Who: Alexandr Alekhin, Nanomagnetism Group
Place: nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
Date: Tuesday, 7 January 2020, 11:00
Ultrafast dynamics at optically-excited magnetic nanostructures
Interaction of femtosecond (fs) laser pulses with magnetic materials result in an immense variety of physical phenomena from different area of physics: nonlinear optics, ultrafast spintronics, acoustics, physics of shock waves and/or laser-induced phase transitions. From a metrological perspective, different time scales associated with those phenomena can be measured with fs temporal resolution in a variety of conventional linear and nonlinear optical pump-probe experiments. However, absorption of fs laser pulse by an opaque magnetic material may trigger different coherent and incoherent processes involving simultaneously electrons, phonons, and magnons, thereby rendering identification of the underlying mechanisms extremely challenging. Complexity of the interpretation of ultrafast optical measurements can be reduced by the investigation of the experimentally accessible monochromatic excitations and their interactions: periodic oscillations of electromagnetic fields at the optical frequencies, elastic deformations at MHz-THz frequencies and perturbations of the magnetic order like the ferromagnetic resonance (FMR) or exchange-coupled magnon modes oscillating at GHz-THz frequencies.
Monochromatic acoustic waves can be generated by fs laser excitation of periodic gratings, either in the so-called transient grating geometry [1-2] or using permanent gratings . Characteristic feature in these experiments is the possibility to excite monochromatic surface acoustic waves (SAWs) with frequencies tunable by the grating periodicity up to a few tens of GHz in the case of sub-wavelength periodic structures which can be treated as meta-surfaces. Proper combination of the grating periodicity and the magnitude of an external magnetic field may fulfill the resonance conditions for the coupling between the FMR frequency and SAWs, thereby resulting in the enhancement of the FMR precession [1-3], with the onset of parametric instabilities . Whereas excitation of large-amplitude FMR precession through the laser-mediated resonant magneto-elastic interactions have not yet been optimized, such possibility would open a new avenue to modulate optical properties of magnetic meta-surfaces and help to develop real-life applications with magnetic meta-surfaces operated on ultrafast time scales [4-5].
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