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Title of the thesis: All-optical helicity-dependent switching in magnetoplasmonic structures

Thesis defender: Yann Le Guen
Opponent: Professor Liliana Buda-Prejbeanu, Grenoble Institute of Technology, France
Custos: Professor Sebastiaan Van Dijken, Aalto University School of Science

As the demand for fast, high-capacity data storage keeps increasing, novel approaches are needed to meet this demand while also aiming for more energy-efficient solutions. One promising field is spintronics, which studies the use of the spin of electrons instead of their charge for information transport, storage and processing.

One recent and expanding approach is the use of ultrafast optical pulses to manipulate the magnetisation of devices. This ultrafast spintronics could lead to high-speed storage devices and is critical to the development of optical computing. This thesis focuses on all-optical helicity-dependent switching, or AO-HDS, which is a principle that allows the control of magnetisation using ultrafast circularly polarised light pulses. The origin and mechanism behind this switching are still debated, with two main effects considered by the community: magnetic circular dichroism (MCD) and the inverse Faraday effect (IFE). Their separation would allow for further optimisations and the integration of this effect in devices.

To tackle this problem, this thesis studies AO-HDS in magnetic plasmonic structures. Plasmonics, which is the study of the electromagnetic fields in metallic nanostructures, allows the confinement and amplification of light at sub-wavelength scales through resonating effects. These plasmonic resonances impact MCD and IFE differently and are used in this work to disentangle their effects on the switching.

Nanodiscs systems made of Co/Pt multilayers were successfully fabricated and the measurement of their optical and MCD spectra show that around the resonance wavelength the absorption, associated to the IFE, reaches a maximum while the MCD crosses zero and changes sign. Wavelength-dependent AO-HDS experiments showed an inversion of the switching behaviour near the resonance wavelength. The shape of the spectra revealed that both IFE and MCD play a role in AO-HDS, with MCD being the dominant effect.

Overall, this work shows the successful integration of AO-HDS in magneto-plasmonic systems and how the plasmonic effect can impact the switching behaviour. This work also highlighted the central role of MCD in AO-HDS. This can be used to optimise future systems by maximising their MCD.

Keywords: Magnetism, Ferromagnet, Magneto-optics, Plasmonics, Nanoparticles, Magnetoplasmonic, Ultra-fast spintronics, Pulsed laser, All-optical helicity-dependent switching

Thesis available for public display 7 days prior to the defence at .