Excitation Intensity-Dependent Quantum Yield of Semiconductor Nanocrystals.

One of the key phenomena that determine the fluorescence of nanocrystals is the nonradiative Auger-Meitner recombination of excitons. This nonradiative rate affects the nanocrystals' fluorescence intensity, excited state lifetime, and quantum yield. Whereas most of the above properties can be directly measured, the quantum yield is the most difficult to assess.

Isotropic three-dimensional dual-color super-resolution microscopy with metal-induced energy transfer.

Over the past two decades, super-resolution microscopy has seen a tremendous development in speed and resolution, but for most of its methods, there exists a remarkable gap between lateral and axial resolution, which is by a factor of 2 to 3 worse. One recently developed method to close this gap is metal-induced energy transfer (MIET) imaging, which achieves an axial resolution down to nanometers. It exploits the distance-dependent quenching of fluorescence when a fluorescent molecule is brought close to a metal surface.

Time-resolved MIET measurements of blood platelet spreading and adhesion.

Human blood platelets are non-nucleated fragments of megakaryocytes and of high importance for early hemostasis. To form a blood clot, platelets adhere to the blood vessel wall, spread and attract other platelets. Despite the importance for biomedicine, the exact mechanism of platelet spreading and adhesion to surfaces remains elusive.

Dual-Color Metal-Induced Energy Transfer (MIET) Imaging for Three-Dimensional Reconstruction of Nuclear Envelope Architecture.

The nuclear envelope, comprising the inner and the outer nuclear membrane, separates the nucleus from the cytoplasm and plays a key role in cellular functions. Nuclear pore complexes (NPCs) are embedded in the nuclear envelope and control transport of macromolecules between the two compartments. Recently, it has been shown that the axial distance between the inner nuclear membrane and the cytoplasmic side of the NPC can be measured using dual-color metal-induced energy transfer (MIET).

Excitation and Emission Transition Dipoles of Type-II Semiconductor Nanorods.

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods.

Three-dimensional single-molecule localization with nanometer accuracy using Metal-Induced Energy Transfer (MIET) imaging.

Our paper presents the first theoretical and experimental study using single-molecule Metal-Induced Energy Transfer (smMIET) for localizing single fluorescent molecules in three dimensions. Metal-Induced Energy Transfer describes the resonant energy transfer from the excited state of a fluorescent emitter to surface plasmons in a metal nanostructure. This energy transfer is strongly distance-dependent and can be used to localize an emitter along one dimension.

Dual-color metal-induced and Förster resonance energy transfer for cell nanoscopy.

We report a novel method, dual-color axial nanometric localization by metal--induced energy transfer, and combine it with Förster resonance energy transfer (FRET) for resolving structural details in cells on the molecular level. We demonstrate the capability of this method on cytoskeletal elements and adhesions in human mesenchymal stem cells. Our approach is based on fluorescence-lifetime-imaging microscopy and allows for precise determination of the three-dimensional architecture of stress fibers anchoring at focal adhesions, thus yielding crucial information to understand cell-matrix mechanics.

Charge-Driven Fluorescence Blinking in Carbon Nanodots.

This study focuses on the mechanism of fluorescence blinking of single carbon nanodots, which is one of their key but less understood properties. The results of our single-particle fluorescence study show that the mechanism of carbon nanodots blinking has remarkable similarities with that of semiconductor quantum dots. In particular, the temporal behavior of carbon nanodot blinking follows a power law both at room and at cryogenic temperatures.

Three-Dimensional Reconstruction of Nuclear Envelope Architecture Using Dual-Color Metal-Induced Energy Transfer Imaging.

The nuclear envelope, comprising the inner and the outer nuclear membrane, separates the nucleus from the cytoplasm and plays a key role in cellular functions. Nuclear pore complexes (NPCs), which are embedded in the nuclear envelope, control transport of macromolecules between the two compartments. Here, using dual-color metal-induced energy transfer (MIET), we determine the axial distance between Lap2β and Nup358 as markers for the inner nuclear membrane and the cytoplasmic side of the NPC, respectively.

Dead-time correction of fluorescence lifetime measurements and fluorescence lifetime imaging.

We present a comprehensive theory of dead-time effects on Time-Correlated Single Photon Counting (TCSPC) as used for fluorescence lifetime measurements, and develop a correction algorithm to remove these artifacts. We apply this algorithm to fluorescence lifetime measurements as well as to Fluorescence Lifetime Imaging Microscopy (FLIM), where rapid data acquisition is necessarily connected with high count rates. There, dead-time effects cannot be neglected, and lead to distortions in the observed lifetime image.

Super-Resolution Optical Fluctuation Bio-Imaging with Dual-Color Carbon Nanodots.

Success in super-resolution imaging relies on a proper choice of fluorescent probes. Here, we suggest novel easily produced and biocompatible nanoparticles-carbon nanodots-for super-resolution optical fluctuation bioimaging (SOFI). The particles revealed an intrinsic dual-color fluorescence, which corresponds to two subpopulations of particles of different electric charges.

Simultaneous Measurement of the Three-Dimensional Orientation of Excitation and Emission Dipoles.

The emission properties of most fluorescent emitters, such as dye molecules or solid-state color centers, can be well described by the model of an oscillating electric dipole. However, the orientations of their excitation and emission dipoles are, in most cases, not parallel. Although single molecule excitation and emission dipole orientation measurements have been performed in the past, no experimental method has so far looked at the three-dimensional excitation and emission dipole geometry of individual emitters simultaneously.

Photoluminescence of a single quantum emitter in a strongly inhomogeneous chemical environment.

We present the results of a comprehensive photoluminescence study of defect centres in single SiO2 nanoparticles. We show that the photo-physical properties of the luminescent centres strongly resemble those of single dye molecules. However, these properties exhibit a large variability from particle to particle due to the different local chemical environment around each centre of each particle.

Photoluminescence of carbon nanodots: dipole emission centers and electron-phonon coupling.

Inorganic carbon nanomaterials, also called carbon nanodots, exhibit a strong photoluminescence with unusual properties and, thus, have been the focus of intense research. Nonetheless, the origin of their photoluminescence is still unclear and the subject of scientific debates. Here, we present a single particle comprehensive study of carbon nanodot photoluminescence, which combines emission and lifetime spectroscopy, defocused emission dipole imaging, azimuthally polarized excitation dipole scanning, nanocavity-based quantum yield measurements, high resolution transmission electron microscopy, and atomic force microscopy.

Single-molecule metal-induced energy transfer (smMIET): resolving nanometer distances at the single-molecule level.

We present a new concept for measuring distance values of single molecules from a surface with nanometer accuracy using the energy transfer from the excited molecule to surface plasmons of a metal film. We measure the fluorescence lifetime of individual dye molecules deposited on a dielectric spacer as a function of a spacer thickness. By using our theoretical model, we convert the lifetime values into the axial distance of individual molecules.

Local refractive index probed via the fluorescence decay of semiconductor quantum dots.

We present a novel approach for convenient tuning of the local refractive index around nanostructures. We apply this technique to study the influence of the local refractive index on the radiative decay time of CdSe/ZnS quantum dots with three distinct emission wavelengths. The dependence of the luminescence decay time on the environment is well described by an effective medium approach.

Probing the radiative transition of single molecules with a tunable microresonator.

Using a tunable optical microresonator with subwavelength spacing, we demonstrate controlled modulation of the radiative transition rate of a single molecule, which is measured by monitoring its fluorescence lifetime. Variation of the cavity length changes the local mode structure of the electromagnetic field, which modifies the radiative coupling of an emitting molecule to that field. By comparing the experimental data with a theoretical model, we extract both the pure radiative transition rate as well as the quantum yield of individual molecules.

Excitation isotropy of single CdSe/ZnS nanocrystals.

We study the dimensionality of the excitation transition dipole moment for single CdSe/ZnS core-shell nanocrystals using azimuthally and radially polarized laser modes. The comparison of measured and simulated single nanocrystal excitation patterns shows that single CdSe/ZnS quantum dots possess a spherically degenerated excitation transition dipole. We show that the dimensionality of the excitation transition dipole moment distribution is the same for all individual CdSe/ZnS nanocrystals, disregarding the difference in core size and irrespective of variations in the local environment.

Optical imaging of excited-state tautomerization in single molecules.

Tautomerism process of single fluorescent molecules was studied by means of confocal microscopy in combination with azimuthally or radially polarized laser beams. During a tautomerism process the transition dipole moment (TDM) of a molecule changes its orientation which can be visualized by the fluorescence excitation image of the molecule. We present experimental and theoretical studies of two porphyrazine-type molecules and one type of porphyrin molecule: a symmetrically substituted metal-free phthalocyanine and porphyrin, and nonsymmetrically substituted porphyrazine.

Three-dimensional orientation of single molecules in a tunable optical lambda/2 microresonator.

A tightly focused radially polarized laser beam forms an unusual bimodal field distribution in an optical lambda/2-microresonator. We use a single-molecule dipole to probe the vector properties of this field distribution by tuning the resonator length with nanometer precision. Comparing calculated and experimental excitation patterns provides the three-dimensional orientation of the single-molecule dipole in the microresonator.

Microcavities: tailoring the optical properties of single quantum emitters.

We present a general review of different microresonator structures and how they can be used in future device applications in modern analytical methods by tailoring the optical properties of single quantum emitters. The main emphasis is on the tunable lambda/2-Fabry-Perot-type microresonator which we used to obtain the results presented in this article. By varying the mirror distance the local mode structure of the electromagnetic field is altered and thus the radiative coupling of fluorescent single quantum emitters embedded inside the resonator to that field is changed, too.

Imaging and spectroscopy of defect luminescence and electron-phonon coupling in single SiO2 nanoparticles.

Silicon nanocrystals were synthesized by CO(2) laser pyrolysis of SiH(4). The fresh silicon nanopowder was oxidized in water to obtain SiO(2) nanoparticles (NPs) exhibiting strong red-orange photoluminescence. Samples of SiO(2) NPs embedded in low concentration in a thin polymer layer were prepared by spin-coating a dedicated solution on quartz cover slides.

Tuning the fluorescence emission spectra of a single molecule with a variable optical subwavelength metal microcavity.

We present experimental and theoretical results on changing the fluorescence emission spectrum of a single molecule by embedding it within a tunable planar microcavity with subwavelength spacing. The cavity length is changed with nanometer precision by using a piezoelectric actuator. By varying its length, the local mode structure of the electromagnetic field is changed together with the radiative coupling of the emitting molecule to the field.

Longitudinal localization of a fluorescent bead in a tunable microcavity with an accuracy of lambda/60.

The exact localization of a quantum emitter in a transparent dielectric medium is an important task in applications of precision confocal microscopy. Therefore we use a planar metallic subwavelength microcavity that can be reversibly tuned across the entire visible range, with the transparent medium between the cavity mirrors. By analyzing the excitation patterns resulting from the illumination of a single fluorescent bead with a radially polarized doughnut mode laser beam we can determine the longitudinal position of this bead in the microcavity with an accuracy of a few nanometers.