Abstract
The interaction between light and matter can give rise to novel topological states. This principle was recently exemplified in Floquet topological insulators, where classical light was used to induce a topological electronic band structure. Here, in contrast, we show that mixing single photons with excitons can result in new topological polaritonic states—or “topolaritons.” Taken separately, the underlying photons and excitons are topologically trivial. Combined appropriately, however, they give rise to nontrivial polaritonic bands with chiral edge modes allowing for unidirectional polariton propagation. The main ingredient in our construction is an exciton-photon coupling with a phase that winds in momentum space. We demonstrate how this winding emerges from the finite-momentum mixing between -type and -type bands in the electronic system and an applied Zeeman field. We discuss the requirements for obtaining a sizable topological gap in the polariton spectrum and propose practical ways to realize topolaritons in semiconductor quantum wells and monolayer transition metal dichalcogenides.
- Received 25 August 2014
DOI:https://doi.org/10.1103/PhysRevX.5.031001
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Published by the American Physical Society
Synopsis
Meet the Topolariton
Published 1 July 2015
Quasiparticles dubbed topological polaritons make their debut in the theoretical world.
See more in Physics
Popular Summary
The fact that light can propagate forward or backward (i.e., reciprocity) is part of our daily life experience: When we see someone, we can also be seen. Recently, scientists have started to explore ways of breaking this seemingly universal paradigm with the goal of creating one-way channels for light that could have applications, for example, as diodes in photonic circuits. Here, we propose a scheme to generate one-way photons by coupling ordinary ones to semiconductor excitons (bound pairs formed from a conduction-band electron and a valence-band hole). We describe how mixed exciton-photon states (or polaritons) can have a nontrivial topology, which can be thought of as “knots” encoded in their gapped band structure.
Our proposal to turn ordinary photons and excitons into topological polaritons relies on a winding in the exciton-photon coupling that provides a nontrivial mixing or “knotting” of exciton and photon bands. These knots have to be undone at the edge of the system, which leads to the presence of special edge modes. Remarkably, these modes are very robust and propagate in a single direction set by the winding of the knots (i.e., they are protected against backscattering). Similar chiral edge modes are well known in electronic systems, a prime example being quantum Hall states. Previous schemes to create analogs of quantum Hall states for light mostly relied on magneto-optical effects, however, limiting their applicability to the microwave regime.
Our scheme is not limited to excitons and photons, but can in principle also be used to generate nontrivial topology by mixing other types of bosonic bands. We expect that our results will be applicable to semiconductor quantum wells and monolayer transition metal dichalcogenides.