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1.
Sci Adv ; 6(3): eaay8717, 2020 01.
Article in English | MEDLINE | ID: mdl-32010774

ABSTRACT

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism.

2.
Struct Dyn ; 4(5): 054501, 2017 Sep.
Article in English | MEDLINE | ID: mdl-28795080

ABSTRACT

We apply time-resolved MeV electron diffraction to study the electron-lattice energy relaxation in thin film Au-insulator heterostructures. Through precise measurements of the transient Debye-Waller-factor, the mean-square atomic displacement is directly determined, which allows to quantitatively follow the temporal evolution of the lattice temperature after short pulse laser excitation. Data obtained over an extended range of laser fluences reveal an increased relaxation rate when the film thickness is reduced or the Au-film is capped with an additional insulator top-layer. This behavior is attributed to a cross-interfacial coupling of excited electrons in the Au film to phonons in the adjacent insulator layer(s). Analysis of the data using the two-temperature-model taking explicitly into account the additional energy loss at the interface(s) allows to deduce the relative strength of the two relaxation channels.

3.
Phys Rev Lett ; 111(16): 167204, 2013 Oct 18.
Article in English | MEDLINE | ID: mdl-24182297

ABSTRACT

Motivated by the recent controversy about the importance of spin-flip scattering for ultrafast demagnetization in ferromagnets, we study the spin-dependent electron dynamics based on a dynamical Elliott-Yafet mechanism. The key improvement to earlier approaches is the use of a modified Stoner model with a dynamic exchange splitting between majority and minority bands. In the framework of our microscopic model, we find a novel feedback effect between the time-dependent exchange splitting and the spin-flip scattering. This feedback effect allows us to reproduce important properties of the demagnetization dynamics quantitatively. Our results demonstrate that in general Elliott-Yafet spin-flip scattering needs to be taken into account to obtain a microscopic picture of demagnetization dynamics.

4.
Phys Rev Lett ; 110(24): 245502, 2013 Jun 14.
Article in English | MEDLINE | ID: mdl-25165936

ABSTRACT

Ion tracks formed in amorphous Ge by swift heavy-ion irradiation have been identified with experiment and modeling to yield unambiguous evidence of tracks in an amorphous semiconductor. Their underdense core and overdense shell result from quenched-in radially outward material flow. Following a solid-to-liquid phase transformation, the volume contraction necessary to accommodate the high-density molten phase produces voids, potentially the precursors to porosity, along the ion direction. Their bow-tie shape, reproduced by simulation, results from radially inward resolidification.

5.
Phys Rev Lett ; 107(16): 165003, 2011 Oct 14.
Article in English | MEDLINE | ID: mdl-22107395

ABSTRACT

The femtosecond dynamics of the electrons in aluminum after an intense extreme ultraviolet pulse is investigated by Monte Carlo simulations. Transient distributions of the conduction band electrons show an almost thermalized, low-energy part and a high-energy tail. Constructing emission spectra from these data, we find excellent agreement with measurements. The radiative decay mainly reflects the colder part of the distribution, whereas the highly excited electrons dominate the bremsstrahlung spectrum. For the latter, we also find good agreement between predicted and measured energy scales.

6.
Opt Express ; 15(26): 17855-62, 2007 Dec 24.
Article in English | MEDLINE | ID: mdl-19551080

ABSTRACT

Control of two basic ionization processes in dielectrics i.e. photo ionization and electron-electron impact ionization on intrinsic time and intensity scales is investigated experimentally and theoretically. Temporally asymmetric femtosecond pulses of identical fluence, spectrum and pulse duration result in different final free electron densities. We found that an asymmetric pulse and its time reversed counterpart address two ionization processes in a different fashion. This results in the observation of different thresholds for surface material modification in sapphire and fused silica. We conclude that control of ionization processes with tailored femtosecond pulses is suitable for robust manipulation of breakdown and thus control of the initial steps of laser processing of high band gap materials.


Subject(s)
Aluminum Oxide/chemistry , Aluminum Oxide/radiation effects , Lasers , Manufactured Materials , Semiconductors , Silicon Dioxide/chemistry , Silicon Dioxide/radiation effects , Ions , Materials Testing
7.
Phys Rev Lett ; 92(18): 187401, 2004 May 07.
Article in English | MEDLINE | ID: mdl-15169529

ABSTRACT

We develop a model describing the free-electron generation in transparent solids under high-intensity laser irradiation. The multiple rate equation model unifies key points of detailed kinetic approaches and simple rate equations to a widely applicable description, valid on a broad range of time scales. It follows the nonstationary energy distribution of electrons on ultrashort time scales as well as the transition to the asymptotic avalanche regime for longer irradiations. The role of photoionization and impact ionization is clarified in dependence on laser pulse duration and intensity.

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