Abstract
Controlling light-matter interactions has applications ranging from sensing to communications. Metamaterials provide a tailorable route to manipulating these light-matter interactions. However, their optical response is usually fixed during fabrication. To modulate their response, it is necessary to design nano elements that can mechanically change their configuration or use materials with a changeable refractive index. Chalcogenide phase change materials (PCMs) can exhibit tuneable optical constants and promise, therefore, a route toward active photonics. In particular, certain PCMs exhibit high optical contrast, fast switching speed, good cyclicality, as well as non-volatile switching, thus are deemed suitable candidates for tuning photonic devices. The objective of this thesis is to answer the research question whether phase change materials are useful for tuning photonic devices. If so, the thesis aims to examines the applicability of different types of phase change materials to different types of photonic devices. Metamaterials using Ge2Sb2Te5 have been designed and fabricated to demonstrate a resonant tuneability over a broad frequency band in the mid-infrared (M-IR) from 3 to 6 µm. By switching the structural state of the Ge2Sb2Te5, the metamaterial’s response can be tuned post-fabrication. This tuneable metamaterial configuration is well suited to the M-IR spectrum but is not suitable for the visible spectrum because Ge2Sb2Te5 strongly absorbs visible light. This isseen when Ge2Sb2Te5 is used to achieve a wide-band high absorption in the visible frequency by combing Ge2Sb2Te5 with plasmonic Al nanograting arrays. Though different colours are achieved, the colours are less bright and vivid due to the high absorption of Ge2Sb2Te5. In order to overcome the high absorption of well-studied PCMs, such as Ge2Sb2Te5, in the visible spectrum, tuneable optical structures based on Sb2S3 was developed. Sb2S3 has a wide optical band gap and exhibits a large iv !Re(n) at visible frequencies. Sb2S3 was exploited in a simple multi-layered optical resonator, and by inducing a phase transition in Sb2S3, the resonant frequency was tuned. The maximum resonance redshift was 110 nm. The phase transition of Sb2S3 was invoked by both optical and electrical heating. This new phase change material has applications that range from displays to beam steering. Ag doping was explored to decrease the crystallisation temperature and crystallite size of Sb2S3. And in the process of studying the properties of Ag-doped Sb2S3, it is serendipitously found that co-sputtering Ag and Sb2S3 could form Agx(Sb2S3)1-x nanostructures automatically. These nanostructures can be grown either on glass, crystalline, and flexible plastic substrates. The Agx(Sb2S3)1-x nanoporous structure exhibits a high absorptance of >70 % in the VIS-NIR spectrum, with a maximum absorptance of 95 % at 430 nm and 90 % at 1540 nm, respectively. The high absorptance is not sensitive to polarisation and incident angle. Numerical simulations suggested that the high absorption stemmed from the coalescence of surface/bulk plasmons and semiconductor band gap absorption. Additionally, these plasmon resonances of the Agx(Sb2S3)1-x nanoporous structure can be used to demonstrate optical label-free biosensing with an extraordinarily high sensitivity of a 1 fM. The simplicity of the one-step fabrication process is ideal for commercial application in biosensors and light absorbers. To conclude, this thesis has answered the question of whether phase change materials are useful for tuning photonic devices and discusses the suitability of different types of phase change materials to different devices. The Ge2Sb2Te5 is an excellent phase change material for tuning non-volatile photonic devices in the infrared, whereas Sb2S3 is more suitable for tuning visible photonics devices. A facile method was serendipitously discovered to fabricate wafer-scale Agx(Sb2S3)1-x nanoporous structures, which are likely to find applications in light absorbers and biosensors.