Dipolar Excitons Research Articles

Transport of Dipolar Excitons at the Type-II Polytype Interface of InP Platelets

Transport of Dipolar Excitons at the Type-II Polytype Interface of InP Platelets

Direct Linearly Polarized Emission in van der Waals LEDs via Flexoelectric Effect

AbstractFollowing the rapid development of information technology, modern polarized light, which is a critical component for display and data transmission, has been in demand for miniaturization and high efficiency, rendering two‐dimensional (2D) semiconductors potential candidates. The traditional polarized light is usually generated by external optical structures or polarizers that influence the scaling and bring up losses. Previous works have reported polarized light emission from inversion‐asymmetric 2D semiconductors such as black phosphorus (BP), black arsenide phosphorus (AsP), and rhenium disulfide (ReS2), however, their emission wavelengths are not in the visible range. Here, a direct emission of linearly polarized light is demonstrated from van der Waals light–emitting diodes (vdWLEDs) via the flexoelectric effects by inducing the non‐uniform strain in monolayer (ML) transition metal dichalcogenides (TMDCs). In this work, the effects of strain including excitonic binding energy and exciton dipole moment distribution is analyzed by the density functional theory (DFT) then we show that linearly polarized photoluminescence (PL) with a degree of linear polarization (DOLP) of ≈17% can be realized at room temperature (RT), and the polarization angle is perpendicular to the direction of the strain‐gradient. By incorporating the strained ML TMDCs into vdWLEDs, electroluminescence (EL) with DOLP of ≈19% can be observed at RT. This work puts forward a direct and universal strategy for fabricating polarized LEDs based on inversion‐symmetric semiconductors.

Exciton Dipole Orientation and Dynamic Reactivity Synergistically Enable Overall Water Splitting in Covalent Organic Frameworks

Exciton Dipole Orientation and Dynamic Reactivity Synergistically Enable Overall Water Splitting in Covalent Organic Frameworks

Bright dipolar excitons in twisted black phosphorus homostructures.

Bright dipolar excitons, which contain electrical dipoles and have high oscillator strength, are an ideal platform for studying correlated quantum phenomena. They usually rely on carrier tunneling between two quantum wells or two layers to hybridize with nondipolar excitons to gain oscillator strength. In this work, we uncovered a new type of bright infrared dipolar exciton by stacking 90°-twisted black phosphorus (BP) structures. These excitons, inherent to the reconstructed band structure, exhibit high oscillator strength. Most importantly, they inherit the linear polarization from BP, which allows light polarization to be used to select the dipole direction. Moreover, the dipole moment and resonance energy can be widely tuned by the thickness of the BP. Our results demonstrate a useful platform for exploring tunable correlated dipolar excitons.

Polariton-assisted incoherent to coherent excitation energy transfer between colloidal nanocrystal quantum dots.

We explore the dynamics of energy transfer between two nanocrystal quantum dots placed within an optical microcavity. By adjusting the coupling strength between the cavity photon mode and the quantum dots, we have the capacity to fine-tune the effective coupling between the donor and acceptor. Introducing a non-adiabatic parameter, γ, governed by the coupling to the cavity mode, we demonstrate the system's capability to shift from the overdamped Förster regime (γ ≪ 1) to an underdamped coherent regime (γ ≫ 1). In the latter regime, characterized by swift energy transfer rates, the dynamics are influenced by decoherence time. To illustrate this, we study the exciton energy transfer dynamics between two closely positioned CdSe/CdS core/shell quantum dots with sizes and separations relevant to experimental conditions. Employing an atomistic approach, we calculate the excitonic level arrangement, exciton-phonon interactions, and transition dipole moments of the quantum dots within the microcavity. These parameters are then utilized to define a model Hamiltonian. Subsequently, we apply a generalized non-Markovian quantum Redfield equation to delineate the dynamics within the polaritonic framework.

Band alignments, conduction band edges and intralayer bandgap renormalisation in MoSe2/WSe2 heterobilayers

Abstract Stacking two semiconducting transition metal dichalcogenide (MX2) monolayers to form a heterobilayer creates a new variety of semiconductor junction with unique optoelectronic features, such as hosting long-lived dipolar interlayer excitons. Despite many optical, transport, and theoretical studies, there have been few direct electronic structure measurements of these junctions. Here, we apply angle-resolved photoemission spectroscopy with micron-scale spatial resolution (µARPES) to determine the band alignments in MoSe2/WSe2 heterobilayers, using in-situ electrostatic gating to electron-dope and thus probe the conduction band edges. By comparing spectra from heterobilayers with opposite stacking orders, that is, with either MoSe2 or WSe2 on top, we confirm that the band alignment is type II, with the valence band maximum in the WSe2 and the conduction band minimum in the MoSe2. The overall band gap is EG = 1.43 ± 0.03 eV, and to within experimental uncertainty it is unaffected by electron doping. However, the offset between the WSe2 and MoSe2 valence bands clearly decreases with increasing electron doping, implying band renormalisation only in the MoSe2, the layer in which the electrons accumulate. In contrast, µARPES spectra from a WS2/MoSe2 heterobilayer indicate type I band alignment, with both band edges in the MoSe2. These insights into the doping-dependent band alignments and gaps of MX2 heterobilayers will be useful for properly understanding and ultimately utilizing their optoelectronic properties.

Exploring the impact of weak measurements on exciton–exciton interactions

In our study of super quantum discord between two excitonic qubits inside a coupled semiconductor quantum dots system, our primary focus is to uncover the impact of weak measurement on its quantum characteristics. To achieve this, we analyze how varying the measurement strength x, affects this super quantum correlation in the presence of thermal effects. Additionally, we assess the effect of this variation on the system’s evolution against its associated quantum parameters; external electric fields, exciton–exciton dipole interaction energy and Förster interaction. Our findings indicate that adjusting x to smaller values effectively enhances the super quantum correlation, making weak measurements act as a catalyst. This adjustment ensures its robustness against thermal effects while preserving the non-classical attributes of the system. Furthermore, our study unveils that the effect of weak measurements on this latter surpasses the quantum effects associated with the system. Indeed, manipulating the parameter x allows the weak measurement to function as a versatile tool for modulating quantum characteristics and controlling exciton–exciton interactions within the coupled semiconductor quantum dots system.

Controlled interlayer exciton ionization in an electrostatic trap in atomically thin heterostructures

Atomically thin semiconductor heterostructures provide a two-dimensional (2D) device platform for creating high densities of cold, controllable excitons. Interlayer excitons (IEs), bound electrons and holes localized to separate 2D quantum well layers, have permanent out-of-plane dipole moments and long lifetimes, allowing their spatial distribution to be tuned on demand. Here, we employ electrostatic gates to trap IEs and control their density. By electrically modulating the IE Stark shift, electron-hole pair concentrations above 2 × 1012 cm−2 can be achieved. At this high IE density, we observe an exponentially increasing linewidth broadening indicative of an IE ionization transition, independent of the trap depth. This runaway threshold remains constant at low temperatures, but increases above 20 K, consistent with the quantum dissociation of a degenerate IE gas. Our demonstration of the IE ionization in a tunable electrostatic trap represents an important step towards the realization of dipolar exciton condensates in solid-state optoelectronic devices.

Quadrupolar and dipolar excitons in symmetric trilayer heterostructures: insights from first principles theory

Excitons in van der Waals heterostructures come in many different forms. In bilayer structures, the electron and hole may be localized on the same layer or they may be separated forming an interlayer (IL) exciton with a finite out-of-plane dipole moment. Using first principles calculations, we investigate the excitons in a symmetric WS2/MoS2/WS2 heterostructure in the presence of a vertical electric field. The excitons exhibit a quadratic Stark shift for low field strengths and a linear Stark shift for stronger fields. This behavior is traced to the coupling of IL excitons with opposite dipole moments, which lead to the formation of quadrupolar excitons at small fields. The formation of quadrupolar excitons is determined by the relative size of the electric field-induced splitting of the dipolar excitons and the coupling between them given by the hole tunneling across the MoS2 layer. For the inverted structure, MoS2/WS2/MoS2, the dipolar excitons are coupled by electron tunneling across the WS2 layer. Because this effect is much weaker, the resulting quadrupolar excitons are more fragile and break at a weaker electric field.

Superfluidity of indirect momentum space dark dipolar excitons in a double layer with anisotropic tilted semi-Dirac bands

Superfluidity of indirect momentum space dark dipolar excitons in a double layer with anisotropic tilted semi-Dirac bands

Highly tunable ground and excited state excitonic dipoles in multilayer 2H-MoSe2

The fundamental properties of an exciton are determined by the spin, valley, energy, and spatial wavefunctions of the Coulomb-bound electron and hole. In van der Waals materials, these attributes can be widely engineered through layer stacking configuration to create highly tunable interlayer excitons with static out-of-plane electric dipoles, at the expense of the strength of the oscillating in-plane dipole responsible for light-matter coupling. Here we show that interlayer excitons in bi- and tri-layer 2H-MoSe2 crystals exhibit electric-field-driven coupling with the ground (1s) and excited states (2s) of the intralayer A excitons. We demonstrate that the hybrid states of these distinct exciton species provide strong oscillator strength, large permanent dipoles (up to 0.73 ± 0.01 enm), high energy tunability (up to ~200 meV), and full control of the spin and valley characteristics such that the exciton g-factor can be manipulated over a large range (from −4 to +14). Further, we observe the bi- and tri-layer excited state (2s) interlayer excitons and their coupling with the intralayer excitons states (1s and 2s). Our results, in good agreement with a coupled oscillator model with spin (layer)-selectivity and beyond standard density functional theory calculations, promote multilayer 2H-MoSe2 as a highly tunable platform to explore exciton-exciton interactions with strong light-matter interactions.

Superlattice Quantum Solid of Dipolar Excitons.

We study dipolar excitons confined at 330mK in a square electrostatic lattice of a GaAs double quantum well. In the dipolar occupation blockade regime, at 3/2 filling, we evidence that excitons form a face-centered superlattice quantum solid. This phase is realized with high purity across 36 lattice sites, in a regime where the mean interaction energy exceeds the depth of the electrostatic lattice confinement. The superlattice solid then closely relates to Wigner crystals.

Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity.

With exceptional quantum confinement, 2D monolayer semiconductors support a strong excitonic effect, making them an ideal platform for exploring light-matter interactions and as building blocks for novel optoelectronic devices. Different from the well-known in-plane excitons in transition metal dichalcogenides (TMD), the out-of-plane excitons in indium selenide (InSe) usually show weak emission, which limits their applications as light sources. Here, by embedding InSe in an anisotropic gap plasmon nanocavity, we have realized plasmon-enhanced linearly polarized photoluminescence with an anisotropic ratio up to ∼140, corresponding to degree of polarization (DoP) of ∼98.6%. Such polarization selectivity, originating from the polarization-dependent plasmonic enhancement supported by the "nanowire-on-mirror" nanocavity, can be well tuned by the InSe thickness. Moreover, we have also realized an InSe-based light-emitting diode with polarized electroluminescence. Our research highlights the role of excitonic dipole orientation in designing nanophotonic devices and paves the way for developing InSe-based optoelectronic devices with polarization control.

Heteroepitaxy in Organic/TMD Hybrids and Challenge to Achieve it for TMD Monolayers: The Case of Pentacene on WS2 and WSe2.

The intriguing photophysical properties of monolayer stacks of different transition-metal dichalcogenides (TMDs), revealing rich exciton physics including interfacial and moiré excitons, have recently prompted an extension of similar investigations to hybrid systems of TMDs and organic films, as the latter combine large photoabsorption cross sections with the ability to tailor energy levels by targeted synthesis. To go beyond single-molecule photoexcitations and exploit the excitonic signatures of organic solids, crystalline molecular films are required. Moreover, a defined registry on the substrate, ideally an epitaxy, is desirable to also achieve an excitonic coupling in momentum space. This poses a certain challenge as excitonic dipole moments of organic films are closely related to the molecular orientation and film structure, which critically depend on the support roughness. Using X-ray diffraction, optical polarization, and atomic force microscopy, we analyzed the structure of pentacene (PEN) multilayer films grown on WSe2(001) and WS2(001) and identified an epitaxial alignment. While (022)-oriented PEN films are formed on both substrates, their azimuthal orientations are quite different, showing an alignment of the molecular L-axis along the and directions. This intrinsic epitaxial PEN growth depends, however, sensitively on the substrates surface quality. While it occurs on exfoliated TMD single crystals and multilayer flakes, it is hardly found on exfoliated monolayers, which often exhibit bubbles and wrinkles. This enhances the surface roughness and results in (001)-oriented PEN films with upright molecular orientation but without any azimuthal alignment. However, monolayer flakes can be smoothed by AFM operated in contact mode or by transferring to ultrasmooth substrates such as hBN, which again yields epitaxial PEN films. As different PEN orientations result in different characteristic film morphologies (elongated mesa islands vs pyramidal dendrites), which can be easily distinguished by AFM or optical microscopy, this provides a simple means to judge the roughness of the used TMD surface.

Evidence for Exciton Crystals in a 2D Semiconductor Heterotrilayer.

Two-dimensional (2D) transition metal dichalcogenides (TMDC) and their moiré interfaces have been demonstrated for correlated electron states, including Mott insulators and electron/hole crystals commensurate with moiré superlattices. Here we present spectroscopic evidence for ordered bosons─interlayer exciton crystals in a WSe2/MoSe2/WSe2 trilayer, where the enhanced Coulomb interactions over those in heterobilayers have been predicted to result in exciton ordering. Ordered interlayer excitons in the trilayer are characterized by negligible mobility and by sharper PL peaks persisting to an exciton density of nex ∼ 1012 cm-2, which is an order of magnitude higher than the corresponding limit in the heterobilayer. We present evidence for the predicted quadrupolar exciton crystal and its transitions to dipolar excitons either with increasing nex or by an applied electric field. These ordered interlayer excitons may serve as models for the exploration of quantum phase transitions and quantum coherent phenomena.

Observation of quadrupolar and dipolar excitons in a semiconductor heterotrilayer.

Van der Waals (vdW) materials have opened up many avenues for discovery through layer assembly, as epitomized by interlayer dipolar excitons that exhibit electrically tunable luminescence, lasing and exciton condensation. Extending interlayer excitons to more vdW layers, however, raises fundamental questions concerning coherence within excitons and coupling between moiré superlattices at multiple interfaces. Here, by assembling angle-aligned WSe2/WS2/WSe2 heterotrilayers, we demonstrate the emergence of quadrupolar excitons. We confirm the exciton's quadrupolar nature by the decrease in its energy of 12 meV from coherent hole tunnelling between the two outer layers, its tunable static dipole moment under an external electric field and the reduced exciton-exciton interactions. At high exciton density, we also see signatures of a phase of oppositely aligned dipolar excitons, consistent with a staggered dipolar phase predicted to be driven by attractive dipolar interactions. Our demonstration paves the way for discovering emergent exciton orderings for three vdW layers and beyond.

Quadrupolar-dipolar excitonic transition in a tunnel-coupled van der Waals heterotrilayer.

Strongly bound excitons determine light-matter interactions in van der Waals heterostructures of two-dimensional semiconductors. Unlike fundamental particles, quasiparticles in condensed matter, such as excitons, can be tailored to alter their interactions and realize emergent quantum phases. Here, using a WS2/WSe2/WS2 heterotrilayer, we create a quantum superposition of oppositely oriented dipolar excitons-a quadrupolar exciton-wherein an electron is layer-hybridized in WS2 layers while the hole localizes in WSe2. In contrast to dipolar excitons, symmetric quadrupolar excitons only redshift in an out-of-plane electric field. At higher densities and a finite electric field, the nonlinear Stark shift of quadrupolar excitons becomes linear, signalling a transition to dipolar excitons resulting from exciton-exciton interactions, while at a vanishing electric field, the reduced exchange interaction suggests antiferroelectric correlations between dipolar excitons. Our results present van der Waals heterotrilayers as a field-tunable platform to engineer light-matter interactions and explore quantum phase transitions between spontaneously ordered many-exciton phases.

Dipole-Dependent Waveguiding in an Anisotropic Metal-Organic Framework.

The interaction between excitons and photons underlies a range of emergent technologies, such as directional light emission, molecular lasers, photonic circuits, and polaritonic devices. Two of the key parameters that impact exciton-photon coupling are the binding energy of excitons and the relative orientations between the exciton dipole and photon field. Tightly bound excitons are typically found in molecular crystals, where nevertheless the angular relationship of excitons with photon fields is difficult to control. Here, we demonstrate directional exciton dipoles and photon fields, anchored by metal-ligand coordination. In a pyrene-porphyrin bichromophoric metal-organic framework (MOF), we observe that the perpendicular arrangement of the pyrene- and porphyrin-based exciton dipoles engenders orthogonal polarizations of their respective emissions. The alignment of the directional exciton and photon fields gives rise to an anisotropic waveguide effect, where the pyrene- and the porphyrin-based emissions show distinct spatial distribution within microplate-shaped MOF crystals. This capability to simultaneously host heterogenous excitonic states and anisotropic photon fields points toward MOFs' yet-to-be-realized potential as a platform for advancing the frontier in the field of exciton-photonics, which centers around engineering emergent properties from the interplay between excitons and photons.

Strong Dipolar Repulsion of One-Dimensional Interfacial Excitons in Monolayer Lateral Heterojunctions.

Repulsive and long-range exciton-exciton interactions are crucial for the exploration of one-dimensional (1D) correlated quantum phases in the solid state. However, the experimental realization of nanoscale confinement of a 1D dipolar exciton has thus far been limited. Here, we demonstrate atomically precise lateral heterojunctions based at transitional-metal dichalcogenides (TMDCs) as a platform for 1D dipolar excitons. The dynamics and transport of the interfacial charge transfer excitons in a type II WSe2-WS1.16Se0.84 lateral heterostructure were spatially and temporally imaged using ultrafast transient reflection microscopy. The expansion of the exciton cloud driven by dipolar repulsion was found to be strongly density dependent and highly anisotropic. The interaction strength between the 1D excitons was determined to be ∼3.9 × 10-14 eV cm-2, corresponding to a dipolar length of 310 nm, which is a factor of 2-3 larger than the interlayer excitons at two-dimensional van der Waals vertical interfaces. These results suggest 1D dipolar excitons with large static in-plane dipole moments in lateral TMDC heterojunctions as an exciting system for investigating quantum many-body physics.

Nonlinear Dispersion Relation and Out‐of‐Plane Second Harmonic Generation in MoSSe and WSSe Janus Monolayers

AbstractJanus transition metal dichalcogenides are an emerging class of atomically thin materials with engineered broken mirror symmetry that gives rise to long‐lived dipolar excitons, Rashba splitting, and topologically protected solitons. They hold great promise as a versatile nonlinear optical platform due to their broadband harmonic generation tunability, ease of integration on photonic structures, and nonlinearities beyond the basal crystal plane. Here, second and third harmonic generation in MoSSe and WSSe Janus monolayers is studied. Polarization‐resolved spectroscopy is used to map the full second‐order susceptibility tensor of MoSSe, including its out‐of‐plane components. In addition, the effective third‐order susceptibility and the second‐order nonlinear dispersion close to exciton resonances for both MoSSe and WSSe are measured at room and cryogenic temperatures. This work sets a bedrock for understanding the nonlinear optical properties of Janus transition metal dichalcogenides and probing their use in the next‐generation on‐chip multifaceted photonic devices.