Charged Exciton Complexes Research Articles

Controlled coherent-coupling and dynamics of exciton complexes in a MoSe2 monolayer

Quantifying and controlling the coherent dynamics and couplings of optically active excitations in solids is of paramount importance in fundamental research in condensed matter optics and for their prospective optoelectronic applications in quantum technologies. Here, we perform ultrafast coherent nonlinear spectroscopy of a charge-tunable MoSe2 monolayer. The experiments show that the homogeneous and inhomogeneous line width and the population decay of exciton complexes hosted by this material can be directly tuned by an applied gate bias, which governs the Fermi level and therefore the free carrier density. By performing two-dimensional spectroscopy, we also show that the same bias-tuning approach permits us to control the coherent coupling strength between charged and neutral exciton complexes.

Intercell moiré exciton complexes in electron lattices.

Excitons, Coulomb-bound electron-hole pairs, play a crucial role in both optical excitation and correlated phenomena in solids. When excitons interact with other quasiparticles, few- and many-body excited states can appear. Here we report an interaction between exciton and charges enabled by unusual quantum confinement in two-dimensional moiré superlattices, which results in many-body ground states composed of moiré excitons and correlated electron lattices. In an H-stacked (60o-twisted) WS2/WSe2 heterobilayer, we found an interlayer moiré exciton whose hole is surrounded by its partner electron's wavefunction distributed among three adjacent moiré traps. This three-dimensional excitonic structure enables large in-plane electrical quadrupole moments in addition to the vertical dipole. Upon doping, the quadrupole facilitates the binding of interlayer moiré excitons to the charges in neighbouring moiré cells, forming intercell charged exciton complexes. Our work provides a framework for understanding and engineering emergent exciton many-body states in correlated moiré charge orders.

Transient quantum beatings of trions in hybrid organic tri-iodine perovskite single crystal

Utilizing the spin degree of freedom of photoexcitations in hybrid organic inorganic perovskites for quantum information science applications has been recently proposed and explored. However, it is still unclear whether the stable photoexcitations in these compounds correspond to excitons, free/trapped electron-hole pairs, or charged exciton complexes such as trions. Here we investigate quantum beating oscillations in the picosecond time-resolved circularly polarized photoinduced reflection of single crystal methyl-ammonium tri-iodine perovskite (MAPbI3) measured at cryogenic temperatures. We observe two quantum beating oscillations (fast and slow) whose frequencies increase linearly with B with slopes that depend on the crystal orientation with respect to the applied magnetic field. We assign the quantum beatings to positive and negative trions whose Landé g-factors are determined by those of the electron and hole, respectively, or by the carriers left behind after trion recombination. These are {g}_{[001]}^{e} = 2.52 and {g}_{[1bar{1}0]}^{e},= 2.63 for electrons, whereas big|{g}_{[001]}^{h}big|,= 0.28 and big|{g}_{[1bar{1}0]}^{h}big|,= 0.57 for holes. The obtained g-values are in excellent agreement with an 8-band K.P calculation for orthorhombic MAPbI3. Using the technique of resonant spin amplification of the quantum beatings we measure a relatively long spin coherence time of ~ 11 (6) nanoseconds for electrons (holes) at 4 K.

Relaxation and darkening of excitonic complexes in electrostatically doped monolayer WSe2 : Roles of exciton-electron and trion-electron interactions

We present photoluminescence measurements in monolayer WSe$_2$, which point to the importance of the interaction between charged particles and excitonic complexes. The theoretical analysis highlights the key role played by exchange scattering, referring to cases wherein the particle composition of the complex changes after the interaction. For example, exchange scattering renders bright excitonic complexes dark in monolayer WSe$_2$ on accounts of the unique valley-spin configuration in this material. In addition to the ultrafast energy relaxation of hot excitonic complexes following their interaction with electrons or holes, our analysis sheds light on several key features that are commonly seen in the photoluminescence of this monolayer semiconductor. In particular, we can understand why the photoluminescence intensity of the neutral bright exciton is strongest when the monolayer is hole-doped rather than charge neutral or electron-doped. Or the reason for the dramatic increase of the photoluminescence intensity of negatively-charged excitons (trions) as soon as electrons are added to the monolayer. To self-consistently explain the findings, we further study the photoluminescence spectra at different excitation energies and analyze the behavior of the elusive indirect exciton.

Imaging Charged Exciton Localization in van der Waals WSe2/MoSe2 Heterobilayers.

Exciton localization in transition-metal dichalcogenide monolayers is behind a variety of interesting phenomena and applications, including broad-spectrum solar cells and single-photon emissions. Strain fields at the periphery of topographically distinct features such as nanoscopic bubbles were recently associated with localized charge-neutral excitons. Here, we use tip-enhanced photoluminescence (PL) to visualize excitons in WSe2/MoSe2 heterobilayers (HBL). We find strong optical emission from charged excitons, particularly positively charged trions, in HBL supported by interlayer charge transfer. Our results reveal strong trion confinement, with a localization length scale comparable to the trion size, at the apex region inside individual nanoscopic bubbles. Nano-PL mapping also shows sub-10-nm spatial variations in the localized trion emission spectra, which stem from atomic-scale potential energy fluctuations. These findings demonstrate the possibility of confining charged exciton complexes that are electrically tunable, opening up further opportunities to probe many-body exciton physics and to explore additional possible sites for strong exciton localization that can lead to quantum emission.

Binding energies and chemical potential of neutral and charged exciton-complexes of transition metal dichalcogenides/anisotropic 2-D quantum dots in magnetic field by exact multi-pole expansion of coulomb correlations

Anisotropy is inevitable fate of charged excitonic complexes due to difference in dielectric constants, effective masses, charges of carriers, magnetic field, applied gate voltage etc along in-plane and out-of-plane directions of transition metal dichalcogenides (TMDCs-MX2; M = Mo, W; X = S, Se, Te) as well as in anisotropic quantum dots. In the present paper, the multi-dynamic attractive and scattering interactions among neutral and charged exciton-complexes (N-excitonic, N = 2,3,4, ….) amongst the various charge carriers, i.e., e − h, e − e, h − h has been elegantly accounted for by the use of Lauricella functions and the subsequent application of Chu-vandermonde identity. An exact variational treatment for such complex and intertwined interactions in presence of anisotropy has been achieved by employing multi-pole expansion. The result of which emerges as a unique single-summed, finite, exact and absolutely terminating integrals. The calculated binding energies for various N-excitonic systems have been found to be consistent with the previous experimental and computational studies. A discussion of energy spectra, chemical potential (μ) and addition energy (Δμ) are presented. The results thus obtained indicate increase in binding energy and bosonic character of the multi-excitonic complexes with magnetic field. The effects of anisotropy are manifested through the interplay between many-body coulomb interactions resulting in the dissolution of degeneracy of excitonic states.

The interplay between excitons and trions in a monolayer of MoSe2

The luminescence and absorption properties of transition metal dichalcogenide monolayers are widely determined by neutral and charged excitonic complexes. Here, we focus on the impact of a free carrier reservoir on the optical properties of excitonic and trionic complexes in a MoSe2 monolayer at cryogenic temperatures. By applying photodoping via a non-resonant pump laser, the electron density can be controlled in our sample, which is directly reflected in the contribution of excitons and trions to the luminescence signal. We find significant shifts of both the exciton and trion energies in the presence of an induced electron gas both in power- and in time evolution (on the second to minute scale) in our photoluminescence spectra. In particular, in the presence of the photo-doped carrier reservoir, we observe that the splitting between excitons and trions can be enhanced by up to 4 meV. This behaviour is phenomenologically explained by an interplay between an increased screening of excitons via electrons in our system and a modification of the Fermi level. We introduce a simple but still quantitative treatment of these effects within a variational approach that takes into account both screening and phase space filling effects.

Modulation of Manganite Nanofilm Properties Mediated by Strong Influence of Strontium Titanate Excitons.

Hole-doped perovskite manganites have attracted much attention because of their unique optical, electronic, and magnetic properties induced by the interplay between spin, charge, orbital, and lattice degrees of freedom. Here, a comprehensive investigation of the optical, electronic, and magnetic properties of La0.7Sr0.3MnO3 thin films on SrTiO3 (LSMO/STO) and other substrates is conducted using a combination of temperature-dependent transport, spectroscopic ellipsometry, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. A significant difference in the optical property of LSMO/STO that occurs even in thick (87.2 nm) LSMO/STO from that of LSMO on other substrates is discovered. Several excitonic features are observed in thin film nanostructure LSMO/STO at ∼4 eV, which could be attributed to the formation of anomalous charged excitonic complexes. On the basis of the spectral weight transfer analysis, anomalous excitonic effects from STO strengthen the electronic correlation in LSMO films. This results in the occurrence of optical spectral changes related to the intrinsic Mott-Hubbard properties in manganites. We find that while lattice strain from the substrate influences the optical properties of the LSMO thin films, the coexistence of strong electron-electron (e-e) and electron-hole (e-h) interactions which leads to the resonant excitonic effects from the substrate plays a much more significant role. Our result shows that the onset of anomalous excitonic dynamics in manganite oxides may potentially generate new approaches in manipulating exciton-based optoelectronic applications.

Wigner molecules and charged excitons in near-field magnetophotoluminescence spectra of self-organized InP/GaInP quantum dots

We used high-spatial-resolution, low-temperature near-field scanning optical microscopy (NSOM) operating at magnetic fields $B=0--10\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ to study the effects of Wigner localization (WL) on emission spectra of single self-organized InP/GaInP quantum dots (QDs) and investigate the stability of singly (trion) and doubly (tetron) charged exciton complexes in the weak quantum confinement regime. Using NSOM measurements together with configuration interaction calculations, we identify the dots having different electron population $\mathrm{N}(\mathrm{N}=1--12)$, quantum confinement $(\ensuremath{\hbar}{\ensuremath{\omega}}_{0}=0.6--8\phantom{\rule{0.16em}{0ex}}\mathrm{meV})$ and size $D(D=70--170\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$. For $\mathrm{N}=2$, we observed a magnetic-field-induced molecular-droplet transition, accompanied by the decomposition of the tetron into a Wigner molecule complex (WMC), and the activation of rotovibronic structure. For $\mathrm{N}=1$, unusually strong vibronic structure resulting from a trion-type WMC was observed. We have shown that magnetic-field-induced shifts of this structure allow measurement of single particle Fock-Darwin levels and angular momentum transitions of the WMC. In addition, we demonstrated the use of NSOM imaging to probe the charge density distribution and observed anomalous dependence of the image size on the quantum confinement, implying a pairing of electrons or formation of whispering gallery modes in the QD. We demonstrated that InP/GaInP QDs, provide a Wigner-Seitz radius $({r}_{s})$ up to 13, and that the measurements of NSOM magneto-optical spectroscopy using these dots makes it possible to study effects arising from strong Coulomb interaction of a few confined electrons (holes).

Excitonic complexes in GaN/(Al,Ga)N quantum dots

Here we report a theoretical investigation of excitonic complexes in polar GaN/(Al,Ga)N quantum dots (QDs). A sum rule between the binding energies of charged excitons is used to calculate the biexciton binding energy. The binding energies of excitonic complexes in GaN/AlN are shown to be strongly correlated to the QD size. Due to the large hole localization, the positively charged exciton energy is found to be always blueshifted compared to the exciton one. The negatively charged exciton and the biexciton energy can be blueshifted or redshifted according to the QD size. Increasing the size of GaN/AlN QDs makes the identification of charged excitons difficult, and the use of an Al0.5Ga0.5N barrier can be advantageous for clear identification. Our theoretical results for the binding energy of exciton complexes are also confronted with values deduced experimentally for InAs/GaAs QDs, confirming our theoretical prediction for charged excitonic complexes in GaN/(Al,Ga)N QDs. Finally, we realize that the trends of excitonic complexes in QDs are significantly related to competition between the local charge separation (whatever its origin) and the correlation effect. Following our findings, entangled photons pairs can be produced in QDs with careful control of their size in order to obtain zero exciton–biexciton energy separation.

Dynamics of excitons in individual InAs quantum dots revealed in four-wave mixing spectroscopy

A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.

Single photon emission up to liquid nitrogen temperature from charged excitons confined in GaAs-based epitaxial nanostructures

We demonstrate a non-classical photon emitter at near infrared wavelength based on a single (In,Ga)As/GaAs epitaxially grown columnar quantum dot. Charged exciton complexes have been identified in magneto-photoluminescence. Photon auto-correlation histograms from the recombination of a trion confined in a columnar dot exhibit sub-Poissonian statistics with an antibunching dip yielding g(2)(0) values of 0.28 and 0.46 at temperature of 10 and 80 K, respectively. Our experimental findings allow considering the GaAs-based columnar quantum dot structure as an efficient single photon source operating at above liquid nitrogen temperatures, which in some characteristics can outperform the existing solutions of any material system.

Relative stability of excitonic complexes in quasi-one-dimensional semiconductors

A configuration space approach is developed to uncover generic stability peculiarities for the lowest energy neutral and charged exciton complexes (biexciton and trion) in quasi-one-dimensional semiconductors. Trions are shown to be more stable than biexcitons in strongly confined structures with small reduced electron-hole masses. Biexcitons are more stable in less confined structures with large reduced electron-hole masses. In semiconducting carbon nanotubes, in particular, the trion binding energy is shown to be greater than that of the biexciton by a factor of $\ensuremath{\sim}1.4$, decreasing with diameter, thus revealing the general physical principles that underlie recent experimental observations.

Exciton footprint of self-assembled AlGaAs quantum dots in core-shell nanowires

Quantum-dot-in-nanowire systems constitute building blocks for advanced photonics and sensing applications. The electronic symmetry of the emitters impacts their function capabilities. Here we study the fine structure of gallium-rich quantum dots nested in the shell of $\mathrm{GaAs}\text{\ensuremath{-}}{\mathrm{Al}}_{0.51} {\mathrm{Ga}}_{0.49}$ As core-shell nanowires. We used optical spectroscopy to resolve the splitting resulting from the exchange terms and extract the main parameters of the emitters. Our results indicate that the quantum dots can host neutral as well as charged excitonic complexes and that the excitons exhibit a slightly elongated footprint, with the main axis tilted with respect to the long axis of the host nanowire. $\mathrm{GaAs}\text{\ensuremath{-}}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ emitters in a nanowire are particularly promising for overcoming the limitations set by strain in other systems, with the benefit of being integrated in a versatile photonic structure.

Charge state control in single InAs/GaAs quantum dots by external electric and magnetic fields

We report a photoluminescence (PL) spectroscopy study of charge state control in single self-assembled InAs/GaAs quantum dots by applying electric and/or magnetic fields at 4.2 K. Neutral and charged exciton complexes were observed under applied bias voltages from −0.5 V to 0.5 V by controlling the carrier tunneling. The highly negatively charged exciton emission becomes stronger with increasing pumping power, arising from the fact that electrons have a smaller effective mass than holes and are more easily captured by the quantum dots. The integrated PL intensity of negatively charged excitons is affected significantly by a magnetic field applied along the sample growth axis. This observation is explained by a reduction in the electron drift velocity caused by an applied magnetic field, which increases the probability of non-resonantly excited electrons being trapped by localized potentials at the wetting layer interface, and results in fewer electrons distributed in the quantum dots. The hole drift velocity is also affected by the magnetic field, but it is much weaker.

Single photon emission at 1.55 μm from charged and neutral exciton confined in a single quantum dash

We investigate charged and neutral exciton complexes confined in a single self-assembled InAs/InGaAlAs/InP quantum dash emitting at 1.55 μm. The emission characteristics have been probed by measuring high-spatial-resolution polarization-resolved photoluminescence and cross-correlations of photon emission statistics at T = 5 K. The photon auto-correlation histogram of the emission from both the neutral and charged exciton indicates a clear antibunching dip with as-measured g(2)(0) values of 0.18 and 0.31, respectively. It proves that these exciton complexes confined in single quantum dashes of InP-based material system can act as true single photon emitters being compatible with standard long-distance fiber communication technology.

Toward tripartite hybrid entanglement in quantum dot molecules

Establishing the hybrid entanglement among a growing amount of matter and photonic quantum bits is necessary for scalable quantum computation and long-distance quantum communication. Here we demonstrate that charged excitonic complexes forming in strongly correlated quantum dot molecules are able to generate tripartite hybrid entanglement under proper carrier quantization. The mixed orbitals of the molecule construct multi-level ground states with sub-meV hole tunneling energy and relatively large electron hybridization energy. We show that appropriate size and interdot spacing keeps the electron particle weakly localized, opening extra recombination channels by correlating ground-state excitons. This allows for creation of higher order entangled states. Nontrivial hole tunneling energy, renormalized by multi-particle interactions, facilitates the realization of the energy coincidence among only certain components of the molecule optical spectrum. This translates to the emergence of favorable spectral components in a multi-body excitonic complex which sustain principal oscillator strengths throughout the electric field-induced hole tunneling process. We particularly analyze whether the level broadening of favorable spin configurations could be manipulated to eliminate the distinguishability of photons.

Hydrostatic pressure effect on photoionization cross section of a trion in quantum dots

It is known experimentally that stable charged excitons can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to solve, even numerically. Here, we use the matrix diagonalization method and compact-density approach to investigate the charged excitons in a two-dimensional parabolic quantum dot. With typical semiconducting GaAs based materials, the photoionization cross section has been examined based on the computed energies and wave functions. We find that the photoionization cross section of charged excitons is strongly affected by the confinement frequency, hydrostatic pressure and temperature of QDs. Our results also show that the photoionization cross section of a negatively charged exciton is larger than that of a positively charged exciton.

Neutral and positively charged excitons in quantum ring

We study theoretically quantized states of the neutral and positively charged exciton complexes confined within a circular narrow ring in the presence of the magnetic field applied along the symmetry axis. We show that in the structural adiabatic limit, when the width of the pattern of the particles pathways within the ring is much smaller than its radius, the wave equations for both complexes are separable and their exact solutions can be found in a form of the Fourier series of one and two variables, respectively. We present results of calculation of the lower energies of complexes as functions of the ring's radius and the magnetic field strength for different values of the electron-to-hole mass ratio. We found that in the molecular adiabatic limit, when this ratio tends to zero and the model describes the corresponding donor complexes, the physical interpretation of the quantum-size effect and the oscillations of energy levels in threading magnetic field revealed for the excitons spectra becomes more transparent.

Charge control in laterally coupled double quantum dots

We investigate the electronic and optical properties of InAs double quantum dots grown on GaAs $(001)$ and laterally aligned along the $[110]$ crystal direction. The emission spectrum has been investigated as a function of a lateral electric field applied along the quantum dot pair mutual axis. The number of confined electrons can be controlled with the external bias, leading to sharp energy shifts which we use to identify the emission from neutral and charged exciton complexes. Quantum tunneling of these electrons is proposed to explain the reversed ordering of the trion emission lines as compared to that of excitons in our system.