Plasmonically-enhanced microbolometric sensing and metasurface phased arrays based on the phase transition of vanadium dioxide

Abstract

Solid-solid phase transition materials are a fascinating and diverse topic in the field of condensed matter physics. These extraordinary materials have exotic electronic properties that allow them to switch between distinct phases, each of which may have very different material properties. The implications of this may present, for example, in terms of their optical transmission, thermal characteristics and electrical behaviour. The allure of these materials has spawned an enormous amount of research which aims to understand, classify, and exploit the transition mechanisms in useful ways. The aim of this thesis is to explore novel applications for vanadium dioxide (VO2), a material with a close-to-room temperature phase transition that can behave either as an insulator or as a metal. The first application is improving the performance of microbolometric infrared detectors by means of integrating plasmonic nanoparticles with VO2. Surface plasmons are coherent oscillation modes of plasma density which can be excited at the interface between a material with negative real permittivity and a material with positive real permittivity, most commonly a metal and dielectric, respectively. In plasmonic nanoparticles, localised surface plasmons can be directly excited by incident electromagnetic radiation. Localised surface plasmon resonance modes arise at specific frequencies depending on the permittivity and geometry of the nanoparticle and its surroundings and, for anisotropic geometries, are also polarisation-sensitive. Resonance amplitude, bandwidth and frequency are therefore highly tuneable by a variety of means. The resonance condition is characterised by concentration of the electric near field and enhanced absorption and scattering cross-section. Plasmonic nanoparticles can thus be used in a variety of ways, for example to control light at the nanoscale, efficiently convert light into heat and modulate forward or backward scattering. The flexibility and utility presented by localised surface plasmons has led to a wealth of research into reconfigurable metamaterials and tuneable plasmonic devices, and applications have been found in such diverse fields as medicine, photodetection, sensing and catalysis. The interplay between plasmonic nanostructures and phase change materials is an emerging field, largely because 1.) phase change materials display strong contrast in optical and electronic properties and their behaviour can be strongly modulated by temperature, electromagnetic fields, mechanical forces etc.; 2.) plasmons can be used to modulate the phase transition behaviour and thus effective properties of the material; 3.) controlling the optoelectronic properties of phase change materials allows modulation of plasmon resonance characteristics; 4.) nanoparticles can be designed as localised heaters, frequency filters and polarimeters. Applying these principles to microbolometric photodetection could improve performance and functionality of devices, thus broadening the low-cost photodetector market and greatly benefitting users. The first investigation involved measurement of the optical transmission spectrum (400 – 900 nm) and electric resistivity of a sample consisting of a silver nanorod array / VO2 / sapphire. Localised surface plasmon resonance was identified by a strong dip in transmission (~30 %) and it was found to correlate with a dip in resistivity (~0.3 Ωcm). The resistance change was attributed to the thermoplasmonic effect, whereby high concentration and dissipation of electromagnetic energy by nanoparticles leads to localised heating in the film. The amplitude, position and shape of plasmon resonance peaks, as identified by transmission spectra, was observed to be strongly polarisation dependent. The second work further investigated the thermoplasmonic effect in embedded gold nanorods between 2 – 16 m by quantifying the suppression of VO2 effective phase transition temperature. Effective phase transition temperature suppression of up to 4 °C was observed and greatest suppression corresponded to highest nanorod density. Transmission spectra showed that localised surface plasmon resonance can affect the effective material properties of VO2 film far from the resonance wavelength. The findings demonstrate a reversible, in-situ method for tuning the effective phase transition temperature, significant for lowering energy requirements and device optimisation. The observation of hysteresis in transmission spectra during the second work, as previously reported in literature, provided the motivation for the third investigation. Hysteresis is an important consideration in device design; it is sought after for memory because it allows multiple states to be stored; however, in sensing it is avoided, because typically sensor readout should not depend on the history of the system. To further investigate hysteresis in VO2 films it was necessary to numerically model its behaviour. A modified version of the Maxwell Garnett effective medium approximation was developed and numerically tested using the Fresnel equations. The modified equation was justified by a novel approach to the VO2 phase transition. Although intended as a phenomenological approach to interpreting hysteresis and intermediate states, experimental validation is required. Potentially the work can be used as a simple-to-implement model for hysteresis modelling in phase change materials. The final project in this thesis used numerical analysis to investigate the potential for VO2 as an active element in a metasurface phased reflectarray designed to control the deflection angle of an incident infrared beam. Ultimately a successful design using justifiable VO2 parameters was not found, but some success was found by adjusting the ‘polarisation factor’ used in the material permittivity model. This suggests that perhaps with a different combination of materials, such a design could be possible and the design process used in future work. Following the electromagnetic numerical analysis, a thermal analysis was conducted to investigate if thermal isolation could help maintain specific temperatures for each element, a feature necessary for this type of beam steering platform. These simulation results suggest that etching trenches 1 μm deep could improve both intra-element temperature variation and overall power efficiency of such a device.

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ORCID for Xu Fang: orcid.org/0000-0003-1735-2654

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