Phys. Rev. X 5, 031001 (2015)

Torsten Karzig and co-workers from the USA and Israel have proposed a type of polariton with a 'winding' phase in momentum space that could enable unidirectional propagation. In their theoretical and numerical study they investigate excitons at the edges of semiconductor systems embedded into a photonic waveguide, using quantum wells or transition metal dichalcogenides (TMDs) and a magnetic field. CdTe quantum wells are proposed as a specific experimental candidate as they enable strong exciton–photon coupling. Another alternative proposed by the authors is monolayers of TMDs. The team describes conditions under which such thin TMDs should offer ∼20 meV exciton–photon coupling, which may enable similar topological gaps as CdTe quantum wells. It should be noted that strong magnetic fields (1.5 T) will be required to achieve such gaps with TMDs. The simulations by Karzig et al. show that when excited, the topological polaritonic edge states propagate chirally. Although immune from backscatter, exciton and photon losses lead to dissipation and decay over time.