Biexcitons In Quantum Dots Research Articles

Long-Lived and Bright Biexcitons in Quantum Dots with Parabolic Band Potentials.

Multiple exciton physics in semiconductor nanocrystals play an important role in optoelectronic devices. This work investigates radially alloyed CdZnSe/CdS nanocrystals with suppressed Auger recombination due to the spatial separation of carriers, which also underpins their performance in optical gain and scintillation experiments. Due to suppressed Auger recombination, the biexciton lifetime is greater than 10 ns, much longer than most nanocrystals. The samples show optical gain, amplified spontaneous emission, and lasing at thresholds <2 excitons per particle. They also show broad gain bandwidth (>500 meV) encompassing 4 amplified spontaneous emission bands. Similarly enabled by slowed multiple exciton relaxation, the samples display strong performance in scintillating films under X-ray illumination. The CdZnSe/CdS samples have fast radioluminescence rise (<80 ps) and decay times (<5 ns), light yields up to 6700 photons·MeV-1, and the demonstrated capacity for incorporation into large area films for scintillation imaging.

Pulse shaping for on-demand emission of single Raman photons from a quantum-dot biexciton

Semiconductor quantum dots embedded in optical cavities are promising on-demand sources of single photons. Here, we theoretically study single photon emission from an optically driven two-photon Raman transition between the biexciton and the ground state of a quantum dot. The advantage of this process is that it allows all-optical control of the properties of the emitted single photon with a laser pulse. However, with the presence of other decay channels and excitation-induced quantum interference, on-demand emission of the single Raman photon is generally difficult to achieve. Here we show that laser pulses with non-trivial shapes can be used to maintain excitation conditions for which with increasing pulse intensities the on-demand regime is reached. To provide a realistic picture of the achievable system performance, we include phonon-mediated processes in the theoretical caluclations. While preserving both high photon purity and indistinguishability, we find that although based on a higher-order emission process, for realistic system parameters on-demand Raman photon emission is indeed achievable with suitably tailored laser pulses.

All-optical tailoring of single-photon spectra in a quantum-dot microcavity system

Semiconductor quantum-dot cavity systems are promising sources for solid-state based on-demand generation of single photons for quantum communication. Commonly, the spectral characteristics of the emitted single photon are fixed by system properties such as electronic transition energies and spectral properties of the cavity. In the present work we study single-photon generation from the quantum-dot biexciton through a partly stimulated non-degenerate two-photon emission. We show that frequency and linewidth of the single photon can be fully controlled by the stimulating laser pulse, ultimately allowing for efficient all-optical spectral shaping of the single photon.

Electromagnetically induced transparency in quantum dot biexciton–exciton cascaded scheme

We numerically investigated the electromagnetically induced transparency in the quantum dot biexciton–exciton scheme. Through calculating the density matrix equation in the steady state, we obtain the electromagnetically induced transparency absorption dip with different pump intensity and biexciton decay rate. The refractive index is also calculated. We analyze the biexciton energy renormalization and pump pulse intensity influencing on the EIT dip and the slow factor. The slow factor decreases from about 3000 to 2000 due to the renormalization. It shows better temperature stability for stronger confinement energy and smaller decay rate compared with quantum well.

Multiexcitonic emission from single elongated InGaAs/GaAs quantum dots

In this work, we present both experimental data and simulations of multiexcitonic emission spectra of single self-assembled elongated In0.3Ga0.7As/GaAs quantum dots. The emission spectra reveal an unusual evolution with the increased excitation power density. First, a biexciton line appears simultaneously with its low energy sideband, the origin of which has already been postulated previously and related to the interaction of a quantum dot biexciton with excitons generated in the surrounding wetting layer. A further increase of the excitation causes a disappearance of the exciton line accompanied with a transformation of the biexciton sharp line and its sideband into a redshifting broad emission band. The latter recalls a typical feature of the transition from excitonic emission into electron-hole plasma called Mott transition, which is possible to occur in wire-like structures under the conditions of very high carrier densities. However, we propose an alternative explanation and show that this behavior can be well explained based on a multilevel rate equation model, indicating that such a dependence of the emission spectra is a fingerprint of a formation of multiexcitonic states. Further, we discuss the importance of various quantum system parameters as the radiative lifetimes or spectral linewidths.

Cavity-assisted emission of polarization-entangled photons from biexcitons in quantum dots with fine-structure splitting

We study the quantum properties and statistics of photons emitted by a quantum-dot biexciton inside a cavity. In the biexciton-exciton cascade, fine-structure splitting between exciton levels degrades polarization-entanglement for the emitted pair of photons. However, here we show that the polarization-entanglement can be preserved in such a system through simultaneous emission of two degenerate photons into cavity modes tuned to half the biexciton energy. Based on detailed theoretical calculations for realistic quantum-dot and cavity parameters, we quantify the degree of achievable entanglement.

Excitonic exchange effects on the radiative decay time of monoexcitons and biexcitons in quantum dots

Electron-hole exchange interaction splits the exciton ground state into ``dark'' and ``bright'' states. The dynamics of those states depends on the internal relaxation time between bright and dark states (spin-flip time), and on the radiative recombination time of the bright states. On the other hand, the calculated values of these recombination times depend not only on the treatment of correlation effects, but also on the accuracy of the electron and hole wavefunctions. We calculate the radiative decay rates for monoexcitons and biexcitons in $(\mathrm{In},\mathrm{As})\mathrm{Ga}∕\mathrm{GaAs}$ self-assembled and colloidal $\mathrm{CdSe}$ quantum dots from atomistic correlated wave functions. We show how the radiative decay time ${\ensuremath{\tau}}_{R}({X}^{0})$ of the monoexciton depends on the spin-flip relaxation time between bright and dark states. In contrast, a biexciton has no bright-dark splitting, so the decay time of the biexciton ${\ensuremath{\tau}}_{R}(X{X}^{0})$ is insensitive to this spin-flip time. This results in ratios ${\ensuremath{\tau}}_{R}({X}^{0})∕{\ensuremath{\tau}}_{R}(X{X}^{0})$ of 4 in the case of fast spin flip, and a ratio of 2 in the case of slow spin flip. For $(\mathrm{In},\mathrm{Ga})\mathrm{As}∕\mathrm{GaAs}$, we compare our results with the model calculation of Wimmer et al. [Phys. Rev. B 73, 165305 (2006)]. When the same spin-flip rates are assumed, our predicted ${\ensuremath{\tau}}_{R}({X}^{0})∕{\ensuremath{\tau}}_{R}(X{X}^{0})$ agrees with that of Wimmer et al., suggesting that our treatment of correlations is adequate to predict the ratio of monoexciton and biexciton radiative lifetimes. Our results agree well with experiment on self-assembled quantum dots when assuming slow spin flip. Conversely, for colloidal dots the agreement with experiment is best for fast spin flip.

Coherent nonlinear optical response of excitons and biexcitons in quantum dots coupled to phonons

AbstractFour wave mixing signals from single dots and dot ensembles involving excitonic and biexcitonic transitions are discussed. The results have been obtained analytically from a four level dot model coupled to longitudinal acoustic phonons. Time integrated four wave mixing amplitudes for quantum dot ensembles exhibit polarization dependent quantum beats that have already been found in experimental data. In four wave mixing signals of single quantum dots we find that the ground state to exciton transition has two contributions. One is identical to the known solution of the two level quantum dot model and leads to symmetric spectra for longer delay times, while the other comprises phase information from the biexciton and results in asymmetric spectra. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical nonlinearity induced by exciton-biexciton coherent effects in InGaAs quantum dots

We report on large optical nonlinear effects induced by coherent interactions between excitons and biexcitons in quantum dots. The absorption spectra in an exciton–biexciton system are measured in InGaAs quantum dots by using single dot spectroscopy. The spectra exhibit unusual shapes that have a transparent region under a strong excitation condition. A pump–probe measurement ensures that the exciton state is erased by a quantum interference effect with exciton and biexciton photoabsorption processes. These results suggest that optical functions such as electromagnetically induced transparency are feasible with excitons and biexcitons in quantum dots.

Second Bound State of Biexcitons in Quantum Dots

The second bound state of the biexcitons in a quantum dot, with orbitalangular momentum L = 1, is reported. By using the method of few-bodyphysics, the binding energy spectra of the second bound state of a biexcitonin a GaAs quantum dot with a parabolic confinement have been calculated as afunction of the electron-to-hole mass ratio and the quantum dot size. Thefact that the biexcitons have a second bound state may aid in the betterunderstanding of their binding mechanism.

Shape and Composition Effects on Excitons and Biexcitons in Quantum Dots

Exciton and biexciton states in realistic quantum dots are investigated theoretically. The few-particle interactions are shown to depend sensitively on the structural properties of the dots. The state energies are obtained by a configuration interaction method added on eight-band k . p calculations of single-particle states. By comparing the calculations to empirical pseudopotential ones, the accuracy of the method is established.

Binding energy of biexcitons in quantum dots

We propose a procedure to solve exactly the Schrödinger equationfor a biexciton composed of two electrons and two holes in a harmonicquantum dot by using the hyperspherical coordinates. The binding energy ofthe ground state has been obtained as a function of the electron-hole massratio. Dependence of the ratio of the binding energy of a biexciton to thatof a exciton on the mass ratio in a quantum dot has been obtained.

The Features of Low-Lying States of the Biexciton in Quantum Dots

The Hamiltonian of a biexciton in a two-dimensional quantum dot with parabolic confinement has been diagonalized to obtain the low-lying eigenenergy values as a function of the electron-to-hole effective mass ratio and the dot size. Using the present result, a series of magic angular momenta of the biexciton dots have been discovered. Theoretical explanations derived from first principles have been formulated.

Narrow-band spectral hole burning in quantum dots

Pump-probe beam measurements performed with semiconductor doped glasses (CdSxSe1−x and CdSe), which are specially prepared to avoid diffusion-limited aggregation processes, revealed a homogeneous width (FWHM) as small as 11 meV (90 cm-1). Narrow-band nonlinear response provides an analysis of absorption fine structures as well as of phase relaxation features of excitons and biexcitons in quantum dots. A drastic influence of the growth dynamics on the optical properties of crystallites embedded in glass matrix is outlined.

Confinement-enhanced biexciton binding energy in semiconductor quantum dots.

Experimental observation of the ground-state biexciton transition in ${\mathrm{CdS}}_{\mathit{x}}$${\mathrm{Se}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$ quantum dots in glass is presented in a three-beam experiment, involving a probe, a pump, and a saturating laser pulse. The observed quantum-dot biexciton ground state has a strongly enhanced binding energy compared to the bulk as theoretically predicted. The biexciton binding energy is measured as a function of quantum-dot size and the results are compared with calculations.