Anisotropic Exciton-Polaritons Reveal Non-Hermitian Topology in van der Waals Materials

Topological band theory has expanded into various domains in applied physics, offering significant potential for future technologies. Recent developments indicate that unique bulk band topology perceived for electrons can be realized in a system of light-matter quasiparticles with reduced crystal symmetry by utilizing tunable light-matter interaction. In this work we realize topologically nontrivial energy band dispersion of exciton-polaritons confined in two-dimensional anisotropic materials inside an optical microcavity and show the emergence of exceptional points (EPs) due to non-Hermitian topology arising from excitonic dipole oscillators with finite quasiparticle lifetime. Fourier-plane imaging reveals two pairs of EPs connected by bulk Fermi arcs for each of the transverse electric and transverse magnetic modes. An anisotropic Lorentz oscillator model captures the exact band dispersion observed in our experiment in a two-dimensional momentum space. Our findings establish anisotropic two-dimensional materials as a platform for exploring non-Hermitian topological physics, with implications for polarization-controlled optical technologies.