In this paper we present a study of an exciton system where electrons and holes are confined in double quantum well structures. The dominating interaction between excitons in such systems is a dipole -dipole repulsion. We show that the tail of this interaction leads to a strong correlation between excitons and substantially affects the behavior of the system. Making use of qualitative arguments and estimates we develop a picture of the exciton -exciton correlations in the whole region of temperature and concentration where excitons exist. It appears that at low concentration degeneracy of the excitons is accompanied with strong multi-particle correlation so that the system cannot be considered as a gas. At high concentration the repulsion suppresses the quantum degeneracy down to temperatures that could be much lower than in a Bose gas with contact interaction.We calculate the blue shift of the exciton luminescence line which is a sensitive tool to observe the exciton -exciton correlations. d and the average separation between excitons are assumed to be larger than the exciton radius a X .

We report on new experiments and theory that unambiguously resolve the recent puzzling observation of large diameter exciton emission halos around a laser excitation spot in two dimensional systems. We find a novel separation of plasmas of opposite charge with emission from the sharp circular boundary between these two regions. This charge separation allows for cooling of initially hot optically generated carriers as they dwell in the charge reservoirs for very long times.

Dipolar excitons are long-lived quasi-particle excitations in semiconductor heterostructure that carry an electric dipole. Cold dipolar excitons are expected to have new quantum and classical multi-particle correlation regimes, as well as several collective phases, resulting from the intricate interplay between the many-body interactions and their quantum nature. Here we show experimental evidence of a few correlation regimes of a cold dipolar exciton fluid, created optically in a semiconductor bilayer heterostructure. In the higher temperature regime, the average interaction energy between the particles shows a surprising temperature dependence, which is evidence for correlations beyond the mean field model. At a lower temperature, there is a sharp increase in the interaction energy of optically active excitons, accompanied by a strong reduction in their apparent population. This is evidence for a sharp macroscopic transition to a dark state, as has been suggested theoretically.

Plasmonic devices with absorbance close to unity have emerged as essential building blocks for a multitude of technological applications ranging from trace gas detection to infrared imaging. A crucial requirement for such elements is the angle independence of the absorptive performance. In this work, we develop theoretically and verify experimentally a quantitative model for the angular behavior of plasmonic perfect absorber structures based on an optical impedance matching picture. To achieve this, we utilize a simple and elegant k-space measurement technique to record quantitative angle-resolved reflectance measurements on various perfect absorber structures. Particularly, this method allows quantitative reflectance measurements on samples where only small areas have been nanostructured, for example, by electron-beam lithography. Combining these results with extensive numerical modeling, we find that matching of both the real and imaginary parts of the optical impedance is crucial to obtain perfect absorption over a large angular range. Furthermore, we successfully apply our model to the angular dispersion of perfect absorber geometries with disordered plasmonic elements as a favorable alternative to current array-based designs.

We consider the design of two-dimensional electrostatic traps for dipolar indirect excitons. We show that the excitons dipole-dipole interaction, combined with the in-plane electric fields that arise due to the trap geometry, constrain the maximal density and lifetime of trapped excitons. We derive an analytic estimate of these values and determine their dependence on the trap geometry, thus suggesting the optimal design for high density trapping as a route for observing excitonic Bose-Einstein condensation.For many years, excitons in semiconductors had been predicted to undergo a phase transition at high enough densities and low enough temperatures to form a BoseEinstein condensate (BEC) 1,2,3 , similar to BEC of atomic gases, already observed a decade ago 8 . This is expected to happen due to the predicted bosonic nature of excitons at densities that still disguise the fermionic nature of their constituents, i.e., the electron and the hole. A few major obstacles have, however, prevented a clear observation of an exciton BEC phase until this day, even though the typical predicted transition temperature is of the order of a kelvin, much hotter than its atomic counterpart, and is available in many labs. Maybe the most crucial obstacle to excitonic BEC is the short exciton intrinsic radiative lifetime (of the order of hundred picosecond) due to electron-hole recombination, which limits the time available for exciton thermalization. Since the initial state of the exciton after optical excitation is out of equilibrium, and full thermalization with the lattice becomes more difficult at low lattice temperatures, the thermalization time turns out to be longer than the intrinsic exciton lifetime. Thus, the temperature of the exciton gas may not reach the required transition temperature to the condensed state, although the lattice temperature may in fact be well below that temperature. In recent years, a promising way to overcome the lifetime issue has emerged. The exciton lifetime can be considerably increased by spatially separating the electron and the hole. This is usually achieved by utilizing a double quantum well (DQW) system 4,5 . The resulting excitons are constructed from electrons in one layer and holes in the other and are known as "spatially indirect excitons". This trick can increase the exciton lifetime by many orders of magnitude (from less than a nanosecond to tens of microseconds) 6 while only slightly reducing the exciton binding energy (due to the 3D nature of the coulomb interaction).It seems that by utilizing these indirect excitons, the major obstacle to BEC has been removed. However, a new problem arises. The indirect excitons are dipolar in nature since they all carry a permanent dipole moment due to the charge separation of the electron and hole in the growth direction, perpendicular to the QW planes. All the dipoles are aligned in the same direction. As a result, there is a strong repulsive dipole-dipole interaction between all excitons. On one hand, this repulsive interaction has an additional...

One of the most important challenges in modern quantum optical applications is the demonstration of efficient, scalable, on-chip single photon sources, which can operate at room temperature. In this paper we demonstrate a room-temperature single photon source based on a single colloidal nanocrystal quantum dot positioned inside a circular bulls-eye shaped hybrid metal-dielectric nanoantenna. Experimental results show that 20% of the photons are emitted into a very low numerical aperture (NA < 0.25), a 20-fold improvement over a free-standing quantum dot, and with a probability of more than 70% for a single photon emission. With an NA = 0.65 more than 35% of the single photon emission is collected. The single photon purity is limited only by emission from the metal, an obstacle that can be bypassed with careful design and fabrication. The concept presented here can be extended to many other types of quantum emitters. Such a device paves a promising route for a high purity, high efficiency, on-chip single photon source operating at room temperature.

The possible phases and the nanoscale particle correlations of two-dimensional interacting dipolar particles is a long-sought problem in many-body physics. Here we observe a spontaneous condensation of trapped two-dimensional dipolar excitons with internal spin degrees of freedom from an interacting gas into a high density, closely packed liquid state made mostly of dark dipoles. Another phase transition, into a bright, highly repulsive plasma, is observed at even higher excitation powers. The dark liquid state is formed below a critical temperature Tc ≈ 4.8 K, and it is manifested by a clear spontaneous spatial condensation to a smaller and denser cloud, suggesting an attractive part to the interaction which goes beyond the purely repulsive dipole-dipole forces. Contributions from quantum mechanical fluctuations are expected to be significant in this strongly correlated, long living dark liquid. This is a new example of a two-dimensional atomic-like interacting dipolar liquid, but where the coupling of light to its internal spin degrees of freedom plays a crucial role in the dynamical formation and the nature of resulting condensed dark ground state.

Strong electrical interactions between hybrid light-matter quasiparticles in semiconductors can lead to new quantum applications.