title: Manipulation of light-emitting properties of 2D materials by photonic nanostructures
Creators: name: Chen, Haitao
Publisher: name: The Australian National University
Year: 2017
Subjects: subject: 2D materials
subject: Nanostructures
subject: Photonics
subject: Plasmonic
subject: Waveguide
subject: Grating
subject: SHG
subject: PL

description: The development of two-dimensional (2D) materials brings up new opportunities for 2D physics and new applications. Among them, semiconductor transition metal dichalcogenides (TMDCs) show quite advanced properties. Especially, in the monolayer limit they become direct-bandgap semiconductors. The monolayer form demonstrates a range of optical effects including photoluminescence (PL), valley polarization and second-harmonic generation (SHG), which are important for future optoelectronic applications. However, the light-emitting efficiency of such monolayer TMDCs is generally low due to the subnanometer light-matter interaction length and defects. To enable their practical applications, new schemes are desirable to enhance the emission efficiency and further control properties such as directionality and polarization. On the other hand, photonic nanostructures show great capability to manipulate light-matter interaction at the nanoscale. Thus, in this thesis, we explore different schemes to enhance and control emission from monolayer TMDCs by integrating them with resonant photonic nanostructures, including plasmonic structures, waveguides and nanoantennas. We have successfully demonstrated integration of monolayer TMDCs into different photonic platforms experimentally and showed great capability to control the emission from them. Associated theoretical work has also been presented. Our work shows potential important applications for future optoelectronic devices. In Chapter 1, we firstly introduce the necessary background of the development of 2D materials and semiconductor optics. Then, we focus on the optoelectronic properties of 2D TMDCs. The prospect of integration of 2D materials with photonic nanostructures is further discussed. In Chapter 2, we show integration of a monolayer MoSe2 onto resonant plasmonic nanoantenna. Comprehensive control of the PL emission from quenching to enhancement is demonstrated by changing the thickness of the spacer between the MoSe2 and nanoantenna. Further simulation results explain the experimental observation well. In Chapter 3, we demonstrate enhanced and directional PL emission from WSe2 monolayer integrated onto a silicon photonics structure. This is achieved by coupling the WSe2 into a multi-resonant grating-waveguide structure which supports several leaky waveguide modes. Further theoretical work explains our experimental results well. In Chapter 4, we propose a plasmonic-TMDCs scheme that could effectively separate the PL emission from different valleys of 2D TMDCs into opposite directions spatially. This is realized by simultaneously exciting both electric dipole and quadrupole modes from two-bar plasmonic antenna, the interference of radiation of these modes induces the spin-locked directionality. In Chapter 5, we demonstrate enhanced SHG from a monolayer MoSe2. This is achieved by integrating the monolayer onto a waveguide and exciting the material through evanescent field of the guided modes. The enhancement is due to the fact that the waveguide geometry dramatically increases the nonlinear interaction length and allows for phase matching. Theoretical calculation further reveals the conversion mechanism in our system and points out further directions. In Chapter 6, we summarize the work and discuss the future prospects in regards to the integration of 2D materials with resonant photonic nanostructures.