Biexciton Binding Energy Research Articles

Coherent Longitudinal-Optical Ground-State Phonon in CdSe Quantum Dots Triggered by Ultrafast Charge Migration

We observe the CdSe longitudinal-optical ground-state phonon in the electron transfer system composed of CdSe quantum dots and methylviologen directly by femtosecond absorption spectroscopy. A significant phase shift indicates that the coherent oscillations are triggered by an ultrafast charge migration, which is the consequence of an electron transfer from the photoexcited quantum dot to the molecular acceptor methylviologen. In contrast, the observed coherent phonons in isolated quantum dots stem from the frequency modulation of the quantum dot excited-state spectrum. From the probe wavelength dependence of the longitudinal-optical phonons in the electronic ground state and excited state it is possible to determine a biexciton binding energy of 35meV.

State-resolved observation in real time of the structural dynamics of multiexcitons in semiconductor nanocrystals

Biexciton processes in semiconductor nanocrystals, e.g. optical gain, are widely treated as arising from structureless biexcitons. We challenge this view by observing the binding energies of hot biexcitons in CdSe nanocrystals. These experiments reveal a previously unobserved spectrum in the biexciton binding energies that correlates with hole configuration. Analysis of this spectrum of binding energies shows that the onset and the decay of optical gain in nanocrystals are dictated by cooling dynamics of the hot biexciton rather than Auger recombination. In addition to the structural dynamics of the biexciton, we show that the capacity to control optical gain bandwidth in these nanocrystals arises from stimulated emission from higher multiexcitons.

Generation of a two-photon state from a quantum dot in a microcavity

We propose and characterize a two-photon (2P) source that emits in a highly polarized, monochromatic and directional beam, realized by means of a quantum dot embedded in a linearly polarized cavity. In our scheme, the cavity frequency is tuned to half the frequency of the biexciton (two excitons with opposite spins) and largely detuned from the excitons thanks to the large biexciton binding energy. We show how the emission can be Purcell enhanced by several orders of magnitude into the 2P channel for available experimental systems.

Well‐width dependence of excitonic properties in organic‐inorganic hybrid quantum well materials

AbstractOptical properties in a single crystal of the organic‐inorganic hybrid perovskite quantum‐well material (C4H9NH3)2(CH3NH3)Pb2Br7 have been investigated. This material has double‐layer sheets of corner‐sharing [PbBr6]4‐ octahedrons constituting the inorganic well. The estimated longitudinal‐transverse (LT) splitting, exciton exchange energy, and biexciton binding energy are about 60 meV, 17 meV and 26 meV respectively. We can also study well‐width dependence in angstroms by comparing with (C4H9NH3)2PbBr4 which has single‐layer sheets comprising the inorganic well. This is the first time the well‐width dependence of excitonic properties in hybrid organic‐inorganic quantum materials is investigated. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Asymptotic exchange coupling of quasi-one-dimensional excitons in carbon nanotubes

An analytical expression is obtained for the biexciton binding energy as a function of the interexciton distance and binding energy of constituent quasi-one-dimensional excitons in carbon nanotubes. This allows one to trace biexciton energy variation and relevant nonlinear absorption under external conditions whereby the exciton binding energy varies. The nonlinear absorption line shapes calculated exhibit characteristic asymmetric (Rabi) splitting as the exciton energy is tuned to the nearest interband plasmon resonance. These results are useful for tunable optoelectronic device applications of optically excited semiconducting carbon nanotubes, including the strong excitation regime with optical nonlinearities.

Biexcitons in semiconducting single-walled carbon nanotubes

We study numerically the nature of biexcitons in semiconducting single-walled carbon nanotubes by exactly solving the two-electron-two-hole problem in the realistic Hamiltonian derived using the $\mathbit{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbit{p}$ and screened Hartree-Fock approximation. Our treatment captures the essential features of narrow-gap semiconductors, i.e., band nonparabolicity, form factors in the density operators, and many-body effects. It turns out that the form factor as well as the screening effect significantly suppresses the biexciton binding energy. The Haynes factor we obtained by including all the above features is less than half the values predicted in the phenomenological model. We expect them to be the general aspects observed in narrow-gap semiconductor systems.

Huge binding energy of localized biexcitons in Al-rich AlxGa1−xN ternary alloys

Excitonic optical properties of Al-rich AlxGa1−xN ternary alloy epitaxial layers have been studied by means of photoluminescence excitation spectroscopy. On the basis of the energy separation between exciton resonance and two-photon biexciton resonance, the binding energy of biexcitons was estimated to be 56±5 and 48±5 meV for the sample with x=0.81 and 0.89, respectively. The biexciton binding energy of 56 meV was approximately three times as large as the biexciton binding energy of 19 meV in AlN. The large enhancement of the biexciton binding energy resulted from the strong localization of biexcitons due to alloy disorder.

The enhanced binding energy for biexcitons in InAs quantum dots

We observed that the biexciton binding energy in InAs quantum rhombic disks (QRDs) is enhanced by twice compared with that for InAs quantum dots (QDs) so far reported around 1.24 μm nearby the telecommunication wavelength. The heterodyne-detected four-wave-mixing detected the exciton-biexciton quantum beat superposed on photon echo decay, giving the biexciton binding energy of 3.4 meV to 3 monolayer (ML) InAs QRDs and 4.1 meV to 4 ML InAs QRDs, respectively. The largest biexciton binding energy of 4.1 meV in InAs QDs is ascribed to increased electron-hole overlap in confined geometry with a minimized strain distribution.

Polaron contributions to the biexciton binding energies in self-assembled quantum dots

The contribution to the biexciton binding energy in quantum dots resulting from the interaction with longitudinal optical phonons is estimated by performing the configuration-interaction calculation of the few-particle states in a simple model of the confining potential and by including the phonon corrections by means of a perturbation theory. It is found that the polaron contribution tends to compensate the Coulomb-related biexciton shift (binding energy) and reduces its value by up to 30%, depending on the material parameters of the system.

Electric Field Induced Removal of the Biexciton Binding Energy in a Single Quantum Dot

We control the electrostatic environment of a single InAsP quantum dot in an InP nanowire with two contacts and two lateral gates positioned to an individual nanowire. We empty the quantum dot of excess charges and apply an electric field across its radial dimension. A large tuning range for the biexciton binding energy of 3 meV is obtained in a lateral electric field. At finite lateral electric field the exciton and biexciton emission overlap within their optical line width resulting in an enhancement of the observed photoluminescence intensity. The electric field dependence of the exciton and biexciton is compared to theoretical predictions and found to be in good qualitative agreement. This result is promising toward generating entangled photon pairs on demand without the requirement to remove the anisotropic exchange splitting from asymmetric quantum dots.

Fine structure and size dependence of exciton and biexciton optical spectra in CdSe nanocrystals

Theory of electronic and optical properties of exciton and bi-exciton complexes confined in CdSe spherical nanocrystals is presented. The electron and hole states are computed using atomistic $sp^3d^5s^*$ tight binding Hamiltonian including an effective crystal field splitting, spin-orbit interactions, and model surface passivation. The optically excited states are expanded in electron-hole configurations and the many-body spectrum is computed in the configuration-interaction approach. Results demonstrate that the low-energy electron spectrum is organized in shells ($s$, $p$, ...), whilst the valence hole spectrum is composed of four low-lying, doubly degenerate states separated from the rest by a gap. As a result, the bi-exciton and exciton spectrum is composed of a manifold of closely lying states, resulting in a fine structure of exciton and bi-exciton spectra. The quasi-degenerate nature of the hole spectrum results in a correlated bi-exciton state, which makes it slowly convergent with basis size. We carry out a systematic study of the exciton and bi-exciton emission spectra as a function of the nanocrystal diameter and find that the interplay of repulsion between constituent excitons and correlation effects results in a change of the sign of bi-exciton binding energy from negative to positive at a critical nanocrystal size.

Excitonic complexes in InGaAs/GaAs quantum dash structures

Optical properties of single self-assembled InGaAs/GaAs quantum dashes have been investigated. The excitation power dependent measurements together with the rate equation model calculations enable to identify the excitonic and biexcitonic emission from one quantum dash. Based on that the biexciton binding energy and the relative exciton to biexciton lifetime ratio have been estimated. The emission spectra have also revealed a fingerprint of Coulomb interaction between excitons confined in the quantum dash and adjacent wetting layer. The conditions required for observing this phenomena and the influence of the wetting layer on the emission of individual quantum dashes have been discussed.

Quantum confinement in MOVPE-grown structures with self-assembled InAs/GaAs quantum dots

In this communication we report on low-temperature, micro-photoluminescence study of quantum confinement in MOVPE-grown structures with InAs/GaAs quantum dots (QDs) with GaAs and/or strain reducing InGaAs/GaAs capping. We focus our attention on sharp emission lines, which appear in both structures at energies up to 80 meV below the wetting line emission. Power-dependent measurements confirmed their attribution to single excitons as well as biexcitons. Negative binding energy of biexcitons with systematic dependence on their energy was observed. It has been proposed that the investigated emission lines result from radiative recombination in flat non-fully developed QDs in the investigated structure. The attribution is confirmed by transmission electron microscopic analysis of investigated structures.

High-excitation and high-resolution photoluminescence spectra of bulk AlN

Photoluminescence spectra of high-quality bulk AlN crystals are reported. In addition to the expected linear luminescence features like free excitons and donor-bound excitons, nonlinear processes like biexcitons and the exciton-exciton scattering band are seen for higher excitation densities. For temperatures above $\ensuremath{\approx}150\text{ }\text{K}$ the electron-hole plasma becomes clearly visible in the spectra. A detailed analysis of the data yields an exciton binding energy of 55 meV, a biexciton binding energy of 28.5 meV, a band gap of 6.089 eV at low temperature, and a band gap of 6.015 eV at room temperature.

Biexciton emission from sol-gel ZnMgO nanopowders

We studied the power-dependent photoluminescence of Zn1−xMgxO nanopowders grown by sol-gel method, at temperature T=100 K. At moderate optical pumping intensity, a nonlinear emission band due to the radiative recombination of free biexcitons was detected. We found that the free biexciton binding energies of Zn1−xMgxO nanopowder (0.01≤x≤0.05) are nearly constant (13.5±1.5 meV).

Impact of shell thickness on exciton and biexciton binding energies of a ZnSe/ZnS core–shell quantum dot

Impact of shell structure on the exciton and biexciton binding energies has been studied in a ZnSe/ZnS core–shell quantum dot using Wentzel–Kramers–Brillouin (WKB) approximation. For excitons, the binding is caused by the Coulombic as well as the confinement potentials while biexciton binding energy is determined by taking into account the exchange and correlation effects. The exciton binding energy was found to increase initially with increasing shell thickness which reaches saturation at larger shell thickness. On the other hand, the biexciton binding energy exhibits a crossover from the bonding to antibonding state with increasing shell thickness for smaller core radius of the quantum dot.

Biexciton binding energy control in site-selected quantum dots

A unique nanotemplate deposition technique is utilized in growth of semiconductor quantum dots which enables precise control over the dot dimensions and nucleation site. Here, we demonstrate tuning of the biexciton binding energy in a single, site-selected InAs/InP quantum dot through manipulation of the nanotemplate dimensions and thus, dot size. A monotonic decrease of the biexciton binding energy from the binding to anti-binding regime through zero is observed with increasing dot size. Piezoelectric fields in large quantum dots are suggested as the mechanism to obtain an unbound biexciton state. The tunability of the biexciton binding energy demonstrated here for a deterministically positioned quantum dot is an important step towards a scalable route in the generation of entangled photon pairs that emit around the telecommunications band of 1.55 μm.

Two-photon lasing by a single quantum dot in a high-Qmicrocavity

We investigate theoretically two-photon processes in a microcavity containing one quantum dot in the strong coupling regime. The cavity mode can be tuned to resonantly drive the two-photon transition between the ground and the biexciton states, while the exciton states are far-off resonance due to the biexciton binding energy. We study the steady state of the quantum dot and cavity field in presence of a continuous incoherent pumping. We identify the regime where the system acts as two-photon emitter and discuss the feasibility and performance of realistic single quantum dot devices for two-photon lasing.

Quantum Dashes and Quantum Rods: Optical Properties and Application Prospects

There are reported the optical properties of novel quasi-zero-dimensional semiconductor structures obtained by self-assembly during the molecular beam epitaxy process. The studies has been focused on strongly asymmetric systems, called quantum dashes, quantum rods or columnar quantum dashes, and their potential applications in light sources or amplifiers for telecommunication and data transmission or single photon sources and other quantum electrodynamics based emitters. There is discussed the linear polarization degree of the edge emission of the columnar structures with respect to their potential for a polarization independent semiconductor optical amplifier at 1.55 μm. Additionally, there have been analyzed the photoluminescence spectra of single quantum dashes and single quantum rods in order to detect the exciton/biexciton emission, determine the biexciton binding energy and predict the exciton to biexciton radiative lifetimes ratio basing on a few level rate equation model.

In(Ga)As/GaAs quantum dots grown on a (111) surface as ideal sources of entangled photon pairs

Self-organized In(Ga)As/GaAs quantum dots (QDs) grown on (111) substrate are proposed as ideal sources for the generation of entangled photon pairs. Due to the threefold rotational symmetry of the (111) surface, QDs with ${C}_{3v}$ symmetry or higher are expected to develop during growth. In contrast to QDs on (001)-oriented substrates, the symmetry of the confinement potential of (111) QDs is not lowered by piezoelectric effects. As a result the excitonic bright splitting vanishes and the $\text{biexciton}\ensuremath{\rightarrow}\text{exciton}\ensuremath{\rightarrow}0$ recombination cascade can be used for the generation of entangled photons. We evaluate the spectroscopic separability of excitonic and biexcitonic emissions as a function of QD size, shape, and composition using the configuration-interaction model in conjunction with eight-band $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory. The piezoelectric field in (111) QDs predominantly aligns along the growth direction and gives rise to vertical charge separation. First- and second-order piezoelectric fields are oriented in opposite directions. The In/Ga ratio inside the QD determines the leading contribution and can be employed to balance both terms in order to achieve a field-free situation with maximal electron-hole overlap. The biexciton binding energy depends on the net piezoelectric potential drop across the QD vertical extension and becomes maximal if the first- and second-order fields outweigh each other within the QD interior.