Experimental observation of exceptional surface in synthetic dimensions with magnon polaritons

Exceptional points (EPs) are singularities of eigen-energies in non-Hermitian systems. Intriguing phenomena have been previously observed around EPs. However, previous demonstrations are limited to 0-dimensional points and 1-dimensional lines. Here we report, to the best of our knowledge, the first experimental observation of an exceptional surface (ES) in a magnon polariton system. Magnon polaritons are hybrid excitations of electromagnetic waves and spin waves, which have recently emerged as a promising candidate for coherent information processing. We took advantage of the excellent tunability in magnon polaritons and introduced a 4-dimensional synthetic space, which enabled the observation of ES. In addition, we also observed an exceptional saddle point in the ES, which exhibits unique anisotropic behaviors in both the real and imaginary part of the eigenfrequencies. Our findings open up new opportunities for high-dimensional control of non-Hermitian systems and novel sensing applications.