Exciton Exchange Interaction Research Articles

Modeling of Exciton Exchange Interaction in GaAs/AlGaAs Quantum Wells

In this work, we study the exchange interactions between two excitons in the GaAs/AlGaAs quantum wells of various widths. We numerically solved the Schrodinger equation for an exciton in a quantum well to find the two-exciton wave functions and to calculate the exchange integral. The results suggest that the strongest interactions between excitons occur in the quantum wells of widths of about 40–50 nm, with the exchange energy being of about of 9 μeV for an exciton density of 1/μm2.

Probing exciton/exciton interactions with entangled photons: Theory.

Quantum entangled photons provide a sensitive probe of many-body interactions and offer a unique experimental portal for quantifying many-body correlations in a material system. In this paper, we present a theoretical demonstration of how photon-photon entanglement can be generated via interactions between coupled qubits. Here, we develop a model for the scattering of an entangled pair of photons from a molecular dimer. We develop a diagrammatic theory for the scattering matrix and show that one can correlate the von Neumann entropy of the outgoing bi-photon wave function with exciton exchange and repulsion interactions. We conclude by discussing possible experimental scenarios for realizing these ideas.

Shedding light on exciton's nature in monolayer quantum material by optical dispersion measurements.

Strong light-matter interactions based on two-dimensional excitons formed in quantum materials such as monolayer transition-metal dichalcogenides have become a major subject of research in recent years. Particularly attractive is the extraordinarily large oscillator strength as well as binding energy of the excitonic quasiparticles in these atomically-thin crystal lattices. Numerous theoretical studies and experiments have been devoted to the exploration of the excitonic systems that could be exploited in future nano-scaled optoelectronic devices. To obtain unique insight into the exciton's characteristics in an archetype monolayer quantum material, we directly measure the quasiparticle energy-momentum dispersion for the first time optically. Our results for h-BN encapsulated single-layer WSe2 clearly indicate an emission regime with a dispersion in the meV range in within the light cone at cryogenic temperatures. The amount of dispersion agrees well with calculations for an exciton-polariton based on the material's monolayer exciton, or energetic modifications caused by exciton exchange interactions predicted for this material family. The measurable dispersion slightly weakens for elevated excitation densities, whereas at elevated temperatures, it even becomes immeasurable. The obtained reduction in dispersion is attributed to an enhanced role of uncorrelated charge carriers as well as the formation of phonon sidebands above 100 K.

K-dependent exchange interaction of the1Sorthoexciton inCu2O

When treating the exchange interaction of Wannier excitons, usually only the leading terms of the analytic and the nonanalytic exchange interaction are considered. However, higher order terms can lead to a splitting of exciton states, for which reason a splitting of the $1S$~exciton in cuprous oxide $\left(\mathrm{Cu_{2}O}\right)$ depending on its total momentum $\hbar K$ has been attributed to a $K$ dependent analytic exchange interaction by Dasbach et al [Phys. Rev. Lett. 91, 107401 (2003)]. Going beyond the common treatment of the exchange interaction, we derive the correct expressions for these $K$ dependent higher order terms using $\boldsymbol{k}\cdot\boldsymbol{p}$ perturbation theory. We prove that the appearance of a $K$ dependent exchange interaction is inseparably connected with a $K$ independent exchange interaction of $P$ and $D$ excitons. We estimate the magnitude of these terms for $\mathrm{Cu_{2}O}$ from microscopic calculations and show that they are far too small to explain the observed $K$ dependent splitting. Instead, this splitting has to be treated in terms of the dispersion of the excitons. Furthermore, we prove the occurence of a coupling between longitudinal and transverse excitons in $\mathrm{Cu_{2}O}$ due to the $K$ dependent nonanalytic exchange interaction.

Exciton fine structure and spin decoherence in monolayers of transition metal dichalcogenides

We study the neutral exciton energy spectrum fine structure and its spin dephasing in transition metal dichalcogenides such as ${\mathrm{MoS}}_{2}$. The interaction of the mechanical exciton with its macroscopic longitudinal electric field is taken into account. The splitting between the longitudinal and transverse excitons is calculated by means of the both electrodynamical approach and $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ perturbation theory. This long-range exciton exchange interaction can induce valley polarization decay. The estimated exciton spin dephasing time is in the picosecond range, in agreement with available experimental data.

Magneto-Optical Studies of Direct Exciton State in Thallous Chloride

The magneto-optical properties of the 1s exciton in TlCl associated with the direct transition at the X point in Brillouin zone are studied theoretically and experimentally. It is found that the triplet exciton state as well as the longitudinal exciton state becomes observable in a certain configuration of the light and the magnetic field. These results are analyzed by the theory based on the exciton exchange interaction. The electron-hole intravalley exchange energies for the two exciton polarizations are obtained as 2.7 and 1.5 meV. The effective g value of exciton parallel to the valley direction is estimated to be 3.4.

Exchange interactions in Cd1−xMnxTe wide quantum wells

The exchange interactions between excitons and manganese ions in wide (80 nm) quantum wells of Cd${}_{1\ensuremath{-}x}$Mn${}_{x}$Te ($x\ensuremath{\approx}0.0015$ and $x\ensuremath{\approx}0.00027$) have been measured for states with center of mass quantum numbers up to 10. The corresponding translational wave vectors are in the range up to $4\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{2.84526pt}{0ex}}$cm${}^{\ensuremath{-}1}$ and in this range we detect no systematic variation of the exchange parameter ${N}_{0}(\ensuremath{\alpha}\ensuremath{-}\ensuremath{\beta})$, which remains constant to within $\ifmmode\pm\else\textpm\fi{}3%$. The result is consistent with previous studies of the wave vector dependence of the electron contribution ${N}_{0}\ensuremath{\alpha}$ but differs from previous observations of the wave vector dependence of exciton exchange interaction in quantum wells with much higher concentrations of manganese.

Exciton Supersolidity in Hybrid Bose-Fermi Systems

We investigate the ground states of a Bose-Einstein condensate of indirect excitons coupled to an electron gas. We show that in a properly designed system the crossing of a roton minimum into the negative energy domain can result in the appearance of the supersolid phase, characterized by periodicity in both real and reciprocal space. Accounting for the spin-dependent exchange interaction of excitons we obtain ferromagnetic supersolid domains. The Fourier spectra of excitations of weakly perturbed supersolids show pronounced diffraction maxima which may be detected experimentally.

Exciton luminescence in In 0.3Ga 0.7As/GaAs quantum well heterostructures

Emission maxima related to the recombination of excitons (e1–hh1, e1–lh1, e2–hh2, and e2–lh2) were observed in photoluminescence spectra of GaAs/In 0.3Ga 0.7As/GaAs quantum wells. The emission bands due to e1–hh1 and e1–lh1 transitions were found to have a doublet character explained by the exchange interaction of excitons in quantum wells. Emission bands due to radiative E b–hh1, E b–lh1 transitions in the buffer GaAs layer are observed in the region of 1.5 eV.

Magnetic-field modulated exciton-exciton interaction in semiconductor microcavities

We study the quantum-well magnetoexcitons by the variational method in the strong coupling regime of excitons with microcavity photons. In such strong coupling regime, we find that the coupling between the exciton internal state and center of mass motion can be neglected. Through the calculations, we find that the magnetic field can reduce the exciton exchange interaction to 30% as Lw=30 nm with Lw being the quantum well width. This is in contrast to the magnetic-field enhancement in the exciton Rabi splitting. In this work, we obtain that a magnetic field can enhance the exciton Rabi splitting up to 1.6 times in the case of Lw=30 nm. They both originate from the local enhancement in the exciton internal state due to the magnetic field. In addition, we analyze the behavior of the magnetic response of the exciton energy, Rabi splitting, and exchange interaction in manipulating the polariton parametric scattering, and widening the microcavity applications.

Magnetooptics and dynamics of a magnetic polaron in semimagnetic CdSe/ZnMnSe quantum dots

The magnetooptics and picosecond dynamics of the radiative recombination of excitons in self-assembled semimagnetic CdSe/ZnMnSe quantum dots is studied at low temperatures. The behavior of individual quantum dots in a magnetic field and with an increase in temperature is indicative of a strong exchange interaction of excitons and magnetic Mn ions giving rise to a quasi-zero-dimensional exciton magnetic polaron. When the exciton energy exceeds the intracenter transition energy in Mn, the energy is rapidly transferred from excitons to Mn ions (faster than 20 ps). In the opposite case, a substantial red shift (∼15 meV) of the emission line maximum is observed during the exciton lifetime ∼500 ps, presenting the dynamics of the formation of a magnetic polaron with a characteristic time of ∼110 ps.

Excitons in nanoscale systems

Nanoscale systems are forecast to be a means of integrating desirable attributes of molecular and bulk regimes into easily processed materials. Notable examples include plastic light-emitting devices and organic solar cells, the operation of which hinge on the formation of electronic excited states, excitons, in complex nanostructured materials. The spectroscopy of nanoscale materials reveals details of their collective excited states, characterized by atoms or molecules working together to capture and redistribute excitation. What is special about excitons in nanometre-sized materials? Here we present a cross-disciplinary review of the essential characteristics of excitons in nanoscience. Topics covered include confinement effects, localization versus delocalization, exciton binding energy, exchange interactions and exciton fine structure, exciton-vibration coupling and dynamics of excitons. Important examples are presented in a commentary that overviews the present understanding of excitons in quantum dots, conjugated polymers, carbon nanotubes and photosynthetic light-harvesting antenna complexes.

Temperature and power-density-dependent inter-shell energy states in InAs/GaAs quantum dots

We have investigated the evolution of exciton state filling in InAs/GaAs quantum dot (QD) structures as a function of the excitation power density by using micro-photoluminescence spectroscopy at different temperatures. In addition to the emission bands of exciton recombination corresponding to the atom-like S, P and D, etc. shells of QDs, it was observed that some extra states P′ between the S and P shells, and D′ between the P and D shells appear in the spectra with increasing number of excitons occupying the QDs at a certain temperature. The emergence of these inter-shell excitonic levels is power density and temperature dependent, which is an experimental demonstration of strong exciton–exciton exchange interaction, state hybridization, and coupling of a multi-exciton system in QDs.

Observation of intershell and hybridized energy states in InAs∕GaAs quantum dots

We have investigated the evolution of exciton state filling as a function of excitation power density in InAs∕GaAs quantum dots (QDs). In addition to the emission bands of exciton recombination corresponding to the atom-like S, P, and D, etc. shells of quantum dots, it was observed that some extra states, P′ (between the S and P shells) and D′ (between the P and D shells), appear in the spectra with increasing number of excitons occupying the QDs. The emergence of these intershell excitonic levels is an experimental demonstration of strong exciton–exciton exchange interaction and coupling as well as state mixing and hybridization of a multiexciton system in quantum dots.

Parametric amplification and polariton liquids in semiconductor microcavities

AbstractParametric amplification in semiconductor microcavities provides an example in which nonlinear optical interactions produced by the exchange interaction of excitons become so large that multiple scattering of polaritons becomes important. Here we review time‐resolved observations of the polariton interactions in a number of different geometries including pumping at either the magic angle, or the bottom of the polariton trap. Situations in which the polariton dispersion is multiply occupied by large populations give rise to k‐dependent energy shifts, modifying the dispersion dynamically, a situation we term the strongly‐interacting ‘polariton liquid’. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin relaxation in CdTe quantum dots

We have measured photoluminescence (PL) spectra and time-resolved PL in CdTe quantum dots under the longitudinal magnetic field up to 10 T. Circular polarization of PL increases with increasing magnetic field, while its linear polarization remains zero under linearly polarized excitation. This behavior cannot be explained by the anisotropic exchange interaction of excitons. Time-resolved PL measurements clarified that this behavior is caused by the suppression of spin relaxation induced by the longitudinal magnetic field. We believe that this behavior is related to the hyperfine interaction of electron spin with magnetic momenta of lattice nuclei.

Wave-vector-dependent exciton exchange interaction.

The exchange interaction for the yellow 1S orthoexciton in Cu2O is derived up to the order K2. The resulting exchange splittings are verified experimentally using high resolution spectroscopy. In agreement with theory the fine structure shows a characteristic dependence on the direction of the wave vector.

K‐dependent exchange interaction of quadrupole excitons in Cu2O

AbstractThe long‐ and short‐range exchange interaction for the quadrupole allowed transition to the yellow 1S orthoexciton in Cu2O is derived. While the three orthoexciton states still remain degenerate, when treating the exchange up to the order of K‐linear terms, a fine structure arises when K‐quadratic terms are included. This splitting is investigated experimentally by high resolution spectroscopy. Exchange contributions of few μeV are resolved and compared to theory. The impact of strain on the exciton fine structure is discussed and evaluated.

Quenching of the exciton-spin relaxation via exchange interaction inGaAs/AlxGa1−xAsquantum wells

We studied the influence of the electron gas on the exciton spin relaxation in GaAs/AlGaAs quantum wells. We observed an increase of the continuous-wave degree of polarization and the exciton spin relaxation time with the electron density. These results are interpreted based on the quenching of the exciton bound state. As a consequence, the spin relaxation changes from one dominated by the long-range exciton exchange interaction for intrinsic excitonic transitions to one dominated by the hole spin relaxation when in the presence of the electron gas. The experimental results are in qualitative agreement with calculations of the exciton spin relaxation time in doped samples.

Transverse magnetic fieldeffects upon the exciton exchange interaction in quantum wells

We calculated the effects of a transverse magneticfield on electron-hole exchange interaction for freeexcitons in semiconductor quantum wells (QWs). The magnetic field isthe source of an anisotropy on the exciton dispersion for motionon the QW plane in directions parallel andperpendicular to the field. Weakening of the exciton bindingenergy and the exchange interaction terms were predicted forthe moving excitons. These effects are better resolved in wideQWs.