Phase-Change Meta-Devices for Tuneable Bandpass Filtering in the Infrared

Abstract

Tuneable light filters, especially those which are compact and fast to tune, are essential in a wide range of technologies, especially for multispectral imaging applications. However, state-of-the-art approaches to create such filters all possess drawbacks, with many wavelength regions poorly served. This thesis attempts to address this problem by combining metasurfaces which support extraordinary optical transmission (ultra-thin band-pass filters) with chalcogenide phase-change materials (adding dynamic tuneability). The optical properties of phase-change materials are very different in their amorphous and crystalline states and switching between such states can be rapidly controlled via thermal excitations. In this work nine different phase-change materials, including alloys of GeTe, GeSbTe, GeSbSeTe and GaLaS, were optically and elementally characterised and assessed for their application-specific suitability. The resulting materials data was used to computationally design and evaluate a range of tuneable infrared filter device designs both optically and thermally. These filters exhibit high transmission (≈80% at best) with large spectral tuning ranges of approximately +50% relative to their shortest wavelength; this range is sufficient to cover entire atmospheric transmission windows. This is the first such combination of phase-change materials and extraordinary optical transmission for application from the visible through to long-wave infrared (14 μm) regions of the spectrum. A rigorous computational study was conducted to produce comprehensive design guidelines for such filters, and confirm the viability of in-situ electrical switching. Several filter devices were experimentally fabricated, and the viability for a number of applications, including tuneable filtering, chemical sensing and infrared displays, was investigated and confirmed computationally.