Controlling the emission of ultra-thin semiconductors by photonic nanostructures

Abstract

Since the discovery of graphene in 2004, ultra-thin materials have attracted a lot of research interests in the last decade. However, the zero-bandgap of graphene limits its application, so more emerging ultra-thin semiconductors with visible direct bandgap such as monolayer transition metal dichalcogenides (TMDC) and Lead iodide (PbI2) nanosheets have been widely studied. Their remarkable optical properties including the visible direct bandgap, robust valley polarisation and strong tunability make them ideal for optoelectronic devices. The unique valley polarisation can also be considered as a new degree of freedom, which enables new opportunities in the field of valleytronics. Manipulating the valley polarisation by the optical method is the key to realise valleytronic devices. However, the atomic thickness limits the light-matter interaction length and efficiency. An important solution to enhance the light-matter interactions, including photoluminescence (PL) and nonlinear emissions from such ultra-thin semiconductors is to couple them to photonic nanostructures.

In this thesis, I will show the emission manipulation of ultra-thin semiconductors by coupling them with different photonic nanostructures including dielectric grating waveguide, plasmonic nanoantennas and metasurface. Our works demonstrate enhancement and control of the emissions and show great potential for the future optoelectronic applications.

In Chapter 1, I will first give an introduction on the basic concepts of semiconductors, emissions and photonic structures, and review on the related works as well. In Chapter 2, we will present the enhanced and polarisation-selective directional PL emission from a monolayer WSe2 coupled to a silicon grating waveguide nanostructure. In Chapter 3, we will present how to spatially separate the PL originating from the different valleys of the monolayer WSe2 by plasmonic nanoantennas. Besides PL, in Chapter 4 I will demonstrate the nonlinear microscopy of two novel ultra-thin semiconductors including PbI2 nanosheets and InP nanomembranes with InAsP quantum wells. In the PbI2 nanosheets subsection, the nonlinear emissions can be controlled by thickness, pump polarisation and wavelength, and substrate. By using the nonlinear harmonic emission, we can also precisely determine their thickness and crystalline orientation. In the InP nanomembrane subsection, I will demonstrate the SHG enhancement based on the resonance mode of InAsP QW. Instead of coupling monolayer with photonic nanostructures, we can fabricate the ultra-thin semiconductors themselves to nanostructures. In Chapter 5, we will show the tunable nonlinear emissions from TMDC metasurfaces by using nonlinear microscopy techniques. Finally, I will summarise my work and give an outlook on the future direction of coupling ultra-thin semiconductors with photonic nanostructures. Show more