Symmetries and broken symmetries play important roles in physics. In particular, they constrain the electromagnetic response of materials and the allowed light-matter interactions. In such a context, symmetry breaking can lead to unique and peculiar physical phenomena. The Hall effects, for example, result from broken symmetries. In the usual linear Hall effects, the time-reversal symmetry is broken by a magnetic bias. Alternatively, nonlinear Hall effects can occur in systems with a broken inversion symmetry rather than broken time-reversal symmetry.
Here we explore several physical platforms that can enable nonreciprocal and non-Hermitian responses based on the nonlinear Hall effect and 2D material layers biased with a static electric field. It will be shown that the electric field bias may create unique physical responses, including regimes of loss and gain controlled by the wave-polarization, asymmetric responses, and others. In this talk, we will present an overview of our on-going work on this topic.
It is experimentally verified that nonreciprocal photonic systems with continuous translation symmetry may have an ill-defined topology. The topological classification of such systems is only feasible when the material response is regularized with a spatial-frequency cutoff. We experimentally demonstrate that adjoining a small air layer to the relevant material interface may effectively imitate an idealized spatial cutoff that suppresses the nonreciprocal response for short wavelengths and regularizes the topology. Furthermore, it is experimentally verified that nonreciprocal systems with an ill-defined topology may be used to abruptly halt the energy flow in a unidirectional waveguide due to the violation of the bulk-edge correspondence. In particular, we report the formation of an energy sink that absorbs the incoming electromagnetic waves with a large field enhancement at the singularity.
Achieving nonreciprocal light propagation is of fundamental importance in photonic devices and systems. Nonreciprocal effects are typically obtained using bulky magneto-optical materials externally biased by a static magnetic field. Notably, it was recently demonstrated that some of these magnetically-biased systems with a broken-time reversal symmetry have nontrivial topological properties and support unidirectional backscattering immune chiral edge modes. Nevertheless, the required external magnetic bias, together with the relatively weak gyrotropic responses achievable at optical frequencies, makes the integration of such elements in nanophotonic systems extremely difficult. Because of this, there has been recently a great effort in the development of magnetic-free solutions that give nonreciprocal responses and are fully compatible with modern highly-integrated photonic systems.
Here we propose a novel route to achieve magnetic-free nonreciprocal subwavelength light propagation. We theoretically demonstrate that by biasing a graphene sheet with a direct electric current it is possible to break the Lorentz reciprocity and have a broadband regime of unidirectional propagation of surface plasmons. Remarkably, we prove that the drift-current biasing also enables enhancing the propagation length of the graphene plasmons. Furthermore, it is shown that the surface plasmons supported by the graphene sheet with a drift current are protected against backscattering from obstacles and imperfections, similar to the “one-way” topologically protected chiral edge modes supported by topological photonic systems. We believe that these findings may open a new and exciting opportunity towards the full integration of nonreciprocal components in nanophotonic systems.
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