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

Population dependence of THz charge carrier mobility and non-Drude-like behavior in short semiconductor nanowires

We investigate THz radiation absorption by charge carriers, focusing on the mobility in nanorods and wires. We show that for short rods the mobility is limited by the high spacing of the charge carrier energy levels, while for longer wires (greater 25 nm) finite dephasing results in considerably higher low frequency mobility. Analyzing the length, temperature and population dependence, we demonstrate that, apart from the temperature dependent dephasing, the mobility becomes strongly charge carrier population dependent. The latter results in no simple linear relationship between carrier density and conductivity. Additionally their thermal distribution determines the mobility, measured in experiments. We further show that Drude or Plasmon models apply only for long wires at elevated temperatures, while for short length quantization results in considerable alterations. In contrast to those phenomenological models, i.e. a negative imaginary part of the frequency-dependent conductivity in a nanosystem can be understood microscopically. Based on the results, we develop guidelines to analyze 1D terahertz conductivity spectra. Our approach provides also a new tool to optimize the mobility by nanowire length as well as to analyze the dephasing, not by conventional wave mixing techniques, but by coherent optical pump-THz probe spectroscopy.TU Berlin, Open-Access-Mittel – 202

Sideband pump-probe technique resolves nonlinear modulation response of PbS/CdS quantum dots on a silicon nitride waveguide

For possible applications of colloidal nanocrystals in optoelectronics and nanophotonics, it is of high interest to study their response at low excitation intensity with high repetition rates, as switching energies in the pJ/bit to sub-pJ/bit range are targeted. We develop a sensitive pump-probe method to study the carrier dynamics in colloidal PbS/CdS quantum dots deposited on a silicon nitride waveguide after excitation by laser pulses with an average energy of few pJ/pulse. We combine an amplitude modulation of the pump pulse with phase-sensitive heterodyne detection. This approach permits to use co-linearly propagating co-polarized pulses. The method allows resolving transmission changes of the order of 10(-5) and phase changes of arcseconds. We find a modulation on a sub-nanosecond time scale caused by Auger processes and biexciton decay in the quantum dots. With ground state lifetimes exceeding 1 mu s, these processes become important for possible realizations of opto-electronic switching and modulation based on colloidal quantum dots emitting in the telecommunication wavelength regime

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

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

Temperature dependent radiative and non-radiative recombination dynamics in CdSe-CdTe and CdTe-CdSe type II hetero nanoplatelets

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 investigate the temperature-dependent decay kinetics of type II CdSe-CdTe and CdTe-CdSe core-lateral shell nanoplatelets. From a kinetic analysis of the photoluminescence (PL) decay and a measurement of the temperature dependent quantum yield we deduce the temperature dependence of the non-radiative and radiative lifetimes of hetero nanoplates. In line with the predictions of the giant oscillator strength effect in 2D we observe a strong increase of the radiative lifetime with temperature. This is attributed to an increase of the homogeneous transition linewidth with temperature. Comparing core only and hetero platelets we observe a significant prolongation of the radiative lifetime in type II platelets by two orders in magnitude while the quantum yield is barely affected. In a careful analysis of the PL decay transients we compare different recombination models, including electron hole pairs and exciton decay, being relevant for the applicability of those structures in photonic applications like solar cells or lasers. We conclude that the observed biexponential PL decay behavior in hetero platelets is predominately due to spatially indirect excitons being present at the hetero junction and not ionized e-h pair recombination

Tuning trion binding energy and oscillator strength in a laterally finite 2D system: CdSe nanoplatelets as a model system for trion properties

We present a theoretical study combined with experimental validations demonstrating that CdSe nanoplatelets are a model system to investigate the tunability of trions and excitons in laterally finite 2D semiconductors. Our results show that the trion binding energy can be tuned from 36 meV to 18 meV with the lateral size and decreasing aspect ratio, while the oscillator strength ratio of trions to excitons decreases. In contrast to conventional quantum dots, the trion oscillator strength in a nanoplatelet at low temperature is smaller than that of the exciton. The trion and exciton Bohr radii become lateral size tunable, e.g. from ∼3.5 to 4.8 nm for the trion. We show that dielectric screening has strong impact on these properties. By theoretical modeling of transition energies, binding energies and oscillator strength of trions and excitons and comparison with experimental findings, we demonstrate that these properties are lateral size and aspect ratio tunable and can be engineered by dielectric confinement, allowing to suppress e.g. detrimental trion emission in devices. Our results strongly impact further in-depth studies, as the demonstrated lateral size tunable trion and exciton manifold is expected to influence properties like gain mechanisms, lasing, quantum efficiency and transport even at room temperature due to the high and tunable trion binding energies.EC/H2020/714876/EU/Photonics in Flatland: Band Structure Engineering of 2D Excitons in Fluorescent Colloidal Nanomaterials/PHOCONATU Berlin, Open-Access-Mittel - 202

Two-photon based pulse autocorrelation with CdSe nanoplatelets

We investigate broadband two-photon absorption autocorrelators based on II–VI semiconductor nanoplatelets as an alternative to common second harmonic generation based techniques. As compared to bulk materials the exceptionally high enhancement of two-photon absorption in these 2D structures results in very efficient two-photon absorption based autocorrelation detected via PL emission. We compare the results with TPA autocorrelation in CdS bulk as well as SHG based autocorrelation in β-barium borate. We show that CdSe nanoplatelet based autocorrelation can exceed the efficiency of conventional methods by two orders in magnitude, especially for short interaction length, and allows a precise pulse-width determination. We demonstrate that very high two-photon absorption cross sections of the nanoplatelets are the basis for this effective TPA autocorrelation. Based on our results with II–VI nanoplatelets efficient broadband autocorrelation with more than ∼100 nm bandwidth and very high sensitivity seems feasible.EC/H2020/714876/EU/Photonics in Flatland: Band Structure Engineering of 2D Excitons in Fluorescent Colloidal Nanomaterials/PHOCONATU Berlin, Open-Access-Mittel - 201