We demonstrate the combination of a hemispherical solid immersion lens with a micro– photoluminescence set–up. Two advantages introduced by the SIL, an improved resolution of 0.4 times the wavelength in vacuum and a 5 times enhancement of the collection efficiency, make it an ideal system for spatially resolved spectroscopy applications. The influence of the air gap between the SIL and the sample surface is investigated in detail. We confirm the tolerance of the set–up to an air gap of several micrometers. Such a system is proven to be ideal system in the studies of exciton transport and polarization dependent single quantum dot spectroscopy.

The spatial density profile of a low-density exciton ensemble in ZnSe quantum wells shows a breathinglike oscillation on a 30-ps time scale. This breathing results from the emission of the first acoustic phonon at the end of the quasiballistic transport phase of the excitons which reverses their direction of propagation. Since the scattering destroys the phase of the excitonic wave function, one can deduce simultaneously the coherence length and the coherence time of excitonic transport by evaluation of the oscillation measured from a single experiment. The breathing, which can be modeled by Monte Carlo simulations, is quenched for rising lattice temperature, i.e., increasing phonon absorption, and in samples with significant disorder. These results were obtained by time-resolved nanophotoluminescence with 5 ps and 250 nm temporal and spatial resolution, respectively.

The transport of quantum-well excitons is investigated in the model system ZnSe on the length scale of the light wavelength and on the timescale of the first inelastic scattering processes. The experiments are based on confocal microscopy including a solid immersion lens in the setup to achieve a spatial resolution of 250nm.The excitons propagate in the quantum-well plane quasi-ballistically for several 100nm only affected by elastic scattering due to interface disorder. The first inelastic scattering process, which destroys the coherence of the exciton wave function, is the interaction with acoustic phonons after about 30ps. This process results in the reversal of the excitonic propagation direction which is directly reflected by a spatial oscillation in the temporal evolution of the photoluminescence spot. From the oscillation one can deduce simultaneously the coherence time and length of the excitonic transport. We describe the transition to diffusive exciton transport with increasing temperature and the influence of strong interface disorder.

We measured the transport dynamics of excitons in ZnSe quantum wells with help of a time-resolved nanophotoluminescence setup. Right after picosecond excitation, the excitons move out of the laser spot, but then they reverse the direction of propagation, and finally spread out again. We attribute this "spatial breathing" of the exciton population to acoustic phonon emission, which is a predominantly backward scattering event. This permits us to detect the first inelastic scattering event after the fast excitation generation, causing the end of the coherent transport regime. Using this method, we can determine simultaneously the coherence time and length of excitons in an 8nm well to 29 ps and 800 nm, respectively. We can reproduce the results with help of a Monte-Carlo simulation of the exciton dynamics in the well. The obtained values for coherence length and time are consistent with other, independent experiments. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We present a quantitative interpretation of the anomalous temperature behavior-the so-called "S-shape dependence"-of the photoluminescence (PL) in quantum films containing nanoislands. Experimental data for CdSe/ZnSe samples are modeled using Monte Carlo simulations of the involved relaxation mechanisms and thus providing a realistic picture of the exciton kinetics. We are able to reproduce simultaneously the temperature dependence of the PL maximum and of the full width at half maximum of the PL with good accuracy. We deduce information about the distribution of the localization centers and identify hopping processes between spatially separated states within one island.

We have studied the temperature dependence of the photoluminescence (PL) spectra of molecular beam epitaxy grown ultrathin Zn1-xCdxSe/ZnSe quantum wells with random and inhomogeneous Cd distributions over cation sublattice within the temperature interval 2-300 K. Depending on the Cd concentration, the PL band maximum position E-max(PL)(T) follows either a "normal" or an "anomalous" (known as "S-shaped") temperature dependence. We have analyzed both dependences in detail for a model of an island ensemble which can be characterized by a single-mode distribution of the most important parameters governing the optical properties of the quantum well. We demonstrate that the anomalous behavior arises due to the strong temperature dependence of the lifetimes of a family of metastable states participating in formation of the PL band at low temperatures. The metastablility of some island states is ascribed to a complex topological structure of the islands. The mechanism of the exciton-phonon interaction responsible for the fast decrease of the lifetime of these states with the increase of temperature has the same origin as the mechanism leading to the vanishing of narrow lines in mu-PL. We also present results of time-resolved experiments which yield the shift of the PL band for hot excitons cooling in a cold lattice.

We demonstrate a far-field nano-photoluminescence setup based on the combination of a hemispherical solid immersion lens (SIL) with a confocal microscope. The spatial resolution is confirmed to be 0.4 times the wavelength in vacuum in terms of half width at half maximum. The collection efficiency is found to be about five times higher than the same microscope without SIL, which is consistent with our theoretical analysis. We investigate in detail the influence of an air gap between the SIL and the sample surface on the system performance, and prove both experimentally and theoretically the tolerance of this far-field system to an air gap of several micrometers. These features make the present setup an ideal system for spatially resolved spectroscopy of semiconductor nanostructures. In particular, we show two examples of such applications in which the present setup is clearly suitable: Studies of excitonic transport in quantum wells and spectroscopy of single quantum dots with emphasis on polarization dependence and weak-signal detection.

We demonstrate an efficient method to simultaneously measure both, the coherence length and time, of quantum-well excitons based on photoluminescence with high spatial and temporal resolution.

We investigate the lateral transport of excitons in ZnSe quantum wells by using time-resolved micro-photoluminescence enhanced by the introduction of a solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps, respectively. Strong deviation from classical diffusion is observed up to 400 ps. This feature is attributed to the hot-exciton effects, consistent with previous experiments under cw excitation. The coupled transport-relaxation process of hot excitons is modelled by Monte Carlo simulation. We prove that two basic assumptions typically accepted in photoluminescence investigations on excitonic transport, namely (i) the classical diffusion model as well as (ii) the equivalence between the temporal and spatial evolution of the exciton population and of the measured photoluminescence, are not valid for low-temperature experiments.

In this contribution, a Monte-Carlo computer simulation of phonon-assisted relaxation processes of excitons in CdSe/ZnSe quantum islands is presented. With the same set of parameters, it was possible to reproduce both the temperature dependence of the energy position and of the full width at half maximum (FWHM) of the luminescence emitted from a ZnCdSe quantum film containing 5 to 10 nm wide Cd-rich islands. With help of the model, the energy dependence of the distribution of the localized states is shown to be weaker than Gaussian. Furthermore, the obtained fitting parameters give evidence for an intra-island excitonic relaxation. Finally, our results, based on a realistic description of the exciton kinetics, are compared with those of a rate equation model.

Reaching and perusing the quantum limit in conventional devices like memory units is the goal of the current research to allow a further increase in device performance while reducing the device size. While lithographic processes are still widely used for device patterning, these techniques result in corrugated surfaces, inducing undesired surface recombination centers. Thus they are not suitable for high quality, high density quasi-zero dimensional systems with lateral dimensions typically below 30 nm. A self-organization of quantum dots is the alternative. The self-organization of colloidal chalcogenide quantum dots (QD) from liquid solutions or in a glass melt is known since the 1930-ies and has lead to applications like optical absorbers and filters, as the average size of the quantum dot ensemble determines the absorption edge. While a very high density and homogeneity of QD sizes can be achieved, the. matrix these QDs are embedded in is generally non-conducting and does not allow to build electrically driven devices.In recent years epitaxial techniques have been developed that allow the embedding of QDs in a semiconducting matrix. These QDs are induced by driving forces that are determined e.g. by the interface energy and the lattice mismatch between the QD and the matrix-material, a perfect crystallinity provided.On the example of CdSe islands embedded in ZnSe we shall discuss the outstanding physical properties of such quasi zero dimensional island structures but also the difficulties in their fabrication.

We report investigations on excitonic transport in ZnSe quantum wells using far-field nano-photoluminescence enhanced by a solid immersion lens. The 250-nm spatial resolution and 5-ps temporal resolution allow the access to non-classical transport regimes. From zero-phonon-line spectroscopy we deduce the spatial distribution of the PL intensity under different cw excitation condition and its temporal evolution after a pulsed excitation, respectively. The periodic quenching of the transport length as function of excitation excess energy and the nonlinear expansion of the PL spot observed in time-resolved experiments reveal the dominance of hot-exciton effects. The experimental data are well modelled by a Monte Carlo simulation. Spatially resolved phonon-sideband spectroscopy is used to observe directly the coherent transport and the energy relaxation of hot excitons during transport. The coherence length is measured to be 300-400 nm at low temperatures and the excitons are found to remain hot during transport on a length scale of several micrometers.

physica status solidi (c)Volume 0, Issue 5 p. 1544-1547 Original Paper Exciton states and energy relaxation in ZnCdSe nano-islands A. Reznitsky, Corresponding Author A. Reznitsky alexander.reznitsky@physik.uni-karlsruhe.de Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaPhone: +49-721 608-7479, Fax: +49-721 608-8480Search for more papers by this authorA. Klochikhin, A. Klochikhin Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany Petersburg Nuclear Physics Institute, 188350 St. Petersburg, RussiaSearch for more papers by this authorH. Priller, H. Priller Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorB. Dal Don, B. Dal Don Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorG. Schwartz, G. Schwartz Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Zhao, H. Zhao Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Kalt, H. Kalt Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorC. Klingshirn, C. Klingshirn Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorS. Permogorov, S. Permogorov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorL. Tenishev, L. Tenishev A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorI. Sedova, I. Sedova A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Sorokin, S. Sorokin A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Ivanov, S. Ivanov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this author A. Reznitsky, Corresponding Author A. Reznitsky alexander.reznitsky@physik.uni-karlsruhe.de Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaPhone: +49-721 608-7479, Fax: +49-721 608-8480Search for more papers by this authorA. Klochikhin, A. Klochikhin Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, Germany Petersburg Nuclear Physics Institute, 188350 St. Petersburg, RussiaSearch for more papers by this authorH. Priller, H. Priller Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorB. Dal Don, B. Dal Don Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorG. Schwartz, G. Schwartz Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Zhao, H. Zhao Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorH. Kalt, H. Kalt Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorC. Klingshirn, C. Klingshirn Institut für Angewandte Physik, Universität Karlsruhe, 76128 Karlsruhe, GermanySearch for more papers by this authorS. Permogorov, S. Permogorov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorL. Tenishev, L. Tenishev A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorI. Sedova, I. Sedova A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Sorokin, S. Sorokin A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this authorS. Ivanov, S. Ivanov A. F. Ioffe Physico-Technical Institute, 194021 St. Petersburg, RussiaSearch for more papers by this author First published: 04 August 2003 https://doi.org/10.1002/pssc.200303227Citations: 6AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract We suggest a new concept of exciton states in nano-islands based on the study of optical spectra, namely the temperature dependence of cw photoluminescence (PL) and PL excitation (PLE), as well as PL spectra and kinetics under ps pulsed excitation. The scheme of the exciton states in nano-islands is as follows: (I) deep spatially isolated ground states, (II) excited meta-stable states the energy relaxation rate of which strongly depends on the temperature, and (III) the states extending over the whole island size. We show that a redistribution of the population between meta-stable and ground states is responsible for the anomalous temperature shift of the PL band in QWs with islands. Citing Literature Volume0, Issue5August 2003Pages 1544-1547 RelatedInformation

We monitor directly non-classical transport of quantum-well excitons. The excitons propagate coherently up to four hundreds nanometers, followed by a hot-exciton transport on a length scale of several micrometers at low temperatures.

We present a novel experimental setup, which introduces a solid-immersion-lens (SIL) into a confocal microphotoluminescence system. This non-destructive method allows us, within a field of view of 35 mum, to reach a spatial resolution of about 200 nm, which is comparable to near-field systems. We demonstrate an enhancement of the collection efficiency by a factor of five in comparison to standard confocal setups. This is very important for single-dot spectroscopy, where the excitation and detection signals are very low. Moreover, we prove that the spatial resolution and collection efficiency of the system are quite insensitive to air gaps which could be formed at the interface between sample and SIL. These two features can be explained with theoretical considerations. For all these reasons, the system is very suitable for the study of single excitonic lines in quantum dots. We show its application in polarization- and temperature-dependent studies.

Combining a low-temperature scanning near-field optical microscope with a picosecond streak camera allows us to measure the complete wavelength-time behavior at one spot on the sample within about 13 min at excitation powers of 100 nW. We use this instrument to measure the variation of relaxation times in disordered single-GaAs quantum wells with sample position.

We report on relaxation of excitons in II-VI heterostructures containing CdSe islands embedded in a ZnSe matrix, and showing different localization properties. We perform spatially resolved photoluminescence (mu-PL), time-resolved PL (TRPL), and time-resolved near-field spectroscopy (n-TRPL) to study the differences between two samples. In the sample showing the weaker localization, we see a correlation between the localized states, which is not obvious in the sample with the stronger localization. We consider as possible explanations for these repeated lines phonon-assisted transport as well as relaxation from excited states into a lower energetic state.

We report on relaxation studies of excitons in quantum islands using time-resolved near-field photoluminescence (n-TRPL). This new method allows us to follow the temporal evolution of an ensemble of localized states with a spatial resolution of about 200 nm. The temporal and spectral resolutions are 5 ps and 1.7 meV, respectively. We study the relaxation mechanisms of excitons in two samples containing CdSe/ZnSe quantum islands. In the first sample, where the excitons are weaker localized, the sharp lines of the n-TRPL are repeated at a constant distance of 25 meV This is not the case in the second sample, where the excitons are stronger localized. We discuss the origin of this correlation, comparing the dynamics of the lines.

We investigatetheexcitonic transportprocessin a ZnSequantumwell by micro– photoluminescence.A solid immersionlens is introducedinto the system,improving the resolutionto 200nmandthecollectionefficiency by afactorof five. Weexploit boththedirect couplingof coldexcitonsto photonsandtheLO–phononassistedexciton–photoncouplingfor the investigationsof coherentandhot exciton effects. Thecoherenttransportregimeextends up to a lengthscaleof 400nm. After the decoherence, the transportprocessis provento be dominatedby hot excitoneffecton a lengthscaleof severalmicrometers.

Photoluminescence of the ground and excited states of type II CdS/ZnSe single quantum wells with different width is investigated for high electron densities and by means of time resolved photoluminescence measurements. For high excitation intensities the filling of states in the QW up to energies of the second excited state can be observed. The PL decay times of the ground state vary from 2-36 ns for different samples and depend strongly on the QW width. This dependence can be explained with the overlap of electron and hole wavefunctions. The PL rise time of the ground state is influenced by inter- and intrasubband relaxation, which is induced by the cooling of hot carriers.