Biexciton Transition Research Articles

Optical Properties of Excitons Confined in a Single CdTe Quantum Dot

Spectroscopy of CdTe single quantum dots reveals intrinsic properties of quantum dots, such as the acoustic phonon broadening mechanism and the fine structure of the exciton. Strong asymmetry of a dot induces both electron–hole exchange splitting of the bright exciton and an anisotropy of its linearly polarized components. Owing to the existence of a unique biexciton state, the biexciton transition reproduces the exciton fine structure in a reverse sense. At increasing excitation density, emission lines of a few exciton complexes corresponding to s- and p-shell transitions are observed.

One-dimensional biexcitons in a single quantum wire

We report the direct observation of one-dimensional (1D) excitonic molecules (biexcitons) in high quality disorder free semiconductor quantum wires. By means of spatially resolved near-field photoluminescence spectroscopy, we detected delocalized 1D excitons through the spatial extension of their homogeneous emission. 1D biexciton transitions are evidenced by their correlation to the 1D excitons as well as their power dependence. The photoluminescence intensity from biexcitons is shown to increase quadratically compared to the linear variation for excitonic emission, in agreement with the mass action law. The biexciton binding energy is measured to be 1.2 meV, and the lineshape is clearly asymmetric, showing the expected low energy tail.

Effect of Polariton-Polariton Scattering on the Two-Photon Transition to Biexcitons in CuCl

Two-photon absorption spectra of the biexciton transition in CuCl have been investigated using a pump-probe technique. Combinations of two circularly-polarized lightbeams with various photon energies are used to examine the relation of the absorption to the transition selection rule. The absorption is found to occur even for the forbidden configuration of polarizations of two beams when the photon energy of the one incident light is close to the exciton resonance. The departure from the selection rule is shown to arise from depolarization of polaritons. The degree of depolarization depends on the polariton density. It is concluded that the elastic scattering between polaritons is the dominant process mediating the two-photon transition under forbidden configuration.

Spectral signatures of χ^(5) processes in four-wave mixing of homogeneously broadened excitons

The influence of fifth-order coherences on the spectrally resolved four-wave mixing response of predominantly homogeneously broadened quasi-two-dimensional excitons is studied. Fifth-order signatures are discussed as a function of spectral position and excitation polarization. An exciton–biexciton beating for positive delay times is the dominant effect, which is pronounced at the exciton–biexciton transition for collinearly polarized excitation and at the exciton transition for cross-linearly polarized excitation. For negative delay times and collinearly polarized excitation a pronounced exciton–biexciton beating at the exciton resonance is observed that is vanishing for long negative delays owing to the faster dephasing in the two-exciton continuum compared with the bound biexciton state. These results are in qualitative agreement with microscopic model calculations that include the coherent dynamics of one- and two-exciton resonances up to the fifth order in the optical field.

Crossover from Exciton to Biexciton Cavity Polaritons

We report novel oscillator strength transfer phenomena induced by the exciton–exciton correlation. A high quality semiconductor microcavity is resonantly excited by intense circularly polarised σ+ pump pulses (100 fs long) and probed at different delays by weak counter-polarised σ— test pulses. We show that the presence of σ+ excitons can completely inhibit the creation of σ— excitons. With increasing σ+ exciton density the oscillator strength of the σ— transition is transferred to the biexciton transition, permitting the first observation of the strong coupling regime between the biexciton transition and the cavity photon mode, i.e., the biexciton polariton states.

Effects of Electronic Fourth-Order Correlations on the Pump-Probe Response of Semiconductors

The pump-and-probe response of a ZnSe quantum well is investigated. In this particular sample the binding energy far exceeds both the exciton damping and the inhomogeneous broadening. A comparison between experimental data and model calculations shows that with co-circularly polarized beams the nonlinear mean-field contributions dominate those induced by four-particle correlations. In contrast, only the four-particle correlations contribute to the signal in counter-circular configuration. For this configuration, the exciton–biexciton interaction dominates the response for resonant excitation of the exciton. The occurrence of exciton–biexciton transitions leads to a renormalization of the period of coherent oscillations at negative pulse delay.

Spin and exchange effects in CdSe/ZnSe quantum dots probed by single-dot spectroscopy

The influence of exchange interaction on the eigenstate symmetry in a variety o CdSe/ZnSe single quantum dots is studied by photoluminescence spectroscopy. The lifting of spin degeneracy of the optically allowed exciton states results in a linearly polarized doublet of both the single exciton and the biexciton transitions, with an energy splitting reflecting the symmetry of the dots. Comparing different single dots, we found that most of the dots are almost symmetric, i.e. have an energy splitting <0.2 meV, while strongly asymmetric dots (i.e. a symmetry lower than D 2d) with energy splittings up to about 1 meV are predominantly oriented along the [1 1 0] and [1 1 ̄ 0] axes of the crystal.

Polariton–biexciton transitions in a ZnSe-based microcavity

The optical third-order nonlinearity of a ZnSe-based microcavity is investigated by the pump-and-probe method. In the specially designed non-monolithic sample the biexciton binding energy exceeds all damping constants and the normal-mode splitting between exciton and cavity photon. For counter-circular polarized beams the nonlinear response exhibits strong oscillatory structures in the spectral vicinity of the polariton–biexciton transition. Comparison to model calculations shows that in this case the coherent nonlinearity is completely dominated by biexciton–exciton interactions beyond the Hartree–Fock approximation.

Eigenstate symmetry probed by biexciton transitions in a single quantum dot

The eigenstate symmetry in CdSe/ZnSe single quantum dots (SQDs) has been studied by low-temperature magnetoluminescence spectroscopy. Regarding both, the fine structure splitting and the polarization properties of the biexciton transition, the influence of exchange and Zeeman interaction on the eigenstate symmetry of the final state of recombination, the ground state of the single exciton, is investigated.

Temperature-dependent line widths of single excitons and biexcitons

The coupling of single zero-dimensional excitons and biexcitons to acoustic and optical phonons is studied in epitaxially grown Zn 1− x Cd x Se/ZnSe quantum structures. From the temperature-dependent line width of the exciton photoluminescence (PL) an enhanced coupling to acoustic phonons is found which we explain by the quantum confinement. The LO-phonon coupling is reduced compared to CdSe bulk material. The temperature dependence of the line width of the biexciton PL is similar, indicating a similar coupling strength of the exciton and the exciton–biexciton transition to the crystal lattice.

Buried single CdTe/CdMnTe quantum dots realized by focused ion beam lithography

Buried single CdTe/CdMnTe quantum dots are realized by implantation-induced intermixing using a focused 100 keV Ga+ ion beam. For an implantation dose of 5×1013 cm−2 and an annealing temperature of 390 °C, a lateral potential depth of about 65 meV is obtained. By means of photoluminescence spectroscopy, the formation of zero-dimensional multiexcitons in single quantum dots is investigated, yielding a biexciton binding energy of about 3.5 meV. In addition, the occurrence of an excited biexciton transition in the photoluminescence spectrum gives clear evidence of a suppressed exciton spin-flip process in quantum dots.

Zeeman splitting of excitons and biexcitons in singleIn0.60Ga0.40As/GaAsself-assembled quantum dots

Self-assembled ${\mathrm{In}}_{0.60}{\mathrm{Ga}}_{0.40}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots have been studied by single dot photoluminescence spectroscopy at $T=1.5\mathrm{K}.$ Emission from the biexciton state is observed, for which we find a binding energy of 3.1 meV. In magnetic field we observe equal Zeeman splittings for the exciton and biexciton spectral lines. The splitting of the biexciton emission is given by the splitting of the exciton in the final state of the biexciton transition. The circular polarization of the quantum dot luminescence decreases with increasing magnetic field, although the spin splitting is clearly larger than the thermal energy. This indicates that the spin relaxation is strongly suppressed in these quantum dots.

Exciton complexes inInxGa1−xAs/GaAsquantum dots

Multiexciton complexes in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots in the weak-confinement regime have been investigated by photoluminescence spectroscopy at $T=2\mathrm{K}.$ The lateral dot sizes varied down to about 50 nm so that the transition from the quantum well to the quantum dot case could be studied. The biexciton binding energy $\ensuremath{\Delta}{(X}_{2})$ increases with decreasing dot size, depending on the height of the confinement potential. The splitting of the biexciton emission by a magnetic field $(Bl~8 \mathrm{T})$ arises from the spin splitting of the exciton in the final state of the optical transition, because the biexciton is a spin-singlet state. A magnetic field reduces $\ensuremath{\Delta}{(X}_{2})$ by approximately the exciton spin splitting. This reduction is due to the decrease of the energy gap between the biexciton transition into the spin ground-state of the exciton and the transition of the spin ground- state exciton. The formation of complexes consisting of three and four excitons becomes possible due to the three-dimensional quantum dot confinement. Because of the Pauli principle the Coulomb correlations in the three-exciton complex result in a net repulsion energy that increases strongly with decreasing dot size. The multiexciton interaction energies are reduced by a magnetic field because the exciton repulsion decreases when the magnetic length becomes smaller than the quantum dot size. In the four-exciton-complex we find an exchange energy splitting of states corresponding to carrier configurations with parallel and antiparallel spins. The energy differences between exciton transitions from excited and ground states are about equal to the corresponding energy splittings of the single-particle electron and hole states. These findings indicate that the strong Pauli repulsion between electrons and holes determines the formation of exciton complexes. Based on these results a shell model for quantum dot multiexciton states is discussed.

Two-photon resonance spectra of biexcitons in PbI 2: Contribution of coherent and incoherent processes

Photoluminescence excitation spectra for biexcitons are investigated in the energy range near the exciton absorption band of PbI 2 at 7 K. Two-photon absorption for the direct generation of biexcitons is found, and the biexciton energy is determined to be 4.9912 eV, which corresponds to the binding energy of 4.6 meV. Spectra of reflection-type degenerate four-wave mixing are also measured in order to extract the two-photon resonance spectrum due to coherent biexciton transition. It is found that the photoluminescence excitation spectra and the four-wave mixing spectra reflect the line width of biexciton state.

Carrier Dynamics Study of CdxZn1-xSe/ZnSe (x = 0.2) Multiple Quantum Wells

In this paper, nonlinear optical properties of CdxZn1-xSe/ZnSe (x = 0.2) multiple quantum wells were studied by low temperature steady-state and transient photoluminescence at high excitation densities. The biexciton transition was observed on the low energy side of the exciton transition. Based on the characteristics of stimulated emission observed in similar structures, we suggest the biexciton transition as the mechanism for stimulated emission. Optical degradation was also studied by room temperature photoluminescence using femtosecond laser pulses as the excitation source. The results confirm the formation of nonradiative recombination centers with a saturating degradation effect after about 10 min of exposure.

Observation of exciton and biexciton processes in CdxZn1−xSe/ZnSe (x=0.2)

Exciton and biexciton transitions have been observed in low-temperature (10 K) photoluminescence from CdxZn1−xSe/ZnSe (x=0.2) multiple quantum well samples grown by molecular beam epitaxy. Transient photoluminescence experiments were conducted to study the dynamics of carrier decay associated with these processes. The formation of exciton and biexciton species is confirmed by examining their energy positions, intensity dependence on excitation power density, spectral line shapes, relative decay lifetimes, and polarization dependence.

Four‐wave‐mixing theory beyond the semiconductor Bloch equations

AbstractFour‐wave‐mixing (FWM) experiments using a dynamical density matrix model of the semiconductor band edge are discussed. Higher‐order correlation functions are retained which are neglected in the commonly used RPA treatment leading to the semiconductor Bloch equations. In order to terminate the hierarchy of the equations of motion for the higher‐order density matrices systematically a truncation scheme controlled by orders in the driving field is applied. For any prescribed order n in the exciting field a closed set of equations is obtained from which the dielectric response up to order n can be calculated exactly. In addition it turns out that in a coherently driven system part of the remaining density matrices become redundant and can be eliminated. Four‐wave‐mixing experiments are dominated by third‐order contributions. Applying the above‐described results one ends up with only two functions in this case. These are the excitonic and the biexcitonic transition densities. As an application of our method the example of a GaAs single quantum well is studied. Two pulses with finite lengths are assumed such that both heavy‐and light‐hole excitons are excited. The influence of the biexciton contribution on the polarization properties of the FWM signal is analyzed and compared with experimental results.

Picosecond induced absorption in ultradisperse amorphous selenium particles

A study was made of the induced absorption in amorphous selenium particles (average radius ~20 nm) embedded in a polymer. A picosecond pump and probe technique was used. The change in the absorption was measured in the spectral range 550–750 nm when the pump radiation was in the form of pulses of 15 ps duration and 2.3 eV photon energy. The induced-absorption spectra were attributed to optical exciton—biexciton transitions. The exciton lifetime was estimated to be 424 ± 32 ps.

Polarization-dependent coherent nonlinear optical response in GaAs quantum wells: Dominant effects of two-photon coherence between ground and biexciton states

Using the unique polarization selection rule for the biexcitonic transition we demonstrate conclusively the dominant effects of the biexciton-induced two-photon coherence on polarization-dependent four-wave-mixing in GaAs quantum wells. We also show that the two-photon coherence is the leading contribution to the self-diffracted four-wave-mixing response in the cross-linearly polarized configuration and can explain the anomalous polarization dependence in four-wave-mixing.

The role of the biexciton in a dynamic density matrix theory of the semiconductor band edge

We describe the dielectric response of the semiconductor band edge in a dynamic density matrix model. Our treatment is based on a set of χ3-relevant constitutive equations involving two-, four- and six-point density matrices. We demonstrate that under certain conditions all contributions to the third order susceptibility can be expressed in terms of excitonic and biexcitonic transitions. As a first application of these χ3-relevant equations we investigate the influence of the biexciton on the optical Stark effect in CuCl. We calculate shifts and lineshapes. Our results turn out to be in excellent agreement with experiments as well as with other theoretical predictions.