Tiago Morgado, from IT in Coimbra, won the Young Scientist Award (YSA) in the Photonics & Electromagnetics Research Symposium (PIERS 2019), held in Rome from 17 - 20 June 2019. This award is given to young scientists under 40 years with a distinguished work in a field of photonics and/or electromagnetism. Tiago Morgado was chosen among several candidates in the field of Metamaterials, Plasmonics and Complex Media with the research work entitled “Drift-induced Nonreciprocal Graphene Plasmonics”. In this research work, it is proposed a new route to break Lorentz reciprocity and achieve unidirectional propagation of light at the nanoscale.

Light propagation in conventional photonic platforms is constrained by the Lorentz reciprocity law. This fundamental principle imposes that the level of light transmission is the same for symmetric propagation directions. Thereby, reciprocal photonic systems are intrinsically bidirectional.

Breaking Lorentz reciprocity and achieve one-way light flows typically requires using ferrimagnetic materials biased by a static magnetic field. However, these solutions are far from being satisfactory, as the required biasing circuits are bulky and challenging to incorporate in highly-integrated photonic systems. Due to this, there has been a great deal of interest in developing magnetless nonreciprocal solutions that can be straightforwardly integrated in nanophotonic systems.

“We have recently proposed a novel paradigm to achieve magnetic-free nonreciprocal light propagation at the nanoscale. Inspired by the nonreciprocal responses provided by moving media, we have theoretically demonstrated that a drift-current biasing of a graphene sheet may mimic a translational motion and may lead to broadband tunable regimes of unidirectional propagation of light. For sufficiently strong drift currents, the plasmon waves supported by the graphene sheet are dragged by the drifting electrons so that they can only propagate along the direction of the drift current, even when there are obstacles placed along the propagation path. Therefore, the drift-induced unidirectional graphene plasmons are strongly immune to backscattering”, explains Tiago Morgado. “Moreover, we have demonstrated that the drift-current biasing may give rise to regimes of optical gain wherein the graphene plasmons are pumped by the drifting electrons. By taking advantage of this gain effect, it is possible to have lossless propagation or even amplification of the plasmon waves in nanostructures formed by a drift-current biased graphene sheet coupled to a resonant slab (e.g., a SiC substrate)”, Tiago Morgado concludes.

These findings open new inroads in nonreciprocal and active nanophotonics and offer exciting opportunities to control the flow of light at the nanoscale.