Long-range Electron-hole Exchange Interaction Research Articles

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.

Long-range electron-hole exchange interaction in aluminum nitride

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large longitudinal-transverse splitting energy obtained in this study suggests that AlN has strong light-matter coupling without quantum-confined structures.

The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state.

Lead halide perovskites have emerged as promising new semiconductor materials for high-efficiency photovoltaics, light-emitting applications and quantum optical technologies. Their luminescence properties are governed by the formation and radiative recombination of bound electron-hole pairs known as excitons, whose bright or dark character of the ground state remains unknown and debated. While symmetry analysis predicts a singlet non-emissive ground exciton topped with a bright exciton triplet, it has been predicted that the Rashba effect may reverse the bright and dark level ordering. Here, we provide the direct spectroscopic signature of the dark exciton emission in the low-temperature photoluminescence of single formamidinium lead bromide perovskite nanocrystals under magnetic fields. The dark singlet is located several millielectronvolts below the bright triplet, in fair agreement with an estimation of the long-range electron-hole exchange interaction. Nevertheless, these perovskites display an intense luminescence because of an extremely reduced bright-to-dark phonon-assisted relaxation.

Exciton valley dynamics in monolayer Mo1-xWxSe2 (x = 0, 0.5, 1)

We study the exciton valley dynamics in monolayers MoSe2, Mo0.5W0.5Se2, and WSe2 by employing helicity-resolved two-color transient reflection spectroscopy. The valley depolarization dynamics as a function of the excitation laser energy is studied systematically at above-resonant excitation of excitons at 10 K. A longer intervalley scattering time is obtained as the excitation energy approaches the A exciton resonance for the three studied materials. The excitation energy dependence of exciton valley relaxation proves that the long-range electron-hole exchange interaction dominates the intervalley scattering in transition metal dichalcogenide monolayers. The longer valley scattering time and higher valley polarization degree commonly observed for WSe2 than for MoSe2 is discussed to result from the interplay between the intervalley electron-hole exchange interaction and dark-bright exciton scattering, where the existence of energetically lower lying dark excitonic states in monolayer WSe2 favors the suppression of the intervalley electron-hole exchange interaction.

Effective Theory of Electron-Hole Exchange in Semiconductor Quantum Dots

Semiconductor self-assembled quantum dots (QDs) are expected to become building blocks of novel optoelectronic devices capable of generating single photons and entangled photon pairs (EPPs) on demand. The major barrier to generation of EPPs is the so-called "bright exciton splitting" originating from the long-range electron-hole exchange interaction. A k · p effective mass theory of electron-hole exchange in semiconductor quantum dots is presented (Phys. Rev. B 81, 045311). The matrix element responsible for the "bright" exciton splitting in the effective exciton Hamiltonian is identified and analyzed. An excitonic fine structure for a model quantum dot with quasi-two-dimensional anisotropic harmonic oscillator (2DLAHO) confining potential is considered as a function of the shape and external fields.

Theory of exciton fine structure in semiconductor quantum dots: Quantum dot anisotropy and lateral electric field

Theory of exciton fine structure in semiconductor quantum dots and its dependence on quantum-dot anisotropy and external lateral electric field is presented. The effective exciton Hamiltonian including long-range electron-hole exchange interaction is derived within the $k\ensuremath{\cdot}p$ effective-mass approximation. The exchange matrix elements of the Hamiltonian are expressed explicitly in terms of electron and hole envelope functions. The matrix element responsible for the ``bright'' exciton splitting is identified and analyzed. An excitonic fine structure for a model quantum dot with quasi-two-dimensional anisotropic harmonic oscillator confining potential is analyzed as a function of the shape anisotropy, size, and applied lateral electric field.

Fine structure of exciton excited levels in a quantum dot with a magnetic ion

The fine structure of excited excitonic states in a quantum dot with an embedded magnetic ion is studied theoretically and experimentally. The developed theory takes into account the Coulomb interaction between charged carriers, the anisotropic long-range electron-hole exchange interaction in the zero-dimensional exciton, and the exchange interaction of the electron and the hole with the $d$ electrons of a Mn ion inserted inside the dot. Depending on the relation between the quantum dot anisotropy and the exciton-Mn coupling, the photoluminescence excitation spectrum has a qualitatively different behavior. It provides a deep insight into the spin structure of the excited excitonic states.

Light scattering on exciton resonance in a semiconductor quantum dot: Exact solution

We find an exact solution to the problem of light scattering on exciton resonance in a semiconductor quantum dot surrounded by a medium with a given dielectric constant. This enables us to obtain the exciton resonant-frequency renormalization due to the long-range electron-hole exchange interaction and exciton radiative lifetime to any order in the ratio of the quantum dot size to the wavelength of light. We find spatial distribution of the electric field both inside and outside the quantum dot.

Theory of exciton doublet structures and polarization relaxation in single quantum dots

The mechanism of exciton doublet structures in quantum dots is identified as the long-range part of the electron-hole exchange interaction which is emphasized by the anisotropic shape of quantum dots. The physical origin of the energetic order of the orthogonally polarized exciton states of each doublet is clarified by inspecting the spatial distribution of the exciton polarization. The key concepts to understand the energetic order are the node configuration of the distribution function of exciton polarization and the dipole-dipole interaction energy originating from the long-range electron-hole exchange interaction. The population relaxation and the polarization relaxation of excitons are studied and the extremely slow polarization relaxation within exciton doublet states is predicted. It is also found that the inter-doublet cross-relaxation between orthogonally polarized exciton states occurs as efficiently as the population relaxation.

Polaritons in semiconductor multiple-quantum-well structures with Förster-type interwell coupling.

When the barrier potential is high enough and/or its thickness large enough resonance tunneling becomes negligible in multiple-quantum-well structures. In this case a possible mechanism to connect the quantum wells is a F\"orster-type interwell coupling that originates from the long-range electron-hole exchange interaction. We theoretically investigate excitons in such a coupled multiple-quantum-well structure subject to an external light field, and show that the resulting polaritons behave in a qualitatively different way than those in uncoupled quantum-well structures. Of particular interest are interbranch polariton transitions and the redistribution of the radiative damping rate between different polariton branches. Numerical calculations are performed for a CdS-coupled double-quantum-well structure with dephasing by phonons taken into account.

Long-range exchange splitting in excitons

The Zeeman levels of a shallow direct exciton in cubic semiconductor are analysed. The long-range electron-hole exchange interaction is taken into account by the linear response dielectric formalism. The frequency wave-vector dispersion relations of the exciton-polariton, in the presence of the homogeneous magnetic field, are derived.