Optical Characterisation of Strain and Defects in 2D Photonic Materials

Abstract

Strain and defect engineering have shown to be powerful tools in modifying optoelectronic properties of semiconductors. This thesis aims to advance the fundamental understanding of electronic and optical properties in material systems with broken inversion symmetries and to use this understanding to engineer in-situ, localized strain fields for tailoring photonic responses at the nanoscale. We will address the fundamental question: How can we characterize the effect of strain and defects in two-dimensional photonic materials? To this end, we open with a review of current strategies in strain engineering, its fundamental consequences on electronic, optical, and magnetic properties, and the state-of-the-art applications of this technology in achieving band-gap-engineered straintronic devices. Touching on the advent of strain engineering for flexoelectricity - a spontaneous material polarization produced by a strain gradient that lifts the inversion symmetry, which can enable a bulk photogalvanic effect, we posit the aspect of meta-valent bonding in materials having a key role in this, by showing that the majority of prime material candidates known to have exhibit large photogalvanic response exhibit this characteristic. The rest of the thesis focuses on characterizing layered metal thio(seleno)phosphates, a family of materials known for their magnetic, electronic, and nonlinear optical properties. We show how the optical properties of these materials can be modulated via different means of defects and strain. These photoactive materials can be pivotal to a future comprising of strain-engineered flexoelectric devices, which take advantage of the bulk photogalvanic effect, to develop a new family of practical, deployable, self-powered, and low-cost photodetectors, and integrated arrays with limits-breaking performance in the UV-to-LWIR spectral bands.