Sebastian Bär

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. In contrast to the emission transition dipole moment, which is oriented in one plane, the excitation transition dipole moment of a single CdSe/ZnS quantum dots possesses an isotropy in three dimensions.

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. We observe a broad scattering of quantum yield values from molecule to molecule, which reflects the strong variation of the local interaction of the observed molecules with their host environment.

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

AbstractWe 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 λ/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. As a result a modification of the optical properties of these quantum emitters can be observed. We present experimental as well as theoretical results illustrating this effect. Furthermore, the developed resonator can be used to determine the longitudinal position of embedded emitters with an accuracy of λ/60 by analyzing the excitation patterns of nano-sized fluorescent polymer spheres after excitation with a radially polarized doughnut mode laser beam. Finally, we will apply this resonator to a biological system and demonstrate the modification of Förster resonant energy transfer (FRET) efficiency by inhibiting the excited state energy transfer from the donor to the acceptor chromophore of a single DsRed protein. FigureEffect of a microresonator on single quantum emitters (from left to right): PI molecule or DsRed protein invesitigated in a microresonator with resulting exciation patterns of the PI molecule after exciation with a radially polarized laser beam or the cavity-controlled emisison spectrum of DSRed in comparison with its free space spectrum (hatched). The background shows the Newton rings of the microrsonator.

Controlling the interaction of photons and single molecules in a λ/2-microresonator

Optical microresonators are structures which confine light to a small region in the range of one wavelength. A practical design for a single-mode microresonator is formed by two silver mirrors enclosing a transparent dielectric medium with single quantum emitters. The radiation of a quantum emitter is coupled to cavity resonances which leads to an optical confinement of the broadband fluorescence. Using a tunable microresonator, we were able to actively change the optical properties of an embedded single molecule. The radiative coupling of the emitter to the electromagnetic field is also determined by the orientation of its transition dipole moment with respect to the cavity normal. We describe here a method to determine the 3D-orientation and position of quantum emitters embedded in the microresonator. In addition, this method can be used to detect the longitudinal position of a fluorescent bead in the microresonator with an accuracy of a few nanometers. We will also present first experiments on Förster Resonance Energy Transfer (FRET) control on the DsRed protein system in a microresonator.

Controlling the optical properties of quantum emitters by optical confinement in a tunable microcavity

Optical microresonators are structures which confine light to a small region in the range of one wavelength. The radiation of a quantum emitter is coupled to cavity resonances which leads to an optical confinement of the broadband fluorescence. A practical design for this single-mode microresonator is formed by two silver mirrors enclosing a transparent dielectric medium with single quantum emitters. In our tunable microresonator, the resonator length can be changed reversibly with piezoelectric elements to a distinct position corresponding to a specific emission wavelength. The local mode structure of the electromagnetic field is changed at this position which results in a redistribution of the fluorescence and a modification of the lifetime for the same single molecule. The radiative coupling of the emitter to the electromagnetic field is also determined by the orientation of its transition dipole moment with respect to the cavity normal. The doughnut laser modes used for illumination of the single molecule allow us by analyzing the excitation patterns to determine its three-dimensional orientation in the microresonator. In addition, these modes provide an excitation pattern which can be used to detect the longitudinal position of a fluorescent bead in the microresonator with an accuracy of a few nanometers.

Controlling the optical properties of single molecules by optical confinement in a tunable microcavity

Optical microcavities are structures which confine light to a small region in the range of one wavelength. The radiation of a quantum emitter is coupled to cavity resonances which leads to an optical confinement of the broadband fluorescence [Fig. 1a, b]. A practical design for this single-mode microcavity is formed by two silver mirrors enclosing a transparent dielectric medium with single quantum emitters. Steiner et al. have shown that the fluorescence spectra and decay lifetimes of single molecules in this Fabry-Perot type microcavity are strongly dependent on the resonator length [1].