Observation of PT-symmetric exceptional point from magnetoelectric bianisotropy

Abstract

Recently, we have witnessed a wave of investigations on PT-symmetric optical systems due to the promising features of tunable optical properties given by exceptional points and PT-phase transitions. Although many exotic optical properties have been demonstrated with PT-symmetry, some of these demonstrated properties are commonly believed not special to PT-symmetry. For example, unidirectional zero reflection [1, 2], which can occur at the exceptional point of PT-symmetry, is such a property. A typical bianisotropic medium (medium with cross coupling between magnetic and electric resonance) yields asymmetric reflection and therefore can also have unidirectional zero reflection when the impedance is only matched in one direction. Thus, the question arises: Can these systems, which naturally give rise to asymmetric light propagation, also have an underlying PT-symmetry? If this is the case, it should also be possible to establish the associated PT-symmetric Hamiltonian for these systems. Here, we propose a microwave transmission-line to simulate the bianisotropic medium [3], which exhibits unidirectional zero reflection at a particular condition. Such a system, seemingly lacking even a mirror symmetry in the lossless situation, is experimentally shown to have a PT-symmetric Hamiltonian by defining a new parity operator with magnetoelectric coupling [4]. The results reveal the necessary “hidden” PT-symmetry of any systems with unidirectional zero reflection and also open up a simple route to realize and explore exceptional point behavior of PT-symmetric system.