Probing Quantized Excitations and Many-body Correlations in Transition Metal Dichalcogenides with Optical Spectroscopy

Layered transition metal dichalcogenides (TMDCs) have attracted great interests in recent years due to their physical properties manifested in different polytypes: Hexagonal(H)-TMDC,which is semiconducting, exhibits strong Coulomb interaction and intriguing valleytronic properties; distorted octahedral(T’)-TMDC,which is semi-metallic, is predicted to exhibit rich nontrivial topological physics. In this dissertation,we employ the polarization-resolved micron-Raman/PL spectroscopy to investigate the optical properties of the atomic layer of several polytypes of TMDC. In the first part for polarization-resolved Raman spectroscopy, we study the lattice vibration of both H and T’-TMDC, providing a thorough understanding of the polymorphism of TMDCs. We demonstrate that Raman spectroscopy is a versatile tool to probe the symmetry as well as the quality of crystals. This becomes quite important for atomic layers of TMDCs which are sensitive to the environment and substrate. In the second part of the dissertation, we focus on fabricating high-quality monolayer tungsten diselenide samples and study their excitonic bound states by photoluminescence, reflection, resonant-Raman spectroscopy, magneto-optical measurements, and time-resolved spectroscopy. We first demonstrate the many-body correlation of the multi-particle bound excitonic states at low temperatures. The PL measurements in magnetic fields demonstrate for the first time the abnormal valleytronic properties of the biexciton and five-particle bound states, exciton-trion in 1L-TMDCs. The time-resolved PL measurement reveals the ultralong lifetime of the several bound states at even lower energy range, establishing a potential platform for further investigation on exciton condensation. In addition to the lower energy bound states, we also explore its excited Rydberg states in high magnetic fields. Surprisingly, we observe for the first time the PL of up to 4s excitons, opening doors to the investigation the physics of Rydberg exciton in a 2D system. For example, we investigate the role of electron-hole exchange-interactions in 2s exciton. Contrary to 1s exciton, the exchange interaction in 2s exciton is strongly suppressed due to its larger size. Consequently, the 2s exciton display superior valley polarization and valley coherence, paving the way to valleytronic applications

China can win!: The new stage in the aggression of Japanese imperialism and the new period in the struggle of the Chinese people

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Spin-orbit interaction of light induced by transverse spin angular momentum engineering

We report the first demonstration of a direct interaction between the extraordinary transverse spin angular momentum in evanescent waves and the intrinsic orbital angular momentum in optical vortex beams. By tapping the evanescent wave of whispering gallery modes in a micro-ring-based optical vortex emitter and engineering the transverse spin state carried therein, a transverse-spin-to-orbital conversion of angular momentum is predicted in the emitted vortex beams. Numerical and experimental investigations are presented for the proof-of-principle demonstration of this unconventional interplay between the spin and orbital angular momenta, which could provide new possibilities and restrictions on the optical angular momentum manipulation techniques on the sub-wavelength scale. This phenomenon further gives rise to an enhanced spin-direction coupling effect in which waveguide or surface modes are unidirectional excited by incident optical vortex, with the directionality jointly controlled by spin-orbit states. Our results enrich the spin-orbit interaction phenomena by identifying a previously unknown pathway between the polarization and spatial degrees of freedom of light, and can enable a variety of functionalities employing spin and orbital angular momenta of light in applications such as communications and quantum information processing