Breaking temporal symmetries in metamaterials and metasurfaces

Metamaterials are artificially structured materials that are engineered to interact with waves in extraordinary ways, leading to unconventional physical phenomena not found in natural materials, such as negative refraction and cloaking. So far, they have been for the most part based on structures that are inherently symmetric upon time reversal. In this work, I will explore the largely uncharted properties of electromagnetic and acoustic metamaterials that are designed to purposely break time-reversal symmetry. I will show how time-reversal symmetry breaking can be exploited to build a novel class of non-reciprocal acoustic and electromagnetic devices, such as isolators and circulators. These devices will then be used as building blocks to construct the acoustic equivalent of topological insulators, a metamaterial that supports one-way phononic transport on its edges with strong topological protection against defects and disorder. I will study the exceptional properties of time-asymmetric systems that fulfill a special kind of space-time symmetry, consisting in taking their mirror image and running time backwards. Known as Parity-Time (PT) symmetry, this property leads to anomalous scattering behaviors, such as unidirectional invisibility and phase compensation. I will demonstrate theoretically and experimentally how PT-symmetric metasurface pairs can replicate electromagnetic phenomena usually associated with bulk metamaterials, including negative refraction, planar focusing and invisibility effects, with the clear advantage of being completely loss-immune and potentially overcoming the bandwidth limitations of passive metamaterials.