飞秒光诱导铽镓石榴石晶体中的磁化响应研究

运用时间分辨抽运-探测光谱技术,研究了磁光晶体铽镓石榴石(TGG)在不同椭圆偏振态的飞秒激光脉冲诱导下的极化和磁化响应.研究表明,当仅存在逆法拉第效应时,探测光旋转角信号和椭圆率信号的变化方向与圆偏振抽运光的旋向相关.这是由于圆偏振光在TGG晶体中产生的瞬态有效磁场的方向依赖于圆偏振光的旋向所致.光诱导磁化过程与材料的性质有关,TGG晶体的顺磁特性决定了其自旋弛豫时间为几十飞秒.由于探测光旋转角信号和椭圆率信号的半高全宽均为500 fs左右,加之信号强度随着抽运光脉冲能量密度的增加呈线性增长,表明TGG晶 关键词： 铽镓石榴石晶体 抽运-探测光谱技术 逆法拉第效应     

C掺杂FePt铁磁薄膜光诱导超快退磁动力学研究

FePt合金薄膜由于具有较强的磁各向异性而在磁信息和磁光信息存储中具有重要的应用.C掺杂可精确调控薄膜的磁各向异性,从而可有效地改变薄膜的矫顽场.通过超短激光脉冲与铁磁薄膜相互作用,可以获得非平衡状态下电子、自旋和晶格等自由度之间的动态耦合参数,这是研究超快磁记录材料的物理基础.本文基于瞬态磁光Kerr效应,研究了两种C掺杂浓度下FePt薄膜的超快磁光响应.实验结果表明:瞬态Kerr信号与外加磁场正相关,磁场反向,Kerr信号反号,而瞬态反射率与外加磁场无关;不同C掺杂的FePt薄膜的矫顽场不同,软磁的退磁时间显著小于硬磁薄膜的退磁时间.我们还观测到超快激光在铁磁薄膜中诱导频率约为49 GHz的相干声学声子,该声子的频率与外加磁场无关.实验结果为设计和研制新型磁光薄膜提供了实验依据.     

聚焦光在单轴晶体中的感应磁化

运用矢量衍射理论,分析了高度聚焦的圆偏振激光脉冲在具有逆法拉第效应的单轴晶体中产生的感应磁化分布.详细研究了界面位置不同对磁化分布的影响,比较了小双折射效应的单轴晶体中的磁化分布与均匀介质中的磁化分布.计算结果发现:随着界面向透镜移动,磁化强度的最大值和磁化斑的最小值位置沿着光轴方向移动.在单轴晶体中的磁化强度大于在均匀介质中的磁化强度.但是,在单轴晶体中的磁化斑的大小比在均匀介质中的小.描述单轴晶体各向异性的光磁常量与各向同性的光磁常量的比值愈大,磁化强度愈大,磁化斑愈小.     

磁性金属颗粒膜磁光-Kerr效应增强现象研究

基于磁性金属颗粒膜Kerr磁光效应的关系,结合电子跃迁和自由电子共振模型及实验数据,分析了金属颗粒膜的磁光Kerr旋转和Kerr椭偏现象,由分析研究表明：在磁性金属颗粒材料中Kerr旋转和Kerr椭偏率光谱(Kerr效应)中的共振峰不是由单一电子的跃迁引起,其中由于等离子体激化元的存在,而引起的等离子体共振行为在此产生了相当的影响. 亦即对磁光Kerr旋转和Kerr椭偏率起作用的是电子跃迁与自由电荷载流子的等离子共振相互作用共同引起的. 这一结论与对较大Kerr效应的4f 化合物的相关实验强烈的Kerr效应和明显的共振结构相一致,这在铥原子化合物（TmS,TmSe）和铈化合物（CeSb,CeSb0.75Te0.25,CeTe.）及L10FePt膜中都已观察到这一现象.     

磁性金属颗粒膜磁光-Kerr效应增强现象研究

基于磁性金属颗粒膜Kerr磁光效应的关系,结合电子跃迁和自由电子共振模型及实验数据,分析了金属颗粒膜的磁光Kerr旋转和Kerr椭偏现象,由分析研究表明:在磁性金属颗粒材料中Kerr旋转和Kerr椭偏率光谱(Kerr效应)中的共振峰不是由单一电子的跃迁引起,其中由于等离子体激化元的存在,而引起的等离子体共振行为在此产生了相当的影响.亦即对磁光Kerr旋转和Kerr椭偏率起作用的是电子跃迁与自由电荷载流子的等离子共振相互作用共同引起的.这一结论与对较大Kerr效应的4f化合物的相关实验强烈的Kerr效应和明显的共振结构相一致,这在铥原子化合物(Tm S,Tm Se)和铈化合物(Ce Sb,Ce Sb0.75Te0.25,Ce Te.)及L10Fe Pt膜中都已观察到这一现象.     

磁光光纤Bragg光栅中圆偏振光的非线性传输特性

提出了磁光光纤Bragg光栅的理论模型,给出了圆偏振光在磁光光纤光栅中传播的非线性耦合模方程. 研究表明,在磁光光纤Bragg光栅中,光栅引起正反传播方向的导波光发生耦合,法拉第效应引起磁圆双折射效应,而非线性效应则将左旋和右旋圆偏振光耦合在一起,它们的共同作用可使双稳态状态发生反转、非线性光控光开关阈值功率降低. 与传统光纤光栅相比,利用左旋和右旋磁圆偏振光之间的交叉相位调制实现的脉冲整形具有磁光偏置可调特性,为基于磁光光纤光栅的动态灵活全光3R再生器的研制提供了理论基础. 关键词： 磁光光纤Bragg光栅 圆偏振光 脉冲整形     

光磁相互作用及自旋极化的光学调控

近年来,超短激光脉冲的发展为人们在极端时间尺度上研究光与物质的相互作用提供了有 效工具。皮秒时间尺度上研究磁有序材料中的自旋动力学过程已经成为凝聚态物理研究的热点, 促进了自旋电子学的发展。本论文基于飞秒激光抽运–探测技术,介绍了半导体及其纳米结构中 光注入自旋极化及其弛豫过程;综述了铁磁性薄膜中的超快退磁,激光诱导磁阻尼进动和逆法拉 第效应; 利用太赫兹脉冲的磁场分量,研究了反铁磁晶体中自旋共振模式的激发,相干控制以 及自旋重取向的探测; 最后介绍了超快光谱用于研究多铁性材料中电子、晶格和自旋间的耦合。 了解不同材料在超快时间尺度上的磁光、光磁效应的最新进展,有助于在磁有序的超快光控制研 究领域做出原创性的工作。     

克尔介质中脉冲宽度对瞬态热光非线性效应的影响

在纳秒时域，研究了激光脉冲宽度变化对克尔介质中瞬态热光非 线性效应的影响.通过不同脉冲宽度τp下得到的二硫化碳 苯胺黑溶液的Z_scan实验结果，对瞬态热光非线性效应和光克尔效应的共存过程进行了分析 .随着脉冲宽度的增加，观察到了热光非线性效应从瞬态到稳态变化过程和Z_scan峰谷特性 的转变.同时，从声波的传播方程和光波的非线性传播方程出发，用数值计算方法模拟了这 一非线性过程，结果表明数值模拟结果和实验结果是相符的. 关键词： 瞬态热光非线性效应 脉冲宽度 Z_scan 克尔效应     

外磁场对(Ga，Mn)As有效g因子的影响

利用时间分辨Kerr旋光技术测量低温下稀磁半导体Ga_0.937Mn_0.063As中光注入极化载流子的自旋进动信号,并观察到自旋极化载流子的有效g因子值随外磁场的增强而增大的反常现象.这归结于磁场导致局域化空穴转化为非局域化空穴,从而使自发磁化强度增强,有效g因子值增大.基于此物理图像,进一步给出了(Ga,Mn)As的有效g因子与外磁场的关系式. 关键词： 时间分辨Kerr旋光测量 Zeeman效应 Ruderman-Kittel-Kasuya-Yosida模型     

非线性克尔效应对飞秒激光偏振的超快调制

研究了近红外飞秒激光的偏振在太赫兹频率的超快调制.利用抽运-探测光谱技术,通过改变两个脉冲之间的延迟时间可以控制光脉冲的旋转角.在Li：NaTb（WO₄）₂磁光晶体中观察到探测光的偏振随延迟时间变化的高速振荡,振荡信号的中心频率为0.19 THz.这种超快偏振调制现象可以解释为,抽运-探测实验构置中,前向传播的抽运光诱导的光学克尔非线性引起被晶体远端表面所反射的背向传播的探测光脉冲偏振面的额外旋转.通过改变抽运光的圆偏振旋性可以控制探测光调制信号的相位和振幅.实验结果表明,非线性光学克尔效应可以作为一种全新的手段,在磁光晶体中实现近红外飞秒激光以太赫兹频率的超快偏振调控.这将在超快磁光调制器等全光器件中得以应用.实验结果将有助于偏振依赖的超快动力学过程的研究.     

High-density all-optical magnetic recording using a high-NA lens illuminated by circularly polarized pulse lights

We propose a method for high-density all-optical magnetic recording. Our analyses, based on the vector diffraction theory, show that owing to the inverse Faraday effect, circularly polarized laser pulses focused by a high numerical aperture (NA) lens can induce a small magnetization domain. For an example, the FWHM of the effective magnetization domain is 0.4646λ when NA=0.85. The magnetization direction is basically perpendicular to the surface of the optic-magneto film within the effective magnetization domain and the switching direction of magnetization can be controlled by the helicity of the incident circularly polarized light. These characteristics are useful to next-generation high-density all-optical magnetic storage.     

Quantum field theory of optical birefringence phenomena

The inverse Faraday effect, in which a magnetization is induced in a solution through which is passed a polarized light beam of arbitrary ellipticity, is discussed on the basis of the S-matrix formulation of optical birefringence. It is shown that the Faraday effect and the inverse Faraday effect are topologically identical problems of diagrammatic perturbation theory and so it follows automatically that the magnetization should be proportional to the Verdet constant. The optical Faraday effect is the circular birefringence induced by an intense circularly polarized beam of light propagated colinearly with the weak measuring beam: the electric vector of the circularly polarized beam interacts with the molecule in a way that resembles the interaction of a static magnetic field. The interrelations of these two effects and the normal Faraday effect the self-rotation of the polarization ellipse of an intense beam are discussed.     

Modeling magnetization oscillations in ferromagnetic cobalt disks exposed to picosecond laser pulses

Magnetic reversal of a ferromagnetic single-domain nanoparticle in the form of a disk contained in a multilayer nanostructure irradiated by linearly and circularly polarized picosecond laser pulses is modeled. It is shown that heating of the disk by the laser light causes a change in its direction of magnetization. This is accompanied by oscillations in the magnetization that decay over 1–16 ns. The frequencies of these oscillations lie in the range of 0.5–26 GHz. The main reason for the magnetic reversal of the disk is a change in the magnetic anisotropy energy during heating. The laser pulses also create a spin-polarized current and an inverse Faraday magneto-optical field in the structure, which influence the duration, amplitude, and frequency of the magnetization oscillations.     

Peculiarities of the inverse Faraday effect induced in iron garnet films by femtosecond laser pulses

The inverse Faraday effect in iron garnet films subjected to femtosecond laser pulses is experimentally investigated. It is found that the magnitude of the observed effect depends nonlinearly on the energy of the optical pump pulses, which is in contradiction with the notion that the inverse Faraday effect is linear with respect to the pump energy. Thus, for pump pulses with a central wavelength of 650 nm and an energy density of 1 mJ/cm², the deviation from a linear dependence is as large as 50%. Analysis of the experimental data demonstrates that the observed behavior is explained by the fact that the optically induced normal component of the magnetization is determined, apart from the field resulting from the inverse Faraday effect, by a decrease in the magnitude of the precessing magnetization under the influence of the femtosecond electromagnetic field.     

Micromagnetic investigation of all-optical switching

This Letter investigates all-optical magnetization switching from micromagnetic perspective. The influence of circularly polarized light on a magnetic sample was considered to be both directly through the inverse Faraday effect and indirectly aided by thermal induced effects by the laser beam. Dependence of all-optical switching on pulse duration, laser intensity and the magneto-optical susceptibility strength is studied. An important aspect of this investigation is the analysis of how the cooling process influences the successive switches, especially through limiting the successive writing time.     

Symmetry properties of three-dimensional magnetization distributions induced by focused circularly polarized lights

We analyze symmetry properties of the three-dimensional magnetization distribution in the optic-magneto film induced by focused circularly polarized lights. The magnetization distributions are derived and evaluated based on the vector diffraction theory and the inverse Faraday effect of the isotropic and nonmagnetically ordered material. It is shown that for any radial symmetrical amplitude, phase, or hybrid amplitude-phase pupil filter, the magnetization distribution of the axial component is circular symmetric but those of the radial and azimuthal components are annular symmetric with regard to the optical axis. All of the three components have a symmetric distribution with regard to the focal plane. The direction of both axial and radial components can be inversed with the helicity of incident circularly polarized light but the direction of azimuthal component is independent of helicity. The axial component has a decisive effect to all-optical magnetic recording, and within the effective axial range, the size of its magnetization domain hardly expands in the transverse direction.     

Generation of an axial magnetic field from photon spin

In circularly polarized light the spins of the photons are aligned. When a short intense pulse of circularly polarized laser light is absorbed by a plasma, a torque is delivered initially to the electron species, resulting primarily in an opposing torque from an induced azimuthal electric field. This electric field, in general, has a curl and leads to the generation of an axial magnetic field. It also is the main means for transferring angular momentum to the ions. The time-dependent magnetic field has a magnitude proportional to the transverse gradient of the absorbed intensity but inversely proportional to the electron density, in contrast to earlier theories of the inverse Faraday effect.