The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

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Ultrafast optical manipulation of magnetic order

Optical Studies of Dynamics in Noble Metal Nanostructures

The dependence of the strength of the electron-phonon coupling and the electron heat capacity on the electron temperature is investigated for eight representative metals, Al, Cu, Ag, Au, Ni, Pt, W, and Ti, for the conditions of strong electron-phonon nonequilibrium. These conditions are characteristic of metal targets subjected to energetic ion bombardment or short-pulse laser irradiation. Computational analysis based on first-principles electronic structure calculations of the electron density of states predicts large deviations (up to an order of magnitude) from the commonly used approximations of linear temperature dependence of the electron heat capacity and a constant electron-phonon coupling. These thermophysical properties are found to be very sensitive to details of the electronic structure of the material. The strength of the electron-phonon coupling can either increase (Al, Au, Ag, Cu, and W), decrease (Ni and Pt), or exhibit nonmonotonic changes (Ti) with increasing electron temperature. The electron heat capacity can exhibit either positive (Au, Ag, Cu, and W) or negative (Ni and Pt) deviations from the linear temperature dependence. The large variations of the thermophysical properties, revealed in this work for the range of electron temperatures typically realized in femtosecond laser material processing applications, have important implications for quantitative computational analysis of ultrafast processes associated with laser interaction with metals.

Electron-phonon coupling and electron heat capacity of metals under conditions of strong electron-phonon nonequilibrium

Electron and lattice dynamics following optical excitation of metals

The kinetics and microscopic mechanisms of laser melting and disintegration of thin Ni and Au films irradiated by a short, from 200 fs to 150 ps, laser pulse are investigated in a coupled atomistic-continuum computational model. The model provides a detailed atomic-level description of fast nonequilibrium processes of laser melting and film disintegration and, at the same time, ensures an adequate description of the laser light absorption by the conduction band electrons, the energy transfer to the lattice due to the electron-phonon coupling, and the fast electron heat conduction in metals. The interplay of two competing processes, the propagation of the liquid-crystal interfaces (melting fronts) from the external surfaces of the film and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for melting of metal films irradiated by laser pulses at fluences close to the melting threshold. The relative contributions of the homogeneous and heterogeneous melting mechanisms are defined by the laser fluence, pulse duration, and the strength of the electron-phonon coupling. At high laser fluences, significantly exceeding the threshold for the melting onset, a collapse of the crystal structure overheated above the limit of crystal stability takes place simultaneously in the whole overheated region within \ensuremath{\sim}2 ps, skipping the intermediate liquid-crystal coexistence stage. Under conditions of the inertial stress confinement, realized in the case of short $\ensuremath{\tau}l~10\mathrm{ps}$ laser pulses and strong electron-phonon coupling (Ni films), the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated films as well as on both homogeneous and heterogeneous melting processes. Anisotropic lattice distortions and stress gradients associated with the relaxation of the laser-induced pressure destabilize the crystal lattice, reduce the overheating required for the initiation of homogeneous melting down to $T\ensuremath{\approx}{1.05T}_{m},$ and expand the range of pulse durations for which homogeneous melting is observed in 50 nm Ni films up to \ensuremath{\sim}150 ps. High tensile stresses generated in the middle of an irradiated film can also lead to the mechanical disintegration of the film.

Combined atomistic-continuum modeling of short-pulse laser melting and disintegration of metal films

The ultrafast magnetization dynamics of thin Ni and Co films on Cu(001) and of polycrystalline Ni surfaces was studied by pump-probe reflection second-harmonic generation, utilizing 150 fs/800 nm laser pulses. In all cases no delay between electron excitation and magnetization breakdown was observed within the experimental time resolution. An upper limit of such delay is 50 fs in case of bulk Ni surfaces. The recovery of magnetization follows the electron temperature relaxation during the first few picoseconds and thereafter cooling by regular thermal diffusion.

Magnetization dynamics of ni and co films on cu(001) and of bulk nickel surfaces

Optical Second-Harmonic Generation (SHG) in reflection from a polycrystalline copper surface in air was studied using femtosecond time-resolved pump and probe measurements at λ=625 nm. The observed time dependence of second-harmonic yield from the probe beam demonstrates, that SHG is a very sensitive technique for measuring transient electron temperatures of metals even when these are covered by an oxide layer. For polycrystalline copper, an electron-phonon energy transfer time of 2 ps was observed, corresponding to a coupling constant of 3.75×1017 W/m3 K at average lattice temperatures of about 500 K. The analysis of experimental data indicates that the time dependence of SHG is governed by the linear dielectric function e which, in turn, is affected by the electron temperature. There is no evidence for a temperature dependence of the nonlinear susceptibility χ(2).

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Ultrafast electron and magnetization dynamics of thin Ni and Co films on Cu(001) observed by time-resolved SHG

Femtosecond pump-probe second-harmonic generation (SHG) and transient linear reflectivity measurements were carried out on polycrystalline Cu, Ag and An in air to analyze whether the electron temperature affects Fresnel factors or nonlinear susceptibilities, or both. Sensitivity to electron temperatures was attained by using photon energies near the interband transition threshold. We find that the nonlinear susceptibility carries the electron temperature dependence in case of Ag and Au, while for Cu the dependence is in the Fresnel factors. This contrasting behavior emphasizes that SHG is nota priori sensitive to electron dynamics at surfaces or interfaces, notwithstanding its cause.

U. Conrad

Papers

Electron and lattice dynamics following optical excitation of metals

Magnetization dynamics of ni and co films on cu(001) and of bulk nickel surfaces

Femtosecond time-resolved reflection second-harmonic generation on polycrystalline copper

Ultrafast electron and magnetization dynamics of thin Ni and Co films on Cu(001) observed by time-resolved SHG

Does femtosecond time-resolved second-harmonic generation probe electron temperatures at surfaces ?