The physical behavior of colloids suspended in a fluid close to a solid wall has been well characterized in the past[1]. Conversely the dynamical behavior of a particle close to a fluid interface is a relatively new field with strong implications in green and food processing [2]. Previous focus was limited to spherical particle’s viscous drag close to the interface [3] and wetting dynamics after interface breaching [4]. In this paper we present experimental results on the dynamics of micrometric colloids(spheres and ellipsoids)close to an interface between water based solution and air prior to the particle’s interfacebreaching. In order to study such dynamics we use a home-made pending-drop setup where particles dispersed in water based solutions (mixture with heavy water and/or with salt at different molarity) sediment towards the interface with air of the drop.The drop curvature is adjusted via the water solution pressure in order to obtain a flat interface at the particle scale. All the particle’s degreesof freedom (i.e. distance from the interface, position in a plane parallel to the interface and, for ellipsoids,orientation) are measured tracking the particle-interface interference patterns using incoherent ledswith three different wavelengths. The typical dynamics of a particle approaching the interface shows an initial sedimentation regimedriven by gravity and viscous drag. Closer to the interface a second regime arises which strongly depend on the solution molarity and density and particle morphology. For both particles morphologies the sedimentation dynamics stops at a distance from the interface of the order of hundreds nanometers. At small molarity (0.5 mM) both type of particles fluctuate around this position during the entire experimental observation time interval (10^3s). Increasing the molarity, ellipsoids more easily then spheres start to escape from this position approaching rapidly the interface till a distance of few nanometers. Such observations point out the importance of the electrostatic and morphology in the interaction with the interface. A calibration of thisinteraction will be presented by taking advantage of the buoyancy driving force changing the solution density. Building up on all these experimental observations, amodel considering the interplay between all the contributing interactions (i.e. gravity with buoyancy, electrostatics and Van der Walls) will be discussed and compared to the experimental results. [1]M. Hosoda, K. Sakai, and K. Takagi, Phys. Rev. E 58, 6275 (1998).[2] V. Sarrot, Z. Huang, D. Legendre, P. Guiraud, Chem. Eng. Sci. 62, 7359–7369 (2007).[3] G. M. Wang, R. Prabhakar and E. M. Sevick, Phys. Rev. Lett. 103, 248303 (2009).[4] A. Wang, W.B. Rogers, V.N. Manoharan, arXiv:1607.08167 [cond-mat.soft].

This special issue contains 9 original review papers, research papers and discussion papers on indirect excitons. An exciton is a Coulomb-correlated electron-hole pair. Frenkel excitons dominate optical properties of organic semiconductors, while Wannier-Mott excitons are responsible for the hydrogen-like absorption spectra of inorganic semiconductors at low temperatures. The interest to the physics of excitons has strongly increased in the new century. This interest is motivated by unique bosonic properties of excitons that lead to the phenomena of exciton-polariton lasing and stimulated scattering, build-up of the spontaneous coherence and polarisation in cold exciton gases. In addition to the rich fundamental physics, excitons offer the perspective of applications in opto-electronic devices such as exciton transistors, switches, optical integrated circuits, etc.The indirect excitons form an important research sub-field of the exciton physics. The reduction of electron-hole overlap in the real space leads to longer exciton radiative lifetimes, which is crucial for the studies of thermolyzed exciton gases. Large spatial dipole moments of indirect excitons are responsible for the strong exciton-exciton repulsion. This prevents formation of bi-excitons and excitonic molecules and facilitates formation of coherent collective excitonic states.The papers contained in this volume are devoted to different aspects of the modern physics of indirect excitons. In the review paper by Leonid Butov the intriguing perspective for implementation of excitonic device concepts is discussed. This review is nicely complemented by the work by Anton Nalitov et al. who propose using indirect excitons for realisation of bosonic cascade lasers emitting the coherent terahertz light. The review of Joe Wilkes and Egor Muljarov discusses the effect of electric and magnetic fields on indirect exciton and microcavity exciton-polaritons. Ursula Wurstbauer et al. present the experimental study of laterally trapped dipolar exciton ensembles at high magnetic fields in the Faraday configuration. Colin Hubert et al. study experimentally the dynamics of indirect excitons in GaAs/AlGaAs double quantum wells grown along the (111) direction. Tao Liu et al. study theoretically the spin dynamics of spatially trapped Bose-Einstein condensates of indirect excitons. Carsten Leding Vendelbjerg and Massimo Rontani present the theoretical research of the spin transfer torque induced onto a nano-magnet in hybrid magnetic nanostructures. Andrea Bertoni et al. address the time-dependent quantum scattering of a spatially indirect exciton by an external potential. Finally, Yotam Mazuz-Harpaz et al. analyse and discuss the recent experimental results on cold dipolar exciton gases in electrostatic traps.The nine groups that contributed to this special issue are the world-leaders in the rapidly growing research field of the physics of indirect excitons. The papers that composed this volume represent the state-of-the-art in this research domain. We hope that this volume may be used by young researchers and PhD students as well as the leading scientists as a reference book for further studies of indirect excitons.

Light emission in polar group-III nitride quantum wells optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Such excitons are characterized by a non-zero dipole moment and long radiative lifetimes, because their constituent electron and hole are spatially separated by the built-in electric field in the growth direction. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.

Light emission in polar group-III nitride quantum wells (QWs) optically pumped at densities significantly below the lasing threshold is usually interpreted in terms of excitons. Whereas thin GaN QWs with fast radiative lifetimes are employed in light-emitting devices, the excitons in thick QWs (typ. 7nm) are characterized by a non-zero dipole moment and long radiative lifetimes, due to the built-in electric field. These features result in many interesting properties of dipolar excitons: they can propagate over large distances and thus cool down to the lattice temperature before recombination, offering the possibility for studies of cold and dense gas of interacting bosons. In this context, two important issues must be addressed: the strength of the exciton-exciton interaction, and the excitonic Mott transition, that sets the maximum density of excitons. Combining spatially and time-resolved photoluminescence in different excitation geometries, we provide a set of experimental data showing that the emission linewidth is proportional to the emission energy in a wide range of carrier densities and temperatures. The proportionality coefficient is almost temperature independent. These results are compared to the existing data in various material systems and discussed in terms of the Mott transition and its criteria, exciton-exciton interaction and its possible contribution to the linewidth, as well as the role of disorder in the density-dependent emission spectra.

Semiconductor coupled quantum wells (CQWs) offer an excellent laboratory for studying both intra-well direct excitons (DX) and inter-well indirect excitons (IX), as well as their interactions and spin dynamics. The presentation will focus on the pump-probe spectroscopy, a powerful tool of nonlinear optics, that we have applied to biased CQW. We will show how DX and IX spin and population dynamics, as well as the spin polarization of residual electrons may be detected via the modulation of reflectivity and Kerr rotation spectra. In particular, the origin of the nonlinearities related to the low oscillator strength IXs will be addressed. Spin relaxation and decoherence mechanisms for DX, IX and electron gas will be presented. Results on IXs created via magnetic Stark effect and the resulting spin dynamics in unbiased quantum wells will be discussed