绕外磁场进动. 在共振射频磁场的作用下, 原子被去极化而重新吸收光子. 通过探测出射光光谱可以测得拉莫尔频率进而得到外磁场的信息. 本文通过运用自制的894 nm 外腔半导体激光器, 建立了激光稳频装置和低噪声磁场测量环境, 实现了一种基于铯原子激光抽运射频共振的磁力仪. 通过磁力仪参数优化以及闭环测量, 磁力仪测量的外磁场达到了19 fT/Hz^1/2的极限灵敏度和1.8 pT/Hz^1/2的本征灵敏度, 空间分辨率小于2 cm. 关键词： 光抽运 塞曼效应 光探测磁共振 磁力仪     

全光学高灵敏度铷原子磁力仪的研究

报道了一种全光学的高灵敏度铷原子磁力仪.其原理是基于激光束与处于微弱磁场中的铷原子的相互作用.这种相互作用与铷原子所处的环境中的磁场有关,因而通过测量透过铷原子气体的激光强度的变化可以获得磁场信息.分析了该磁力仪的工作原理,建立了相应的实验装置,并对其性能进行了测试.结果表明实验结果与理论相符合.进一步研究了影响磁力仪灵敏度的一些因素,提出了优化各参数来提高磁力仪灵敏度的方法.     

抽运-检测型原子磁力仪对电流源噪声的测量

用于在宽量程范围内标定原子磁力仪的灵敏度的复现磁场通常由精密电流源和标准线圈产生,电流源噪声将直接影响原子磁力仪在宽量程范围内标定的灵敏度.本文基于抽运-检测型原子磁力仪首先提出抑制复现磁场漂移的磁补偿方法,其次开展宽量程范围内电流源的噪声和原子磁力仪的灵敏度之间依赖关系的研究.研究结果表明,抽运-检测型原子磁力仪的灵敏度主要由电流源噪声决定,因此可用特定磁场下的灵敏度估算电流源在对应输出电流条件下的电流噪声.本文研究对弱磁传感器灵敏度指标的标定、高精度电流源的研制、磁感应强度计量和电流计量的协同发展都具有参考价值.     

铷-氙气室原子磁力仪系统磁场测量能力的标定

本文针对微弱磁场精密测量问题,在自主研制的铷-氙气室原子磁力仪系统上,探讨了两种磁场测量的方式,分别实现了对交流磁场与静磁场的测量,并对它们的磁场测量能力进行了实验标定.交流磁场测量原理是基于测量外磁场对~(87)Rb原子极化的影响,实验标定结果为在2100 Hz频率范围内磁场测量的灵敏度约为1.5 pT/Hz~(1/2),带宽约为2.8 kHz;静磁场测量原理是基于测量铷-氙气室内超极化~(129)Xe的拉莫进动频率,实验上首先测得超极化~(129)Xe的横向、纵向弛豫时间分别约为20.6和21.5 s,然后通过标定给出静磁场测量精度约为9.4 pT,测量范围超过50μT.相比无自旋交换弛豫原子磁力仪,该磁力仪在同一体系内实现了交流磁场与静磁场的测量,且交流磁场测量具有更大的带宽,静磁场测量可在地磁场下正常工作,将有望应用于地磁测量、基础物理等方面的研究.     

共振线极化光实现原子矢量磁力仪的理论研究

共振线偏振光激发原子张量磁矩,本文理论研究在矢量磁场和射频场的共同作用下,张量磁矩进动的模型,求解刘维尔方程获得透射光时域完整解析解,包括直流、一次和二次谐波分量.研究发现:当进动的拉比频率Ω1/(22~(1/2))时,两谐波间的干涉效应使直流分量和一次谐波对称成分的单吸收峰劈裂成双峰,裂距((Ω~2+Ω~4-Ω~2-1)~(3/2))~(1/2),一次谐波反对称成分在共振处产生干涉条纹.研究结果显示,谐波间的干涉也可导致直流分量和二次谐波线宽仅为一次谐波线宽的38%,且存在磁场取向临界点,在不同的取向区间分别利用直流及两谐波共振信号辨析磁场变化,可获得最佳测磁灵敏度;同时还可通过共振时直流分量及两谐波对称成分振幅来确定磁场与激光极化方向的夹角,利用两谐波反对称成分相移的差值来确定待测磁场在垂直光极化方向投影与射频场方向的夹角,进而实现结构简单的张量磁矩进动型矢量磁力仪.这种磁力仪适合构成磁力仪阵列,可用于磁定位、水下磁异常源的检测和地磁导航等领域.     

结构改进的厘米尺寸谐振腔的磁场传感特性

基于光力谐振腔的磁力仪在应用时主要受限于灵敏度和检测带宽两个指标.本文设计了一种厘米尺寸的回音壁模式谐振腔结构,可探测6 Hz至1 MHz频率范围内的交变磁场,在无磁屏蔽、室温环境下、无直流偏置磁场时,其最佳灵敏度在123.8 kHz可达530 pT·Hz~(–1/2),探测带宽和最佳灵敏度分别为同尺寸谐振腔的11倍和1.67倍.该磁场传感器仅需100μW的光功率,功耗很低.后续通过优化系统噪声、提升器件磁场响应能力等手段可进一步提升其传感性能,有望在电力系统故障监测和医学诊断等领域发挥其应用潜力.     

光泵原子磁力仪及其应用

光泵原子磁力仪是利用激光极化原子气体,外磁场与原子相互作用使原子光学性质发生变化,进而改变探测光的偏振方向或光强。原子磁力仪精度已超过超导磁力仪,但是体积重量比超导磁力仪小。给出了碰撞混合以及驰豫与极化速率以及最终极化度的定量关系,总结了原子磁力仪国内外研究进展以及在导航领域的应用。     

双光束抽运对全光Cs原子磁力仪灵敏度的影响

原子磁力仪的灵敏度直接取决于抽运光对原子的极化率。分析了Cs原子气室中充入13 332.2 Pa He缓冲气体,两束抽运光频率分别锁定在Cs原子D1线F=4F=3和F=3F=4共振跃迁线时,基态和激发态各磁子能级上粒子数变化的动力学过程,发现在抽运光的持续作用下Cs原子基态F=3和F=4各磁子能级上的粒子被完全抽运至|F=4,mF=4态,Cs原子极化率达到最大值。同时,实验结果证明双光束抽运有效提高了磁力仪响应特性曲线的信噪比,从而使原子磁力仪的灵敏度提高了34%。     

磁致伸缩材料弱磁场响应特性的实验研究

针对磁致伸缩材料在弱磁场传感器领域的应用需要,采用迈克耳逊干涉原理实验测量了零应力条件下Tb-Dy-Fe材料和Fe-Ga合金的磁场响应灵敏度,以及不同应力下Fe-Ga合金的磁场响应特性和温度响应特性.实验结果表明：在零应力,外加磁场16 mT条件下,Fe-Ga合金的磁场响应灵敏度远高于Tb-Dy-Fe材料,更合适作为弱磁场传感器敏感材料;同时,在1.2 MPa预应力和26 mT偏置磁场下,Fe-Ga合金材料具有较好的磁场响应灵敏度和较大的饱和磁致伸缩系数,因而处在最佳工作状态.所得到的材料的磁场和温度响应曲线可作为弱磁场传感器参量设计的参考依据.     

Dark and Bright Solitons of Extraordinary Waves in Ferromagnetic Systems

It is shown that the slowly varying amplitudes of both high and low frequency extraordinary waves propagating perpendicular to the dc magnetic field in ferromagnetic systems are described by the nonlinear SchrEdinger equation. The extraordinary waves are modulationally stable (unstable) and may evolve as dark (bright) solitons as the frequency ω satisfies ω₀ < ω < ω_b or ω > ω₁(0 < ω < ω₀ or ω_a ≤ ω < ω₁), where ω_a, ω_b, ω₀ and w₁ are given in the text.     

Rashba自旋-轨道相互作用影响下量子盘中强耦合磁极化子性质的研究

本文基于Lee-Low-Pines幺正变换法,采用Tokuda改进的线性组合算符法研究了Rashba自旋-轨道相互作用效应下量子盘中强耦合磁极化子的性质.结果表明,磁极化子的相互作用能Eint的取值随量子盘横向受限强度ω0、外磁场的回旋频率ωc、电子-LO声子耦合强度α和量子盘厚度L的变化均与磁极化子的状态性质密切相关;磁极化子的平均声子数N随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在Rashba自旋-轨道相互作用效应影响下磁极化子的有效质量将劈裂为m*+,m*-两种,它们随ωc,ω0和α的增加而增大,随L的增加而振荡减小;在研究量子盘中磁极化子问题时,电子-LO声子耦合和Rashba自旋-轨道相互作用效应的影响不可忽略,但Rashba自旋-轨道相互作用和极化子效应对磁极化子的影响只有在电子运动的速率较慢时显著.     

Plasma effects in a free electron laser

The introduction of a plasma and a strong guide magnetic field in a free electron laser (FEL) slows down the phase velocity of radiation, significantly reducing the requirements on beam energy for generating frequencies below the electron-cyclotron frequency (ω₁≲ω_c). Around plasma resonance (ω₁~ω_p), the FEL mode couples to two-stream instability (TSI), attaining a large growth rate, comparable to that of the wiggler-free TSI. At plasma densities comparable to beam density, the beam-induced local depression in the electron density of the plasma acts as a waveguide for guiding any high-frequency radiation when the beam current is ≳17 kA     

WKB solution for wave propagation in a time-varying magnetoplasmamedium: longitudinal propagation

The interaction of a right circularly polarized electromagnetic wave of frequency ω₀ with a switched-on-time-varying plasma medium, in the presence of a static magnetic field, is considered. Longitudinal propagation is assumed and ion motion is neglected. The electron density is assumed to vary slowly with time, and the solution is obtained through a WKB approximation. The main effect of switching the time-varying medium is the splitting of the original (incident) wave of frequency ω₀ into three new waves with time-varying frequencies. An exponentially increasing electron density profile is considered to illustrate the solution. The distinguishing feature of the presence of the static magnetic field is the creation of the third wave. The initial value of the instantaneous frequency ω₃ of this wave is equal to the gyrofrequency ω_b, and the final value depends on ω_b, ω₀, and the final value of the plasma frequency. ω₃ decreases with time for the profile under consideration     

Equilibrium dynamics of the sub-Ohmic spin-boson model under bias

Using the bosonic numerical renormalization group method, we studied the equilibrium dynamical correlation function C(ω) of the spin operator σ_z for the biased sub-Ohmic spin-boson model. The small-ω behavior C(ω) ∝ω~s is found to be universal and independent of the bias ε and the coupling strength α(except at the quantum critical point α = αc and ε = 0). Our NRG data also show C(ω) ∝χ~2ω~s for a wide range of parameters, including the biased strong coupling regime(ε = 0 and α α_c), supporting the general validity of the Shiba relation. Close to the quantum critical point αc,the dependence of C(ω) on α and ε is understood in terms of the competition between ε and the crossover energy scale ω_0~*of the unbiased case. C(ω) is stable with respect to ε for ε《ε~*. For ε》ε~*, it is suppressed by ε in the low frequency regime. We establish that ε~*∝(ω_0~*)~(1/θ)holds for all sub-Ohmic regime 0≤s 1, with θ = 2/(3s) for 0 s≤1/2 and θ = 2/(1 + s) for 1/2 s 1. The variation of C(ω) with α and ε is summarized into a crossover phase diagram on the α–ε plane.     

Oscillating two-stream instability of a plasma wave in a plasmachannel

A long wavelength Langmuir wave (ω₀, k₀), propagating through a parabolic plasma density channel, can decay into a low-frequency mode (ω,k&oarr;) and two short wavelength Langmuir wave sidebands (ν_1,2,k&oarr;_1,2 ), via two-stream instability where ω_1,2=ω∓ω₀ and k&oarr;_1,2=k&oarr;∓k&oarr;₀. Depending on the mode number n, the growth rate maximizes in the range γ_max≃0.1ω_pi-0.4ω_pi for the range of k from 0.1(ω_pi/c_s) to 0.2(ω_pi/c_x) for ν₀~ν_th where ν₀ and ν_th are the oscillatory and thermal velocities of electrons, ω_pi is the ion plasma frequency on the axis, and c_s is the sound speed. The growth rate increases with the width a of the plasma density channel. It decreases with the mode number. The instability may be relevant to laser based charged particle accelerators     

Analysis of the deflection system for a magnetic-field-immersedmagnicon amplifier

A linear analysis of the electron-beam deflection system in a magnicon amplifier is presented. The system consists of identical cavities, one driven and the remainder passive, separated by a drift space and immersed in an axial magnetic field. The cavities contain a rotating TM₁₁₀ mode. The length of each cavity is πν _z/ω, and that of the drift space is πν_z/ω_c, where ω is the RF frequency, ω_c is the relativistic gyrofrequency in the guide field, and ν_z is the mean axial velocity of the beam electrons. The linearized electron orbits are obtained for arbitrary initial axial velocity, radial coordinate, and magnetic field. The small-signal gain and the phase shift are determined. The special case where ω_c/ω=2 has unique features and is discussed in detail. For the NRL magnicon design, a power gain of 10 dB per passive cavity is feasible. Results from numerical modeling of a magnicon with two passive cavities are presented. Operation of the output cavity at the fundamental and higher harmonics of the input drive frequency is briefly discussed     

Cold-Fluid Analysis of the Space-Charge Effects in an Electromagnetically Pumped Free-Electron Laser with an Axial-Guide Magnetic Field

The space-charge effects are analysed and five kinds of fluid waves are obtained in the e-beam density and the axial-current density for an electromagnetically pumped free-electron laser with an axial-guide magnetic field by using the cold-fluid theory. To enhance the gain, the guide field Bo must be in this regime: -2(ω_υ + cβ_x0k_ω) < Ω < -(ω_υ + cβ_x0k_ω), where Ω₀ = eB₀/mr₀ and ω_υ(k_ω) is the frequency (wave number) of the wiggler.     

Second harmonic stimulated Raman scattering of laser radiation in aplasma

A high-power linearly polarized laser propagating through a plasma produces oscillatory electron velocity at the second harmonic due to-the ν&oarr;×B&oarr; force, This velocity couples a Langmuir wave (ω, k&oarr;) and an electromagnetic wave (ω₁, k&oarr;₁), where ω₁=ω-2ω₀ , k-2k&oarr;₀ and ω₀, k&oarr;₀ are frequency and wavenumber of the laser pump, causing second harmonic Raman scattering. The growth rate is maximum for side scattering. This process can occur above the quarter critical density, unlike the first harmonic stimulated Raman scattering which occurs below the quarter critical density     

氢化杂质和厚度效应对高斯势量子点中二能级体系量子跃迁的影响

在计及氢化杂质和厚度效应下,分别选取抛物线型限定势阱和高斯函数型限定势阱描写盘型量子点中电子的横向限定势和纵向限定势,采用Lee-Low-Pines-Pekar变分法推导出量子点中电子的基态和第一激发态能量本征值和本征函数,以此为基础,构造了一个二能级结构,并基于二能级体系理论,讨论了电子在磁场作用下的量子跃迁.结果表明,高斯函数型限定势比抛物线型限定势更能精准反映量子点中真实的限定势;量子点的厚度对电子的跃迁概率的影响不凡;电声耦合强度、介电常数比、磁场的回旋频率、高斯函数型限定势阱的阱深和阱宽等对电子基态与第一激发态声子平均数、能量以及量子跃迁的影响显著.