We explore attractive dipolar interaction in indirect excitons (IXs). For one layer of IXs in a single pair of coupled quantum wells (CQW), the out-of-plane IX electric dipoles lead to repulsive dipolar interaction between IXs. The attractive dipolar interaction between IXs is realized in a 2-CQW heterostructure with two IX layers in two separated CQW pairs. We found both in experimental measurements and theoretical simulations that increasing density of IXs in one layer causes a monotonic energy reduction for IXs in the other layer. We also found an in-plane shift of a cloud of IXs in one layer towards a cloud of IXs in the other layer. This behaviour is qualitatively consistent with attractive dipolar interaction. The measured IX energy reduction and IX cloud shift are higher than the values given by the correlated liquid theory.
We report experiments showing that spatially indirect excitons confined in a wide single quantum well can exhibit macroscopic spatial coherence. Extended coherence is spontaneously established in the regime where indirect excitons form a distinctive ring shaped pattern fragmented into microscopic beads. These contain a large concentration of indirect excitons at sub-Kelvin temperatures, the excitons spatial coherence being the greatest in the vicinity of the fragments.
We report a two-dimensional artificial lattice for dipolar excitons confined in a GaAs double quantum well. Exploring the regime of large fillings per lattice site, we verify that the lattice depth competes with the magnitude of excitons repulsive dipolar interactions to control the degree of localisation in the lattice potential. Moreover, we show that dipolar excitons radiate a narrow-band photoluminescence, with a spectral width of a few hundreds of micro-eV at 340 mK, in both localised and delocalised regimes. This makes our device suitable for explorations of dipolar excitons quasi-conden…
We propose a scheme of continuous tunable THz emission based on dipolaritons --- mixtures of strongly interacting cavity photons and direct excitons, where the latter are coupled to indirect excitons via tunnelling. We investigate the property of multistability under continuous wave (CW) pumping, and the stability of the solutions. We establish the conditions of parametric instability, giving rise to oscillations in density between the direct exciton and indirect modes under CW pumping. In this way we achieve continuous and tunable emission in the THz range in a compact single-crystal device. …
We experimentally investigate the lateral diffusion of dipolar excitons in coupled quantum wells in two (2D) and one (1D) dimensions. In 2D, the exciton expansion obeys non-linear temporal dynamics due to the repulsive dipole pressure at a high exciton density, in accordance with recent reports. In contrast, the observed 1D expansion behaves linearly in time even at high exciton densities. The corresponding 1D diffusion coefficient exceeds the one in 2D by far and depends linearly on the exciton density. We attribute the findings to screening of quantum well disorder by the dipolar excitons.
Dipolariton is a voltage controlled mixture of direct and indirect excitons in asymmetric double quantum wells coupled by resonant carrier tunneling, and a microcavity photon. We calculate the voltage dependence of the spin-singlet and spin-triplet interaction parameters $α_{1}$ and $α_{2}$. Both parameters can reach values one order of magnitude larger than that of exciton-polaritons thanks to the strong interaction between indirect excitons. We show that the variation of the indirect exciton fraction of the dipolaritons induces a changes of sign of $α_{2}$ increasing voltage: the interaction…
Dipolaritons are mixed light-matter quasiparticles formed in double quantum wells embedded in microcavities. Due to resonant coupling between direct and indirect excitons via electronic tunnelling, dipolaritons possess large dipole moments. Resonant excitation of the cavity mode by a short pulse of light induces oscillations of the indirect exciton density with a characteristic frequency of Rabi flopping. This results in oscillations of classical Hertz dipoles array which generate supperradiant emission on a terahertz (THz) frequency. Resulting THz signal may be enhanced using the supplementar…
While the interaction potential between two dipoles residing in a single plane is repulsive, in a system of two vertically adjacent layers of dipoles it changes from repulsive interaction in the long range to attractive interaction in the short range. Here we show that for dipolar excitons in semiconductor heterostructures, such a potential may give rise to bound states if two such excitons are excited in two separate layers, leading to the formation of vertically coupled dipolar exciton molecules. Our calculations prove the existence of such bound states and predict their binding energy as a …
We propose a scheme for the controllable preparation of a cold indirect exciton condensate using dipolaritonic setup with an optical pumping. Dipolaritons are bosonic quasiparticles which arise from the coupling between cavity photon (C), direct exciton (DX), and indirect exciton (IX) modes, and appear in a double quantum well embedded in a semiconductor microcavity. Controlling the detuning between modes of the system, the limiting cases of exciton-polaritons and indirect excitons can be realized. Our protocol relies on the initial preparation of an exciton polariton condensate for the far bl…
We demonstrate an efficient switching between strong and weak exciton-photon coupling regimes in microcavity-embedded asymmetric double quantum wells, controlled by an applied electric field. We show that a fine tuning of the electric field leads to drastic changes in the polariton properties, with the polariton ground state being red-shifted by a few meV and having acquired prominent features of a spatially indirect dipolar exciton. We study the properties of dipolar exciton polaritons, called dipolaritons, on a microscopic level and show that, unlike recent findings, they are not dark polari…
We discuss neutral and charged complexes (biexciton and trion) formed by indirect excitons in layered quasi-two-dimensional semiconductor heterostructures. Indirect excitons -- long-lived neutral Coulomb-bound pairs of electrons and holes of different layers -- have been known for semiconductor coupled quantum wells and are recently reported for van der Waals heterostructures such as bilayer graphene and transition metal dichalcogenides. Using the configuration space approach, we derive the analytical expressions for the trion and biexciton binding energies as functions of the interlayer dista…
The possible phases and the nano-scale 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 $T_c \approx 4.8K$, and it is manifested by …
Magnetoluminescence of spatially indirect dipolar excitons collected in 25 nm GaAs/AlGaAs single quantum well within a lateral potential trap has been studied in perpendicular magnetic field in Faraday geometry. The paramagnetic spin splitting of the luminescence line of the heavy-hole excitons in the trap centre is completely compensated at magnetic fields below critical value, around 2 Tesla. The effect of spin-splitting compensation is caused by the exchange interaction in dense exciton Bose gas which is in qualitative agreement with the existing theoretical concepts. In the range of low ma…
In twisted homobilayer transition metal dichalcogenides, intra- and inter-layer valley excitons hybridize with the layer configurations spatially varying in the moiré. The ground state valley excitons are trapped at two high-symmetry points with opposite electric dipoles in a moiré supercell, forming a honeycomb superlattice of nearest-neighbor dipolar attraction. We find that the spatial texture of layer configuration results in a luminescence anomaly of the moiré trapped excitons, where a tiny displacement by interactions dramatically increases the brightness and changes polarization from ci…
Photogenerated excitonic ensembles confined in coupled GaAs quantum wells are probed by a complementary approach of emission spectroscopy and resonant inelastic light scattering. Lateral electrostatic trap geometries are used to create dense systems of spatially indirect excitons and excess holes with similar densities in the order of 10$^{11}$ cm$^{-2}$. Inelastic light scattering spectra reveal a very sharp low-lying collective mode that is identified at an energy of 0.44 meV and a FWHM of only ~50 $μ$eV. This mode is interpreted as a plasmon excitation of the excess hole system coupled to t…
While photons in free space barely interact, matter can mediate interactions between them resulting in optical nonlinearities. Such interactions at the single-quantum level result in an on-site photon repulsion, crucial for photon-based quantum information processing and for realizing strongly interacting many-body states of light. Here, we report repulsive dipole-dipole interactions between electric field tuneable, localized interlayer excitons in MoSe$_2$/WSe$_2$ heterobilayer. The presence of a single, localized exciton with an out-of-plane, non-oscillating dipole moment increases the energ…
We report on the principle and realization of an excitonic device: a ramp that directs the transport of indirect excitons down a potential energy gradient created by a perforated electrode at constant voltage. The device provides an experimental proof of principle for controlling exciton transport with electrode density gradients. We observed that the exciton transport distance along the ramp increases with increasing exciton density. This effect is explained in terms of disorder screening by repulsive exciton-exciton interactions.
Interaction of dipolar polaritons can be efficiently tuned by means of a shape resonance in their excitonic component. Provided the resonance width is large, a squeezed population of strongly interacting polaritons may persist on the repulsive side of the resonance. We derive an analytical expression for the polariton coupling constant and show that it may have very large values in typical experimental conditions. Our arguments provide a new direction for the quest of interactions in quantum photonics.
Dipolaritons are a three-way superposition of photon, a direct exciton, and an indirect exciton that are formed in coupled quantum well microcavities. As is the case with exciton-polaritons, dipolaritons have a self-interaction due to direct and exchange effects of the underlying electrons and holes. Here we present a theoretical description of dipolaritons and derive simple formulas for their basic parameters. In particular, we derive the effective dipolariton-dipolariton interaction taking into account of exchange effects between the excitons. We obtain a simple relation to describe the effe…
Exciton-dipolaritons are investigated as a platform for realizing working elements of a polaritronic transistor. Exciton-dipolaritons are three-way superposition of cavity photons, direct and indirect excitons in a bilayer semiconducting system embedded in an optical microcavity. Using the forced diffusion equation for dipolaritons, we study the room-temperature dynamics of dipolaritons in a transition metal dichalcogenide (TMD) heterogeneous bilayer. Specifically, we considered a MoSe$_2$-WS$_2$ heterostructure, where a Y-shaped channel guiding the dipolariton propagation is produced. We demo…
Dipolar (or spatially indirect) excitons (IXs) in semiconductor double quantum well (DQW) subjected to an electric field are neutral species with a dipole moment oriented perpendicular to the DQW plane. Here, we theoretically study interactions between IXs in stacked DQW bilayers, where the dipolar coupling can be either attractive or repulsive depending on the relative positions of the particles. By using microscopic band structure calculations to determine the electronic states forming the excitons, we show that the attractive dipolar interaction between stacked IXs deforms their electronic …
Van der Waals heterostructures composed of transition metal dichalcogenide monolayers (TMDs) are characterized by their truly rich excitonic properties which are determined by their structural, geometric and electronic properties: In contrast to pure monolayers, electrons and holes can be hosted in different materials, resulting in highly tunable dipolar manyparticle complexes. However, for genuine spatially indirect excitons, the dipolar nature is usually accompanied by a notable quenching of the exciton oscillator strength. Via electric and magnetic field dependent measurements, we demonstra…
The photoluminescence dynamics of a microscopic gas of indirect excitons trapped in coupled quantum wells is probed at very low bath temperature (approximately 350 mK). Our experiments reveal the non linear energy relaxation characteristics of indirect excitons. Particularly, we observe that the excitons dynamics is strongly correlated with the screening of structural disorder by repulsive exciton-exciton interactions. For our experiments where two-dimensional excitonic states are gradually defined, the distinctive enhancement of the exciton scattering rate towards lowest energy states with in…
We consider dipolar interactions between heteronuclear molecules in low-dimensional geometries. The setup consists of two one-dimensional tubes. We study the stability of possible few-body complexes in the regime of repulsive intratube interaction, where the binding arises from intertube attraction. The stable dimers, trimers, and tetramers are found and we discuss their properties for both bosonic and fermionic molecules. To observe these complexes we propose an optical non-destructive detection scheme that enables in-situ observation of the creation and dissociation of the few-body complexes…
We derive upper and lower bounds on the Casimir--Polder force between an anisotropic dipolar body and a macroscopic body separated by vacuum via algebraic properties of Maxwell's equations. These bounds require only a coarse characterization of the system---the material composition of the macroscopic object, the polarizability of the dipole, and any convenient partition between the two objects---to encompass all structuring possibilities. We find that the attractive Casimir--Polder force between a polarizable dipole and a uniform planar semi-infinite bulk medium always comes within 10% of the …
