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Physical Review Letters

Optical Traps for Dark Excitons

Monique Combescot, M. G. Moore, Carlo Piermarocchi

Published May 20, 2011• 10 citations
Publisher

Abstract

We propose a mechanism for optical trapping of dark excitons by linearly polarized unabsorbed standing waves, with a potential depth of the order of a few meV. Since this trapping, based on carrier exchanges with virtual excitons coupled to unabsorbed photons, equally acts on bright and dark states, Bose-Einstein condensation of excitons--which occurs in dark states--must appear as dark spots in a cloud of bright excitons, at the trap potential minima, when the temperature decreases.

DOI
10.1103/physrevlett.106.206404

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We propose a mechanism for optical trapping of dark excitons by linearly polarized unabsorbed standing waves, with a potential depth of the order of a few meV. Since this trapping, based on carrier exchanges with virtual excitons coupled to unabsorbed photons, equally acts on bright and dark states, Bose-Einstein condensation of excitons--which occurs in dark states--must appear as dark spots in a cloud of bright excitons, at the trap potential minima, when the temperature decreases.
We study spatially indirect excitons confined in a 10 microns wide electrostatic trap. At sub-Kelvin bath temperatures, we show that the optical emission from the trap is anomalously weak in a regime where the excitons photoluminescence is homogeneously broadened and narrow-band. Particularly, we observe a loss of photoluminescence below a few Kelvin which violates classical expectations. Through a careful calibration of our experiments, we show that the photoluminescence loss can only be interpreted as the signature for Bose-Einstein stimulation of excitons from bright to dark states.
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The formation of a dense Bose-Einstein condensate in dark spin states of two-dimensional dipolar excitons is shown to be driven by a dynamical transition to the long-lived dark states. The condensate is stabilized by strong dipole-dipole interactions up to densities high enough for a dark quantum liquid to form. The persistence of dark condensation was observed in recent experiments. A model describing the non-equilibrium dynamics of externally driven coupled dark and bright condensates reproduces the step-like dependence of the exciton density on the pump power or on temperature. This unique condensate dynamics demonstrates the possibility of observing new unexpected collective phenomena in coupled condensed Bose systems, where the particle number is not conserved.
We predict that the collision of two fully dark exciton condensates produces bright interference fringes. So, quite surprisingly, the collision of coherent dark states makes light. This remarkable effect, which is many body in essence, comes from the composite boson nature of excitons, through the fermion exchanges they can have which transform dark states into bright states. The possibility of optically detecting quantum coherence in a regime where the system is hidden by its total darkness was up to now considered as hopeless.
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 onedimensional 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 at the same atomic state, which is included here through a bosonization procedure with kinematic interactions.Interestingly these interactions couple dark and bright excitons, and 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 found to be correlated and are represented as dark bogolon-excitons.The results can be adapted for any super-lattice of active materials, e.g., of organic molecules.