Exploration of plasmonic antennas, for sub-wavelength magneto-optical Kerr imaging

Abstract

This thesis outlines work performed with the intention of producing a novel near- field magneto-optical scanning microscope. This scanning microscope utilises a near field probe, produced through modification of existing atomic force micro- scopy (AFM) probes. In order to achieve the goal of strongly sub-wavelength res- olution imaging of magneto-dynamics, studies of planar plasmonic structures, and their interaction with magnetic materials were made. This was done in order to gain a better understanding of the complex interaction between plasmonic anten- nas, and magnetic materials. Investigations of planar systems, began with finite element modelling of the magneto optical Kerr effect (MOKE) effect, and its in- teraction with plasmonic structures. Initial modelling demonstrated the suitability of the commercial finite element modelling software ”COMSOL Multiphysics" for modelling magneto-optical effects. A series of plasmonic antennas were in- troduced to this model. The simplest of these (a gold disc of 140nm diameter) showed enhancement of the MOKE signal at resonance of up to 40x. A cut- cross antenna (consisting of two crossed cavities of 20nm width, and variable length and depth), which had been selected as a promising candidate for high field confinement, showed a generally smaller enhancement. However the field distribution from these structures was more suitable for the eventual near field microscopy applications. Similar real world structures were fabricated in mul- tilayer stacks consisting of P t(3nm)/ 4x[Co(0.5nm)/ P t(3nm)]/ T a2 O5 (various)/ 5 Au(100nm) layers. This was accomplished by focussed ion beam lithography (FIB) lithography through the gold layer. Magneto-optical characterisation of these structures was not possible, and this was believed to be due to FIB induced gal- lium poisoning of the magnetic layers. A gold floating technique was pursued in order to circumvent the gallium poisoning. New structures fabricated on gold films were shown to be resonant at optical wavelengths, through bright field trans- mission spectroscopic characterisation. However the floating technique in com- bination with the FIB beam produced capillaries that adhere to the underside of the gold film, which again prevent magneto-optical characterisation. Concurrent to the development of planar antenna structures, a platform for performing near field optical measurements of magnetic materials utilising an AFM, and modified probes was developed. This platform was used to obtain time resolved images of permalloy elements with a spatial resolution comparable to that achieved with a diffraction limited laser spot. A number of potential techniques for AFM probe modification that could be used to produce strongly sub-wavelength resolution time-resolved imaging have been explored.