Ultrastrong coupling of topologically protected edge and bulk magnetoplasmons

In the ultrastrong light-matter coupling regime, the ability to confine the electric field to subwavelength scales and define an electromagnetic mode is essential. This task is typically achieved through dedicated cavities, such as THz metal resonators. Here, we instead introduce ultrastrong coupling directly between distinct magnetoplasmon excitations, leveraging solely the spatial symmetry of a confined electronic medium in the presence of a magnetic field. By patterning an AlGaN/GaN single quantum well hosting a two-dimensional electron gas into micrometer-scale patches, we tailor the mutual interactions between topologically protected edge and bulk magnetoplasmons, achieving coupling strengths of up to 0.1. The progressive breaking of rotational symmetry determines the coupling rules and leads to the emergence of energy gaps and unique chiral-mode hybridization. Our results highlight a path towards exploiting the symmetry class and topology of the magnetoplasmon wavefunction as a new parameter space for ultrastrong light-matter interaction without a dedicated cavity.