Date of Degree

9-2019

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Vinod Menon

Committee Members

Carlos Meriles

Gabriele Grosso

Sriram Ganeshan

Stephane Kena-Cohen

Subject Categories

Condensed Matter Physics | Optics

Keywords

exciton polariton, 2D material, TMDs, strong coupling, light matter interaction

Abstract

Strong interaction between photons and excitons in semiconductors results in the formation of half-light half-matter quasiparticles termed exciton-polaritons. Owing to their hybrid character, they inherit the strong interparticle interaction from their excitonic (matter) component via Coulomb interaction while the photonic component lends the small mass (~105 times lighter than free electrons) and long propagation distances. Additionally, exciton-polaritons also carry properties of the host material excitons such as spin and valley polarization and can be probed via the photons that leak out of the cavities since the photon carries all the information owing to conservation laws. Since the first demonstration of exciton-polaritons in GaAs, they have been used to demonstrate a wide array of fundamental phenomena and potential applications ranging from Bose-Einstein like condensation to analog Hamiltonian simulators and chip-scale interferometers. Recently the two-dimensional transition metal dichalcogenides (TMDs) owing to their large exciton binding energies, oscillator strength and valley degree of freedom have emerged as a very attractive platform to realize exciton-polaritons at elevated temperatures. The TMDs are a group of van der Waals semiconductor material that transit from indirect to direct bandgap in monolayer (2D) limit. The reduced dielectric screening in the 2D limit leads to a strong binding energy and oscillator strength, resulting in strong light matter interaction. The direct bandgap in these materials occur at the K and K’ points in momentum space. Owing to the broken inversion symmetry in the monolayer limit, the K and K’ points are inequivalent and show interband transitions with opposite optical selection rules for circularly polarized light. This thesis focuses on the exciton polaritons in TMDs monolayers. Specifically, we show that the valley property of exciton polariton can be addressed by its exciton component and light-matter coupling strength is controllable by external doping. We demonstrate the transition from strong to weak coupling via electron doping (5*1012 cm-2) resulting in reduction in oscillator strength by a factor of 10. We also present experimental results on exciton-polaritons realized using excited states in TMDs systems. More importantly, we show the nonlinear polariton-polariton interaction, mainly stemming from the phase space filling in the 2D system, is ~ 15 folds higher for the 2S exciton polariton than that in the 1S case, which could be useful for nonlinear polaritonic application. Finally, we introduce the electrical excitation of exciton polariton in TMDs monolayers using a tunnel junction based van der Waals heterostructure embedded in a microcavity. The device uses tunnel injection via hBN barriers, graphene contacts and WS2 as the emissive layer. The spectroscopic investigations and device demonstrations using exciton-polaritons in 2D TMDs reported in this thesis presents a first step towards using TMDs for polaritonic circuits and quantum nonlinear photonic applications.

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