Fluorescent dyes are the basis for a broad range of modern techniques in life and material sciences. Consequently, there
is a pressing need for the development of new classes of NIR fluorophores in recent years. Pyrrolopyrrole Cyanines
(PPCys) are a novel class of NIR chromophores that were first presented in 2007 by Fischer and coworkers. Their
optical properties are marked by strong and narrowband NIR absorptions, strong NIR fluorescence and hardly any
absorption in the visible range. The absorption maxima can be tuned over a broad range while high fluorescence
quantum yields are maintained. PPCys are attractive candidates for labelling applications or as selective NIR absorbers.
Moreover, PPCys exhibit very high photostability. Due to these outstanding photophysical properties, PPCys are heading
into a promising future as NIR dyes.
A two branch Er:fiber laser was developed for coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The
compact and highly stable light source allows for fast single-frequency CARS microspectroscopy with a wide tuning
range from 1150 cm<sup>-1</sup> up to 3800 cm<sup>-1</sup>. Single-pass frequency conversion enables easy tunability. The spectral selectivity
of the system is shown using polymer beads. Imaging of biological samples is demonstrated on <i>C. elegans </i>and yeast
cells. Modification of the light source allows for broadband background-free CARS microspectroscopy. Impulsive
excitation of molecular resonances is achieved using an 11 fs pulse at 1210 nm. Broadband excitation gives access to
molecular resonances from 0 cm<sup>-1</sup> up to 4000 cm<sup>-1</sup>. Time-delayed narrowband probing at 775 nm enables sensitive and
high-speed spectral detection of the CARS signal, free of nonresonant background with a resolution of 10 cm<sup>-1</sup>.
A simple scheme for video-rate wide-field coherent anti-Stokes Raman scattering (CARS) microscopy is presented. The method is based on collinear nonphase-matching illumination. The mechanisms leading to CARS signal generation are investigated. We find that refraction-mediated phase-matching is the main effect. Video-rate wide-field CARS microscopy of polystyrene beads and CARS wide-field images of C. elegans embryos are shown, and the capabilities and the limitations of the scheme are discussed.
A single photon source which generates transform limited single photons is highly desirable for applications in quantum optics. Transform limited emission guarantees the indistinguishability of the emitted single photons. This, in turn brings groundbreaking applications in linear optics quantum information processing within an experimental reach. Recently, self-assembled InAs quantum dots and trapped atoms have successfully been demonstrated as such sources for highly indistinguishable single photons.
Here, we demonstrate that nearly transform limited zero-phonon-line (ZPL) emission from single molecules can be obtained by using vibronic excitation. Furthermore we report the results of coincidence detection experiments at the output of a Michelson-type interferometer. These experiments reveal Hong-Ou-Mandel correlations as a proof of the indistinguishability of the single photons emitted consecutively from a single molecule. Therefore, single molecules constitute an attractive alternative to single InAs quantum dots and trapped atoms for applications in linear optics quantum information processing.
Experiments were performed with a home-built confocal microscope keeping the sample in a superfluid liquid Helium bath at 1.4K. We investigated terrylenediimide (TDI) molecules highly diluted in hexadecane (Shpol'skii matrix). A continuous wave single mode dye laser was used for excitation of vibronic transitions of individual molecules. From the integral fluorescence, the ZPL of single molecules was selected with a spectrally narrow interference filter. The ZPL emission was then sent to a scanning Fabry-Perot interferometer for linewidth measurements or a Michelson-type interferometer for coincidence detection.
Coherent Anti-Stokes Raman Scattering (CARS)-microscopy has in recent years developed as a promising microscopical
technique for label-free microscopy of living cells. We propose a new concept, spectral focusing, for
highly efficient coherent anti-Stokes Raman scattering (CARS) microscopy. It allows optimal use of the excitation
energy of femtosecond laser pulses in terms of generated CARS signal against a low background. This
is accomplished by introducing a linear chirp in the excitation pulses. The temporal delaying of the excitation
pulses can be used to record vibrational spectra of a sample. Despite the inherently broad spectral width of the
excitation pulses, the technique enables resolution of spectral features 60 times narrower than the bandwidth of
the probe light. First applications of this technique are exemplified with CARS of micron sized crystallites of
sodium nitroprusside, a commonly used hypotensive agent.
Fluorescence correlation spectroscopy (FCS) has turned out to be a useful tool for investigations of the population of dark states of the GFP. Using common FCS techniques, it is however not possible to reveal the nature of the dark state. We developed a simultaneous two-color excitation scheme which allows us to collect information about the dark state absorption spectra. Apart from the identification of dark states, two-color FCS opens up ways for the detailed investigation of the photodynamics of molecules with multiple dark states. It will be shown how the increase in fluorescence signal obtained by double resonance excitation can be exploited for obtaining brighter images in live cell and single molecule microscopy. The experimental setup employed for the two-color excitation can easily be extended to generate non-linear vibrational contrast. First examples of this new Coherent Anti-Stokes Raman Scattering (CARS) correlation spectroscopy will be presented.