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

High-harmonic generation by focusing a femtosecond laser onto a gas is a well-known method of producing coherent extreme-ultraviolet (EUV) light. This nonlinear conversion process requires high pulse intensities, greater than 10(13) W cm(-2), which are not directly attainable using only the output power of a femtosecond oscillator. Chirped-pulse amplification enables the pulse intensity to exceed this threshold by incorporating several regenerative and/or multi-pass amplifier cavities in tandem. Intracavity pulse amplification (designed not to reduce the pulse repetition rate) also requires a long cavity. Here we demonstrate a method of high-harmonic generation that requires no extra cavities. This is achieved by exploiting the local field enhancement induced by resonant plasmons within a metallic nanostructure consisting of bow-tie-shaped gold elements on a sapphire substrate. In our experiment, the output beam emitted from a modest femtosecond oscillator (100-kW peak power, 1.3-nJ pulse energy and 10-fs pulse duration) is directly focused onto the nanostructure with a pulse intensity of only 10(11) W cm(-2). The enhancement factor exceeds 20 dB, which is sufficient to produce EUV wavelengths down to 47 nm by injection with an argon gas jet. The method could form the basis for constructing laptop-sized EUV light sources for advanced lithography and high-resolution imaging applications.

High-harmonic generation by resonant plasmon field enhancement

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

The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

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Ultrafast lasers—reliable tools for advanced materials processing

Electron and lattice dynamics following optical excitation of metals

Femtosecond laser pulses were applied to study the energy deposition depth and transfer to the lattice for Au, Ni, and Mo films of varying thickness. The onset of melting, defined here as damage threshold, was detected by measuring changes in the scattering, reflection and transmission of the incident light. Experiments were done in multi-shot mode and single-shot threshold fluences were extracted by taking incubation into account. Since melting requires a well-defined energy density, we found the threshold depends on the film thickness whenever this is smaller than the range of electronic energy transport. The dependence of the threshold fluence on the pulse length and film thickness can be well described by the two-temperature model, proving that laser damage in metals is a purely thermal process even for femtosecond pulses. The importance of electron–phonon coupling is reflected by the great difference in electron diffusion depths of noble and transition metals.

The role of electron–phonon coupling in femtosecond laser damage of metals

Damage threshold dependence on electron–phonon coupling in Au and Ni 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.

J. Güdde

Papers

Electron and lattice dynamics following optical excitation of metals

The role of electron–phonon coupling in femtosecond laser damage of metals

Damage threshold dependence on electron–phonon coupling in Au and Ni films

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

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