Exciton-electron Interaction Research Articles

Fermi polaron fine structure in strained van der Waals heterostructures

The fine structure of attractive Fermi polarons in van der Waals heterostructures based on monolayer transition metal dichalcogenides in the presence of elastic strain is studied theoretically. The charged excitons (trions), three particle bound states of two electrons and a hole or two holes and an electron, do not show any strain-induced fine structure splitting compared to neutral excitons whose radiative doublet is split by the strain into linearly polarized components. The correlation of the trions with Fermi sea holes gives rise to attractive Fermi polarons. We show that this results in a fine structure splitting of the polaron into states polarized along the main axes of the strain tensor. This effect is related to the bosonic statistics of Fermi polarons. We develop a microscopic theory of the effect and calculate the strain-induced splitting of Fermi polarons for both tungsten- and molybdenum-based monolayers, identifying the role of inter- and intravalley exciton–electron interactions. The fine structure splitting of the attractive Fermi polaron is proportional to both the excitonic splitting and the Fermi energy. The Fermi polaron fine structure in bilayers is briefly analyzed, and the role of electron and trion localization in moiré potentials is discussed.

Coherent Phonon-Mediated Many-Body Interaction in Monolayer WSe2

Due to the strong Coulomb interaction, the optical and electrical properties of two-dimensional transition metal dichalcogenides (TMDCs) are greatly determined by the emergence of many-body complexes such as excitons or trions. To fully realize the potential functionalities of these atomically thin materials, a comprehensive understanding of their many-body interaction mechanism is essential. Here, using the advanced femtosecond two-dimensional electronic spectroscopy technique combined with broadband transient absorption spectroscopy, a strong electron-exciton coupling effect in monolayer WSe2 following the ultrafast photoexcitation is revealed. We demonstrate that such many-body complexes can be generated effectively through the band-edge optical excitation, with a ∼1.5 ps stabilization process. The coherent optical phonon plays a dominant role in this electron-exciton interaction, and the coherence of the electron (exciton)-phonon coupling can last for ∼4.5 ps. This finding offers new insight into the formation mechanism of photoinduced many-body complexes in TMDCs.

Superfluid drag between excitonic polaritons and superconducting electron gas

The Andreev-Bashkin effect, or superfluid drag, is predicted in a system of Bose-condensed excitonic polaritons in optical microcavity coupled by electron-exciton interaction with a superconducting layer. Two possible setups with spatially indirect dipole excitons or direct excitons are considered. The drag density characterizing a magnitude of this effect is found by many-body calculations with taking into account dynamical screening of electron-exciton interaction. For the superconducting electronic layer, we assume the recently proposed polaritonic mechanism of Cooper pairing, although the preexisting thin-film superconductor should also demonstrate the effect. According to our calculations, the drag density can reach considerable values in realistic conditions, with excitonic and electronic layers made from GaAs-based quantum wells or two-dimensional transition metal dichalcogenides. The predicted nondissipative drag could be strong enough to be observable as induction of a supercurrent in the electronic layer by a flow of polariton Bose condensate.

Optical Signatures of Periodic Magnetization: The Moiré Zeeman Effect.

Detecting magnetic order at the nanoscale is of central interest for the study of quantum magnetism in general, and the emerging field of moiré magnets in particular. Here, we analyze the exciton band structure that arises from a periodic modulation of the valley Zeeman effect. Despite long-range electron-hole exchange interactions, we find a sizable splitting in the energy of the bright circularly polarized exciton Umklapp resonances, which serves as a direct optical probe of magnetic order. We first analyze quantum moiré magnets realized by periodic ordering of electron spins in Mott-Wigner states of transition metal dichalcogenide monolayers or twisted bilayers: we show that spin valley-dependent exciton-electron interactions allow for probing the spin-valley order of electrons and demonstrate that it is possible to observe unique signatures of ferromagnetic order in a triangular lattice and both ferromagnetic and Néel order in a honeycomb lattice. We then focus on semiclassical moiré magnets realized in twisted bilayers of ferromagnetic materials: we propose a detection scheme for moiré magnetism that is based on interlayer exchange coupling between spins in a moiré magnet and excitons in a transition metal dichalcogenide monolayer.

Emergence of a Sharp Quantum Collective Mode in a One-Dimensional Fermi Polaron

The Fermi-polaron problem of a mobile impurity interacting with fermionic medium emerges in various contexts, ranging from the foundations of Landau's Fermi-liquid theory to electron-exciton interaction in semiconductors, to unusual properties of high-temperature superconductors. While classically the medium provides only a dissipative environment to the impurity, quantum picture of polaronic dressing is more intricate and arises from the interplay of few- and many-body aspects of the problem. The conventional expectation for the dynamics of Fermi polarons is that it is dissipative in character, and any excess energy is rapidly emitted away from the impurity as particle-hole excitations. Here we report a strikingly different type of polaron dynamics in a one-dimensional system of the impurity interacting repulsively with the fermions. When the total momentum of the system equals the Fermi momentum, there emerges a sharp collective mode corresponding to long-lived oscillations of the polaronic cloud surrounding the impurity. This mode can be observed experimentally with ultracold atoms using Ramsey interferometry and radio-frequency spectroscopy.

Energy relaxation of quantum dot hot electrons in hybrid quantum dot—Bose–Einstein condensate system

The theory of electron energy relaxation in a hybrid structure consisting of quantum dot interacting with a two dimensional exciton gas in Bose–Einstein condensate (BEC) regime is developed. A new type of the relaxation mechanism in the presence of BEC is introduced and theoretically analyzed. It is shown that, in the first order of electron-exciton interaction, two microscopic processes of energy relaxation appear. The first one is related to the emission of a single Bogoliubov excitation (bogolon) by an electron, whereas the second process is associated with the emission of two bogolons. It is shown that the second type processes dominate in the QD electron energy relaxation.

Enhanced Scattering between Electrons and Exciton-Polaritons in a Microcavity.

The interplay between strong light-matter interactions and charge doping represents an important frontier in the pursuit of exotic many-body physics and optoelectronics. Here, we consider a simplified model of a two-dimensional semiconductor embedded in a microcavity, where the interactions between electrons and holes are strongly screened, allowing us to develop a diagrammatic formalism for this system with an analytic expression for the exciton-polariton propagator. We apply this to the scattering of spin-polarized polaritons and electrons, and show that this is strongly enhanced compared with exciton-electron interactions. As we argue, this counterintuitive result is a consequence of the shift of the collision energy due to the strong light-matter coupling, and hence this is a generic feature that applies also for more realistic electron-hole and electron-electron interactions. We furthermore demonstrate that the lack of Galilean invariance inherent in the light-matter coupled system can lead to a narrow resonancelike feature for polariton-electron interactions close to the polariton inflection point. Our results are potentially important for realizing tunable light-mediated interactions between charged particles.

Optical Signatures of Periodic Charge Distribution in a Mott-like Correlated Insulator State

The elementary optical excitations in two dimensional semiconductors hosting itinerant electrons are attractive and repulsive polarons -- excitons that are dynamically screened by electrons. Exciton-polarons have hitherto been studied in translationally invariant degenerate Fermi systems. Here, we show that electronic charge order breaks the excitonic translational invariance and leads to a direct optical signature in the exciton-polaron spectrum. Specifically, we demonstrate that new optical resonances appear due to spatially modulated interaction between excitons and electrons in an incompressible Mott state. Our observations demonstrate that resonant optical spectroscopy provides an invaluable tool for studying strongly correlated states, such as Wigner crystals and density waves, where exciton-electron interactions are modified by the emergence of new electronic charge or spin order.

Quantum dot in a hybrid structure with dipolar excitons

Electron states in a quantum dot (QD) located near a 2D system of dipolar excitons are perturbed by fluctuations of the exciton density caused by the electron-exciton interaction. This results in the frequency changes of electron transitions in a QD. The frequency depends on the exciton density, as well as on the exciton gas phase state. In the present work, the shifts of the two lowest QD energy levels are found both in the normal state of the exciton system and for the Bose-Einstein condensation (BEC) regime.

Exciton-exciton interaction and cascade relaxation of excitons in colloidal CdSe nanoplatelets

The exciton-exciton interaction and phonon-induced cascade relaxation of free electrons and holes determine the dynamic of stationary resonantly excited excitons in CdSe/CdS nanoplatelets. Along with saturation of the heavy-hole and light-hole exciton transitions explained by the phase space filling effect the exciton-exciton interaction increases and starts to play a significant role in excitons dynamic. The lowest four band structure of CdSe/CdS nanoplatelets at the Γ point of Brillouin zone and its modification with CdS shell thickness changing are experimentally investigated for the first time. Bleaching of the two additional to the heavy- and light-hole exciton transitions with the energies higher than the absorbed photons energy can occur at room temperature due to rapid cascade relaxation of free electrons and holes formed as a result of the exciton-exciton interaction. To analyze the recombination and interaction of excitons in the colloidal nanoplatelets under high excitation densities the rate equations describing the exciton-exciton and exciton-electron interactions are applied.

Nonlinear optics in the fractional quantum Hall regime.

Engineering strong interactions between optical photons is a challenge for quantum science. Polaritonics, which is based on the strong coupling of photons to atomic or electronic excitations in an optical resonator, has emerged as a promising approach to address this challenge, paving the way for applications such as photonic gates for quantum information processing1 and photonic quantum materials for the investigation of strongly correlated driven-dissipative systems2,3. Recent experiments have demonstrated the onset of quantum correlations in exciton-polariton systems4,5, showing that strong polariton blockade6-the prevention of resonant injection of additional polaritons in a well delimited region by the presence of a single polariton-could be achieved if interactions were an order of magnitude stronger. Here we report time-resolved four-wave-mixing experiments on a two-dimensional electron system embedded in an optical cavity7, demonstrating that polariton-polariton interactions are strongly enhanced when the electrons are initially in the fractional quantum Hall regime. Our experiments indicate that, in addition to strong correlations in the electronic ground state, exciton-electron interactions leading to the formation of polaron-polaritons8-11 have a key role in enhancing the nonlinear optical response of the system. Our findings could facilitate the realization of strongly interacting photonic systems, and suggest that nonlinear optical measurements could provide information about fractional quantum Hall states that is not accessible through their linear optical response.

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.

The direct observation of electron backflow in an organic heterojunction formed by two n-type materials.

Many physical processes such as exciton interfacial dissociation, exciton interfacial recombination, and exciton-electron and exciton-hole interactions coexist at the interface of organic solar cells (OSC). In this study, the direction of free charge generation is defined as the direction from the interface to the side where free charges are left. For a p-n type device, the direction of free electron (hole) generation from exciton dissociation at the donor/accepter (D/A) interface is the same as the subsequent transportation direction under the built-in electric field. However, the direction of free electron (hole) generation from exciton-exciton recombination across the D/A interface is opposite to the direction of free charge transportation. Both free charges generated from exciton interfacial dissociation and recombination are contributed to the photocurrent for a p-n type device. In a device with a heterojunction formed by two n-type materials (here it is defined as an n-n type device), the direction of free electron (hole) generation from exciton recombination across the interface is also the same as the subsequent free charge transportation. At the same time, there are also some free electrons (free holes) generated by exciton interfacial dissociation. The direction of free charge generation from exciton dissociation for this n-n type device is also opposite to the direction of free charge transportation. However, only free charges generated from exciton interfacial recombination are contributed to the photocurrent for an n-n type device. But so far there has been no direct experimental evidence to prove the above theories. In this work, an NPB interfacial layer with a high LUMO was introduced in an n-n type OSC to inhibit the backflow of electrons, which are generated from exciton dissociation at the heterojunction formed by two n-type materials, enhancing the device performance accordingly. This work is conducive to interfacial engineering in an OSC to further improve its performance.

Exciton-exciton interaction in transition-metal dichalcogenide monolayers

We study theoretically the Coulomb interaction between excitons in transition metal dichalcogenide (TMD) monolayers. We calculate direct and exchange interaction for both ground and excited states of excitons. The screening of the Coulomb interaction, specific to monolayer structures, leads to the unique behavior of the exciton-exciton scattering for excited states, characterized by the non-monotonic dependence of the interaction as function of the transferred momentum. We find that the nontrivial screening enables the description of TMD exciton interaction strength by approximate formula which includes exciton binding parameters. The influence of screening and dielectric environment on the exciton-exciton interaction was studied, showing qualitatively different behavior for ground state and excited states of excitons. Furthermore, we consider exciton-electron interaction, which for the excited states is governed by the dominant attractive contribution of the exchange component, which increases with the excitation number. The results provide a quantitative description of the exciton-exciton and exciton-electron scattering in transition metal dichalcogenides, and are of interest for the design of perspective nonlinear optical devices based on TMD monolayers.

Fermi polaron-polaritons in charge-tunable atomically thin semiconductors

The dynamics of a mobile quantum impurity in a degenerate Fermi system is a fundamental problem in many-body physics. The interest in this field has been renewed due to recent ground-breaking experiments with ultra-cold Fermi gases. Optical creation of an exciton or a polariton in a two-dimensional electron system embedded in a microcavity constitutes a new frontier for this field due to an interplay between cavity-coupling favoring ultra-low mass polariton formation and exciton-electron interactions leading to polaron or trion formation. Here, we present cavity spectroscopy of gate-tunable monolayer MoSe$_2$ exhibiting strongly bound trion and polaron resonances, as well as non-perturbative coupling to a single microcavity mode. As the electron density is increased, the oscillator strength determined from the polariton splitting is gradually transferred from the higher-energy repulsive-exciton-polaron resonance to the lower-energy attractive-polaron manifold. Simultaneous observation of polariton formation in both attractive and repulsive branches indicate a new regime of polaron physics where the polariton impurity mass is much smaller than that of the electrons. Our findings shed new light on optical response of semiconductors in the presence of free carriers by identifying the Fermi polaron nature of excitonic resonances and constitute a first step in investigation of a new class of degenerate Bose-Fermi mixtures.

Trion formation dynamics in monolayer transition metal dichalcogenides

We report charged exciton (trion) formation dynamics in doped monolayer transition metal dichalcogenides (TMDs), specifically molybdenum diselenide (MoSe2), using resonant two-color pump-probe spectroscopy. When resonantly pumping the exciton transition, trions are generated on a picosecond timescale through exciton-electron interaction. As the pump energy is tuned from the high energy to low energy side of the inhomogeneously broadened exciton resonance, the trion formation time increases by ~ 50%. This feature can be explained by the existence of both localized and delocalized excitons in a disordered potential and suggests the existence of an exciton mobility edge in TMDs. The quasiparticle formation and conversion processes are important for interpreting photoluminescence and photoconductivity in TMDs.

Evidences of photocurrent generation by hole–exciton interaction at organic semiconductor interfaces

The charge–exciton interaction at the donor/acceptor interface plays a significant role in the exciton dissociation processes, and thus influences the performance of organic solar cells. In this work, the evidences of photocurrent generation via hole–exciton interaction (HEI) at the organic semiconductor interface in organic solar cells, which is the counterpart of photocurrent generated by electron–exciton interaction, is demonstrated. A heterojunction, composed of copper phthalocyanine (CuPc) and fullerene (C60), is used to provide free holes that interact with the excitons supplied by perfluorinated hexadecafluorophthalo-cyaninatozinc (F16ZnPc). The fact that photocurrent generation via HEI is well evidenced by: (1) a short circuit current of 0.38mAcm−2; (2) the jump of an external quantum efficiency (EQE) around 800nm after adding a bias light; (3) the EQE variations under bias light of different wavelengths and light intensities; and (4) the superlinear dependence of the photocurrent on the light intensity.

Photoinduced Injection Enhancement in Fullerene-Based Organic Solar Cell Originates from Exciton–Electron Interaction

The dependence of photocurrent of a typical copper phthalocyanine (CuPc)/C60 device on external negative bias under monochromatic light illumination is reported. With increase of the negative bias, the photocurrent originating from excitons in the CuPc layer is quickly saturated. On the contrary, the photocurrent originating from excitons in C60 layer does not reach saturation but increases continuously with negative bias. This abnormal enhancement shows that the behavior of excitons in C60 layer is completely different from that in the typical donor material CuPc. The triplet exciton–electron interaction in C60 is supposed to cause the photoinduced hole injection enhancement from cathode, resulting in the observed phenomenon. More experiments designed have confirmed this mechanism.

Lifetime of quasiparticles in a hybrid electron-exciton system

The lifetimes (damping) of one-body and collective excitations in a hybrid system consisting of spatially separated layers of a two-dimensional electron gas and a gas of indirect excitons have been calculated at zero temperature in the presence of the Bose-Einstein condensate of excitons. It has been shown that the electron-exciton interaction leads to a considerable shortening of the lifetime of electrons as compared to the electron-electron interaction and to the appearance of a nonzero (linear in the wave vector) damping of plasmons. The interaction of the exciton Bose gas with the electron layer induces damping of Bogoliubov phonons in the exciton Bose gas, which is, however, much lower than their intrinsic (Belyaev) damping.

Exciton absorption of the GaAs semiconductor crystals under optical pumping to the conduction band

Modifications of the exciton structure of the fundamental absorption edge in the GaAs crystals is experimentally studied at T = 1.7 K using the optical pumping at a photon energy that is significantly greater than the band gap. An increase in the amplitude of the fundamental state of the exciton is observed at a stable maximum energy. The dependence of the integral absorption on the pump intensity is interpreted in the framework of the concept of the excitonic polariton using the dissipative scattering of the exciton by free electrons that are generated by the pumping radiation. The constant of the electron-exciton interaction can be estimated with the aid of the solution to the inverse problem for initial pump levels. The integral absorption of the fundamental exciton state at liquid-helium temperatures can be used to characterize the purity of an epitaxial layer. The reasons for the lower saturation level of the integral absorption that is significantly less than the calculated level determined by the exciton oscillator strength need to be further studied.