Electron-hole Exchange Interaction Research Articles

Magnetic field-driven photoluminescence modulation in graphene oxide quantum dots: Insights into enhancement mechanisms

The electron-hole exchange interaction governs the exciton dynamics in semiconductor quantum dots, which modulates the photoluminescence (PL) properties in various applications. Generally, the exciton dynamics exhibit significant dependence on temperature and magnetic field. Here, we demonstrate that the electron-hole exchange interaction in graphene oxide quantum dots (GOQDs) is enhanced by magnetic field at low temperatures, resulting in increased PL. Specifically, the exciton states can split into singlet-bright and triplet-dark states due to the strong electron-hole exchange interaction. The bright-dark splitting increases below 30 K and under a magnetic field up to 40 T, which suppresses the non-radiative transitions of triplet-dark state. Therefore, PL intensity and lifetimes also increase below 30 K. This study enriches the understanding of the fundamental magneto-optical properties in carbon-based quantum dots.

Anomalous Behavior in Dark-Bright Splitting Impacts the Biexciton Binding Energy in (BA)2(MA)n-1PbnBr3n+1 (n = 1-3).

Two-dimensional Ruddlesden-Popper series are an excellent system for tuning physical properties of the perovskite by controlling the layer number (n). For instance, bandgap and exciton binding energies of the series gradually increase upon reducing n via enhanced quantum and dielectric confinements. Here, we present findings that challenge the anticipated trend in electron-hole exchange interaction within (BA)2MAn-1PbnBr3n+1 (n = 1-3), which causes spin-dependent exciton level splitting into bright and dark states, where the latter is partially visible near the surface of the Br-based two-dimensional Ruddlesden-Popper series. Contrary to expectations, the smallest gap between bright and dark exciton levels is observed from n = 2 at 10 K. This anomaly results in the strongest biexciton binding between two dark excitons occurring at n = 2, rather than at n = 1 as initially hypothesized. The observed anomaly arises from a phase transition induced by octahedral tilting occurring only for n = 2 near 100 K as confirmed by temperature-dependent optical and X-ray diffraction measurements. Our results show that Coulomb interaction need not vary gradually with n, which can impact the optoelectronic properties of the Ruddlesden-Popper series.

Longitudinal-transverse splitting and fine structure of Fermi polarons in two-dimensional semiconductors

Interaction of excitons with resident charge carriers in semiconductors gives rise to bound three-particle complexes, trions, whose optical response is conveniently described in the framework of many-body correlated Fermi polaron states. These states are formed as a result of correlation of photocreated trion with the Fermi sea hole and possess the angular momentum component of ±1 depending on the helicity of the photon. We study theoretically the energy spectrum fine structure of Fermi polarons in two-dimensional semiconductors based on transition metal dichalcogenides. We demonstrate both by the symmetry analysis and microscopic calculation that the Fermi polarons with nonzero in-plane wavevector k are split, similarly to the neutral exciton states, into the linearly polarized longitudinal and transverse, with respect to the k, states. The origin of this longitudinal-transverse splitting is the long-range electron-hole exchange interaction that can be also described as the interaction of Fermi polarons with their induced electromagnetic field. The effective Hamiltonian describing the Fermi polaron fine structure is derived, and its parameters are determined from the microscopic model.

Twist angle-dependent valley polarization switching in heterostructures.

The twist engineering of moiré superlattice in van der Waals heterostructures of transition metal dichalcogenides can manipulate valley physics of interlayer excitons (IXs), paving the way for next-generation valleytronic devices. However, the twist angle-dependent control of excitonic potential on valley polarization is not investigated so far in electrically controlled heterostructures and the physical mechanism underneath needs to be explored. Here, we demonstrate the dependence of both polarization switching and degree of valley polarization on the moiré period. We also find the mechanisms to reveal the modulation of twist angle on the exciton potential and the electron-hole exchange interaction, which elucidate the experimentally observed twist angle-dependent valley polarization of IXs. Furthermore, we realize the valley-addressable devices based on polarization switch. Our work demonstrates the manipulation of the valley polarization of IXs by tunning twist angle in electrically controlled heterostructures, which opens an avenue for electrically controlling the valley degrees of freedom in twistronic devices.

Theory of acoustic-phonon involved exciton spin flip in perovskite semiconductors

We present a theory of the acoustic phonon assisted spin-flip Raman scattering (SFRS), or resonant photoluminescence with the spin flip of a photoexcited exciton localized in a bulk cubic-phase perovskite semiconductor. We consider the spin-flip transitions between the ground-state exciton spin sublevels in external magnetic field B and discuss the variation of their probability rate and polarization selection rules with the increase of B. The transitions are treated as two-quantum processes with the virtual to and fro transfer of the electron in the electron-hole pair between the bottom and first excited conduction bands. The transfer occurs due to both the electron-hole exchange interaction and the electron-phonon interaction. The theoretical results allow one to distinguish the phonon assisted Raman scattering from (a) the resonant Raman scattering with the combined spin flip of the localized resident electron and hole and (b) the biexciton-mediated SFRS analyzed previously.

Large exchange-driven intrinsic circular dichroism of a chiral 2D hybrid perovskite

In two-dimensional chiral metal-halide perovskites, chiral organic spacers endow structural and optical chirality to the metal-halide sublattice, enabling exquisite control of light, charge, and electron spin. The chiroptical properties of metal-halide perovskites have been measured by transmissive circular dichroism spectroscopy, which necessitates thin-film samples. Here, by developing a reflection-based approach, we characterize the intrinsic, circular polarization-dependent complex refractive index for a prototypical two-dimensional chiral lead-bromide perovskite and report large circular dichroism for single crystals. Comparison with ab initio theory reveals the large circular dichroism arises from the inorganic sublattice rather than the chiral ligand and is an excitonic phenomenon driven by electron-hole exchange interactions, which breaks the degeneracy of transitions between Rashba-Dresselhaus-split bands, resulting in a Cotton effect. Our study suggests that previous data for spin-coated films largely underestimate the optical chirality and provides quantitative insights into the intrinsic optical properties of chiral perovskites for chiroptical and spintronic applications.

Effect of finite spin-orbit splitting on the electron-hole exchange interaction in excitons confined in semiconductor nanocrystals

Effect of finite spin-orbit splitting on the electron-hole exchange interaction in excitons confined in semiconductor nanocrystals

Spectral widths and Stokes shifts in InP-based quantum dots.

InP-based quantum dots (QDs) have Stokes shifts and photoluminescence (PL) line widths that are larger than in II-VI semiconductor QDs with comparable exciton energies. The mechanisms responsible for these spectral characteristics are investigated in this paper. Upon comparing different semiconductors, we find the Stokes shift decreases in the following order: InP > CdTe > CdSe. We also find that the Stokes shift decreases with core size and decreases upon deposition of a ZnSe shell. We suggest that the Stokes shift is largely due to different absorption and luminescent states in the angular momentum fine structure. The energy difference between the fine structure levels, and hence the Stokes shifts, are controlled by the electron-hole exchange interaction. Luminescence polarization results are reported and are consistent with this assignment. Spectral widths are controlled by the extent of homogeneous and inhomogeneous broadening. We report PL and PL excitation (PLE) spectra that facilitate assessing the roles of homogeneous and different inhomogeneous broadening mechanisms in the spectra of zinc-treated InP and InP/ZnSe/ZnS particles. There are two distinct types of inhomogeneous broadening: size inhomogeneity and core-shell interface inhomogeneity. The latter results in a distribution of core-shell band offsets and is caused by interfacial dipoles associated with In-Se or P-Zn bonding. Quantitative modeling of the spectra shows that the offset inhomogeneity is comparable to but somewhat smaller than the size inhomogeneity. The combination of these two types of inhomogeneity also explains several aspects of reversible hole trapping dynamics involving localized In3+/VZn2- impurity states in the ZnSe shells.

Composed Effects of Electron-Hole Exchange and Near-Field Interaction in Quantum-Dot-Confined Radiative Dipoles

Conservation of polarization is an important requirement for reliable single-photon emitters, which, in turn, are essential building blocks for light-based quantum information processing. In this work, we study the exciton-spin dynamics in a double quantum dot under the combined effects of electron-hole exchange and Förster resonance energy transfer. By means of numerical solutions of the quantum master equation, we simulate the time-dependent spin polarization for two neighboring dots. According to our results, under some conditions, the depolarization caused by the electron-hole exchange may be slowed by the near field-induced interdot energy transfer, suggesting a new mechanism to extend the exciton coherence time. This opens doors to alternative schemes for improved solid-state quantum light sources.

Optical Alignment and Optical Orientation of Excitons in CdSe/CdS Colloidal Nanoplatelets.

Optical alignment and optical orientation of excitons are studied experimentally on an ensemble of core/shell CdSe/CdS colloidal nanoplatelets. Linear and circular polarization of photoluminescence during resonant excitation of excitons is measured at cryogenic temperatures and with magnetic fields applied in the Faraday geometry. The developed theory addresses the optical alignment and optical orientation of excitons in colloidal nanocrystals, taking into account both bright and dark exciton states in the presence of strong electron-hole exchange interaction and the random in-plane orientation of nanoplatelets within the ensemble. Our theoretical analysis of the obtained experimental data allows us to evaluate the exciton fine structure parameters, the g-factors, and the spin lifetimes of the bright and dark excitons. The optical alignment effect enables the identification of the exciton and trion contributions to the emission spectrum, even in the absence of their clear separation in the spectra.

Mechanisms of interlayer exciton emission and giant valley polarization in van der Waals heterostructures

We perform a theoretical investigation about the valley dynamics of interlayer excitons (IXs) in $\mathrm{Mo}{\mathrm{S}}_{2}/\mathrm{W}{\mathrm{S}}_{2}$ heterostructure with AB stacking configuration grown on ferromagnetic substrate. We find that the interlayer charge transfer process changes dramatically the IX emission as well as its valley polarization (VP). The magnetic proximity effect (MPE) exerted by ferromagnetic substrate, on the other hand, generates a zero-field valley Zeeman splitting, which suppresses the depolarization induced by electron-hole exchange interaction. Remarkably, unlike usual exciton-phonon scattering, which is harmful to the VP, the phonon-assisted intervalley scatterings between two split IX states in the different valleys foster an IX population imbalance in these states, giving rise to a giant VP. A combination of these experimentally tunable physical quantities (interlayer charge transfer rate, MPE intensity and exciton-phonon coupling strength) provides a promising tool for intriguing emerging magneto-optical emissions and their VPs. Considering a large family of layered materials, this study sheds light on the path for development of van der Waals heterostructures with detectable IX emissions and giant VP.

Dielectric effects, crystal field, and shape anisotropy tuning of the exciton fine structure of halide perovskite nanocrystals

We present a theoretical study of the excitonic band edge states applied to the more commonly studied inorganic perovskite nanocrystals: ${\mathrm{CsPbBr}}_{3}$. We highlight the key role played by the electron-hole exchange interaction, including long-range and short range terms, in the positioning and splitting of dark and bright exciton sublevels, and discuss the influence of dielectric confinement, crystal field and shape anisotropy effects on the excitonic fine structure. The electron-hole exchange interaction splits the four excitonic states into three bright excitonic states at higher energy and a singlet dark state at lower energy. We find that, whatever the crystal phase, the bright-dark splitting is weakly sensitive to shape anisotropy. However, a strong enhancement, of about 50%, is obtained at the maximum of the dielectric contrast. For two crystalline symmetries, we particularly analyze how the bright exciton triplet energies vary, taking into account the states polarizations and considering the directions along which shape elongation/contraction applies. The main effect of the dielectric contrast is to increase significantly the exciton energies, for all the configurations. For a cubic crystal---${\mathrm{O}}_{h}$ symmetry---the bright exciton triplet degeneracy can be partially or fully lifted with anisotropic shape along one or two directions, respectively. For a tetragonal crystal---${\mathrm{D}}_{4h}$ symmetry---the partial triplet degeneracy can be completely lifted by a single distortion from the cubic shape along a direction perpendicular to the tetragonal axis but a partial degeneracy is maintained for small distortions along the tetragonal axis. The amplitude of bright exciton splittings are basically driven by the shape anisotropy and is almost insensitive to the dielectric contrast. On the basis of this model, we discuss recent results on ${\mathrm{CsPbBr}}_{3}$ nanocrystals photoluminescence.

Intervalley electron-hole exchange interaction and impurity-assisted recombination of indirect excitons in WS2 and WSe2 monolayers

The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible for a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the emission spectrum.

Strain engineering of quantum confinement in WSe2 on nano-roughness glass substrates

Strain engineering is a powerful tool for generating single-photon emitters in monolayer (ML) transition metal dichalcogenides. Here, we report on a simple method for generating sharp emission lines (linewidths ≈ 150–500 μeV) in a monolayer (ML) WSe2 on nano-roughness regions of Tb3+-borogermanate glasses. We performed a polarization-resolved magneto-luminescence study in WSe2/glass at low temperature. Remarkably, we observed several stable and linearly polarized doublet emission peaks in strained regions that are associated with a fine structure splitting due to the anisotropic electron–hole exchange interaction with g-factors of ∼8.4–9.8. Our results indicate that strain engineering on glass substrates is a promising tool for generating quantum dot-like emitters in ML WSe2 for possible integration with photonics systems for quantum information technology.

Valley-magnetophonon resonance for interlayer excitons

Heterobilayers consisting of MoSe2 and WSe2 monolayers can host optically bright interlayer excitons with intriguing properties such as ultralong lifetimes and pronounced circular polarization of their photoluminescence due to valley polarization, which can be induced by circularly polarized excitation or applied magnetic fields. Here, we report on the observation of an intrinsic valley-magnetophonon resonance for localized interlayer excitons promoted by invervalley hole scattering. It leads to a resonant increase of the photoluminescence polarization degree at the same field of 24.2 Tesla for H-type and R-type stacking configurations despite their vastly different excitonic energy splittings. As a microscopic mechanism of the hole intervalley scattering we identify the scattering with chiral TA phonons of MoSe2 between excitonic states mixed by the long-range electron hole exchange interaction.

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.

A Steady-State Approach for Studying Valley Relaxation Using an Optical Vortex Beam.

Spin-valley coupling in monolayer transition-metal dichalcogenides gives rise to valley polarization and coherence effect, limited by intervalley scattering caused by exciton-phonon, exciton-impurity, and electron-hole exchange interactions (EHEIs). We explore an approach to tune the EHEI by controlling the exciton center of mass momentum (COM) utilizing the photon distribution of higher-order optical vortex beams. By virtue of this, we have observed exciton-COM-dependent valley depolarization and decoherence, which gives us the ability to probe the valley relaxation time scale in a steady-state measurement. Our steady-state technique to probe the valley dynamics can open up a new paradigm to explore the physics of excitons in two-dimensional systems.

Chiral Excitonics in Monolayer Semiconductors on Patterned Dielectrics.

Monolayer transition metal dichalcogenides feature tightly bound bright excitons at the degenerate valleys, where electron-hole Coulomb exchange interaction strongly couples the valley pseudospin to the momentum of the exciton. Placed on a periodically structured dielectric substrate, the spatial modulation of the Coulomb interaction leads to the formation of exciton Bloch states with real-space valley pseudospin texture displayed in a mesoscopic supercell. We find this spatial valley texture in the exciton Bloch function is pattern locked to the propagation direction, enabling nano-optical excitation of directional exciton flow through the valley selection rule. The left-right directionality of the injected exciton current is controlled by the circular polarization of excitation, while the angular directionality is controlled by the excitation location, exhibiting a vortex pattern in a supercell. The phenomenon is reminiscent of the chiral light-matter interaction in nanophotonics structures, with the role of the guided electromagnetic wave now replaced by the valley-orbit coupled exciton Bloch wave in a uniform monolayer, which points to new excitonic devices with nonreciprocal functionalities.

Trions in MoS2 are quantum superpositions of intra- and intervalley spin states

We report magneto-photoluminescence spectroscopy of gated MoS$_2$ monolayers in high magnetic fields to 28 T. At B = 0T and electron density $n_s\sim 10^{12}cm^-2$, we observe three trion resonances that cannot be explained within a single-particle picture. Employing ab initio calculations that take into account three-particle correlation effects as well as local and non-local electron-hole exchange interaction, we identify those features as quantum superpositions of inter- and intravalley spin states. We experimentally investigate the mixed character of the trion wave function via the filling factor dependent valley Zeeman shift in positive and negative magnetic fields. Our results highlight the importance of exchange interactions for exciton physics in monolayer MoS$_2$ and provide new insights into the microscopic understanding of trion physics in 2D multi-valley semiconductors for low excess carrier densities.

Electron–hole exchange interaction in a spherical quantum dot with regard material deformation and polarization charges

Electron–hole exchange interaction in a spherical quantum dot with regard material deformation and polarization charges