145 research outputs found
Effect of iron thicknesses on spin transport in a Fe/Au bilayer system
This paper is concerned with a theoretical analysis of the behavior of
optically excited spin currents in bilayer and multilayer systems of
ferromagnetic and normal metals. As the propagation, control and manipulation
of the spin currents created in ferromagnets by femtosecond optical pulses is
of particular interest, we examine the influence of different thicknesses of
the constituent layers for the case of electrons excited several electronvolts
above the Fermi level. Using a Monte-Carlo simulation framework for such highly
excited electrons, we first examine the spatio-temporal characteristics of the
spin current density driven in a Fe layer, where the absorption profile of the
light pulses plays an important role. Further, we examine how the combination
of light absorption profiles, spin-dependent transmission probabilities, and
iron layer thicknesses affect spin current density in a Fe/Au bilayer system.
For high-energy electrons studied here, the interface and secondary electron
generation have a small influence on spin transport in the bilayer system.
However, we find that spin injection from one layer to another is most
effective within a certain range of iron layer thicknesses
An adaptive model for the optical properties of excited gold
We study the temperature-dependent optical properties of gold over a broad
energy spectrum covering photon energies below and above the interband
threshold. We apply a semi-analytical Drude-Lorentz model with
temperature-dependent oscillator parameters. Our approximations are based on
the distribution of electrons over the active bands with a density of states
provided by density functional theory. This model can be easily adapted to
other materials with similar band structures and can also be applied to the
case of occupational nonequilibrium. Our calculations show a strong enhancement
of the intraband response with increasing electron temperature while the
interband component decreases. Moreover, our model compares well with density
functional theory-based calculations for the reflectivity of highly excited
gold and reproduces many of its key features. Applying our methods to thin
films shows a sensitive nonlinear dependence of the reflection and absorption
on the electron temperature. These features are more prominent at small photon
energies and can be highlighted with polarized light. Our findings offer
valuable insights for modeling ultrafast processes, in particular, the pathways
of energy deposition in laser-excited samples
Laser ablation of silicon with THz bursts of femtosecond pulses
In this work, we performed an experimental investigation supported by a theoretical analysis, to improve knowledge on the laser ablation of silicon with THz bursts of femtosecond laser pulses. Laser ablated craters have been created using 200 fs pulses at a wavelength of 1030 nm on silicon samples systematically varying the burst features and comparing to the normal pulse mode (NPM). Using bursts in general allowed reducing the thermal load to the material, however, at the expense of the ablation rate. The higher the number of pulses in the bursts and the lower the intra-burst frequency, the lower is the specific ablation rate. However, bursts at 2 THz led to a higher specific ablation rate compared to NPM, in a narrow window of parameters. Theoretical investigations based on the numerical solution of the density-dependent two temperature model revealed that lower lattice temperatures are reached with more pulses and lower intra-burst frequencies, thus supporting the experimental evidence of the lower thermal load in burst mode (BM). This is ascribed to the weaker transient drop of reflectivity, which suggests that with bursts less energy is transferred from the laser to the material. This also explains the trends of the specific ablation rates. Moreover, we found that two-photon absorption plays a fundamental role during BM processing in the THz frequency range
Optical properties of gold after intense short-pulse excitations
Intense ultrashort laser pulses can create highly excited matter with extraordinary properties. Experimental and theoretical investigations of these extreme conditions are very complex and usually intertwined. Here, we report on a theoretical approach for the electron scattering rates and the optical properties in gold at elevated temperatures. Our theory is based on the degree of occupancy of the conduction band as well as inputs from ab initio simulations and experimental data. After the electron system has reached a quasi-equilibrium, the occupancy is fully determined by the electron temperature. Thus, our approach covers the important relaxation stage after fast excitations when the two-temperature model can be applied. Being based on the electronic structure of solids, the model is valid for lattice temperatures up to melting but the electron temperature might exceed this limit by far. Our results agree well with recent experimental data for both the collision frequencies and the conductivity of highly excited gold. Scattering of sp-electrons by d-electrons is found to be the dominant damping mechanism at elevated electron temperatures and depends strongly on the number of conduction electrons, hence, revealing the microscopic origin of the conductivity change after heating. The supportive benchmarks with experiments are very valuable as the underlying scattering rates determine a number of other transport, optical and relaxation properties of laser-excited matter
Wavelength-dependent reflectivity changes on gold at elevated electronic temperatures
Upon the excitation by an ultrashort laser pulse the conditions in a material
can drastically change, altering its optical properties and therefore the
relative amount of absorbed energy, a quan- tity relevant for determining the
damage threshold and for developing a detailed simulation of a structuring
process. The subject of interest in this work is the d-band metal gold which
has an absorption edge marking the transition of free valence electrons and an
absorbing deep d-band with bound electrons. Reflectivity changes are observed
in experiment over a broad spectral range at ablation conditions. To understand
the involved processes the laser excitation is modeled by a com- bination of
first principle calculations with a two-temperature model. The description is
kept most general and applied to realistically simulate the transfer of the
absorbed energy of a Gaussian laser pulse into the electronic system at every
point in space at every instance of time. An electronic temperature-dependent
reflectivity map is calculated, describing the out of equilibrium reflectivity
during laser excitation for photon energies from 0.9 - 6.4 eV, including inter-
and intra-band transi- tions and a temperature-dependent damping factor. The
main mechanisms are identified explaining the electronic temperature-dependent
change in reflectivity: broadening of the edge of the occu- pied/unoccupied
states around the chemical potential , also leading to a shift of the
and an increase of the collision rate of free s/p-band electrons with
bound d-band holes
Latent ion tracks in amorphous silicon
We present experimental evidence for the formation of ion tracks in amorphous Si induced by swift heavy-ion irradiation. An underlying core-shell structure consistent with remnants of a high-density liquid structure was revealed by small-angle x-ray scattering and molecular dynamics simulations. Ion track dimensions differ for as-implanted and relaxed Si as attributed to differentmicrostructures andmelting temperatures. The identification and characterization of ion tracks in amorphous Si yields new insight into mechanisms of damage formation due to swift heavy-ion irradiation in amorphous semiconductors
Ultrafast optically induced spin transfer in ferromagnetic alloys
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
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