All-optical helicity-dependent switching in magnetoplasmonic structures

Abstract

As the demand for high-speed storage keeps increasing, its associated energy consumption has become a global concern. An alternative to traditional charge-based high-speed storage comes in the form of spintronic devices. By storing information in a non-volatile manner using magnetisation, these devices cut down on the energy needed to maintain data. This concept is already implemented in hard-disk drives and magneto-resistive RAM. A new technological solution in the form of ultra-fast light-induced magnetisation control has the potential to further increase writing speed and energy efficiency.

Research in this emerging field has resulted in the discovery of all-optical helicity-dependent switching (AO-HDS), which enables the local control of the magnetisation in ferromagnetic thin films through the use of multiple circularly polarised femtosecond laser pulses. The mechanism behind this phenomenon is still under investigation. Two effects with similar observable impacts are being considered: magnetic circular dichroism (MCD) and the inverse Faraday effect (IFE). Plasmonic systems resonantly couple the oscillation of electrons at a metal surface to an electromagnetic field. This gives them the ability to confine light to sub-wavelength-scale particles, leading to a significant enhancement of light-matter interactions. Metallic nano-particles such as nano-disks, exhibit plasmonic resonances that can be further amplified by arranging them into periodic arrays. The resonance wavelength can be precisely tuned by adjusting the disk shape, size, and array periodicity.

This thesis investigates the origin of AO-HDS through the design, simulation, fabrication and characterisation of periodic and aperiodic distributions of Co/Pt multilayer nano-disks. These magneto-plasmonic systems, which incorporate magnetic materials in plasmonic designs, feature distinct resonances in their magneto-optical response. Optical and magneto-optical spectrometry reveals that, at the plasmonic resonance wavelength, Kerr and Faraday rotations, linked to the IFE, reach a maximum, while the ellipticities, associated with MCD, cross zero and change sign. This contrasting behaviour enables spectral separation of these two effects.

Wavelength-dependent AO-HDS experiments demonstrate partial magnetisation switching. For aperiodic systems, the final magnetisation crosses zero and changes sign near the plasmonic resonance wavelength. This behaviour is characteristic of MCD, highlighting its central role in AOHDS. However, a shift in the zero-crossing between the switching and MCD spectra suggests an additional IFE contribution.

As part of this work, the laser setup was automated and enhanced, leading to the development of a novel technique for visualising and characterising arbitrary shaped laser spots. This advancement enables more precise and systematic measurements of all-optical switching state diagrams.