Hilmar Gugel

Closing the spectral gap : the transition from fixed-parameter fluorescence to tunable devices in confocal microscopy

Modern microscopy in life sciences is ruled by development and exploration of new dyes and stains (probes for histochemical staining, quantum dots, fluorescent proteins etc.) on one side, and technological improvements and innovations for fluorescence microscopy-especially high resolution and optical sectioning microscopy-on the other side. Concerning the technical innovations, several ingenious inventions have been made available for confocal microscopy. First, the acousto optical tunable filter, which allows switching and dimming of laser lines. Second the spectral detector, employing mirror sliders in front of the detectors which allow continuous tuning of the spectral emission band detected by the sensor. Third, the most challenging task: a substitute to the classical beam splitter-the device which is restricting fluorescence microscopy most. This was solved by introduction of the acousto optical beam splitter. The very last device which is still lacking flexibility is the laser source, operating only at non-equidistant frequencies and requiring a set of quite different laser sources as gas lasers, solid state lasers or diode lasers. A new approach by supercontinuum light sources is presented and discussed, which significantly enhances flexibility and coverage of the excitation spectra of typical, rare and natural fluorochromes.

Confocal imaging at the nanoscale with two-color STED microscopy

STED microscopy enables confocal imaging of biological samples with a resolution that is not limited by diffraction. It provides new insights in various fields of biology, such as membrane biology, neurobiology and physiology. Its three dimensional sectioning ability allows the acquisition of high resolution image stacks. Furthermore, STED microscopy enables the recording of dynamic processes and live cell images. We present two-color imaging in confocal STED microscopy with a single STED wavelength. Pulsed and continuous wave lasers in the visible and near infra-red wavelengths range are used for stimulated emission. The resolution enhancement is demonstrated in comparison to confocal imaging with biological specimens.

New developments in STED Microscopy

STED microscopy has gained recognition as a method to break the diffraction limit of conventional light microscopy. Despite being a new technique, STED is already successfully implemented in life science research. The resolution enhancement is achieved by depleting fluorescent markers via stimulated emission. The performance is significantly dependent on the laser source and the fluorescence markers. Therefore the use of novel fluorescent markers in conjunction with the right laser system was the main focus of our research. We present new developments and applications of STED microscopy, unraveling structural details on scales below 90nm and give an overview of required specifications for the solid state laser systems.

Compact 4pi beam scanning microscope for imaging cells with a sevenfold increase in axial resolution

Confocal microscopy is the method of choice in biological 3D-imaging, however, the axial resolution is limited to ~500 nm. During the last decade it has been successfully demonstrated that the axial resolution can be substantially improved with 4Pi microscopy. We report a 4Pi microscope realized as a fast beam scanning system consisting of a 4Pi-module linked to a state-of-the-art confocal microscope. As a result, the advantages of the confocal system such as scanning, sensitive multicolor detection and imaging speed are combined with the superior resolution of 4Pi microscopy. This novel microscope is eminently suited for biological applications. It is designed both for single-photon and for two-photon picosecond excitation, and also enables joint coherent illumination and detection, i.e. 4Pi type C. The superior PSF and OTF of the system enable 80 nm axial resolution in cells mounted in aqueous media. We present the optical design of the system and demonstrate an up to 7-fold improved optical sectioning in live cells.