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At low temperatures, indirect excitons formed at the in-plane electron-hole interface in a coupled-quantum-well structure undergo a spontaneous transition into a spatially modulated state. We report on the control of the instability wavelength, measurement of the dynamics of the exciton emission pattern, and observation of the fluctuation and commensurability effect of the exciton density wave. We found that fluctuations are suppressed when the instability wavelength is commensurate with defect separation along the exciton density wave. The commensurability effect is also found in numerical simulations within the model describing the exciton density wave in terms of an instability due to stimulated processes. © 2015 American Physical Society.

We report the observation of spin currents in a coherent gas of indirect excitons. The realized long-range spin currents originate from the formation of a coherent gas of bosonic pairs - a new mechanism to suppress the spin relaxation. The spin currents result in the appearance of a variety of polarization patterns, including helical patterns, four-leaf patterns, spiral patterns, bell patterns, and periodic patterns. We demonstrate control of the spin currents by a magnetic field. We also present a theory of coherent exciton spin transport that describes the observed exciton polarization patterns and indicates the trajectories of the spin currents. © 2013 American Physical Society.

We report on the study of spatiotemporal self-organization of carriers in photoexcited GaAs quantum wells. Propagating interfaces between electron-rich and hole-rich regions are seen as expanding and collapsing exciton rings in exciton emission patterns. The interfaces preserve their integrity during expansion, remaining as sharp as in the steady state, which indicates that the dynamics is controlled by carrier transport. The front propagation velocity is measured and compared to theoretical model. The measurements of expanding and collapsing exciton rings afford a contactless method for probing the electron and hole transport. © 2010 The American Physical Society.

We evaluate the effect of diffraction-limited resolution of the optical system on the estimates of the coherence length of two-dimensional excitons deduced from the interferometric study of the exciton emission. The results are applied for refining our earlier estimates of the coherence length of a cold gas of indirect excitons in coupled quantum wells. We show that the apparent coherence length is well approximated by the quadratic sum of the actual exciton coherence length and the diffraction correction given by the conventional Abbe limit divided by π. In practice, accounting for diffraction is necessary only when the coherence length is smaller than about one wavelength. The earlier conclusions regarding the strong enhancement of the exciton coherence length at low temperatures remain intact. © 2008 The American Physical Society.

We investigated emission from single CdSe nanocrystals in poly(methyl methacrylate). A fraction of the nanocrystals exhibited switching between two energy states which had similar total intensities but distinctly different spectra. We found that the characteristic frequency of this spectral shift increased with the pump power. By using the dynamic shift in the spectral position of emission peaks, we were able to correlate peaks from the same nanocrystal. The measured correlation is consistent with assignment of low energy lines to phonon replicas. © 2007 The American Physical Society.

The macroscopically ordered exciton state (MOES)-a periodic array of beads with spatial order on a macroscopic length-appears in the external exciton rings in coupled GaAs (Al,Ga)As quantum wells at low temperatures below a few Kelvin. Here, we report on the experimental study of the interaction in the MOES. The exciton photoluminescence energy varies in concert with the intensity along the circumference of the ring, with the largest energy found in the brightest regions. This shows that the MOES is characterized by the repulsive interaction and is not driven by the attractive interaction. © 2007 The American Physical Society.

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas. © 2006 The American Physical Society.

We consider in-plane electrostatic traps for indirect excitons in coupled quantum wells, where the traps are formed by a laterally modulated gate voltage. An intrinsic obstacle for exciton confinement in electrostatic traps is an in-plane electric field that can lead to exciton dissociation. We propose a design to suppress the in-plane electric field and, at the same time, to effectively confine excitons in the electrostatic traps. We present calculations for various classes of electrostatic traps and experimental proof of principle for trapping of indirect excitons in electrostatic traps. © 2006 American Institute of Physics.