Indirect Excitons Research Articles

Robust circular polarization of indirect Q-K transitions in bilayer 3R−WS2

Valley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer $3R\ensuremath{-}\mathrm{W}{\mathrm{S}}_{2}$. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: the spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.

Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators.

Electrons and holes residing on the opposing sides of an insulating barrier and experiencing an attractive Coulomb interaction can spontaneously form a coherent state known as an indirect exciton condensate. We study a trilayer system where the barrier is an antiferromagnetic insulator. The electrons and holes here additionally interact via interfacial coupling to the antiferromagnetic magnons. We show that by employing magnetically uncompensated interfaces, we can design the magnon-mediated interaction to be attractive or repulsive by varying the thickness of the antiferromagnetic insulator by a single atomic layer. We derive an analytical expression for the critical temperature T_{c} of the indirect exciton condensation. Within our model, anisotropy is found to be crucial for achieving a finite T_{c}, which increases with the strength of the exchange interaction in the antiferromagnetic bulk. For realistic material parameters, we estimate T_{c} to be around 7K, the same order of magnitude as the current experimentally achievable exciton condensation where the attraction is solely due to the Coulomb interaction. The magnon-mediated interaction is expected to cooperate with the Coulomb interaction for condensation of indirect excitons, thereby providing a means to significantly increase the exciton condensation temperature range.

Indirect Excitons and Polarization of Dielectric Nanoparticles

Indirect excitons (IX) are widely studied for the purpose of fundamental physics such as Bose–Einstein condensate formation and to develop new devices for photovoltaics, conversion, accumulation, a...

Quantum anomalous valley Hall effect for bosons

We predict the emergence and quantization of the valley Hall current of indirect excitons residing in noncentrosymmetric two-dimensional materials in the absence of an external magnetic field. Thus we report on the quantum anomalous valley Hall effect (QAVHE) for bosons. The crucial ingredients of its existence are the finite anomalous group velocity of exciton motion and the presence of the Bose-Einstein condensate, from which the particles are pushed out by external illumination to contribute to the stationary nonequilibrium current, which exhibits a quantized behavior as a function of the frequency of light.

Fine Structure and Spin Dynamics of Linearly Polarized Indirect Excitons in Two-Dimensional CdSe/CdTe Colloidal Heterostructures

Heterostructured two-dimensional colloidal nanoplatelets are a class of material that has attracted great interest for optoelectronic applications due to their high photoluminescence yield, atomically tunable thickness, and ultralow lasing thresholds. Of particular interest are laterally heterostructured core-crown nanoplatelets with a type-II band alignment, where the in-plane spatial separation of carriers leads to indirect (or charge transfer) excitons with long lifetimes and bright, highly Stokes shifted emission. Despite this, little is known about the nature of the lowest energy exciton states responsible for emission in these materials. Here, using polarization-controlled, steady-state, and time-resolved photoluminescence measurements, at temperatures down to 1.6 K and magnetic fields up to 30 T, we study the exciton fine structure and spin dynamics of archetypal type-II CdSe/CdTe core-crown nanoplatelets. Complemented by theoretical modeling and zero-field quantum beat measurements, we find the bright-exciton fine structure consists of two linearly polarized states with a fine structure splitting ∼50 μeV and an indirect exciton Landé g-factor of 0.7. In addition, we show the exciton spin lifetime to be in the microsecond range with an unusual B-3 magnetic field dependence. The discovery of linearly polarized exciton states and emission highlights the potential for use of such materials in display and imaging applications without polarization filters. Furthermore, the small exciton fine structure splitting and a long spin lifetime are fundamental advantages when envisaging CdSe/CdTe nanoplatelets as elementary bricks for the next generation of quantum devices, particularly given their ease of fabrication.

Magnetic brightening, large valley Zeeman splitting, and dynamics of long-lived A and B dark excitonic states in monolayer WS2

We theoretically investigate the magnetic brightening, valley Zeeman splitting, and valley dynamics of long-lived A and B dark excitons (A-DEs and B-DEs) in monolayer ${\mathrm{WS}}_{2}$, subjected to tilted magnetic fields. Biexciton states and indirect intervalley excitons, with the latter composed of a hole in the $K$ valley and an electron in the $\mathrm{\ensuremath{\Lambda}}$ valley, are also taken into account. We reveal that both in-plane and out-of-plane field components greatly affect the magnetic brightening process due to correlations among different excitonic quasiparticles but in distinct roles. Specifically, the in-plane component primarily enables the dark-state brightening, while the perpendicular one not only modifies the bright-dark splitting inside a given valley, but also lifts the valley degeneracy of either bright or dark excitonic states. Moreover, in the presence of a tilted magnetic field with a nonzero in-plane component, we observe that both A-DEs and B-DEs can be brightened, with the latter requiring a much stronger field for a certain intensity of light emission. Remarkably, we find that the valley splitting of A-DEs is around twice as large as that of A bright excitons but vanishes for B-DEs, as a result of the combined effect of the spin and orbital magnetic moments on the energy shifts of the two valleys. In addition, as the tilt angle of the field with respect to the normal direction of ${\mathrm{WS}}_{2}$ sheet increases from ${0}^{\ensuremath{\circ}}$ to ${180}^{\ensuremath{\circ}}$, the bright-dark splittings for A and B excitons exhibit an opposite behavior, but both feature an ``X''-like pattern when the field is oriented in-plane. Finally, it is observed that the temperature greatly affects the light emission of bright and brightened dark states, while the corresponding temperature dependence is distinct for A and B excitons.

Cavity-control of interlayer excitons in van der Waals heterostructures

Monolayer transition metal dichalcogenides integrated in optical microcavities host exciton-polaritons as a hallmark of the strong light-matter coupling regime. Analogous concepts for hybrid light-matter systems employing spatially indirect excitons with a permanent electric dipole moment in heterobilayer crystals promise realizations of exciton-polariton gases and condensates with inherent dipolar interactions. Here, we implement cavity-control of interlayer excitons in vertical MoSe2-WSe2 heterostructures. Our experiments demonstrate the Purcell effect for heterobilayer emission in cavity-modified photonic environments, and quantify the light-matter coupling strength of interlayer excitons. The results will facilitate further developments of dipolar exciton-polariton gases and condensates in hybrid cavity – van der Waals heterostructure systems.

Indirect to direct exciton transition by laser irradiance in a type II core/ shell quantum dot

Effect of laser irradiance on indirect exciton in Type II CdTe/CdSe Core/Shell Quantum Dot (CSQD) is investigated theoretically through diamagnetic susceptibility (χdia). CO2 laser source of wavelength 10 μm with 30 THz frequency is used as an external perturbation. Using variational method, χdia of exciton is computed for different laser intensity (α0) under different shell thickness. Drastic variation in χdia is observed on enhancing α0. This is due to the wide spatial extent of the carriers by the high screening effect of LASER. Transition of localized exciton from core to the shell regime is pictured via the probability density function of exciton. Further, the ground state energy of hole and the electron of this CSQD is determined for various shell radius (Rs). It is inferred that the incident of LASER strongly alters the ground state energy of the carriers. The evolution of direct exciton from indirect exciton by laser irradiance is realized. To conclude, the external laser perturbation plays a crucial role in improving the device performance by stabilizing excitonic states.

Exciton Gas Transport through Nanoconstrictions.

An indirect exciton is a bound state of an electron and a hole in spatially separated layers. Two-dimensional indirect excitons can be created optically in heterostructures containing double quantum wells or atomically thin semiconductors. We study theoretically the transmission of such bosonic quasiparticles through nanoconstrictions. We show that the quantum transport phenomena, for example, conductance quantization, single-slit diffraction, two-slit interference, and the Talbot effect, are experimentally realizable in systems of indirect excitons. We discuss similarities and differences between these phenomena and their counterparts in electronic devices.

Rydberg spectroscopy of indirect excitons.

Measuring the 1s–2p splitting of direct and indirect excitons in van der Waals heterostructures allows their binding energy and dynamics to be determined.

Ultrafast charge transfer in a type-II MoS2-ReSe2 van der Waals heterostructure.

We fabricated a van der Waals heterostructure by stacking together monolayers of MoS2 and ReSe2. Transient absorption measurements were performed to study the dynamics of charge transfer, indirect exciton formation, and indirect exciton recombination. The results show that the heterostructure form a type-II band alignment with the conduction band minimum and valance band maximum located in the MoS2 and ReSe2 layers, respectively. By using different pump-probe configurations, we found that electrons could efficiently transfer from ReSe2 to MoS2 and holes along the opposite direction. Once transferred, the electrons and holes form spatially indirect excitons, which have longer recombination lifetimes than excitons in individual monolayers. These results provide useful information for developing van der Waals heterostructure involving ReSe2 for novel electronic and optoelectronic applications.

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.

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.

Attraction of indirect excitons in van der Waals heterostructures with three semiconducting layers

We study a capacitor made of three monolayers of transition metal dichalcogenide (TMD) separated by hexagonal Boron Nitride (hBN). We assume that the structure is symmetric with respect to the central layer plane. The symmetry includes the contacts: if the central layer is contacted by the negative electrode, both external layers are contacted by the positive one. As a result a strong enough voltage $V$ induces electron-hole dipoles (indirect excitons) pointing towards one of the external layers. Antiparallel dipoles attract each other at large distances. Thus, the dipoles alternate in the central plane forming a 2D antiferroelectric with negative binding energy per dipole. The charging of a three-layer device is a first order transition, and we show that if $V_1$ is the critical voltage required to create a single electron-hole pair and charge this capacitor by $e$, the macroscopic charge $Q_c = eSn_c$ ($S$ is the device area) enters the three-layer capacitor at a smaller critical voltage $V_{c} < V_{1}$. In other words, the differential capacitance $C(V)$ is infinite at $V = V_{c}$. We also show that in a contact-less three-layer device, where the chemically different central layer has lower conduction and valence bands, optical excitation creates indirect excitons which attract each other, and therefore form antiferroelectric exciton droplets. Thus, the indirect exciton luminescence is red shifted compared to a two-layer device.

Numerical modeling of indirect excitons in double quantum wells in an external electric field

The ground state of an exciton in the GaAs-based double quantum well structure in an external electric field is calculated by the direct numerical solution of the three-dimensional Schrödinger equation. A formation of the indirect exciton is demonstrated by the study of the localization of the exciton wave function in the double quantum well structure for different electric field strengths. A relatively slow radiative decay rate of the indirect exciton is observed.

Erratum: Spatially resolved and time-resolved imaging of transport of indirect excitons in high magnetic fields [Phys. Rev. B 95 , 235308 (2017)

Erratum: Spatially resolved and time-resolved imaging of transport of indirect excitons in high magnetic fields [Phys. Rev. B <b>95</b> , 235308 (2017)

Ultrafast Energy Transfer of Both Bright and Dark Excitons in 2D van der Waals Heterostructures Beyond Dipolar Coupling.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have shown great potential in ultrathin and flexible optoelectronic and photonics devices. Besides emissive bright excitons, they also possess rich non-emissive dark excitons including momentum-forbidden indirect excitons and spin-forbidden triplet-like excitons, which could be dominant species under optical or electrical excitation in 2D optoelectronic and photonic devices. Efficient harvesting of both bright and dark excitons from TMDs and understanding the exciton-transfer mechanism consequently are not only of fundamental interest but also a technological challenge. Here, by combining steady-state photoluminescence spectroscopy and ultrafast transient reflectance spectroscopy, we show efficient exciton harvesting by ultrafast energy transfer in WSe2/MoTe2 van der Waals heterostructures, leading to the photoluminescence enhancement of MoTe2. The energy transfer occurs with near-unity yield and in an ultrafast (∼200 fs) manner for both bright and dark excitons, suggesting a dominant Dexter-type energy-transfer process consisting of simultaneous transfer of both electron and hole in van der Waals coupled 2D layers at ultimate proximity. This result is beyond the conventional dipole-dipole coupling mechanism typically assumed at 2D interfaces and offers a path to high speed and enhanced light harvesting and emission applications based on 2D heterostructures.

Perfect squeezing of terahertz light by two quantum wells using a squeezed vacuum reservoir

The nonclassical properties of a microcavity confining two quantum wells irradiated by a coherent light source and interacting with a squeezed vacuum reservoir are investigated. By deriving analytical expressions of the intensity power spectrum, the squeezing spectrum, and the second-order correlation function of the output field, we show that the proposed scheme generates perfect squeezing. The obtained results are quite different from the single-quantum-well cavity system where, under the same conditions, squeezing does not exceed 50%, and the transmitted light is bunched. It turns out that the presence of indirect excitons in the cavity strongly modifies the quantum dynamics of the system.

Simulation of exciton condensate-mediated quantum transport in a double-monolayer transition metal dichalcogenide system

We have developed and employed a quantum transport simulation method to study the transport properties in the presence of a spatially indirect exciton condensate between energy-gapped two-dimensional semiconductors, with contact to each end of each layer. This system differs from previous works on bilayer exciton transport properties in the consideration of the energy gap as well as the associated massive (parabolic band structure) electrons and holes. We observe transport properties in the presence of the condensate including the elimination of current counterflow---otherwise phenomenologically analogous to near-perfect Coulomb drag---absent overlap of the conduction and valence bands of opposite layers, as well as substantially greater critical interlayer current for a given assumed bare interlayer coupling.

Hot electron science in plasmonics and catalysis: what we argue about.

Hot electron photochemistry has made strong claims for improved control of chemical reactions. Here I discuss these claims in the light of a plethora of model experiments and theories, asking what are the key issues to solve. I particularly highlight the need to understand nanoscale thermal hot-spots, thermal gradients, and thermal transport, as well as the conventional optical confinement in plasmonics. I note how the 'direct electron transfer' process seems to dominate, and resembles well known 'indirect excitons' in semiconductor quantum wells. I believe a crucial advance still required is a prototype nano-confined geometry which allows reactants and products to access a well-controlled metallic atomic surface.