Dissertations, Theses, and Capstone Projects

Date of Degree

9-2023

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Vinod Menon

Committee Members

Matthew Sfeir

Gabriele Grosso

Euclides Almeida

Arka Majumdar

Subject Categories

Condensed Matter Physics | Optics | Quantum Physics

Keywords

Light matter interaction, exciton-polaritons, 2D materials, Fluorescent dyes and proteins, Bose Einstein condensate

Abstract

Strong light-matter coupling in solid state systems is an intriguing process that allows one to exploit the advantages of both light and matter. In this context, microcavities have become essential platforms for studying the strong coupling regime, where hybrid light-matter states known as exciton-polaritons form, leading to enhanced light matter interaction, modified material properties, and novel quantum phenomena. In this thesis, we explore the phenomenology of exciton-polaritons in strained TMD microcavities, 2D perovskites, fluorescent proteins and organic dyes encompassing thermalization, polariton lasing, and the observation of nonlinear effects.

Transition metal dichalcogenides (TMDs) have emerged as a remarkable class of two- dimensional materials with unique electronic and optical properties. In recent years, the incorporation of strain engineering into TMD-based systems has paved the way for unprecedented tunability of their electronic structure and excitonic properties. Here, we present our findings on enhancing nonlinear polariton interactions at room temperature by a factor of 10 in strained monolayer WSe2 using nanopillars.

In addition to their attractive technological applications in photovoltaics and light emitters, the perovskite family of semiconductors has recently emerged as an excellent excitonic material for fundamental studies. Specifically, the 2D hybrid organic-inorganic perovskite (HOIP) offers the added advantage of room temperature investigations owing to their large exciton binding energy. In this work, we strongly couple excitons in 2D HOIP crystals to planar microcavity photons sustaining exciton-polaritons under ambient conditions resulting in a Rabi splitting of 290 meV. Dark excitons directly pump the polariton branch along its dispersion in resonance with the Stokes shifted emission state (radiative pumping), creating a high density of polaritons at higher in-plane momentum (k||). We further probe the non- linear polariton dispersion dynamics at varying input laser fluence, which indicates efficient polariton-polariton scattering and decay to k|| = 0 from higher k||.

Fluorescent proteins (FPs) have recently emerged as a serious contender for realizing ultra-low threshold room temperature exciton-polariton condensation and lasing. Our contribution investigates the thermalization of FP microcavity exciton-polaritons upon optical pumping under ambient conditions. We realize polariton cooling using a new FP molecule, called mScarlet, coupled strongly to the optical modes in a Fabry–Perot cavity. Interestingly, at the threshold excitation energy (fluence) of 9 nJ/pulse (15.6 mJ/cm2), we observe an effective temperature, Teff 350 ± 35 K close to the lattice temperature indicative of strongly thermalized exciton-polaritons at equilibrium. This efficient thermalization results from the interplay of radiative pumping facilitated by the energetics of the lower polariton branch and the cavity Q-factor. Direct evidence for dramatic switching from an equilibrium state into a metastable state is observed for the organic cavity polariton device at room temperature via deviation from the Maxwell-Boltzmann statistics at k|| = 0 above the threshold.

Exciton-polaritons are attractive platforms for creating macroscopic coherent states like Bose Einstein like condensates (BECs). Exciton-polaritons based on organic molecules are of particular interest for realizing such states at room temperature while offering the promise of synthetic tunability. However, the demonstrations of such condensates have been limited to a few specific molecular systems. Here we report a universal platform for realizing molecular polariton condensates using commercial dyes that solves long standing material challenges. This solution is made possible using a new and programmable molecular material called small-molecule, ionic isolation lattices (SMILES) with the potential to incorporate a wide array of molecular fluorophores. We show exciton- polaritons condensation in rhodamine by incorporating it into SMILES lattice placed in a planar microcavity.

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