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Physics of femtosecond laser-excited magnetic meta-surfaces

Astelehena, Apirilak 15, 2019 - 11:00
nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
Vasily Temnov, CNRS-Le Mans Univ. , France
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Femtosecond laser interactions with magnetic materials result in an immense variety of physical phenomena from different area of physics: nonlinear optics, magnetism, 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 femtosecond temporal resolution in a variety of conventional linear and nonlinear-optical pump-probe experiments. However, in a typical pump-probe experiment, absorption of an ultrashort, femtosecond laser pulse by an opaque magnetic material simultaneously triggers a variety of coherent and incoherent dynamics of elementary excitations (electrons, phonons, magnons etc.). evolving on several, sometimes comparable time scales, rendering the identification of the underlying physical phenomena extremely challenging. 
One of the ways to reduce the complexity of ultrafast optical measurements is to study the experimentally accessible monochromatic excitations and their interactions. To be more specific, here we are talking about the periodic oscillations of electromagnetic fields at the (fundamental, second harmonic, third harmonic etc.) optical frequencies, elastic deformations (surface acoustic waves) at MHz-THz frequencies and time-dependent perturbations of the magnetic order (ferromagnetic resonance, magneto-static or exchange-coupled magnon modes) oscillating at GHz-THz frequencies. 
Apart from a trivial case of oscillating optical fields, the temporal periodicity of fs-laser-induced magnetic and acoustic dynamics is not granted. For example, the absorption of an ultrashort optical pulse in a magnetic material results in a famous but aperiodic phenomenon of ultrafast demagnetization [1]. However, under specific conditions dictated by the orientation of an external magnetic field ultrafast demagnetization can trigger ferromagnetic resonance (FMR) precession and oscillating spin-wave resonances at elevated frequencies in ferromagnetic thin films [2,3]. 

At the same time fs-laser excitation of opaque materials is signified by the thermo-elastic generation of single-cycle acoustic pulses with picosecond time duration [4], which may reach giant strain amplitudes up to 1%, strong enough to induce the nonlinear lattice dynamics at the nano-scale [5] or even switch magnetization in magnetostrictive thin films [6] and ferromagnetic nanostructures [7]. 
Monochromatic acoustic waves can be generated by fs-laser excitation of periodic gratings, either in the so-called transient grating geometry [8-9] or using permanent gratings [10-12]. The characteristic feature in these experiments is the possibility to excite monochromatic surface acoustic waves (SAWs) with frequencies tunable by the grating periodicity and going up to a few tens of GHz when using deeply sub-wavelength periodic structures with periods of the order of 100 nm. This sub-wavelength spatial periodicity for magneto-acoustic studies represents the link between ultrafast magneto-acoustics and (magneto-)optics of meta-surfaces. The intrinsic possibility to bring the FMR-frequency in resonance with acoustic waves, for example using a proper combination of grating periodicity and the magnitude of an external magnetic field, can result in the resonant enhancement of FMR precession [8-9,12], with the onset of parametric instabilities [9]. 
Whereas the experimental conditions to obtain large-amplitude FMR precession through the fs-laser mediated resonant magneto-elastic interactions have not yet been optimized, such possibility would open a new avenue to modulate the optical properties of magnetic meta-surfaces. Given the case that the static nonlinear magneto-optical and/or magneto-plasmonic effects are giant as compared to the linear ones [13-16], it makes sense to do beyond the time-resolved measurements based on linear magneto-optical effects and probe the dynamics of resonant magneto-acoustic interactions [8-10,12] with nonlinear magneto-optical detection schemas [13-16], hoping to develop real-life applications with magnetic meta-surfaces modulated on ultrafast time scales.

Whereas the discussed phenomena have been investigated on periodic nanostructures produced my lithographic techniques, their true potential can be explored while studying fs-laser produced periodic nanostructures on magnetic materials. Many physical properties (not accessible by conventional high-resolution imaging techniques) such as hidden periodicities of buried interfaces, sub-surface inhomogeneities of elastic properties and magnetic anisotropies, linear and nonlinear optical diffraction efficiencies etc. could be extracted from the magneto-optical and magneto-acoustic measurements. 
The work was financially supported by Strategie internationale Pays de la Loire "NNN-Telecom", ANR-DFG "PPMI-NANO" (ANR-15-CE24-0032 and DFG SE2443/2), PRC CNRS-RFBR "Acousto-magneto-plasmonics", Deutsche Forschungsgemeinschaft (AL2143/2-1). 
[1] E. Beaurepaire at al., Phys. Rev. Lett.76: 4250, 1996. [2] M. van Kampen et al., Phys. Rev. Lett. 88: 227201, 2002. [3] R. Salikhov et al., Phys. Rev. B 99: 104412, 2019.  [4] C. Thomsen et al., Phys. Rev. B 34: 4129, 1986. [5] V. V. Temnov et al., Nature Comm. 4: 1468, 2013. [6] O. Kovalenko et al., Phys. Rev. Lett. 110: 266602, 2013. [7] V.S. Vlasov et al., Phys. Rev. B, 2019 (under review) [8] J. Janusonis et al., Phys. Rev. B 94: 024415, 2016. [9] C. Chang et al., Phys. Rev. B 95: 060409, 2017. [10] A. Comin et al., Phys. Rev. Lett. 97 : 217201, 2006. [11] C. Giannetti et al., Phys. Rev. B 76 : 125413, 2007. [12] C. Chang et al., Phys. Rev. Applied 10 : 034068, 2018. [13] I. Razdolski et al., ACS Photonics 3: 179, 2016. [14] V.V. Temnov et., J. of Optics 18, 0903002, 2016. [15] L. Michaeli et al., Phys. Rev. Lett.118: 243904, 2017. [16] M. Tran et al., Phys. Rev. B 98: 245425, 2018.

Host: P. Vavassori

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