Dissertations, Theses, and Capstone Projects

Date of Degree

9-2021

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Vinod Menon

Committee Members

Li Ge

Matthew Y. Sfeir

Gabriele Grosso

Stephane Kena Cohen

Subject Categories

Condensed Matter Physics | Optics | Quantum Physics

Keywords

Spectroscopy, two-dimensional materials, light-matter interactions

Abstract

Technology has been accelerating at breakneck speed since the first quantum revolution, an era that ushered transistors and lasers in the late 1940s and early 1960s. Both of these technologies relied on a matured understanding of quantum theories and since their inception has propelled innovation and development in various sectors like communications, metrology, and sensing. Optical technologies were thought to be the game changers in terms of logic and computing operations, with the elevator pitch being "computing at speed of light", a fundamental speed limit imposed by this universe’s legal system (a.k.a physics). However, it was soon realized that that all-optical logic was not as frugal as their electronic counterparts when it came to power consumption, efficiency, and compactness, and was hence confined to mostly academic explorations. However, after the advent of commercial optical fiber networks and optical amplifiers, light or electromagnetic waves have been the preferred means of communication. But with the rapid advancement of laser technology the world is now moving towards the second quantum revolution which, quoting Ray Simmonds from NIST is “The second quantum revolution is where you’re really using quantum mechanics to do everything for you, such as entangling individual qubits to transmit information. You’re engineering the quantum mechanics itself to do something, not, ‘Oh, I have a widget that has these special properties because of quantum mechanics’.”. This thesis is a humble step towards the realization of that goal. The protagonists in this thesis are exciton-polaritons formed in two-dimensional semiconductors called Transition Metal Dichalcogenides (TMDC). TMDC has been in the limelight since the last decade because of their exceptional optical and optoelectronic properties- which are virtuous enough to warrant their own "tronics" dubbed "Valleytronics" and lucrative enough for scientific publishing houses to confer TMDCs its own franchise. We begin by introducing microcavity exciton-polariton as a powerful means to control the valley coherence, an intrinsic property of TMDC excitons, without the requirements of very high magnetic fields or strong electric fields. We explore the usage of pseudomagnetic fields inside microcavities to rotate the plane of polarization of the emitted linearly polarized photoluminescence of these TMDC materials. But polaritons created by classical sources like a laser are usually incoherent due to the very stochastic/quantum nature of spontaneous emission. To explore the valley coherence as a platform that would one day potentially be technologically relevant, one can take one of the two ways: make a coherent system of polaritons or operate the system at a single valley (qubit). Which brings us to the second topic we tackle in this thesis: pre-requisites towards achieving creation of spontaneous coherence in TMDC exciton-polaritons via Bose Einstein condensation. We study the thermalization properties of exciton-polaritons when the lowest energy of the polariton state is resonant with the Trion state. We then graduate into exploring non-linear properties facilitated in TMDC polaritons systems. We begin by demonstrating relaxation of symmetry requirements for second order non-linear response for centrosymmetric bulk TMDC exciton-polaritons. This modification of physical property of the material has been granted due to the asymmetric electric fields formed in these self hybridized exciton-polariton systems that effectively decouples the individual layers to generate intense second harmonic generation. We then achieve strong polariton-polariton interactions in interlayer excitons in bilayer MoS2, thus making our way into truly quantum technologies. With a potential to realize strongly correlated photons we report a 10 fold enhancement in the polariton interaction strength as compared to the the exciton-polariton formed by its intralayer counterpart. The spectroscopic investigations using exciton-polaritons in 2D TMDCs reported in this thesis present a first step towards using TMDCs for polaritonic circuits and quantum nonlinear photonic applications.

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