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. 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 lambda/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.

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. Using an argon ion laser at 488 nm with higher-order laser modes (azimuthally and radially polarized doughnut modes) for excitation, the three-dimensional orientation of the nanoparticles' transition dipole moment was investigated in a confocal microscope. The linear transition dipole moment was found to be rather stable and randomly oriented. However, dynamical effects such as fluorescence intermittency and transition dipole moment flipping could also be observed. The spectral analysis of single SiO(2) NPs revealed double-peak spectra consisting of a narrow zero-phonon line and a broader phonon band being associated with the excitation of longitudinal optical phonons in the SiO(2) NP.

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.

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. Because mode structure and coupling are both frequency dependent, this leads to a renormalization of the emission spectrum of the molecule. We develop a theoretical model for these spectral changes and find excellent agreement between theoretical prediction and experimental results.

Tight focusing of laser beams in a lambda/2-microcavity.

We evaluate the field distribution in the focal spot of the fundamental Gaussian beam as well as radially and azimuthally polarized doughnut beams focused inside a planar metallic sub-wavelength microcavity using a high numerical aperture objective lens. We show that focusing in the cavity results in a much tighter focal spot in longitudinal direction compared to free space and in spatial discrimination between longitudinal and in-plane field components. In order to verify the modeling results we experimentally monitor excitation patterns of fluorescence beads inside the lambda/2-cavity and find them in full agreement to the modeling predictions. We discuss the implications of the results for cavity assisted single molecular spectroscopy and intra-cavity single molecular imaging.