Observation of Defect States in PT-Symmetric Optical Lattices

We provide the first experimental demonstration of defect states in parity-time ($\mathcal{P}\mathcal{T}$) symmetric mesh-periodic potentials. Our results indicate that these localized modes can undergo an abrupt phase transition in spite of the fact that they remain localized in a $\mathcal{P}\mathcal{T}$-symmetric periodic environment. Even more intriguing is the possibility of observing a linearly growing radiation emission from such defects provided their eigenvalue is associated with an exceptional point that resides within the continuum part of the spectrum. Localized complex modes existing outside the band-gap regions are also reported along with their evolution dynamics.