The effect of ultra-strong coupling between magnetoplasma modes in a square of two-dimensional electrons

We have explored plasma excitations in a square-shaped two-dimensional electron system based on a high-quality AlGaAs/GaAs heterostructure. Our study has demonstrated the anticrossing of the fundamental (1, 0) and (1, 2) plasmon modes. Based on the obtained magnetodispersion data, we have determined that the Rabi frequency (mode splitting) reaches 33% of the unperturbed mode frequency. This finding suggests the realization of an ultra-strong coupling regime, which has promising applications in sensing and spectroscopy.