Exciton-magnon polaritonics in 2D cavities

Principal investigator

Florian Dirnberger, TU Munich [webpage]

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Abstract

In my Emmy Noether group (funded by DFG), we pursue two principal directions in two active PhD projects.

In the first project, mono- and bilayers of two-dimensional magnetic semiconductors with strong excitonic effects embedded in hBN are placed into optical microcavities to induce strong coupling of excitons to confined cavity photons. The resulting hybrid excitations – exciton-polaritons – dominate the optical properties of such crystals. The primary goal of the project is to drive these polaritons into the regime of exciton-polariton condensation, creating one of the first optical lasers made from monolayer or few-layer magnetic materials. Such a laser is expected to inherit unique properties from the coupling of excitons and polaritons to the magnetic order that occurs at low temperatures. Studying dynamics related to the condensation process itself (stimulated polariton scattering) and its interaction with magnetic excitations called magnons addresses fundamental questions of quasiparticles interactions.

In the second project, we will combine optical exciton spectroscopy with methods commonly used in spin wave spectroscopy. Microscopic coplanar waveguides will be fabricated by lithography for the resonant excitation of coherent magnons in hBN-encapsulated twisted bilayers of the van der Waals antiferromagnet CrSBr. Particularly important is the microwave excitation of propagating magnon modes. The primary goal is to study the interaction of these propagating magnons with local densities of excitons to potentially simulate quantum wave physics based on the highly coherent magnon excitation. To shape exciton distributions in real space, we either rely on spatial light modulators, which are frequently employed, e.g., in experiments on ultracold atoms or polariton condensates, or use twisted bilayers and the resulting moire potentials. The research should address deeply fundamental questions about the interactions of excitons with magnons.

Beyond that, we are also planning projects to study proximity effects in vdW heterostructures of single layer graphene and single layer 2D magnets (e.g., Graphene/CrSBr), as well as in vdW heterostructures of transition metal-dichalcogenides and 2D magnets (e.g., WSe2/CrSBr).

Typical optical methods used in our group are standard photoluminescence and reflectance spectroscopy, Fourier imaging, time-resolved detection and magneto-spectroscopy. Samples fabricated by mechanical exfoliation and transfer techniques are embedded in microcavities, e.g., by thin film deposition techniques or by transfer onto commercial highly reflective substrates. Other methods involve microwave techniques using coplanar waveguides and high-frequency excitation schemes.