Dark excitons outnumber bright ones

Noriaki Horiuchi

Hashem Zoubi

Dark and bright excitons are shown to appear naturally in a linear atomic super-lattice with two atoms per unit cell.In bringing the super-lattice into a strong coupling regime with a one-dimensional nanophotonic waveguide, bright excitons and photons are coherently mixed to form polaritons. Treating excitons as bosons implies a mechanism that forbids two excitations from being in the same atomic state, which is included here through a bosonization procedure with kinematic interactions.Interestingly these interactions couple dark and bright excitons, which we exploit as a new tool for exciting dark states in a controllable way.We suggest a pump-probe experiment where two polaritons scatter into two dark excitons that are found to be correlated and are represented as dark bogolon excitons.The results can be adapted for any super-lattice of active materials; for example, of organic molecules.

Yuchen Han, David Hsieh

V. V. Konotop, Sachio Takeno

Dark and bright soliton solutions (excitons) in one-dimensional chains of two-level atoms with exciton-exciton, dipole-dipole, and exchange interactions are obtained. It is shown that in some limiting cases evolution equations resulting from model Hamiltonians are reduced to the exactly integrable Ablowitz-Ladik model. Bloch oscillations of the excitons in inhomogeneous magnetic field and effects originated by the energy shift are discussed. An example of a multidimensional dark exciton is represented.

Monique Combescot, Shiue-Yuan Shiau

Chapter 19 addresses bosonic condensation in composite bosons. It first considers the coboson ground-state energy, which is analytically known by now in the case of N Cooper pairs interacting via the reduced BCS potential but not yet known for N Wannier or Frenkel excitons, owing to the complexity of the Coulomb potential. Still, the chapter provides hints on how to derive the density linear term of the N-exciton ground-state energy in the case of Wannier excitons. It also discusses the role of the exciton composite nature in condensate momentum fragmentation, spin fragmentation, and condensate brightness. Indeed, the excitons that have the lowest energy and thus are the ones expected to condense are not coupled to light; so, the exciton condensate must be dark. However, a bright exciton condensate can appear, coherent with the dark exciton condensate, above a density threshold, owing to coupling induced by carrier exchange between dark excitons and bright excitons.

Hermann Haken, Haken, Hermann