Tunable Exciton-Hybridized Magnon Interactions in a Layered Semiconductor

Geoffrey Diederich, University of Washington

October 7, 2022

The interaction between distinct excitations in solids is of both fundamental interest and technological importance. One example of such interactions is coupling between an exciton, a Coulomb bound electron-hole pair, and a magnon, a collective spin excitation. The recent emergence of van der Waals magnetic semiconductors, which host both strong light-matter interaction due to excitons and potentially long-lived coherent magnons, provides a powerful platform for exploring these exciton-magnon interactions and their fundamental properties, such as strong correlation, as well as their photo-spintronic and quantum transduction applications. In this talk, I will present recent work in which we demonstrate strong magnon-exciton coupling in the 2D van der Waals (vdW) antiferromagnetic (AFM) semiconductor CrSBr, as well as precise control of the coherent magnon-exciton interactions. We show that coherent magnons launched by above-gap optical excitation modulate the interlayer electronic hybridization and excitonic energies. Transient reflectance spectroscopy/microscopy based on direct excitonic sensing reveals AFM magnons with coherence times up to 6.6 ns. The magnon-exciton coupling persists to the 2D bilayer limit and we observe coherent AFM magnons in compensated (even) and uncompensated (odd) layers without and with net magnetization, respectively. We further show that by controlling the direction of applied magnetic fields relative to the crystal axes, and thus the rotational symmetry of the magnetic system, we can tune not only the exciton coupling to the bright magnon, but also to an optically dark mode via magnon hybridization. The exciton-magnon coupling and associated magnon dispersion curves can be further modulated by applying a uniaxial strain. At the critical strain, a dispersionless dark magnon band emerges. Our results demonstrate unprecedented control of the opto-mechanical-magnonic coupling, and a step towards the predictable and controllable implementation of hybrid quantum magnonics.