Christian Strelow

Optical Modes Excited by Evanescent-Wave-Coupled PbS Nanocrystals in Semiconductor Microtube Bottle Resonators

We report on optical modes in rolled-up microtube resonators that are excited by PbS nanocrystals filled into the microtube core. Long ranging evanescent fields into the very thin walled microtubes cause strong emission of the nanocrystals into the resonator modes and a mode shift after a self-removal of the solvent. We present a method to precisely control the number, the energy and the localization of the modes along the microtube axis.

Diameter scaling of the optical band gap in individual CdSe nanowires.

The diameter dependence of the optical band gap of single CdSe nanowires (NWs) is investigated by a combination of atomic force microscopy, scanning fluorescence microscopy, and transmission electron microscopy. We find a good congruence of the experimental data to calculations within the effective mass approximation taking into account quantization, exciton Coulomb interaction, and dielectric mismatch. The experimental data are furthermore compared to different theoretical approaches. We discuss the influence of alternating wurtzite and zinc blende segments along the NWs on their optical properties.

Large array of single, site-controlled InAs quantum dots fabricated by UV-nanoimprint lithography and molecular beam epitaxy

We present the growth of single, site-controlled InAs quantum dots on GaAs templates using UV-nanoimprint lithography and molecular beam epitaxy. A large quantum dot array with a period of 1.5 µm was achieved. Single quantum dots were studied by steady-state and time-resolved micro-photoluminescence experiments. We obtained single exciton emission with a linewidth of 45 µeV. In time-resolved experiments, we observed decay times of about 670 ps. Our results underline the potential of nanoimprint lithography and molecular beam epitaxy to create large-scale, single quantum dot arrays.

Metal-Semiconductor Nanoparticle Hybrids Formed by Self-Organization: A Platform to Address Exciton-Plasmon Coupling.

Hybrid nanosystems composed of excitonic and plasmonic constituents can have different properties than the sum of of the two constituents, due to the exciton-plasmon interaction. Here, we report on a flexible model system based on colloidal nanoparticles that can form hybrid combinations by self-organization. The system allows us to tune the interparticle distance and to combine nanoparticles of different sizes and thus enables a systematic investigation of the exciton-plasmon coupling by a combination of optical spectroscopy and quantum-optical theory. We experimentally observe a strong influence of the energy difference between exciton and plasmon, as well as an interplay of nanoparticle size and distance on the coupling. We develop a full quantum theory for the luminescence dynamics and discuss the experimental results in terms of the Purcell effect. As the theory describes excitation as well as coherent and incoherent emission, we also consider possible quantum optical effects. We find a good agreement of the observed and the calculated luminescence dynamics induced by the Purcell effect. This also suggests that the self-organized hybrid system can be used as platform to address quantum optical effects.

Clustering of CdSe/CdS Quantum Dot/Quantum Rods into Micelles Can Form Bright, Non-blinking, Stable, and Biocompatible Probes.

We investigate clustered CdSe/CdS quantum dots/quantum rods, ranging from single to multiple encapsulated rods within amphiphilic diblock copolymer micelles, by time-resolved optical spectroscopy. The effect of the clustering and the cluster size on the optical properties is addressed. The clusters are bright and stable and show no blinking while retaining the fundamental optical properties of the individual quantum dots/quantum rods. Cell studies show neither unspecific uptake nor morphological changes of the cells, despite the increased sizes of the clusters.

Quantum-confined emission and fluorescence blinking of individual exciton complexes in CdSe nanowires.

One-dimensional semiconductor nanostructures combine electron mobility in length direction with the possibility of tailoring the physical properties by confinement effects in radial direction. Here we show that thin CdSe quantum nanowires exhibit low-temperature fluorescence spectra with a specific universal structure of several sharp lines. The structure strongly resembles the pattern of bulk spectra but show a diameter-dependent shift due to confinement effects. Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in one and the same spectrum. Time- and space-resolved optical spectroscopy are combined with high-resolution transmission electron microscopy of the very same quantum nanowires to establish a detailed structure-property relationship. Extensive numerical simulations strongly suggest that excitonic complexes involving donor and acceptor sites are the origin of the feature-rich spectra.

Synthesis of Carbon Nanowalls and Few-Layer Graphene Sheets on Transparent Conductive Substrates

Abstract We report on the growth of carbon nanowalls and few-layer graphene sheets on glass substrates coated with different transparent conductive oxides or gold. The growth is accomplished by a capacitively-coupled radio-frequency plasma-enhanced chemical vapor deposition setup with a gas mixture of C2H2, H2, and Ar or He. This system allows the synthesis of thin vertical carbon nanosheets without catalyst at mild reaction conditions due to the low plasma density, which is preferable for sensitive substrate materials. In fact, the electrical properties of the transparent conductive indium tin oxide and fluorine-doped tin oxide stay intact upon carbon growth. Carbon nanowalls and few-layer graphene sheets on transparent conductive oxides are promising candidates for solar-cell applications while these nanostructures on gold coated glass can act as catalyst support material.

The influence of temperature on the photoluminescence properties of single InAs quantum dots grown on patterned GaAs

We report the temperature-dependent photoluminescence of single site-controlled and self-assembled InAs quantum dots. We have used nanoimprint lithography for patterning GaAs(100) templates and molecular beam epitaxy for quantum dot deposition. We show that the influence of the temperature on the photoluminescence properties is similar for quantum dots on etched nanopatterns and randomly positioned quantum dots on planar surfaces. The photoluminescence properties indicate that the prepatterning does not degrade the radiative recombination rate for the site-controlled quantum dots.

Light Confinement in Microtubes

We review recent developments in the field of light confinement in semiconductor microtube resonators fabricated by utilizing the self-rolling mechanism of strained bilayers. We discuss resonant optical modes in the framework of a waveguide model that naturally explains the occurrence of two-dimensional ring modes by constructive interference of light azimuthally guided by the tube wall. Experiments show that diverse geometries of a microtube have strong impact on the emission properties, including preferential and directional emission, as well as on a three-dimensional light confinement. We show that by lithographically structuring the microtube, it is possible to reach a three-dimensional confinement in a fully controlled way. The evolving confined modes can be described by an intuitive model using an expanded waveguide approach together with an adiabatic separation of the circulating and the axial light propagation.

Hybrid systems of AlInP microdisks and colloidal CdSe nanocrystals showing whispering-gallery modes at room temperature

We report on the realization of hybrid systems composed of passive optical microdisk resonators prepared from epitaxial layer systems and nanocrystal quantum emitters synthesized by colloidal chemistry. The AlInP disk material allows for the operation in the visible range, as probed by CdSe-based nanocrystals. Photoluminescence spectra at room temperature reveal sets of whispering-gallery modes consistent with finite-difference time-domain simulations. In the experiments, a special sample geometry renders it possible to detect resonant optical modes perpendicular to the disk plane.