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In functional magnetic nanostructures the acoustic, magnetic and plasmonic excitations can co-exist and interact on the nanometer spatial and ultrafast time scales. Optical spectroscopy with femtosecond laser pulses highlights a variety of nontrivial spatio-temporal dynamics, which are not only used to monitor individual excitations in real time, but also study interaction mechanisms between them, often observed in frequency mixing phenomena. Whereas the second harmonic generation (SHG), sum- and different frequency mixing in the optical frequency range originate from the chi(2)-nonlinearities, in acoustics and magnetism they are often dominated by parametric resonances, where system parameters are modulated at frequencies comparable to the natural oscillation frequencies, typically in the MHz-GHz range. Keeping in mind the intrinsic differences in the physical nature and frequency range of these phenomena, here we discuss, in a comparative manner, two examples of frequency mixing in magneto-plasmonics [1,2] and magneto-acoustics [3,4]. The ability to experimentally tune both systems through Surface Plasmon Resonance (SPR) and Ferromagnetic Resonance (FMR) as well as to theoretically describe these resonant interactions within the framework of phenomenological models based on the Lorentz oscillator, represent the key idea behind this presentation. In nonlinear magneto-plasmonic experiments, the Kretschmann configuration SPR resonances for different optical wavelengths occur at different angles Fig. (a,b), offering the unique possibility to match the fundamental and SHG resonances. In magneto-plasmonic Au/Co/Ag/glass samples the plasmonically assisted SHG also depends on the direction of magnetization M in ferromagnetic cobalt, which can be reversed with a weak external magnetic field. A simple model utilizing the resonant plasmonic enhancement of the chi(2)-susceptibility confirms the experimental observation that magnetic effects are most pronounced between the SHG and fundamental SPR resonances [1,2]. In the second experiment [3,4] the magnetization in a Ni/glass sample is excited by two distinct transient surface acoustic waves (SAW and SSLW). Magnetic tuning of the FMR frequency in resonance to their SHG, sum- and difference frequencies demonstrates the full variety of frequency mixing phenomena Fig. (c). In contrast to nonlinear optics, the frequency mixing is dominated by the parametric effect in the externally driven FMR oscillator. An analytical model based on the resonant enhancement of frequency-mixed signals explains the experimental observations [4]. A practical application of these findings to magneto-elastic switching in bi-stable systems, i.e. nickel nanomagnets, is under way [5]. Whereas the use of phenomenological, non-microscopic modeling may fail to capture fine details, it represents a useful tool for experimentalists measuring macroscopic quantities such as magnetization dynamics and/or optical nonlinearities. The presented methodology can further exploited to design experimental architectures based of coupled resonances of different nature (acoustic, SPR, FMR, excitons in semiconductor quantum dots, nanomagnets) providing a playground for future optical metrology and the physics of nano-devices [5,6]. References: [1] I. Razdolski et al., ACS Photonics 3, 179 (2016) [2] V.V. Temnov et al., J. Opt. 18, 093002 (2016) [3] J. Janusonis et al., Phys. Rev. B 94, 024415(2016) [4] C.L. Chang et al., Phys. Rev. B 95, 060409 (2017) [5] V.S. Vlasov et al., Multifunctional Materials (to appear in 2018) [6] D.A. Kuzmin et al., Nanophotonics 7, 597 (2018)

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Vasily Temnov, Uni LeMans
HS II
Contact: Paul van Loosdrecht