Parity-time symmetric nonlocal metasurfaces: Focusing and imaging through balanced loss and gain

Light focusing and imaging represent some of the oldest applications of optics. Despite centuries of developments, however, conventional optical systems still suffer from aberrations of different types and are limited to plane-to-plane imaging, issues that are inherent to the conventional laws of reflection and refraction [1]. Metamaterials and metasurfaces have offered a route to partially overcome these limitations and go beyond traditional lens designs [2]. However, metamaterial lenses have been found to be extremely sensitive to material losses due to their typical resonant nature. This is particularly evident in ‘perfect lenses’ based on slabs with a negative index of refraction. Recently, there has been increasing interest in the possibility to realize relevant metamaterial functionalities, such as negative refraction and cloaking, based on loss-immune platforms that suitably include active elements [3-5].