Phase-change photonic meta-devices for active filtering and modulation

Abstract

This thesis presents the development (design, fabrication and characterisation) of two actively tunable devices for amplitude modulation of light. Chalcogenide phase-change materials, the optical properties of which can be tuned thermally through a variety of methods, are used as the active component. The large contrast in refractive index between the amorphous and crystalline states of these materials allows for substantial change in the optical response of the devices. They also have other desirable properties such as non-volatile operation, good switching endurance and the ability to switch on very small time scales.

The first class of devices developed here were novel hybrid metal-dielectric metasurfaces for selective dual-band amplitude modulation at telecommunication frequencies. These devices consist of a metal back plane and an array of nano-cubic resonators consisting of a layer of phase-change material (Ge2Sb2Te5) sandwiched between two layers of silicon. Switching the phase-change material layer between the amorphous and crystalline states changes the resonant properties of the device, shifting the reflectance minimum from the O-band (1310 nm) to the C-band (1550 nm). Devices were designed and optimised using finite element simulations. The final fabricated devices performed well with extinction ratios ranging from -5.0 to -9.4 dB with low insertion losses, ranging from 0.2 to 2.3 dB.

A study was conducted on the adverse effects of metal diffusion in these devices using different bottom plane configurations. The use of ultra-thin layers of Si3N4 as barrier layers was investigated as a remedy to diffusion. These barrier layers were found to effectively eliminate the issues without negatively impacting the optical performance of the devices. Further to this, thin film samples with and without barrier layers were fabricated in order to investigate the diffusion of Au into Ge2Sb2Te5. TEM cross-sectional imaging and EDS analysis were used to characterise material constituents through the thin film samples and therefore assess the extent of the diffusion present.

The second class of devices were novel thin-film based amplitude-only spatial light modulators. The modulators operate in reflection and were designed to have minimal effect on the phase of the reflected light, while maximising the modulation of the amplitude of the light reflected. The device consists of a Ti bottom layer, a GeTe phase-change material middle layer, and a ITO capping layer. Test spatial crystalline patterns were successfully written to devices using a series of laser scans and the phase response of the device was measured using a purpose built off-axis digital holography interferometer. The experimental results show the modulation depth of these devices reaches 0.38 and the averaged phase difference from an amorphous to crystalline section was found to be less than pi/50, which agrees well with simulation. This design opens up a potential route to ultra-fast spatial light modulation and, when paired with phase-only spatial light modulators, could have widespread applications in wavefront shaping.