Exceptional points and their coalescence of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi mathvariant="script">PT</mml:mi></mml:math>-symmetric interface states in photonic crystals

Abstract

Interface modes that exist in the band gap of Hermitian systems are well studied. This work looks at the interface modes localized at the loss-gain boundary of 𝓟𝓣-symmetric photonic crystals. These interface bands exhibit zigzag dispersions that exist only in open systems, and the turning points are found to be exceptional points, which are characteristic of non-Hermitian systems. By building a non-Hermitian Hamiltonian model and an effective medium theory, the authors analyze the evolution of the interface bands as the system parameters change.