Long-range transport in excitonic dark states in coupled quantum wells

Ultrafast exciton fluid flow in an atomically thin MoS2 semiconductor

Excitons (coupled electron-hole pairs) in semiconductors can form collective states that sometimes exhibit spectacular nonlinear properties. Here, we show experimental evidence of a collective state of short-lived excitons in a direct-bandgap, atomically thin MoS2 semiconductor whose propagation resembles that of a classical liquid as suggested by the nearly uniform photoluminescence through the MoS2 monolayer regardless of crystallographic defects and geometric constraints. The exciton fluid flows over ultralong distances (at least 60 μm) at a speed of ~1.8 × 107 m s-1 (~6% the speed of light). The collective phase emerges above a critical laser power, in the absence of free charges and below a critical temperature (usually Tc ≈ 150 K) approaching room temperature in hexagonal-boron-nitride-encapsulated devices. Our theoretical simulations suggest that momentum is conserved and local equilibrium is achieved among excitons; both these features are compatible with a fluid dynamics description of the exciton transport.

Diffusion-controlled annihilation A+B→0: Coalescence, fragmentation, and collapse of nonidentical A-particle islands submerged in the B-particle sea

We present a systematic analysis of diffusion-controlled interaction and collapse of two nonidentical spatially separated d-dimensional A-particle islands in the B-particle sea at propagation of the sharp reaction front A+B→0 at equal species diffusivities. We show that at a sufficiently large initial distance between the centers of islands 2ℓ and a relatively large initial ratio of island-to-sea concentrations, the evolution dynamics of the island-sea-island system demonstrates remarkable universality and, depending on the system dimension, is determined unambiguously by two dimensionless parameters Λ=N_{0}^{+}/N_{Ω} and q=N_{0}^{-}/N_{0}^{+}, where N_{0}^{+} and N_{0}^{-} are the initial particle numbers in the larger and smaller of the islands, respectively, and N_{Ω} is the initial number of sea particles in the volume Ω=(2ℓ)^{d}. We find that at each fixed 0<q≤1, there are threshold values Λ_{★}(q) and Λ_{s}(q)≥Λ_{★}(q) that depend on the dimension and separate the domains of individual death of each of the islands Λ<Λ_{★}(q), coalescence and subsequent fragmentation (division) of a two-centered island Λ_{★}(q)<Λ<Λ_{s}(q), and collapse of a single-centered island formed by coalescence Λ>Λ_{s}(q). We demonstrate that regardless of d, the trajectories of the island centers are determined unambiguously by the parameter q, and we reveal a detailed picture of the evolution of islands and front trajectories with an increase in Λ, focusing on the scaling laws of evolution at the final collapse stage and in the vicinity of starting coalescence and fragmentation points.

Photo-induced macro/mesoscopic scale ion displacement in mixed-halide perovskites: ring structures and ionic plasma oscillations

Contrary to the common belief that the light-induced halide ion segregation in a mixed halide alloy occurs within the illuminated area, we find that the Br ions released by light are expelled from the illuminated area, which generates a macro/mesoscopic size anion ring surrounding the illuminated area, exhibiting a photoluminescence ring. This intriguing phenomenon can be explained as resulting from two counter-balancing effects: the outward diffusion of the light-induced free Br ions and the Coulombic force between the anion deficit and surplus region. Right after removing the illumination, the macro/mesoscopic scale ion displacement results in a built-in voltage of about 0.4 V between the ring and the center. Then, the displaced anions return to the illuminated area, and the restoring force leads to a damped ultra-low-frequency oscillatory ion motion, with a period of about 20-30 h and lasting over 100 h. This finding may be the first observation of an ionic plasma oscillation in solids. Our understanding and controlling the "ion segregation" demonstrate that it is possible to turn this commonly viewed "adverse phenomenon" into novel electronic applications, such as ionic patterning, self-destructive memory, and energy storage.

Organic Nanostructured Materials for Sustainable Application in Next Generation Solar Cells

Meeting our current energy demands requires a reliable and efficient renewable energy source that will bring balance between power generation and energy consumption. Organic photovoltaic cells (OPVs), perovskite solar cells and dye-sensitized solar cells (DSSCs) are among the next-generation technologies that are progressing as potential sustainable renewable energy sources. Since the discoveries of highly conductive organic charge-transfer compounds in the 1950s, organic semiconductors have captured attention. Organic photovoltaic solar cells possess key characteristics ideal for emerging next-generation technologies such as being nontoxic, abundant, an inexpensive nanomaterial with ease of production, including production under ambient conditions. In this review article, we discuss recent methods developed towards improving the stability and average efficiency of nanostructured materials in OPVs aimed at sustainable agriculture and improve land-use efficiency. A comprehensive overview on developing cost-effective and user-friendly organic solar cells to contribute towards improved environmental stability is provided. We also summarize recent advances in the synthetic methods used to produce nanostructured active absorber layers of OPVs with improved efficiencies to supply the energy required towards ending poverty and protecting the planet.

Microcavity-like exciton-polaritons can be the primary photoexcitation in bare organic semiconductors

Strong-coupling between excitons and confined photonic modes can lead to the formation of new quasi-particles termed exciton-polaritons which can display a range of interesting properties such as super-fluidity, ultrafast transport and Bose-Einstein condensation. Strong-coupling typically occurs when an excitonic material is confided in a dielectric or plasmonic microcavity. Here, we show polaritons can form at room temperature in a range of chemically diverse, organic semiconductor thin films, despite the absence of an external cavity. We find evidence of strong light-matter coupling via angle-dependent peak splittings in the reflectivity spectra of the materials and emission from collective polariton states. We additionally show exciton-polaritons are the primary photoexcitation in these organic materials by directly imaging their ultrafast (5 × 10⁶m s^-1), ultralong (~270 nm) transport. These results open-up new fundamental physics and could enable a new generation of organic optoelectronic and light harvesting devices based on cavity-free exciton-polaritons.

Twist-angle-dependent interlayer exciton diffusion in WS2-WSe2 heterobilayers

The nanoscale periodic potentials introduced by moiré patterns in semiconducting van der Waals heterostructures have emerged as a platform for designing exciton superlattices. However, our understanding of the motion of excitons in moiré potentials is still limited. Here we investigated interlayer exciton dynamics and transport in WS₂-WSe₂ heterobilayers in time, space and momentum domains using transient absorption microscopy combined with first-principles calculations. We found that the exciton motion is modulated by twist-angle-dependent moiré potentials around 100 meV and deviates from normal diffusion due to the interplay between the moiré potentials and strong exciton-exciton interactions. Our experimental results verified the theoretical prediction of energetically favourable K-Q interlayer excitons and showed exciton-population dynamics that are controlled by the twist-angle-dependent energy difference between the K-Q and K-K excitons. These results form a basis to investigate exciton and spin transport in van der Waals heterostructures, with implications for the design of quantum communication devices.

Diffusion-controlled coalescence, fragmentation, and collapse of d-dimensional A-particle islands in the B-particle sea

We present a systematic analysis of diffusion-controlled interaction and collapse of two identical spatially separated d-dimensional A-particle islands in the B-particle sea at propagation of the sharp reaction front A+B→0 at equal species diffusivities. We show that at a sufficiently large initial distance between the centers of islands 2ℓ compared to their characteristic initial size and a relatively large initial ratio of island to sea concentrations, the evolution dynamics of the island-sea-island system is determined unambiguously by the dimensionless parameter Λ=N_{0}/N_{Ω}, where N_{0} is the initial particle number in the island and N_{Ω} is the initial number of sea particles in the volume Ω=(2ℓ)^{d}. It is established that (a) there is a d-dependent critical value Λ_{★} above which island coalescence occurs; (b) regardless of d the centers of each of the islands move toward each other along a universal trajectory merging in a united center at the d-dependent critical value Λ_{s}≥Λ_{★}; (c) in one-dimensional systems Λ_{★}=Λ_{s}, therefore, at Λ<Λ_{★} each of the islands dies individually, whereas at Λ>Λ_{★} coalescence is completed by collapse of a single-centered island in the system center; (d) in two- and three-dimensional systems in the range Λ_{★}<Λ<Λ_{s} coalescence is accompanied by subsequent fragmentation of a two-centered island and is completed by individual collapse of each of the islands. We discuss a detailed picture of coalescence, fragmentation, and collapse of islands focusing on evolution of their shape and on behavior of the relative width of the reaction front at the final collapse stage and in the vicinity of starting coalescence and fragmentation points. We demonstrate that in a wide range of parameters, the front remains sharp up to a narrow vicinity of the coalescence, fragmentation, and collapse points.

Precursors to Exciton Condensation in Quantum Hall Bilayers

Tunneling spectroscopy reveals evidence for interlayer electron-hole correlations in quantum Hall bilayer two-dimensional electron systems at layer separations near, but above, the transition to the incompressible exciton condensate at total Landau level filling ν_{T}=1. These correlations are manifested by a nonlinear suppression of the Coulomb pseudogap which inhibits low energy interlayer tunneling in weakly coupled bilayers. The pseudogap suppression is strongest at ν_{T}=1 and grows rapidly as the critical layer separation for exciton condensation is approached from above.

Strongly interacting dipolar-polaritons

Exciton-polaritons are mutually interacting quantum hybridizations of confined photons and electronic excitations. Here, we demonstrate a system of optically guided, electrically polarized exciton-polaritons ("dipolaritons") that displays up to 200-fold enhancement of the polariton-polariton interaction strength compared to unpolarized polaritons. The magnitude of the dipolar interaction enhancement can be turned on and off and can be easily tuned over a very wide range by varying the applied polarizing electric field. The large interaction strengths and the very long propagation distances of these fully guided dipolaritons open up new opportunities for realizing complex quantum circuitry and quantum simulators, as well as topological states based on exciton-polaritons, for which the interactions between polaritons need to be large and spatially or temporally controlled. The results also raise fundamental questions on the origin of these large enhancements.

Enhanced Absorption by Linewidth Narrowing in Optically Excited Type-II Semiconductor Heterostructures

We experimentally report a surprising linewidth narrowing of the direct exitonic 1  s heavy-hole transition in a type-II quantum well system. This narrowing, which builds up on a pico- to nanosecond timescale, causes a transient enhanced absorption at the spectral peak position of the excitonic resonance. We discuss how this effect depends on experimental parameters such as excitation density, temperature, and barrier width. We cannot attribute this effect to known physical mechanisms.

Exciton Diffusion and Halo Effects in Monolayer Semiconductors

We directly monitor exciton propagation in freestanding and SiO_{2}-supported WS_{2} monolayers through spatially and time-resolved microphotoluminescence under ambient conditions. We find a highly nonlinear behavior with characteristic, qualitative changes in the spatial profiles of the exciton emission and an effective diffusion coefficient increasing from 0.3 to more than 30  cm^{2}/s, depending on the injected exciton density. Solving the diffusion equation while accounting for Auger recombination allows us to identify and quantitatively understand the main origin of the increase in the observed diffusion coefficient. At elevated excitation densities, the initial Gaussian distribution of the excitons evolves into long-lived halo shapes with μm-scale diameter, indicating additional memory effects in the exciton dynamics.

Experimental Study of the Exciton Gas-Liquid Transition in Coupled Quantum Wells

We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, T_{C}=4.8  K, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the density-temperature phase diagram over the temperature range 0.1-4.8 K. We find that the latent heat increases linearly with temperature at T≲1.1  K, similarly to a Bose-Einstein condensate transition, and becomes constant at 1.1≲T<4.8  K. Resonant Rayleigh scattering measurements reveal that the disorder in the sample is strongly suppressed and the diffusion coefficient sharply increases with decreasing temperature at T<T_{C}, allowing the liquid to spread over large distances away from the excitation region. We suggest that our findings are manifestations of a quantum liquid behavior.

Diffusion-controlled formation and collapse of a d-dimensional A-particle island in the B-particle sea

We consider diffusion-controlled evolution of a d-dimensional A-particle island in the B-particle sea at propagation of the sharp reaction front A+B→0 at equal species diffusivities. The A-particle island is formed by a localized (point) A-source with a strength λ that acts for a finite time T. We reveal the conditions under which the island collapse time t_{c} becomes much longer than the injection period T (long-living island) and demonstrate that regardless of d the evolution of the long-living island radius r_{f}(t) is described by the universal law ζ_{f}=r_{f}/r_{f}^{M}=sqrt[eτ|lnτ|], where τ=t/t_{c} and r_{f}^{M} is the maximal island expansion radius at the front turning point t_{M}=t_{c}/e. We find that in the long-living island regime the ratio t_{c}/T changes with the increase of the injection period T by the law ∝(λ^{2}T^{2-d})^{1/d}, i.e., increases with the increase of T in the one-dimensional (1D) case, does not change with the increase of T in the 2D case and decreases with the increase of T in the 3D case. We derive the scaling laws for particles death in the long-living island and determine the limits of their applicability. We demonstrate also that these laws describe asymptotically the evolution of the d-dimensional spherical island with a uniform initial particle distribution generalizing the results obtained earlier for the quasi-one-dimensional geometry. As striking results, we present a systematic analysis of the front relative width evolution for fluctuation, logarithmically modified, and mean-field regimes, and we demonstrate that in a wide range of parameters the front remains sharp up to a narrow vicinity of the collapse point.

Bose-Einstein condensation and indirect excitons: a review

We review recent progress on Bose-Einstein condensation (BEC) of semiconductor excitons. The first part deals with theory, the second part with experiments. This Review is written at a time where the problem of exciton Bose-Einstein condensation has just been revived by the understanding that the exciton condensate must be dark because the exciton ground state is not coupled to light. Here, we theoretically discuss this missed understanding before providing its experimental support through experiments that scrutinize indirect excitons made of spatially separated electrons and holes. The theoretical part first discusses condensation of elementary bosons. In particular, the necessary inhibition of condensate fragmentation by exchange interaction is stressed, before extending the discussion to interacting bosons with spin degrees of freedom. The theoretical part then considers composite bosons made of two fermions like semiconductor excitons. The spin structure of the excitons is detailed, with emphasis on the crucial fact that ground-state excitons are dark: indeed, this imposes the exciton Bose-Einstein condensate to be not coupled to light in the dilute regime. Condensate fragmentations are then reconsidered. In particular, it is shown that while at low density, the exciton condensate is fully dark, it acquires a bright component, coherent with the dark one, beyond a density threshold: in this regime, the exciton condensate is 'gray'. The experimental part first discusses optical creation of indirect excitons in quantum wells, and the detection of their photoluminescence. Exciton thermalisation is also addressed, as well as available approaches to estimate the exciton density. We then switch to specific experiments where indirect excitons form a macroscopic fragmented ring. We show that such ring provides efficient electrostatic trapping in the region of the fragments where an essentially-dark exciton Bose-Einstein condensate is formed at sub-Kelvin bath temperatures. The macroscopic spatial coherence of the photoluminescence observed in this essentially dark region confirms this conclusion.

Enabling valley selective exciton scattering in monolayer WSe2 through upconversion

Excitons, Coulomb bound electron–hole pairs, are composite bosons and their interactions in traditional semiconductors lead to condensation and light amplification. The much stronger Coulomb interaction in transition metal dichalcogenides such as WSe₂ monolayers combined with the presence of the valley degree of freedom is expected to provide new opportunities for controlling excitonic effects. But so far the bosonic character of exciton scattering processes remains largely unexplored in these two-dimensional materials. Here we show that scattering between B-excitons and A-excitons preferably happens within the same valley in momentum space. This leads to power dependent, negative polarization of the hot B-exciton emission. We use a selective upconversion technique for efficient generation of B-excitons in the presence of resonantly excited A-excitons at lower energy; we also observe the excited A-excitons state 2s. Detuning of the continuous wave, low-power laser excitation outside the A-exciton resonance (with a full width at half maximum of 4 meV) results in vanishing upconversion signal.

Monolayer transition metal dichalcogenides host excitons, bound electron-hole pairs that play a pivotal role in optoelectronic applications relying on strong light-matter interaction. Here, the authors unveil the spectroscopic signature of boson scattering of two-dimensional excitons in monolayer WSe₂.

Revealing and Characterizing Dark Excitons through Coherent Multidimensional Spectroscopy

Dark excitons are of fundamental importance in a broad range of contexts but are difficult to study using conventional optical spectroscopy due to their weak interaction with light. We show how coherent multidimensional spectroscopy can reveal and characterize dark states. Using this approach, we identify parity-forbidden and spatially indirect excitons in InGaAs/GaAs quantum wells and determine details regarding lifetimes, homogeneous and inhomogeneous linewidths, broadening mechanisms, and coupling strengths. The observations of coherent coupling between these states and bright excitons hint at a role for a multistep process by which excitons in the barrier can relax into the quantum wells.

Controlling the Electronic Structures and Properties of in-Plane Transition-Metal Dichalcogenides Quantum Wells

In-plane transition-metal dichalcogenides (TMDs) quantum wells have been studied on the basis of first-principles density functional calculations to reveal how to control the electronic structures and the properties. In collection of quantum confinement, strain and intrinsic electric field, TMD quantum wells offer a diverse of exciting new physics. The band gap can be continuously reduced ascribed to the potential drop over the embedded TMD and the strain substantially affects the band gap nature. The true type-II alignment forms due to the coherent lattice and strong interface coupling suggesting the effective separation and collection of excitons. Interestingly, two-dimensional quantum wells of in-plane TMD can enrich the photoluminescence properties of TMD materials. The intrinsic electric polarization enhances the spin-orbital coupling and demonstrates the possibility to achieve topological insulator state and valleytronics in TMD quantum wells. In-plane TMD quantum wells have opened up new possibilities of applications in next-generation devices at nanoscale.

Anomalous Light Cones and Valley Optical Selection Rules of Interlayer Excitons in Twisted Heterobilayers

We show that, because of the inevitable twist and lattice mismatch in heterobilayers of transition metal dichalcogenides, interlayer excitons have sixfold degenerate light cones anomalously located at finite velocities on the parabolic energy dispersion. The photon emissions at each light cone are elliptically polarized, with the major axis locked to the direction of exciton velocity, and helicity specified by the valley indices of the electron and the hole. These finite-velocity light cones allow unprecedented possibilities for optically injecting valley polarization and valley current, and the observation of both direct and inverse valley Hall effects, by exciting interlayer excitons. Our findings suggest potential excitonic circuits with valley functionalities, and unique opportunities to study exciton dynamics and condensation phenomena in semiconducting 2D heterostructures.

Spontaneous formation and nonequilibrium dynamics of a soliton-shaped Bose-Einstein condensate in a trap

The Bose-stimulated self-organization of a quasi-two-dimensional nonequilibrium Bose-Einstein condensate in an in-plane potential is proposed. We obtained the solution of the nonlinear, driven-dissipative Gross-Pitaevskii equation for a Bose-Einstein condensate trapped in an external asymmetric parabolic potential within the method of the spectral expansion. We found that, in sharp contrast to previous observations, the condensate can spontaneously acquire a solitonlike shape for spatially homogeneous pumping. This condensate soliton performs oscillatory motion in a parabolic trap and, also, can spontaneously rotate. Stability of the condensate soliton in the spatially asymmetric trap is analyzed. In addition to the nonlinear dynamics of nonequilibrium Bose-Einstein condensates of ultracold atoms, our findings can be applied to the condensates of quantum well excitons and cavity polaritons in semiconductor heterostructure, and to the condensates of photons.

Electronic structures of in-plane two-dimensional transition-metal dichalcogenide heterostructures

In-plane two-dimensional MoS₂/WS₂ and MoSe₂/WS₂ heterostructures have been identified to show type-II band alignment and interface band bending.

Temperature-dependent photoluminescence in light-emitting diodes

Temperature-dependent photoluminescence (TDPL), one of the most effective and powerful optical characterisation methods, is widely used to investigate carrier transport and localized states in semiconductor materials. Resonant excitation and non-resonant excitation are the two primary methods of researching this issue. In this study, the application ranges of the different excitation modes are confirmed by analysing the TDPL characteristics of GaN-based light-emitting diodes. For resonant excitation, the carriers are generated only in the quantum wells, and the TDPL features effectively reflect the intrinsic photoluminescence characteristics within the wells and offer certain advantages in characterising localized states and the quality of the wells. For non-resonant excitation, both the wells and barriers are excited, and the carriers that drift from the barriers can contribute to the luminescence under the driving force of the built-in field, which causes the existing equations to become inapplicable. Thus, non-resonant excitation is more suitable than resonant excitation for studying carrier transport dynamics and evaluating the internal quantum efficiency. The experimental technique described herein provides fundamental new insights into the selection of the most appropriate excitation mode for the experimental analysis of carrier transport and localized states in p-n junction devices.

Semiconductor microcavities: towards polariton lasers

In this review paper we address one of the most rapidly developing new domains of semiconductor optics: light-matter coupling in semiconductor microcavities. Using the non-local dielectric response theory and transfer matrix technique, we show how two-dimensional confinement of a photonic mode coupled to an exciton resonance results in the appearance of two branches of exciton-polaritons, quasi-particles combining properties of photons and excitons. We obtain the dispersion relations of polaritons in microcavities and derive a condition for strong-weak coupling threshold. We show that being bosons, exciton-polaritons are subject to Bose-condensation which might result in emission of a coherent and monochromatic light in the strong coupling regime. A source of such coherent light is referred to as a polariton laser. We show that polariton lasers have theoretically no threshold and require essentially new basic physics as compared to conventional lasers described by Einstein theory. We give examples of model polariton laser structures expected to work at room temperature and overview the main difficulties on the way to producing these new opto-electronic devices.

Thermodynamic model of the macroscopically ordered exciton state

We explain the experimentally observed instability of cold exciton gases and the formation of a macroscopically ordered exciton state in terms of a thermodynamic model accounting for the phase fluctuations of the condensate. We show that the temperature dependence of the exciton energy exhibits fundamental scaling behavior with the signature of the second order phase transition.

Interplay between periodicity and nonlinearity of indirect excitons in coupled quantum wells

Inspired by a recent experiment of localization-delocalization transition (LDT) of indirect excitons in lateral electrostatic lattices (Remeika et al 2009 Phys. Rev. Lett. 102 186803), we theoretically investigate the interplay between periodic potential and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that the model involving both attractive two-body and repulsive three-body interactions can lead to a natural account for the LDT of excitons across the lattice when reducing lattice amplitude or increasing particle density. In addition, the observations that the smooth component of the photoluminescent energy increases with increasing exciton density and that the exciton interaction energy is close to the lattice amplitude at the transition are also qualitatively explained. Our model provides an alternative way of understanding the underlying physics of the exciton dynamics in lattice potential wells.

Drift-diffusion model of the fragmentation of the external ring structure in the photoluminescence pattern emitted by indirect excitons in coupled quantum wells

Under optical excitation, coupled quantum wells are known to reveal fascinating features in the photoluminescence pattern originating from dipole orientated indirect excitons. The appearance of an external ring has been attributed to macroscopic charge separation in the quantum well plane. We present a classical model of nonlinear diffusion to account for the observed fragmentation of the external ring into a periodic array of islands. The model incorporates the Coulomb interactions between electrons, holes, and indirect excitons. At low temperatures, these interactions lead to pattern formation similar to the experimentally observed ring fragmentation. The fragmentation is found to persist to temperatures above the quantum degeneracy temperature of indirect excitons.

Exciton phase transitions in semiconductor quantum wells with disc-shaped electrode

Phase transitions in a system of indirect excitons in semiconductor double quantum wells are studied for a set-up when one of the electrodes is of finite size and, in particular, has the shape of a disc. At voltage a region under the rim of the disc is created where excitons have lower energy, thus providing a macroscopic trap attractive for excitons while being repulsive for charged particles. The theory of the formation of patterns of the excitonic condensed phase under the disc is built based on the assumption of the existence of the inter-exciton range where the interaction between them is attractive. The finite value of the exciton lifetime is taken into account serving as a limiting factor for the size of the islands of the condensed phase. The calculations reveal complex restructuring of the patterns of the spatial distribution of exciton density with increasing pumping intensity: from the structureless gaseous phase to separate islands of the condensed phase within the gaseous phase, then to islands of the gaseous phase in the bulk of the condensed phase and finally to the continuous condensed phase.

Transition to a Bose–Einstein condensate and relaxation explosion of excitons at sub-Kelvin temperatures

Quasiparticles in quantum many-body systems have essential roles in modern physical problems. Bose–Einstein condensation (BEC) of excitons in semiconductors is one of the unobserved quantum statistical phenomena predicted in the photoexcited quasiparticles in many-body electrons. In particular, para-excitons in cuprous oxide have been studied for decades because the decoupling from the radiation field makes the coherent ensemble a purely matter-like wave. However, BEC has turned out to be hard to realize at superfluid liquid helium-4 temperatures due to a two-body inelastic collision process. It is therefore essential to set a lower critical density by further lowering the exciton temperature. Here we cool excitons to sub-Kelvin temperature and spatially confine them to realize the critical number for BEC. We show that BEC manifests itself as a relaxation explosion as has been discussed in atomic hydrogen. The results indicate that dilute excitons are purely bosonic and BEC indeed occurs.

Bose–Einstein condensation of excitons in thermal equilibrium is a predicted quantum statistical phenomenon that has been difficult to observe. Yoshioka et al. cool trapped excitons to sub-Kelvin temperatures and show that condensation manifests itself as a relaxation explosion as has been observed for atomic hydrogen.

Strong charge-transfer excitonic effects and the Bose-Einstein exciton condensate in graphane

Using first principles many-body theory methods (GW+Bethe-Salpeter equation) we demonstrate that the optical properties of graphane are dominated by localized charge-transfer excitations governed by enhanced electron correlations in a two-dimensional dielectric medium. Strong electron-hole interaction leads to the appearance of small radius bound excitons with spatially separated electron and hole, which are localized out of plane and in plane, respectively. The presence of such bound excitons opens the path towards an excitonic Bose-Einstein condensate in graphane that can be observed experimentally.

Angle-resolved photoluminescence spectrum of the exciton condensate in electron-hole semiconductor bilayers

The electron-hole semiconductor bilayer system is one of the most promising systems to search for exciton superfluid. The exciton superfluid is metastable and will eventually decay through emitting photons. Here we show that the angle-resolved photon spectrum, momentum distribution curve, energy distribution curve, and quasiparticle excitation spectrum in the exciton superfluid show many unique and unusual features not shared by any other atomic or condensed matter systems. Observing all these salient features in the photoluminescence experiments can provide convincing evidence for exciton superfluid in electron-hole semiconductor bilayers. We also comment on relevant experimental data in both exciton and exciton-polariton systems and also suggest possible future experiments to test our theoretical predictions.

Ordered structures of exciton condensed phases in the presence of an inhomogeneous potential

A theory of the separation of a system of indirect excitons into a condensed and a gaseous phase with the formation of regular patterns of alternating phases in inhomogeneous external fields is developed. The model of spinodal decomposition of phase transitions, generalized for systems of unstable particles, is used. The theory is applied to the study of the non-uniform distribution of the exciton density in a double quantum well under a slot cut in a metallic electrode. It is shown that in a certain range of exciton generation rates a chain of light-emitting islands periodically localized along the slot is developed. With increasing width of the slot, the chain splits into two parallel chains shifted by a half period with respect to each other. By creating a biased external potential along the slot, the periodical pattern could be forced to move along the slot. The effect of the motion of the islands should manifest itself as time oscillations of the intensity of the light emitted from a fixed point.

Photoluminescence ring formation in coupled quantum wells: excitonic versus ambipolar diffusion

In this Letter, we study the diffusion properties of photoexcited carriers in coupled quantum wells around the Mott transition. We find that the diffusion of unbound electrons and holes is ambipolar and is characterized by a large diffusion coefficient, similar to that found in p-i-n junctions. Correlation effects in the excitonic phase are found to significantly suppress the carriers' diffusion. We show that this difference in diffusion properties gives rise to the appearance of a photoluminescence ring pattern around the excitation spot at the Mott transition.

Berry phase effect on the exciton transport and on the exciton Bose-Einstein condensate

With the exciton lifetime much extended in semiconductor quantum-well structures, the exciton transport and Bose-Einstein condensation have become a focus of research in recent years. We reveal a momentum-space gauge field in the exciton center-of-mass dynamics due to Berry phase effects. We predict a spin-dependent transport of the excitons analogous to the anomalous Hall and Nernst effects for electrons. We also predict spin-dependent circulation of a trapped exciton gas and instability in an exciton condensate in favor of vortex formation.

Mott transition of excitons in coupled quantum wells

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.

Spin rings in semiconductor microcavities

New effects of self-organization and polarization pattern formation in semiconductor microcavities, operating in the nonlinear regime, are predicted and theoretically analyzed. We show that a spatially inhomogeneous elliptically polarized optical cw pump leads to the formation of a strongly circularly polarized ring in real space. This effect is due to the polarization multistability of cavity polaritons which was recently predicted. The possible switching between different stable configurations allows the realization of a localized spin memory element, suitable for an optical data storage device.

Long-range coherence of interacting Bose gas of dipolar excitons

Experiments connected with dipolar exciton Bose condensation in lateral traps are reviewed. Observations of long-range coherence of condensate in ring electrostatic traps in Schottky-diode heterostructures with double and single quantum wells are presented and discussed.

Anti-trapping of indirect excitons by a current filament

In order to explain the photoluminescence (PL) of indirect excitons collected from localized spots in experiments with a defocused laser excitation of coupled quantum wells (QWs), we model the in-plane carrier transport and charge distribution with a set of drift-diffusion, Poisson and thermalization equations. The quantum statistical corrections are included in our description via a generalized Einstein relationship and quantum mass action law. The PL spots are attributed to transverse current filaments crossing the structure and injecting electrons in the coupled QWs. The accumulated electron charge forms an anti-trap for indirect excitons. Our model quantitatively reproduces the whole set of the relevant experimental data.

Experimental methods and analysis of cold and dense dipolar exciton fluids

We review various aspects of our recent work on dipolar excitons in double quantum well systems. We describe and analyse different possible avenues for obtaining high density and cold dipolar exciton fluids that may enable an observation of quantum phase transitions in excitonic systems. These avenues include free dipolar exciton fluids, dipolar exciton fluids in electrostatic traps and in excitonic rings. We present our experimental and modelling work on the exciton dynamics in such systems, and discuss our current view of the advances made and the challenges that remain in this fast evolving and promising field of research.

Cold exciton gases in coupled quantum well structures

Cold exciton gases can be implemented in coupled quantum well structures. In this contribution, we review briefly the recent works on spontaneous coherence of cold excitons and on trapping of cold excitons with laser light.

Quantum degenerate exciton-polaritons in thermal equilibrium

We study the momentum distribution and relaxation dynamics of semiconductor microcavity polaritons by angle-resolved and time-resolved spectroscopy. Above a critical pump level, the thermalization time of polaritons at positive detunings becomes shorter than their lifetime, and the polaritons form a quantum degenerate Bose-Einstein distribution in thermal equilibrium with the lattice.

Bose–Einstein condensation of exciton polaritons

Phase transitions to quantum condensed phases--such as Bose-Einstein condensation (BEC), superfluidity, and superconductivity--have long fascinated scientists, as they bring pure quantum effects to a macroscopic scale. BEC has, for example, famously been demonstrated in dilute atom gas of rubidium atoms at temperatures below 200 nanokelvin. Much effort has been devoted to finding a solid-state system in which BEC can take place. Promising candidate systems are semiconductor microcavities, in which photons are confined and strongly coupled to electronic excitations, leading to the creation of exciton polaritons. These bosonic quasi-particles are 10(9) times lighter than rubidium atoms, thus theoretically permitting BEC to occur at standard cryogenic temperatures. Here we detail a comprehensive set of experiments giving compelling evidence for BEC of polaritons. Above a critical density, we observe massive occupation of the ground state developing from a polariton gas at thermal equilibrium at 19 K, an increase of temporal coherence, and the build-up of long-range spatial coherence and linear polarization, all of which indicate the spontaneous onset of a macroscopic quantum phase.

Nonequilibrium quantum condensation in an incoherently pumped dissipative system

We study spontaneous quantum coherence in an out of an equilibrium system, coupled to multiple baths describing pumping and decay. For a range of parameters describing coupling to, and occupation of the baths, a stable steady-state condensed solution exists. The presence of pumping and decay significantly modifies the spectra of phase fluctuations, leading to correlation functions that differ both from an isolated condensate and from a laser.

Ring-shaped luminescence pattern in biased quantum wells studied as a steady-state reaction front

Under certain conditions, focused laser excitation in semiconductor quantum well structures can lead to a charge separation and a circular reaction front, which is visible as a ring-shaped photoluminescence pattern. The diffusion-reaction equations governing the system are studied here with the aim of a detailed understanding of the steady state. The qualitative asymmetry in the sources for the two carriers is found to lead to unusual effects which dramatically affect the steady-state configuration. Analytic expressions are derived for carrier distributions and interface positions for a number of cases. These are compared with steady-state information obtained from simulations of the diffusion-reaction equations.

Long-distance diffusion of excitons in double quantum well structures

In this Letter we report on lateral diffusion measurements of excitons at low temperature in double quantum wells of various widths. The structure is designed so that excitons live up to 30 micros and diffuse up to 500 microm. Particular attention is given to establishing that the transport occurs by exciton motion. The deduced exciton diffusion coefficients have a very strong well width dependence, and obey the same power law as the diffusion coefficient for electrons.

Spontaneous coherent phase transition of polaritons in CdTe microcavities

We report on evidence for polariton condensation out of a reservoir of incoherent polaritons. Polariton population and first-order coherence are investigated by spectroscopic imaging of the far-field emission of a CdTe-based microcavity under nonresonant pumping. With increasing pumping power, stimulated emission with thresholdlike behavior and spectral narrowing is observed in the strong exciton-photon coupling regime. We show that it comes from a narrow ring in k space, exhibiting enhanced spatial and angular coherence at the stimulation onset.