title: Mechanical, Optical and Optomechanical Engineering of Novel Atomically Thin Semiconductors
Creators: name: Liu, Boqing
Publisher: name: The Australian National University
Year: 2024

description: In optical imaging, an extended optical phase can enhance image contrast and resolution, leading to clearer imaging outcomes. However, due to the suppressed phase accumulation effects resulting from the atomic thinness of monolayer TMD materials, the short optical path length hinders the development of wavefront engineering. To address this, a pioneering approach involving the application of a special dielectric material coating on substrates and capitalising on the interfacial adhesion ability of monolayer TMD films is introduced. This innovative approach achieves a remarkable nearly pi phase jump in monolayer TMDs, enabling the realization of supercritical lenses with outstanding optics performance across a broadband wavelength range. Strain manipulation can effectively modify the bandgap structure in 2D semiconductors to tune the linear optical behaviours such as Photoluminescence. However, the strain-induced nonlinear response has received limited attention. The layered-dependent second harmonic generation (SHG) response of TMDs exhibits high sensitivity to intrinsic strain caused by temperature changes. Indeed, temperature dependent SHG reveals divergent trends between monolayers and specific odd layers (3L, 5L, 7L, etc.) of TMDs. Notably, monolayer samples experience the enhancement as the lattice expands due to higher temperatures, while other odd layers display a reduction in SHG response. However, the strain induced by the thermal effect has the limited contribution to further enhancing the SHG performance of monolayer TMDc. Large pressurized monolayer TMD domes have been successfully fabricated, providing a versatile platform for generating varying and substantial strain conditions. The domes' hemispherical geometry, containing the significant internal gas pressure, has led to a remarkable enhancement in SHG emission, reaching two orders of magnitude at room temperature and one order of magnitude even at -190 Celsius degree. Furthermore, the amplified SHG responses and distinctive strain fields within enable the exploration of the influence of biaxial strain on anisotropic SHG response. Given the enormous potential of TMD dome structure for future nonlinear optical devices, few-layered individual bubbles, along with 1D and 2D TMDc dome networks, have been introduced. The dome network comprises multiple individual domes grow from distinct basal planes of the TMD flake. In the bubble network system, the variant Plateau's law is observed in this stable nano-scaled solid film system. The discrete joint angles result from the significant differences in effective surface tension over the layer-dependent stiffness and are influenced by the thickness-dependent interlayer adhesion energy. This dome network system holds promise for creating 1D or 2D nanostructures with significant property variations for nano-opto-mechanics. In addition to accumulating optical phase for SHG, ensuring the right thickness to prevent optical cavity effects is vital for nonlinear optical performance of 2D materials. Despite the anticipated exceptional second-order susceptibility tensor in 2D tellurium, the SHG emission efficiency is significantly influenced by the flake thickness. To address this, a thinning process based on the hydrogen plasma etching mechanism is employed to finely optimise the thickness of 2D Te flakes, successfully achieving the extraordinary SHG behaviours. Overall, this thesis focuses on utilising mechanical properties of TMDc and 2D elemental semiconductor materials to precisely tune and enhance the elastic and inelastic optical behaviours. The relevant potential applications and future challenges based on the physics and engineering protypes are also thoroughly considered and discussed.