Optics, dynamics and transport of exciton polaritons in atomically thin semiconductors

• Applicant: Professor Dr. Ermin Malic
• Subject Area: Theoretical Condensed Matter Physics
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 524612380

When atomically thin transition metal dichalcogenides (TMD) are integrated into suitable optical microcavities, their tightly-bound excitons can hybridize with cavity photons to form exciton polaritons. These quasi-particles inherit properties from their constituent parts, potentially combining the spatial coherence and long propagation lengths of photons with the tunability and nonlinearity of material-based excitations. The large oscillator strength of TMD excitons leads to a massive Rabi splitting, and the large binding energy allows for room-temperature exciton-polaritonics. A microscopic understanding of polariton relaxation is essential for interpreting optical spectroscopy measurements, as well as understanding phenomena, such as Bose Einstein condensation and valley polarization retention. To date, much of the theory of TMD polaritons has relied on simple phenomenological rate equations for population dynamics and relaxation. Furthermore, the impact of momentum-indirect excitons has been neglected. In this project, we propose to develop a sophisticated many-particle theory based on the density matrix formalism, combined with a Hopfield approach, to treat on microscopic footing the strong and very-strong coupling regimes of light-matter interaction in TMD monolayers as well as twisted TMD homo- and heterobilayers. Our research plan consists of two main goals: Gain microscopic insights into the interplay of excitons, photons, and phonons within the strong coupling regime for TMD monolayers and twisted bilayers, and learn how to control the exciton-phonon-polariton physics by changing the twist angle, cavity length, temperature, and mirror reflectivity. We will use a combined Wannier-Hopfield approach to develop a model of polariton relaxation that considers the full exciton energy landscape. This will allow us to reveal the time- and energy-resolved many-particle processes behind the formation, thermalization and decay of both intra- and interlayer exciton polaritons. Understand the impact of the very-strong coupling regime on exciton optics, dynamics, and transport in TMDs. Here, the light-exciton interaction is strong enough to couple excitons to the electron-hole continuum. Due to their large oscillator strength, TMDs offer an ideal platform to study this widely unexplored regime. We will develop a new microscopic model, crucially including both bound exciton states and the unbound electron-hole transitions, to describe the light-induced modification of the internal exciton wavefunction, and seek experimental signatures of this novel physics. Our long-standing expertise in modelling many-particle physics and light-matter interactions places us in a strong position to tackle the challenges of this proposal. Our study will provide a major advance for microscopic understanding of the strong and very-strong coupling regime in the technologically promising class of atomically thin nanomaterials.

• DFG Programme: Research Grants