Femtosecond magneto-optical Kerr microscopy

We have developed a magneto-optical Kerr microscope that allows us to measure the ultrafast magnetization dynamics of ferromagnetic nanostructures. The magneto-optical signal can be recorded in a confocal reflection geometry with an accurate selection of the polarization. The magnetization dynamics is obtained from pump-probe measurements using frequency nondegenerate collinear pump and probe beams with a temporal resolution of 180 fs. Both probe and pump beams are focused to their diffraction limit, leading to an overall spatial resolution of 600 nm. The efficiency of the apparatus is tested by investigating the magnetization dynamics of individual CoPt(3) disks with a submicrometer diameter and a thickness of 15 nm.     

Femtosecond spectroscopy of relaxation processes in metals and high-T c superconductors

Spectral dependences of charge carrier relaxation rates, γ _e-e (?ω) and γ _e-ph (?ω), were observed in Au and Cu films and YBa₂Cu₃O_7-δ high-T _c superconductor films. The relaxation rates decreased substantially in the spectral region corresponding to interband transitions to the Fermi level region (?ω_Au = 2.45 eV, ?ω_Cu = 2.15 eV, and ? ω₁ = 1.89 eV and ?ω₂ = 2.08 eV for YBa₂Cu₃O_7-δ). This relaxation deceleration opens up possibilities for developing a new method, based on the spectral dependences of relaxation rates, for the determination of the Fermi level position and the parameters of electron-electron and electron-phonon interactions on the one hand and for studying deviations from the Fermi-liquid behavior in strongly correlated electronic systems. The linear γ _e-e (?ω) ∝ |?ω? E _F| spectral dependence was observed for a YBa₂Cu₃O_7-δ film near ?ω₂ = 2.08 eV, which may be evidence of a non-Fermi-liquid behavior of the electronic subsystem.     

Femtosecond laser-induced nanostructure-covered large-scale waves on metals

Through femtosecond (fs) laser pulse irradiation (pulse duration: 65 fs, central wavelength: 800 nm, and repetition rate: 250 Hz), we investigate the morphological evolution of fs laser-induced periodic surface structure on Au and Pt, called a nanostructure-covered large-scale wave (NC-LSW) with a period of tens of microns, densely covered by iterating stripe patterns of nanostructures and microstructures. We show that the surface morphology of NC-LSW crucially depends on the fluence of the laser, the number of irradiating pulses, and the incident beam angle. Our experimental observations allow us to establish a three-step model for the NC-LSW formation: the formation of laser-induced surface unevenness, inhomogeneous energy deposition due to the interference between the incident light and the scattered field, and nonuniform energy deposition due to shielding by the peaks of LSW.     

Femtosecond laser-induced blazed periodic grooves on metals

In this Letter, we generate laser-induced periodic surface structures (LIPSSs) on platinum following femtosecond laser pulse irradiation. For the first time to our knowledge, we study the morphological profile of LIPSSs over a broad incident angular range, and find that the morphological profile of LIPSSs depends significantly on the incident angle of the laser beam. We show that LIPSS grooves become more asymmetric at a larger incident angle, and the morphological profile of LIPSSs formed at an incident angle over 55° eventually resembles that of a blazed grating. Our study suggests that the formation of the blazed groove structures is attributed to the selective ablation of grooves through the asymmetric periodic surface heating following femtosecond pulse irradiation. The blazed grooves are useful for controlling the diffraction efficiency of LIPSSs.     

Femtosecond time-resolved absorption processes in lithium niobate crystals

Femtosecond pump pulses are strongly attenuated in lithium niobate owing to two-photon absorption; the relevant nonlinear coefficient beta(p) ranges from approximately 3.5 cm/GW for lambda(p) = 388 nm to approximately 0.1 cm/GW for 514 nm. In collinear pump-probe experiments the probe transmission at the double pump wavelength 2lambda(p) = 776 nm is controlled by two different processes: A direct absorption process involving pump and probe photons (beta (r) = 0.9 cm/GW) leads to a pronounced short-duration transmission dip, whereas the probe absorption by pump-excited charge carriers results in a long-duration plateau. Coherent pump-probe interactions are of no importance. Hot-carrier relaxation occurs on the time scale of < or approximately equal to 0.1 ps.     

Femtosecond dynamics of adsorbate charge transfer processes

Received: 21 March 1997/Accepted: 12 August 1997     

Femtosecond dynamics of primary processes in visual pigment rhodopsin

The dynamics of the coherent photoisomerization of the 11-cis-retinal in bovine rhodopsin is studied by femtosecond time-resolved laser absorption spectroscopy with a resolution of 30 fs. Rhodopsin is excited with 500-, 535-, and 560-nm femtosecond pulses to produce different initial Franck-Condon states with different vibrational energies of the molecule in its electronically excited state. The time evolution of the photoinduced differential absorption spectra of rhodopsin is measured upon excitation with a femtosecond pulse in a spectral range from 400 to 720 nm. Oscillations in the time-resolved absorption of the rhodopsin photoproducts, such as photorhodopsin with a vibrationally excited all-trans-retinal and in its initial-state rhodopsin with a vibrationally excited 11-cis-retinal, are examined. It is demonstrated that these oscillations reflect the dynamics of coherent vibrational wavepackets. The Fourier transform of these oscillatory components yields frequencies, amplitudes, and initial phases of various vibrational modes involved in the motion the wavepackets in both photoproducts. The main vibrational modes manifest themselves at frequencies of 62 and 160 cm^?1 for photorhodopsin and 44 and 142 cm^?1 for initial-state rhodopsin. It is shown that these vibrational modes are directly involved in the coherent reaction under the study, with their amplitudes in the power spectrum produced by the Fourier transform of the kinetic curves being dependent on the wavelength of rhodopsin excitation.     

Femtosecond laser induced periodic large-scale surface structures on metals

The periodic ripple structures on wolfram and titanium surfaces are induced experimentally by linear polarized femtosecond laser pulses at small incident angles. The structural features show a material difference in the s- and p-polarized laser irradiation. The interspace between the ripples increases significantly for p-polarized laser irradiation when it exceeds a threshold angle, and the ripples' periodicities are larger than the wavelength of the incident p-polarized femtosecond laser; however, no significant change in the period of the ripples is observed with increasing incident angle for s-polarized laser irradiation. To explain these phenomena we propose a resonant absorption mechanism, by which the experimental observations can be interpreted.     

Femtosecond dynamics in ferromagnetic metals investigated with soft x-ray resonant emission

We present measurements of the magnetic circular dichroism in x-ray resonant emission in the perpendicular geometry (circularly polarized x rays at normal incidence to the magnetization) for L(2,3) the absorption region in Fe, Co, and Ni metal. The results show that spin-dependent screening of the core hole takes place within the scattering time scale, which is supported by the absence of the effect in ionic systems. This allows an assessment of the time scale for the screening process (up to a few femtoseconds). The process is almost complete within the scattering time for Fe and Co, but this is not the case for the narrow band metal Ni which shows a much slower dynamics.     

Femtosecond study of surface structure and composition and time-resolved spectroscopy in metals

Surface structuring and compositioning in aluminum alloy 2024-T3 were demonstrated using a femtosecond pulse laser. Surface nanostructuring was developed as a function of laser parameters and the surface micrographs of the scanning electron microscopy were characterized as a function of incident laser fluence. Surface compositioning was performed by selectively removing the elements on the surface of the sample. Femtosecond studies of highly excited electrons were performed by a pump–probe technique, and the thermalization time was found to be in a range of 1.5–3 ps, increasing with incident fluence. The time-resolved measurement is well matched to the numerical calculation. Received: 6 September 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-405/744-6811, E-mail: dou@okstate.edu     

Femtosecond laser ablation of metals:a molecular dynamics simulation study

Using molecular dynamics (MD) methods combining with two-step radiation heating model, the mechanisms of ablation and the thermodynamic states at Ni surface under femtosecond laser irradiation are investigated. Simulation results show that the main mechanisms of ablation are evaporation and tensile stresses generated inside the target. The velocity of stress wave is predicted to be nearly equal to sound velocity. The rates of ablation at different fluences obtained from simulations are in good agreement with experimental data. Superheating phenomenon is also discovered.     

Relaxation processes in metals at high strain rates

Mathematical modeling is used for experiments involving the loading of plates by plane shock waves to study the relaxation of shear stresses during the high-rate deformation of metallic materials. It is established that the characteristic relaxation times vary broadly — from fractions of a nanosecond to several microseconds. Such variation is indicative of a change in the mechanism responsible for relaxation. As a result, there is a difference between the quasi-equilibrium shear stresses in the elastic precursor and the same stresses behind the shock front. Metallic materials remain capable of resisting plastic deformation behind the front. Structural irregularities created by high-rate deformation result in localization of plastic flow at the microscopic level, which in turn causes the parameters of the stress-strain state at this level to differ from the corresponding parameters on the macroscopic scale.Siberian Physico-Technical Institute, affiliated with Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 82–90, August, 1995.     

Theoretical study of defect formation processes in metals

Abstract

The equilibrium solute atmosphere around a straight edge dislocation in interstitial solid solutions has been investigated. A long-range deformation interaction among impurities is accounted for. Quantitative estimations have been given for the example of carbonaceous martensite. The impurity concentrations in an atmosphere around the dislocation core are calculated for a given temperature in dependence on its mean value in the specimen. For a dislocation with an impurity atmosphere stationary fluxes of interstitial atoms and vacancies on the dislocation are calculated; a concentration dependence of impurity parameters indicating a dislocation capture efficiency of the self-interstitial atoms and vacancies and the parameter of dislocation preference B are received; a radiation-induced deformation rate (swelling and creep) is determined.     

Magnetic properties and magneto-optical spectroscopy of Heusler alloys based on transition metals and Sn

The formation and magnetic properties of Heusler compounds of the general formula X₂YSn were studied, where X represents a 3d transition metal or Cu and where Y represents a second 3d transition metal or a metal of group IV A, VA and VI A of the periodic table. The lattice constant was determined for all Heusler compounds studied. The Co moment in the Co₂YSn compounds was not found to scale in a sample manner to the electronegativity difference E φ^* between Co and the Y components as was previously observed in the series Co₂YAl and Co₂YGa. All compounds of the type Ni₂YSn were found to be Pauli paramagnetic when Y is non-magnetic metal. Compounds having a Curie temperature above room temperature were investigated by means of magneto-optical polar Kerr-effect spectroscopy. Experimental indications were obtained for charge transfer transitions in the series Co₂TiSn, Co₂HfSn.     

Computational modelling of thermophysical processes in the light metals industry

This survey focuses on the aluminium industry, mostly on process aspects as opposed to metallurgical aspects. It covers recent work on process models involving fluid flow and heat transfer, and extends to all important categories of processes encountered in the primary aluminium industry, from raw materials and reduction to cast shop and recycling. This includes a wide variety of processes from precipitators, calciners, rotary kilns, baking furnaces, reduction cells, casting and mixing furnaces to recycling furnaces and metal filtration. A review is carried out on the modelling work, the applications, and the needs expressed not only in analysis and design but also in process control, optimization and supervision, as well as operator training. A summary is given of the problems perceived, mainly in the field of model parameters and model validation. Indications on future trends are also given. Conclusions are drawn from the survey of this fast-expanding body of knowledge that suggests tough challenges as well as unprecedented opportunities. Suggestions are made as to how some of those challenges could be met.