Nonlinear magneto-optical Kerr effects

The polarization state of a second-harmonic wave after reflection from a semi-infinite, optically isotropic magnetic medium is considered for the three characteristic uniformmagnetization directions corresponding to the linear magneto-optical Kerr effects. Expressions for the complex amplitudes of the wave field which specify the nonlinear Kerr effects, viz., the polar, meridional, and equatorial effects, are obtained in a first approximation with respect to the magnetization. The dependences of these effects on the angle of incidence of the inducing wave obtained as a result of a numerical experiment are presented. Analytical formulas are found for them at small angles of incidence. A comparative analysis of the linear and nonlinear Kerr effects is made. Zh. éksp. Teor. Fiz. 116, 141–156 (July 1999)     

Femtosecond magneto-optical processes in metals

Ultrafast magneto-optical techniques, using femtosecond laser pulses, are a powerful tool for investigating the magnetization dynamics in metallic materials. We review some recent developments in this field, which allow progress in the understanding of the fundamental mechanisms that govern the magnetization changes in ferromagnetic structures. Some fundamental aspects of the magneto-optical response in the frequency and time domain are first considered. Then, we analyze the dynamics of the charges and spins following the optical excitation of quasiparticles in different ferromagnetic thin films. This dynamics is discussed by considering the processes that are involved in the ultrafast magneto-optical response.     

克尔磁光效应的经典理论分析

本文引入与自旋-轨道相互作用等有关的有效场概念,运用经典电磁场理论,推导了磁光克尔旋转的明确表达式.证明了各种磁光介质的极向克尔旋转θ_k的实部θ′_k和虚部θ″_k均与有效场H_i成正比,在铁磁性介质中近似与vM成正比,M为磁化强度,在亚铁磁性等介质中近似与(?)v_iM_i成正比,M_i为i次点阵的磁化强度.同时还证明了近似的横向克尔旋转θ_k~t与H_i~2正比;θ_k和θ_k~t的温度特性取决于H_i的温度特性.     

Femtosecond photoemission microscopy

We developed a novel experiment for time-resolved photoemission microscopy by combining a commercial photoemission electron microscope (PEEM) with a pulsed Ti:sapphire laser oscillator. The laser system, the setup of the delay stage for pump-probe experiments, and the interface between the PEEM and the laser system are discussed. We use self-organization of Ag islands and nanowires on Si(1 1 1) and 4° vicinal Si(0 0 1) to generate structures with a plasmon resonance that matches the photon energy of our laser (?ω = 3.1 eV after frequency doubling). In two-photon photoemission (2PPE) the photoemission yield then directly visualizes the plasmons in the nanostructures. Accordingly, the photoemission yield depends on the size and shape of the nanostructures, and on the polarization of the laser pulses as well. In Ag nanowires, we observe surface plasmon polariton (SPP) waves by a beating that is formed by interference of the SPP wave and the incident laser light. In a pump-probe experiment, we can directly visualize the propagation of the SPP on a femtosecond time scale.     

Nonlinear intensity-related magneto-optical Kerr effects in the planar geometry

The reflection of an electromagnetic wave at the second-harmonic frequency from a semi-infinite optically isotropic magnetic medium for the directions of uniform magnetization corresponding to the meridional and equatorial Kerr effects is considered. Using the Green’s tensor function technique, in the first approximation in magnetization, the expressions for complex amplitudes of the field are obtained for the s and p polarizations of the incident beam and their superpositions. It is shown that in the latter case, the meridional effect becomes intensity-related. Dependences of the intensity-related meridional and equatorial Kerr effects on the angle characterizing polarization of the pump wave are obtained by numerical calculations. A comparative analysis of the linear and nonlinear Kerr effects is made.     

Femtosecond time-resolved interferometric microscopy

We describe optical interferometry with 100 fs time resolution and a spatial resolution of approximately one micrometer. Using a pump-probe scheme and a 2D-Fourier transformation algorithm, we are able to retrieve from the interferograms very small changes in the phase and the amplitude of the reflected probe pulses. The performance of the technique is illustrated by measurements of transient and permanent surface modifications of crystalline GaAs after exposure to intense femtosecond laser pulses with fluences near the ablation threshold. PACS 78.47.+p; 42.65.Re; 42.15.Eq; 68.35.Ja     

Femtosecond stimulated Raman microscopy

A novel type of non-linear Raman microscopy, femtosecond stimulated Raman microscopy (FSRM), is introduced. It employs femtosecond white light pulses and intense picosecond pulses which are derived from a femtosecond laser/amplifier system. The pulses are coupled into a microscope set-up and induce a stimulated Raman process at the focus. The Raman interaction spectrally modulates the white light. These modulations are read-out in multi-channel fashion and allow recording of a complete Raman spectrum of the focal region. By raster-scanning the sample, complete Raman images can be obtained. Raman images of polystyrene beads in water demonstrate the feasibility of the approach. PACS 42.65.Dr; 42.30.Va; 42.65.Re     

Features of the simplest model of second-harmonic magneto-optical Kerr effects

A generalised formalism of characteristic matrices is presented to analyse second-harmonic magneto-optical Kerr effects in an optically anisotropic centrosymmetric ferromagnetically ordered multilayer if its response can be described in terms of electric polarisation. Features of the model associated with ideal (infinitely thin) interfaces are highlighted. These are due to both the existence of two versions of unconventional boundary conditions and an inevitable conventional approach to defining the surface polarisations through the fundamental electric field and surface-susceptibility tensors. New analytical results for linear and second-harmonic Kerr effects are shown to be advantageous for developing an effective algorithm for their numerical simulation. The linear approximation with respect to magnetisation is pursued, thereby also making our results suitable for investigating a great variety of magneto-optical effects and (in the second-harmonic case) effects related to anisotropy. Received: 16 October 2001 / Published online: 24 April 2002     

Enhancement of the magneto-optical Kerr rotation in nonlinear optical response

Using an electronic theory we calculate the magneto-optical Kerr rotation in Second-Harmonic Generation (SHG). We derive Fresnel's formulas for the nonlinear optical response. Mathematical details of the derivation are given. For the longitudinal and polar Kerr configuration and for arbitrary angles of incidence it is found that the Kerr angle in SHG may be enhanced by up to one order of magnitude compared to the linear Kerr angle. This enhancement is caused by interband and intraband transitions (plasmons) which in the linear case suppress the Kerr rotation in the optical range. Our results will be useful for a microscopic study of 2D magnetism.Paper presented at the 129th WE- Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Femtosecond Kerr ellipsometry in polydiacetylene solutions: Two-photon effects

We describe a pump-probe femtosecond Kerr experiment in polydiacetylene solutions (4BCMU, yellow form). The Kerr ellipsometry technique permits a separate determination of the real and of the imaginary part of anisotropic index changes. The wavelength (620 nm) of the pump pulses lies in the transparency region of polydiacetylene while the probe pulses have a broad white spectrum. The largest contribution to the Kerr signal is a quasi-resonant stimulated anti-Stokes Raman absorption. A flat two-photon transition spectrum with 4×10^–3 cm/MW is observed close to 4 eV. The two-photon excited states have a life time close to 12 ps and they present a broad absorption band in the 2 eV range.     

Magnetization reversal measurements in mesoscopic amorphous magnets by magneto-optical Kerr effect

A magneto-optical setup based on the transverse Kerr effect has been designed to study the magnetization reversal processes by vector magnetometry in arrays of magnetic nanostructures with a reduced total volume. This system allows the measurement of both the parallel and perpendicular to the field components of the magnetization. It has been used to analyze the behavior of amorphous Co _x Si_1-x lines fabricated by electron beam lithography that present a very well defined shape induced uniaxial anisotropy. When the field is applied near to the hard direction, coherent rotation processes are found to occur with a collapse of this reversal mode at fields very close to the hard axis that allows to estimate the very low anisotropy dispersion of these samples. The analysis of the vector hysteresis loops reveals that the magnetization switches via an incoherent process that starts prior to the Stoner-Wohlfarth instability and that can be described in terms of a localized curling-like reversal mode.Received: 16 June 2004, Published online: 24 September 2004PACS: 75.75. + a Magnetic properties of nanostructures - 75.60.Jk Magnetization reversal mechanisms - 75.50.Kj Amorphous and quasicrystalline magnetic materials     

Femtosecond Kerr-gated wide-field fluorescence microscopy

We present a Kerr-gated microscope capable of collecting diffraction-limited 2D fluorescence images with sub-100 fs time resolution. The concept is based on the insertion of a solid-state optical Kerr gate into a wide-field microscope. In addition to the considerably improved temporal resolution, the wide-field design allows for simultaneous tracking of several objects and ultrafast fluorescence lifetime imaging of doped and heterogeneous surfaces. The ultrafast fluorescence dynamics of gold nanoparticles is presented as an example of the capabilities of the instrument.     

Spin polarized carrier injection driven magneto-optical Kerr effect in Cr-doped ZnO nanorods

Hydrothermal growth of Cr doped ZnO nanorods (NRs) thin films was grown on glass substrates. The strong ferromagnetic (FM) and antiferromagnetic (AFM) ordering was observed in 3% and 7% of Cr doped samples. The optical excitation of the Cr doped ZnO NRs is responsible for injection of the spin-polarized carriers in the 3% Cr doped ZnO NRs through longitudinal magneto-optical Kerr effect (MOKE). We determined a negative anisotropic magnetoresistance ($～23\text{\%}$) originated from spin orbit coupling due to sp-d exchange interaction. we calculated the process of photon induced inverse spin Hall angle (${\theta }_{\mathrm{ISHE}}～3.94\times {10}^{?2}$) close to the MOKE saturated rotation angle (${\theta }_k～0.046$) for 3% Cr: ZnO. These results can open a new path of optical spin detectors for next-generation spintronic device technology.     

Resonant magneto-optical Kerr effect induced by hybrid plasma modes in ferromagnetic nanovoids

With nanovoids buried in Co films, resonant structures were observed in spectra of polar magneto-optical Kerr effect(MOKE), where both a narrow bandwidth and high intensity were acquired. Through changing the thickness of Co films and the lattice of voids, different optical modes were introduced. For a very shallow array of voids, the resonant MOKE was induced by Ag plasma edge resonance, for deeper ones, hybrid plasma modes, such as void plasmons in the voids, surface lattice plasmons between the voids, and the co-action of them, etc. resulted in resonant MOKE. We found that resonant MOKE resulted from the void plasmons resonance which possesses the narrowest bandwidth for the lowest absorption of voids. The simulated electromagnetic field(EF) distribution consolidated different effects of these three optical modes on resonant MOKE modulation. Such resonant polar MOKE possesses high sensitivity, which might pave the way to on-chip MO devices.     

自旋-轨道耦合对磁性绝缘体磁光Kerr效应的影响

采用单原子能级跃迁模型,导出在同时考虑自旋交换劈裂和自旋轨道耦合时磁光Kerr旋转的微观表达式,并就四能级跃迁情况,研究了磁光效应随原子基态及激发态能级自旋轨道耦合常数的变化规律.结果表明:磁光Kerr旋转角与自旋轨道耦合劈裂能量不成正比;单原子能级自旋轨道耦合常数为正或中间激发态自旋轨道耦合常数为负时,有利于提高磁光Kerr旋转. 关键词： 磁光Kerr效应 自旋轨道耦合 线性响应核 劈裂     

Local magneto-optical Kerr effect imaging by scanning near-field optical microscope in reflection-mode

<正>A method for local magneto-optical Kerr effect imaging based on a home-made scanning near-field optical microscope working in reflection mode is presented.Shear force detection is carried out by using a symmetric piezoelectric bimorph sensor,which provides an easy way not only for probe-surface distance control but also for imaging.Polarization-preserving fiber probes used as a local optical detector are fabricated with a heating-pulling technique and the probes' polarization properties are measured.Shear force topographic and near-field magneto-optical images of magneto-optical disk taken with the proposed method are shown.