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Controlling light with light: exploiting fast free-electron nonlinearities in plasmonic metamaterials for the control of light polarisation

Student thesis: Doctoral ThesisDoctor of Philosophy

The manipulation of the properties of light at rates faster than possible with current electronic approaches is a potential revolutionary step in many areas of technology, including increasing data transfer speeds and investigating femtosecond timescale chemical processes. The Kerr-type nonlinearity provides a way to alter the optical response of metal-based nanostructures at ultrafast timescales using light. A control light pulse, of sufficient intensity, can be used to excite the electron gas of the metal altering its Fermi distribution. This excitation changes the permittivity such that a delayed signal pulse, respective to the control, experiences a different optical environment relative to the state before excitation. To enhance the efficiency of the process plasmonic metamaterials, displaying high field enhancement and designed dispersion, can be employed to achieve large optical changes at relatively modest control light intensities. In this thesis the control of the polarisation of light is investigated by combining free-electron nonlinear effects with a strongly anisotropic plasmonic metamaterial based on a self-assembled array of gold nanorods embedded in an alumina matrix. As demonstrated by experiment, this approach can achieve greater than 60ff rotation of the output polarisation ellipse on control light excitation with a switching rate of 0.3 THz: an order of magnitude faster than commercially available polarisation modulators, such as Faraday rotators or Pockels cells. The effect can be used both in transmission and reflection configurations. Without external excitation, the signal pulse is found to alter its own polarisation state, depending on the signal intensity, due to nonlinear self-action, with important consequences for nonlinear pulse shaping. The dynamic behaviour of the metamaterial structure has also shown to be tuneable by taking advantage of the anisotropic geometry of the nanorods influencing the electron temperature dissipation. Using this approach material limitations can be circumvented allowing fast active polarisation control in nanophotonic devices.
Original languageEnglish
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Award date1 May 2019

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