Dipolar Excitons Research Articles

Dynamical formation of a strongly correlated dark condensate of dipolar excitons

Strongly interacting bosons display a rich variety of quantum phases, the study of which has so far been focused in the dilute regime, at a fixed number of particles. Here we demonstrate the formation of a dense Bose-Einstein condensate in a long-lived dark spin state of 2D dipolar excitons. A dark condensate of weakly interacting excitons is very fragile, being unstable against a coherent coupling of dark and bright spin states. Remarkably, we find that strong dipole-dipole interactions stabilize the dark condensate. As a result, the dark phase persists up to densities high enough for a dark quantum liquid to form. The striking experimental observation of a step-like dependence of the exciton density on the pump power is reproduced quantitatively by a model describing the nonequilibrium dynamics of driven coupled dark and bright condensates. This unique behavior marks a dynamical condensation to dark states with lifetimes as long as a millisecond, followed by a brightening transition at high densities.

Trapping Dipolar Exciton Fluids in GaN/(AlGa)N Nanostructures.

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the in-plane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

On the parabolicity of dipolar exciton traps and their population of excess charge carriers

We study spatially trapped ensembles of dipolar excitons in coupled quantum wells by means of photoluminescence and photocurrent spectroscopy. The photogenerated excitons are confined in very clean GaAs double quantum well structures and electrostatically trapped by local gate electrodes. We find that the common approach of electrostatic trap geometries can give rise to an in-plane imbalance of charge carriers especially when an over-barrier excitation is utilized. The excess charge carriers can give rise to an effective parabolic confinement potential for the excitons. In photoluminescence spectra, we identify the emission of both neutral indirect excitons and states influenced by the excess charge carrier density. We find that the charge imbalance in the excitonic ensemble strongly influences the radiative lifetimes of both. Our findings shine a new light on the properties of trapped dipolar exciton ensembles. This is of significant relevance to common interpretations of experimental results in terms of signatures for the formation of ‘dark’ and ‘gray’ excitonic condensates.

Interaction between Electrons and Dipole Excitons in Two-Dimensional Systems (Scientific Summary)

Theoretical studies of effects caused by the interaction of indirect dipole excitons between each other and with a two-dimensional electron gas, which were supported by the Russian Foundation for Basic Research, have been reviewed. Particular attention is focused on situations where an exciton gas is in a state similar to a Bose- Einstein condensate. Excitons in electrostatic traps, polaron effects in hybrid electron-exciton systems, the response of a hybrid system to an external electromagnetic perturbation, and the drag of the exciton gas by a current in the two-dimensional electron gas have been considered. It has been shown that some electronic effects in hybrid systems can be used to identify a phase transition in the exciton system.

Attractive Dipolar Coupling between Stacked Exciton Fluids

The interaction between aligned dipoles is long-ranged and highly anisotropic: it changes from repulsive to attractive depending on the relative positions of the dipoles. We report on the observation of the attractive component of the dipolar coupling between excitonic dipoles in stacked semiconductor bilayers. We show that the presence of a dipolar exciton fluid in one bilayer modifies the spatial distribution and increases the binding energy of excitonic dipoles in a vertically remote layer. The binding energy changes are explained by a many-body polaron model describing the deformation of the exciton cloud due to its interaction with a remote dipolar exciton. The results open the way for the observation of theoretically predicted new and exotic collective phases, the realization of interacting dipolar lattices in semiconductor systems as well as for engineering and sensing their collective excitations.

Molecular Orientation Effects in Organic Light‐Emitting Diodes

AbstractIt is well known that by horizontally aligning the transition dipole moments of exciton dipoles in the emitter films of organic light‐emitting diodes (OLEDs), a larger fraction of the radiative power can escape from the OLED stack, increasing the light outcoupling efficiency by up to 50 % compared to the isotropic counterparts. In this account, we review recent advances in understanding this phenomenon, with a special focus on the practical strategies to control the molecular orientation in vacuum‐deposited films of thermally activated delayed fluorescent (TADF) dyes. The role of molecular orientation in efficient OLED design is discussed, which has been experimentally proven to increase the external quantum efficiency exceeding 30 %. We outline the future challenges and perspectives in this field, including the potential to extend the concept to the solution‐processed films. Finally, the development of multiscale computer simulations is reviewed to assess their potential as a complementary approach to systematically screening OLED molecules in silico.

Coulomb drag of excitons in Bose-Fermi systems

We develop a microscopic theory of the Coulomb drag effect in a hybrid system consisting of spatially separated two-dimensional quantum gases of degenerate electrons and dipolar excitons. We consider both the normal-phase and condensate regimes of the exciton subsystem and investigate the cross-mobility of the system being the kinetic coefficient, which couples the static electric field applied to the electron layer with the particle density current (flux) in the exciton subsystem. We study the temperature dependence of the cross-mobility and its dependence on the interlayer separation. We show that exciton-exciton interaction plays a dramatic role. If the exciton gas is in the normal phase, then the screening of interlayer interaction by the exciton subsystem results in an exponential damping of the cross-mobility with the decrease of temperature, while at low temperatures, the interactions result in a robust bosonic transport due to the emergence of the Bogoliubov quasiparticles.

Defect Proliferation at the Quasicondensate Crossover of Two-Dimensional Dipolar Excitons Trapped in Coupled GaAs Quantum Wells.

We study ultracold dipolar excitons confined in a 10 μm trap of a double GaAs quantum well. Based on the local density approximation, we unveil for the first time the equation of state of excitons. Specifically, in this regime and below a critical temperature of about 1K, we show that for a local density n∼(2-3)×10^{10} cm^{-2} a coherent quasicondensate phase forms in the inner region of the trap, encircled by a more dilute and normal component in the outer rim. Remarkably, this spatial arrangement correlates directly with the concentration of defects in the exciton density, which is strongly decreased in the quasicondensed region, consistent with a superfluid phase. Thus, our observations point towards a Berezinskii-Kosterlitz-Thouless crossover for two-dimensional excitons.

Optical probing in a bilayer dark-bright condensate system

We consider a bilayer system of two-dimensional Bose-Einstein-condensed dipolar dark excitons (upper layer) and bright ones (bottom layer). We demonstrate that the interlayer interaction leads to a mixing between excitations from different layers. This mixing leads to the appearance of a second spectral branch in the spectrum of bright condensate. The excitation spectrum of the condensate of dark dipolar excitons then becomes optically accessible during luminescence spectra measurements of the bright condensate, which allows one to probe its kinetic properties. This approach is relevant for experimental setups, where detection via conventional techniques remains challenging; in particular, the suggested method is useful for studying dark dipolar excitons in transition metal dichalcogenide monolayers.

Polaron Shift of the Levels of a Quantum Wire in a Hybrid Structure with a Bose—Einstein Condensate

The wavefunction of the transverse motion of electrons in a quantum wire located near a two-dimensional layer of dipole excitons is perturbed by fluctuations of the exciton density through the electron-exciton interaction. This polaron effect is responsible for the shift of levels of the wire and can be observed through a change in the intersubband transition frequency. In this work, the polaron shift of the main intersubband transition frequency under the conditions of the Bose-Einstein condensation of excitons, as well as for the normal state of the degenerate Bose gas, is calculated.

New effects in and control of exciton systems in quasi-two-dimensional structures

New effects in systems of quantum dipoles are discussed, such as anisotropic superfluidity in external fields, strong correlations, crystallization, and the supersolid phase. Roton instability effects typical of strongly correlated Bose systems but also manifesting themselves in weakly interacting systems of titled dipoles are analyzed. Among the interesting physical realizations of the systems under consideration are dipole excitons in single or coupled quantum wells under a strong transverse electric field and in van der Waals heterostructures of new 2D materials, such as transition metal dichalcogenides (TMDCs). The use at ultralow temperatures of polar molecules or atoms with permanent or external-field-induced dipoles is also interesting, as are Rydberg atoms in an external electric field.

Complete-series excitonic dipole emissions in few layer ReS2 and ReSe2 observed by polarized photoluminescence spectroscopy

Excitonic emission is a key factor for a two-dimensional (2D) semiconductor to be fabricated in light-emitting device for optoelectronics application. Especially when the 2D material has a specific in-plane optical anisotropy, its excitons would become polarized dipole emitters. In this study, the whole series excitonic emissions of few-layer ReS2 and ReSe2 have been detailed characterized using polarized micro-photoluminescence (μPL) measurement. There are totally five excitonic emissions denoted as E1ex, E2ex, E3ex, ES1ex and ES2ex can be detected by the μPL experiment for both the few-layer ReS2 and ReSe2. The angular dependencies of the polarized excitonic emissions with respect to b axis (Re4 diamond-chain direction) are determined. The emission feature E1ex is the band-edge exciton while it shows orthogonal polarization with respect to both E2ex and E3ex in layered ReX2 (X = S, Se). The formation of Re4 diamond chains (DCs) in the 1 T′-phase ReS2 and ReSe2 will also assist the development of in-plane built-in electric field and forming dipoles around the DC regions. The built-in electric field effect greatly enhances the higher ES1ex and ES2ex excitonic features as evident in the photo-modulated reflectance spectra of the layered ReX2 with in-plane electrical field modulation. The binding energy and transition origin of all the excitons are also analyzed and discussed.

Enhanced Interactions between Dipolar Polaritons.

Nonperturbative coupling between cavity photons and excitons leads to the formation of hybrid light-matter excitations, termed polaritons. In structures where photon absorption leads to the creation of excitons with aligned permanent dipoles, the elementary excitations, termed dipolar polaritons, are expected to exhibit enhanced interactions. Here, we report a substantial increase in interaction strength between dipolar polaritons as the size of the dipole is increased by tuning the applied gate voltage. To this end, we use coupled quantum well structures embedded inside a microcavity where coherent electron tunneling between the wells creates the excitonic dipole. Modifications of the interaction strength are characterized by measuring the changes in the reflected light intensity when polaritons are driven with a resonant laser. The factor of 6.5 increase in the interaction-strength-to-linewidth ratio that we obtain indicates that dipolar polaritons could constitute an important step towards a demonstration of the polariton blockade effect, and thereby to form the building blocks of many-body states of light.

Dipolar and charged localized excitons in carbon nanotubes

We study both experimentally and theoretically the fundamental interplay of exciton localization and polarization in semiconducting single-walled carbon nanotubes. From Stark spectroscopy of individual carbon nanotubes at cryogenic temperatures, we identify localized excitons as permanent electric dipoles with dipole moments of up to $1\phantom{\rule{3.33333pt}{0ex}}e\AA{}$. Moreover, we demonstrate field-effect doping of localized excitons with an additional charge which results in defect-localized trions. Our findings, in qualitative agreement with theoretical calculations, not only provide fundamental insight into the microscopic nature of localized excitons in carbon nanotubes, they also signify their potential for sensing applications and may serve as guidelines for molecular engineering of exciton-localizing quantum dots in other atomically thin semiconductors including transition metal dichalcogenides.

Spontaneous Exciton Valley Coherence in Transition Metal Dichalcogenide Monolayers Interfaced with an Anisotropic Metasurface.

The control of the exciton intervalley coherence renders transition metal dichalcogenides monolayers promising candidates for quantum information science. So far, generating intervalley coherence has the need for an external coherent field. Here, we theoretically demonstrate spontaneous generation (i.e., without any external field) of exciton intervalley coherence. We achieve this by manipulating the vacuum field in the vicinity of the monolayer with a designed polarization-dependent metasurface, inducing an anisotropic decay rate for in-plane excitonic dipoles. Harnessing quantum coherence and interference effects in two-dimensional materials may provide the route for novel quantum valleytronic devices.

Dipolar Biexcitons in Lateral Traps in Si/SiGe/Si Heterostructures

Si/Si1–xGex/Si heterostructures with large-scale (micrometer-size) lateral potential fluctuations at the upper SiGe/Si-cap heterointerface are grown. These potential fluctuations are caused by partial strain relaxation in the SiGe layer. Low-temperature photoluminescence (PL) spectra show that these fluctuations form lateral traps where photoexcited nonequilibrium charge carriers are accumulated and bind into dipolar excitons, which ultimately recombine. At temperatures below 6 K, a new narrow line with a width considerably less than that of the dipolar exciton PL line emerges in the spectra as the level of excitation increases. It is shown that this line is associated with the recombination of dipolar biexcitons in large-scale traps.

Observation of interlayer excitons in MoSe2 single crystals

Interlayer excitons are observed coexisting with intralayer excitons in bi-layer, few-layer, and bulk MoSe2 single crystals by confocal reflection contrast spectroscopy. Quantitative analysis using the Dirac-Bloch-Equations provides unambiguous state assignment of all the measured resonances. The interlayer excitons in bilayer MoSe2 have a large binding energy of 153 meV, narrow linewidth of 20 meV, and their spectral weight is comparable to the commonly studied higher-order intralayer excitons. At the same time, the interlayer excitons are characterized by distinct transition energies and permanent dipole moments providing a promising high temperature and optically accessible platform for dipolar exciton physics.

Resonant Photon Drag of Dipolar Excitons

The theory of the photon drag of dipolar excitons in double-quantum-well nanostructures is presented. It is shown that the exciton-drag flux density features a resonant behavior if the photon frequency is close to some transition frequency in the discrete exciton spectrum. When the structure is irradiated with polarized light, the resonant enhancement of the drag current occurs when the photon energy coincides with the energy of an excited level of the exciton internal motion and the components of the angular momentum of internal motion in the initial and final states differ by one. The proposed effect can be used to control exciton transport in nanostructures based on a two-dimensional exciton gas.

Strong Nonlinear Optical Response in the Visible Spectral Range with Epsilon‐Near‐Zero Organic Thin Films

AbstractEnhanced Kerr nonlinearities are observed in metamaterials such as conducting oxides and doped inorganic semiconductor thin films showing epsilon‐near‐zero (ENZ) response in the infrared region. However, to achieve ENZ in the visible, artificial metamaterials with more complex nanostructures have to be specifically designed. Here, using sodium [5,6‐dichloro‐2‐[[5,6‐dichloro‐1‐ethyl‐3‐(4‐sulphobutyl)‐benzimidazol‐2‐ylidene]‐propenyl]‐1‐ethyl‐3‐(4‐sulphobutyl)‐benzimidazolium hydroxide] and [2,4‐bis[8‐hydroxy‐1,1,7,7‐tetramethyljulolidin‐9‐yl]squaraine] organic thin films, ENZ responses between 450 and 620 nm are demonstrated. Both nonlinear refractive index and nonlinear absorption coefficient are enhanced by more than two orders of magnitude in the ENZ spectral region. These optical effects in the visible spectral range come from the strongly dispersive permittivity of molecular aggregates resulting from the coupling of excitonic transition dipoles. These findings open the path toward a next generation of high‐performance solution‐processable organic nonlinear optical materials with ENZ properties that can be tuned by molecular engineering.

Coulomb Drag of Dipole Excitons in a Hybrid Exciton–Electron System

The effect of Coulomb drag on a gas of dipole excitons in spatially separated two-dimensional quantum wells containing electron and exciton gases is studied theoretically. The Coulomb drag of excitons can be used to control exciton transport in transistor structures whose active element is a two-dimensional gas of dipole excitons. Expressions for the exciton cross conductivity as a function of temperature are obtained for the diffusion and ballistic transport regimes. For each regime, the limiting cases in terms of the ratio of the Coulomb interaction screening length to the distance between the gases are analyzed. It is shown that, at temperatures exceeding considerably the exciton-gas degeneracy temperature, the cross conductivity is independent of the temperature, while in the opposite case it vanishes exponentially.