Post-fabrication tuning of Hybrid Plasmonic Nanostructures with VO2

Abstract

The development of hybrid plasmonic nanostructures and metamaterials to control electromagnetic radiation on the nanoscale has been at the centre of scientific research for the past few decades. Utilising the surface plasmon resonances arising from noble metal nanoparticles is attractive due to the diverse range of applications within the field of nanophotonics. However, the plasmonic response of these nanostructures is heavily dependent on factors set during the fabrication process, such as the combination of materials used as well as there structural dimensions. The inability to dynamically tune the optical response of such nanostuctures post-fabrication imposes serious limitations on potential applications within optoelectronic systems. To overcome this, tunable plasmonic elements consisting of metallic nanoparticles on a thin layer of vanadium dioxide VO2, a phase change material, are considered. VO2 is an attractive option as a phase change material due to its large reversible transition from a semiconducting phase to a metallic phase at the critical temperature of 68? C, close to room temperature.

The VO2 phase change is used to dynamically tune the plasmonic response of coupled nanoparticles within the visible and near IR spectral regions.

The modulation of the reflectance, transmittance and absorptance spectra of VO2 thin films with thicknesses between 5 nm - 200 nm is investigated with significant blueshifts in the optical response within the visible spectral range. By implementing a metallic backreflecting layer a larger contrast in the optical spectra of the VO2 thin film is demonstrated with a large change in the CIE colour in each case. Plasmon resonance wavelength shifts of over 600 nm are observed in the near-IR spectral region upon the transition of the VO2 layer from the semiconducting phase to the metallic phase. This is significantly larger than the wavelength shifts reported in this spectral range using other phase change materials. Additionally, significant increases in the scattering cross sections are observed for VO2 film thicknesses between 30 - 50 nm as well as enhancement of the plasmonic hotspots surrounding Au nanoparticles. Au Bowtie Dimer structures demonstrate a 3.6-fold increase in the E-field intensity within the dimer gap accompanied by a reshaping of the spatial profile of the E-field enhancement providing a mechanism for tunable metasurfaces.

Finally, a method to dynamically tune luminescence enhancement in the near infrared spectral range by coupling metal nanostructures to VO2 thin films. A numerical model is employed to calculate the emission of quantum emitters in a hybrid system comprising a single silver (Ag) nanodisc on top of a thin layer of VO2. Through the optimisation of this system, a luminescence enhancements greater than 4 can be seen in the metallic VO2 compared to the semiconducting phase allowing for compensation of thermal quenching of up to 70% between room temperature and 70? C. This renders the hybrid system a promising candidate for improved photon management in optoelectronic devices where elevated temperatures minimize the efficiency of such devices.

The research presented in this thesis demonstrates how hybrid plasmonic nanostructures, incorporating a thin layer of VO2, can allow for dynamic modulation of reflectance, transmittance, absorptance and scattering spectra as well as enhancement of the localised E-field and emission of coupled quantum emitters.