Single-molecule near-field optical energy transfer microscopy with dielectric tips.

The fluorescence lifetime and the fluorescence rate of single molecules are recorded as a function of the position of a Si3N4 atomic force microscopy tip with respect to the molecule. We observe a decrease of the excited state lifetime and the fluorescence rate when the tip apex is in close proximity to the molecule. These effects are attributed to the fact that the dielectric tip converts non-propagating near-fields to propagating fields within the dielectric tip effectively quenching the fluorescence. The spatial extension of the quenching area is of subwavelength dimensions. The results are discussed in terms of molecular fluorescence in a system of stratified media. The experiment provides surprising new insights into the interactions between a fluorescent molecule and a dielectric tip. The methodology holds promise for applications in ultra high-resolution near-field optical imaging at the level of single fluorophores.

Probing confined fields with single molecules and vice versa.

Single dye molecules are used as local probes to map the spatial distribution of the squared electric field components in the focus of a high numerical aperture lens. Simulated field distributions are quantitatively verified by experimentally obtained fluorescence excitation maps. We show that annular illumination can be used to engineer the field distribution in the focus at a dielectric/air interface such that electric field components in all directions acquire comparable magnitudes. The 3D orientation of molecular absorption dipoles can be determined by comparing measured to simulated image patterns. The presence of longitudinal electric field components in a focus is of particular interest in tip-enhanced scanning near-field optical microscopy.

Statistical analysis of single-molecule colocalization assays.

In chemical assays, specific molecular recognition events result in close physical proximity of two molecular species, e.g., ligands and receptors. Microscopy techniques that are able to image individual molecules allow for achieving a positional accuracy far beyond the resolution limit Therefore, independent position determination, e.g., by dual-color microscopy, becomes possible, permitting determination of intermolecular distances beyond the resolution limit. Nonzero measured distances occur due to experimental inaccuracies in case of a recognition event or due to accidental close proximity between ligand-receptor pairs. Using general statistical considerations, finite measured distances between single ligand-receptor pairs are directly translated into probabilities for true molecular recognition or mere accidental proximity. This enables a quantitative statistical analysis of single recognition events. It is demonstrated that in a general assay, even in the presence of strong unspecific background, the probability for a certain diagnosis and a measure for its reliability can be extracted from the observation of a few binding events. The power of the method is demonstrated at the example of a single-molecule DNA hybridization assay. Our findings are of major importance for future assay miniaturization and assaying with minute amounts of analyte.

Total dephasing-rephasing balancing in Stark-pulse-modulated photon echoes.

Using Stark-pulse-modulated photon echoes, we observe a novel type of rephasing phenomenon in a Eu(3+) - and Pr(3+) -codoped Y(2)SiO(5) crystal. By adjusting the Stark pulse during the rephasing and dephasing periods one can observe a full recovery of the photon echo, corresponding to perfect dephasing-rephasing balancing of the perturbations. We propose to use this effect as a spectroscopic technique to distinguish between reversible and irreversible Stark interactions.

Single-molecule spectroscopy.

Recent experimental and theoretical studies concerning single-molecule spectroscopy in solids are discussed. Pure quantum effects--such as photon bunching, antibunching, and spectral jumps--and more classical phenomena--such as near-field excitation, saturation, ac/dc Stark shifts, spectral diffusion, two-photon excitation, and customary spectroscopic analysis--are considered. The emphasis of this review is on physical results and their interpretation. This is preceded by a general introduction, where fundamentals of single-molecule spectroscopy are explained.

Hologram recording in DuPont photopolymer films by use of pulse exposure.

Reflection hologram recording in DuPont HRF-800X071-20 photopolymer films has been demonstrated by use of pulsed laser exposure with pulse lengths of 25 ns. An expected weak reflectance of the recorded mirror holograms could be significantly increased by preillumination. Although pulsed preillumination enhanced only the reflectance, continuous incoherent preillumination significantly increased both the diffraction efficiency (which reached ~80%) and the sensitivity (which reached an increase of ~100 times, thus approaching the sensitivity of cw recording). The results are compared with those for hologram recordings obtained with cw exposure under the same processing conditions. Spectral absorbance curves of recorded holograms are presented, and possible mechanisms of hologram formation are discussed.

Data compression in frequency-selective materials using frequency-swept excitation pulses.

We demonstrate the temporal compression of photon echoes in frequency-selective materials by application of frequency-swept excitation pulses. Experimental results in Pr(3+):Y(2)SiO(5) for two- and three-pulse photon echoes are presented and compared with theory. A possible application to temporal reduction of optical data streams is shown.

Noncollinear parametric generation in LiIO(3) and beta-barium borate by frequency-doubled femtosecond Ti:sapphire laser pulses.

In LiIO(3) and BBO crystals the wave-matching conditions for femtosecond noncollinear parametric light generation at lambda = 390 nm pumping wavelength are investigated. In the LiIO(3) crystal simultaneous phase- and group-velocity-matching angles are determined. Parametric generation occurred at 0.45-2.9-mu;m wavelengths by pumping with the second harmonic of 150-fs Ti:sapphire laser pulses and is in qualitative agreement with calculated directions in both crystals.

Second-harmonic generation of amplified femtosecond Ti:sapphire laser pulses.

We study theoretically and experimentally second-harmonic generation (SHG) of 150-fs-duration amplified Ti:sapphire laser pulses at a wavelength of 780 nm in the nonlinear crystal KDP of different lengths and at different power densities as high as 150 GW cm(-2). The experimentally observed SHG conversion efficiency does not exceed 50%. It is shown theoretically that one possible process limiting the SHG efficiency at low as well as at high intensities is the modulation of the phase of the fundamental wave. In addition, continuum generation is observed at high intensities and can decrease the SHG efficiency.

Recording of 6000 holograms by use of spectral hole burning.

Experiments verifying a new method of storing spectral hole-burning holograms, which yields reduced cross talk as compared with standard spectral hole-burning holograms, have been conducted. Results demonstrating the reduced width of this type of hologram in both frequency and the applied electric-field dimension are presented. Analytic solutions for the spectral width and diffraction efficiency of these holograms are presented. Using this exposure technique, we have recorded 6000 holograms in a single spectral hole-burning sample.

Frequency and phase swept holograms in spectral hole-burning materials.

A new hologram type in spectral hole-burning systems is presented. During exposure, the frequency of narrow-band laser light is swept over a spectral range that corresponds to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is adjusted as a function of frequency-the phase sweep function. Because of the phase-reconstructing properties of holography, this recording technique programs the sample as a spectral amplitude and phase filter. We call this hologram type frequency and phase swept (FPS) holograms. Their properties and applications are summarized, and a straightforward theory is presented that describes all the diffraction phenomena observed to date. Thin FPS holograms show strongly asymmetric diffraction into conjugated diffraction orders, which is an unusual behavior for thin transmission holograms. Investigations demonstrate the advantages of FPS holograms with respect to conventional cw recording techniques in freq ncymultiplexed data storage. By choosing appropriate phase sweep functions, various features of holographic data storage can be optimized. Examples for cross-talk reduction, highest diffraction efficiency, and maximal readout stability are demonstrated. The properties of these FPS hologram types are deduced from theoretical considerations and confirmed by experiments.

Hole burning, Stark effect, and data storage: 2: holographic recording and detection of spectral holes.

The properties of holographic recording and detection of spectral holes in the frequency and electric-field dimension are investigated. To optimize the storage properties of optical memory devices, based on spectral hole burning and holography, cross-talk effects between adjacent holograms have to be minimized. These interactions depend on the relative phases of the holograms chosen during the recording stage. Using free-base chlorin (2,3-dihydroporphyrin) in polyvinyl butyral as host at a temperature of 1.7 K, the influence of the relative phase difference between holograms is demonstrated in both the frequency and the electric-field dimension. Experimental results are presented for rows and columns of holograms stored either in the laser frequency or the electric-field dimension and compared to transmission data. Using both dimensions a 10 x 10 matrix of holograms has been stored within the range of a single wave number.

Spectral hole buming and molecular computing.

The new concept of molecular computing based on spectral hole burning, the interaction of molecular energy levels with an externally applied electric field and the interferometric properties of holography, is presented. Data stored in the form of 2-D arrays are directly combined in parallel without the use of an external processor.

Two-photon holographic recording with continuous-wave lasers in the 750-1100-nm range.

Two-photon holographic recording with cw laser sources in the 750-1100-nm spectral range is accomplished using recording materials consisting of an alpha-diketone dissolved in a poly-alpha-cyanoacrylate host. Gating of the holographic sensitivity by irradiation with incoherent UV radiation is demonstrated. Hologram diffraction efficiencies as large as 10% are achieved.

A low temperature investigation of the intermediates of the photocycle of light-adapted bacteriorhodopsin. Optical absorption and fluorescence measurements.

Optical absorption and emission measurements have been made on samples of light-adapted purple membrane of Halobacterium halobium at temperatures ranging from 77 K to room temperature. As a result of these experiments a set of equations is given which described thermal and photochemical reactions interrelating various intermediates of the reaction cycle of the chromophore of light-adapted bacteriorhodopsin (BR). Further some specific problems connected to these intermediates have been investigated. Thus the room temperature emission spectrum of bacteriorhodopsin has been found to exhibit a Stokes shift of 3430 cm-1 only, if low excitation intensities are used. The recently detected intermiediate P-BR can be shown to convert thermally into bacteriorhodopsin following a first-order decay with the activation energy delta E = 2.4 +/- 0.2 kcal/mol. The thermal decay of K-BR consists of two exponentials if measured on purple membrane suspensions in a mixture of H2O and glycerol (1 : 1, v/v). A simple procedure is given for trapping the intermediate L-BR at 170 K in a very pure form. M-BR is shown to consist of two species, MI-BR and MII-BR. They are characterized by similar optical absorption spectra but different thermal stability. Further the oscillator strengths corresponding to the long wavelength absorption bands of the intermediates bacteriorhodopsin, K-, L, MI- and MII-BR have been calculated. They have been discussed with respect to the question which of the corresponding absorption spectra show the characteristics of isomerism of the chromophore or simply solvatochromism.