Raphael Gutbrod

Tight focusing of laser beams in a λ/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 λ/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.

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.

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

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].

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.