Magneto-optical Kerr effect with non-uniform optical polarisation

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The magneto-optical Kerr effect (MOKE) is widely used to characterise the magnetic behaviour or state of materials. Uniform beam polarisations are invariably used in MOKE measurements, whether as a parallel beam for thin film measurements or a focussed beam for measurements of small magnetic structures. Observing MOKE from in-plane magnetisation requires an oblique angle of incidence, with higher signals generally obtained with larger angles of incidence. This leads to a compromise being reached between spatial resolution and signal. Tightly focussed light beams are not purely transverse and can have a component of longitudinal electric field in the vicinity of the focus. Radially polarised light beams can significantly increase this effect and have a much reduced focal spot size for the longitudinal field component. We propose to use the longitudinal electric field associated with tightly focussed radially-polarised laser beams to perform normal-incidence MOKE measurements on materials with in-plane magnetisation. This will allow exquisite spatial resolution of magnetic thin films and nanostructures, plus the unprecedented ability to depth-resolve magnetisation. Optimising this will require a new framework to relate the relevant physics of MOKE with non-uniform optical polarisation. The three advantages of radial-polarisation MOKE (R-MOKE) are:1. An order of magnitude increase in the longitudinal component of electric field (which couples to the in-plane magnetisation to generate a Kerr signal)2. A reduction of the transverse spot size affording higher spatial resolution3. The unprecedented ability to depth-resolve magnetisation in MOKE measurements.