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Title: Manipulation of optical coupling between plasmonic nanocavities and 2D excitons
Authors: Lo, Tsz Wing
Degree: Ph.D.
Issue Date: 2021
Abstract: The large binding energies of excitons (up to 500 meV) in two-dimensional (2D) transition metal dichalcogenides (TMDCs) enable thermally robust excitonic responses even at room temperature, which is highly desired for many optoelectronic applications. In particular, the hexagonal lattice of 2D TMDCs exhibits strong spin-orbital coupling (SOC) and inversion symmetry breaking, which gives rise to pronounced valley coherence for important valleytronic applications. The strong SOC and inversion symmetry breaking in TMDCs also results in energy splitting of their conduction bands (CBs), leading to the existence of optically bright (XO) and dark intra-valley excitons (XD). The bright excitons originate from the combination of electrons and holes with antiparallel spins, and can be coupled directly with in-plane polarized photons. On the other hand, the dark excitons originate from the combination of electrons and holes with parallel spins, and have a near-zero in-plane dipole moment and thus a considerably longer lifetime than bright excitons, resulting in an optically inactive property under conventional normal-incidence illumination. Despite these fascinating optical properties, the low quantum yield (QY) of photoluminescence (PL) in TMDCs has significantly limited their optical and optoelectronic applications. Many studies have discovered that defect-mediated non-radiative recombination restricts the QY of pristine exfoliated TMDC monolayers (ML) to ~1 %. In this thesis, by coupling various plasmonic nanostructures with 2D TMDCs, I discovered and discriminated different physical mechanisms responsible for the nanoscale light-matter interaction between 2D excitons and plasmonic nanocavities, and achieved significantly enhanced PL from both bright and dark excitons. Firstly, I studied the strong coupling phenomena between single gold core-silver shell nanocuboids (Au@Ag NC) and the bright excitons of WS2 and MoS2 monolayers under thermal tuning. By fitting the temperature-dependent dark-field scattering spectra of the Au@Ag-WS2 and Au@Ag-MoS2 systems with a classical coupled oscillator model (COM), I observed that the thermal evolution of their coupling strengths were opposite. Such a counter-intuitive observation revealed an indirect coupling channel between the plasmon mode of the Au@Ag NC and the bright and dark excitons of WS2 and MoS2, which is often ignored in previous studies on TMDC-based plasmon-exciton coupling interactions.
Secondly, I employed a gold (Au) nanoparticle-on-mirror (NPoM) nanocavity to enhance the PL emission of bright excitons and explored the enhancement mechanism. In this study, a CVD-grown MoS2 ML was transferred to an alumina (Al2O3)-coated gold mirror through a standard wet transfer method, and the sample was then covered with another Al2O3 layer before drop-casting of Au nanospheres to form Al2O3-MoS2-Al2O3-sandwiched Au NPoM nanocavities. The thicknesses of both Al2O3 spacers were precisely controlled by the number of cycles of atomic layer deposition (ALD) to optimize the plasmon-enhanced PL from MoS2, showing a 7-fold PL intensity enhancement at 5 nm thick Al2O3. Finite-difference time-domain (FDTD) simulations were conducted to quantify the contribution of plasmonic near-field enhancement, antenna efficiency and Purcell effect on the enhanced bright exciton emission. Lastly, I used a single plasmonic NPoM nanocavity to induce significant radiation from the spin-forbidden dark excitons of WSe2 at room temperature, and unravelled an interesting mechanism based on polarization-dependent plasmon-exciton coupling. In this study, the NPoM nanocavity was utilized to sandwich a mechanically exfoliated pristine WSe2 ML that supported a higher-lying bright exciton state and a lower-lying dark exciton state. To ensure the formation of a compact NPoM nanocavity, a template-stripped gold film was used to as a flat metal mirror supporting good adhesion with a WSe2 ML, which strongly quenched the bright exciton emission but, in the meanwhile, largely enhanced the dark exciton emission when coupled to an Au nanosphere on top. The sample was then exposed to directional argon ion bombardment in an inductively charged plasma (ICP-RIE) etching machine. Since the metal nanoparticle functioned as a shadow mask to shelter the underlying WSe2 ML during the etching process, only the WSe2 outside the nanoparticle region was removed. PL spectroscopic measurements of the etched system showed that the PL intensity from the dark excitons was comparable with the bright excitons when resonantly interacting with the gap plasmon cavity mode. I used a double-Lorentz fitting on the PL spectra, suggesting a 60 meV energy difference between these two excitonic states. The numerical aperture (NA) dependent PL and enhanced radiation decay rate unambiguously verified the out-of-plane dipole nature of the dark exciton states. The results presented in this thesis provide an effective paradigm for manipulating the electromagnetic coupling between surface plasmons and 2D excitons through ultra-compact plasmonic nanocavities at room temperature. This study paves way for further understanding the excitonic dynamics of 2D TMDCs and employing them in quantum information and nanoscale optoelectronic devices.
Subjects: Plasmons (Physics)
Nanostructured materials
Semiconductors -- Optical properties
Hong Kong Polytechnic University -- Dissertations
Pages: xi, 119 pages : color illustrations
Appears in Collections:Thesis

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