Ultrafast Dynamics of Carrier Multiplication in Quantum Dots

A quantum-kinetic approach to the ultrafast dynamics of carrier multiplication in semiconductor quantum dots is presented. We investigate the underlying dynamics in the electronic subband occupations and the time-resolved optical emission spectrum, focusing on the interplay between the light-matter and the Coulomb interaction. We find a transition between qualitatively differing behaviors of carrier multiplication, which is controlled by the ratio of the interaction induced time scale and the pulse duration of the exciting light pulse. On short time scales, i.e., before intra-band relaxation, this opens the possibility of detecting carrier multiplication without refering to measurements of (multi-)exciton lifetimes.Comment: 12 pages, 7 figures, submitte

Quantum kinetic exciton-LO-phonon interaction in CdSe

Oscillations with a period of ∼150 fs are observed in the four-wave mixing (FWM) signal of bulk CdSe and interpreted in terms of non-Markovian exciton–LO-phonon scattering. The experiments show evidence of phonon quantum kinetics in semiconductors of strong polar coupling strength and high exciton binding energy. By comparison of the spectral and temporal response of the FWM signal in bulk CdSe and CdSe quantum dots, we demonstrate the influence of continuum states on the interference of electron-hole pair polarizations coupled via an LO phonon

Tuning intraband and interband transition rates via excitonic correlation in low-dimensional semiconductors

We show that electron-hole correlation can be used to tune interband and intra- band optical transition rates in semiconductor nanostructures with at least one weakly confined direction. The valence-to-conduction band transition rate can be enhanced by a factor (L/aB) N – with L the length of the weakly confined direction, aB the exciton Bohr radius and N the dimensionality of the nanostructure – while the rate of intraband and inter-valence-band transitions can be slowed down by the inverse factor, (aB/L) N . Adding a hitherto underexplored degree of freedom to engineer excitonic transition rates, this size dependence is of interest for various opto-electronic applica- tions. It also offers an interpretation of the superlinear volume scaling of two-photon absorption (TPA) cross-section recently reported for CdSe nanoplatelets, thus laying foundations to obtain unprecedented TPA cross sections, well above those of conven- tional two-photon absorbers. Further, our concept explains the background of the validity of the universal continuum absorption approach for the determination of par- ticle concentrations via the intrinsic absorption. Potential applications of our approach include low excitation intensity confocal two-photon imaging, two-photon autocorre- lation and cross correlation with much higher sensitivity and unprecedented temporal resolution as well as TPA based optical stabilization and optimizing of inter-subband transition rates in quantum cascade lasers

Colloidal synthesis and optical properties of type-II CdSe-CdTe and inverted CdTe-CdSe core-wing heteronanoplatelets

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.We developed colloidal synthesis to investigate the structural and electronic properties of CdSe-CdTe and inverted CdTe-CdSe heteronanoplatelets and experimentally demonstrate that the overgrowth of cadmium selenide or cadmium telluride core nanoplatelets with counterpartner chalcogenide wings leads to type-II heteronanoplatelets with emission energies defined by the bandgaps of the CdSe and CdTe platelets and the characteristic band offsets. The observed conduction and valence band offsets of 0.36 eV and 0.56 eV are in line with theoretical predictions. The presented type-II heteronanoplatelets exhibit efficient spatially indirect radiative exciton recombination with a quantum yield as high as 23%. While the exciton lifetime is strongly prolonged in the investigated type-II 2D systems with respect to 2D type-I systems, the occurring 2D giant oscillator strength (GOST) effect still leads to a fast and efficient exciton recombination. This makes type-II heteronanoplatelets interesting candidates for low threshold lasing applications and photovoltaics

p-State Luminescence in CdSe Nanoplatelets: The Role of Lateral Confinement and an LO Phonon Bottleneck

We report excited state emission from p-states at excitation fluences well below ground state saturation in CdSe nanoplatelets. Size dependent exciton ground state-excited state energies and dynamics are determined by three independent methods, time-resolved photoluminescence (PL), time-integrated PL and Hartree renormalized k$\cdot$p calculations -- all in very good agreement. The ground state-excited state energy spacing strongly increases with the lateral platelet quantization. Our results suggest that the PL decay of CdSe platelets is governed by an LO-phonon bottleneck, related to the reported low exciton phonon coupling in CdSe platelets and only observable due to the very large oscillator strength and energy spacing of both states

Strong amplitude-phase coupling in submonolayer quantum dots

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 109, 201102 (2016) and may be found at https://doi.org/10.1063/1.4967833.Submonolayer quantum dots promise to combine the beneficial features of zero- and two-dimensional carrier confinement. To explore their potential with respect to all-optical signal processing, we investigate the amplitude-phase coupling (α-parameter) in semiconductor optical amplifiers based on InAs/GaAs submonolayer quantum dots in ultrafast pump-probe experiments. Lateral coupling provides an efficient carrier reservoir and gives rise to a large α-parameter. Combined with a high modal gain and an ultrafast gain recovery, this makes the submonolayer quantum dots an attractive gain medium for nonlinear optical signal processing