Scientists of the Faculty of Physics (research group of Assoc. Prof. Tatiana Murzina) in collaboration with the group of Prof. Alexey Kimel (Radboud Univ., the Netherlands) have studied the dynamics of the laser-induced anisotropy in a 2D lattice of metal nanoparticles on a dielectric substrate.
Temporal characteristics of optical response of magnetic nanostructures are of high interest due to wide perspectives of applications as well as because of a large variety of new interesting effects that exist in such structures. From practical point of view, the tendency of an increase of the recording and reading speed of magnetic memories requires to study the dynamics of their magnetic response with high temporal resolution. Optical pump-probe technique has recommended itself as a powerful diagnostics tool on femto- and picosecond timescales. This method is based on the usage of two laser beams, the first high-power one (pump) gets a system out of the equilibrium, while the second low-power (probe) pulse probes the state of a magnetic system after a short strictly-controlled time interval.
Recently a group of scientists from Physics Department of Lomonosov Moscow State University and Radbound University (Nijmehen, the Netherlands) studied the dynamics of optical response of a 2D square array of cobalt nanodots on a fused quartz substrate. The dynamics of polarization plane rotation of the radiation after its interaction with such a structure was investigated. The period of a 2D lattice was 1.4 microns, the lateral size of single dot being 600 nm and its height of 30 nm. The pump-probe polarization-resolved studies were performed using an output of a high-power femtosecond laser set-up. As a result, we have fond that several mechanisms determine the polarization state of the structure, each of them being characterized by a definite relaxation time. Namely, on subpicosecond delay time the key role is played by the laser-induced birefringence induced by the fast Kerr-type nonlinearity and by ultrafast demagnetization of metal nanodots. At the timescales of about hundreds of picoseconds the optical response is determined by the magnetization recovery and acoustic excitations within the metal nanoparticles. For the delay time longer than 500 ps the polarization plane rotation is determined by the excitation of acoustic waves in the quartz substrate, the period of these waves being induced by the periodic 2D structure. Performed studies of the dynamics of these effects helps for a better understanding of physics of metal nanostructures.
The results of this work have been published in the paper I. Razdolski, V.L. Krutyanskiy, T.V. Murzina, Th. Rasing, A.V. Kimel, Femtosecond laser-induced optical anisotropy in a 2D lattice of magnetic dots. Phys. Rev. B 89, 064306 (2014).