460 research outputs found
Spatial and temporal pulse shaping for lateral and depth resolved two-photon excited fluorescence contrast
We report combined temporal and spatial laser pulse shaping to perform lateral and depth dependent two-photon excited fluorescence of dyes. For generating the specific spatially and temporally phase tailored pulses a temporal pulse shaper and a subsequent spatial pulse shaper are employed. Simultaneous spatial and temporal shaping is presented for two-photon excited fluorescence by applying temporal third order phase functions on spatially different light field components. Moreover, the prospects of spatial shaping are demonstrated by applying various lateral two-photon fluorescence pattern. In particular, a depth dependent excitation of different dyes is performed which leads to a high axially resolved fluorescence contrast. The introduced spatial and temporal shaping technique provides new perspectives for biophotonic imaging applications
Spatial and temporal laser pulse shaping for two color excitation
Spatial and temporal laser pulse shaping is reported for two adjacent ranges of the laser spectrum. Thereto, two-photon excited fluorescence of dyes is measured for tailored pulses having two different spectral components which possess differently modulated spatial shapes. These particularly designed pulses are formed by using a 4f-temporal liquid crystal pulse shaper followed by a 2D-spatial shaper setup. Increased fluorescence contrasts between different dyes in a cuvette are recorded by selective phase shaping of the two adjacent spectral components. Moreover, two-photon excitation of the two spectral ranges from partially overlapping beams leads to spatially localized fluorescence in the overlap region. This is controlled by utilizing antisymmetric phase functions and can be applied to yield more complex two-photon excited fluorescence structures. The developed temporal and spatial shaping method of two-photon processes has valuable perspectives for optical and biophotonic applications
Simultaneous phase, amplitude, and polarization control of femtosecond laser pulses
We present a serial pulse shaper design which allows us to shape the phase,
amplitude, and polarization of fs laser pulses independently and
simultaneously. The capabilities of this setup are demonstrated by
implementing a method for generating parametrically tailored laser pulses.
This method is applied on the ionization of NaK molecules by feedback loop
optimization, employing a temporal sub pulse encoding. Moreover, we introduce
and characterize a further development of this common path pulse shaper scheme
for full control of all light field parameters
an experimental and theoretical study
MALDI mass spectrometry in combination with post-source decay (PSD) analysis
is a fast and easy to apply method for peptide sequencing. In this study, the
PSD technique was used to investigate the influence of the adaption of one,
two, and three caesium cations to angiotensin II in the gas phase. The PSD
spectra of caesium-aggregated angiotensin II show far less fragmentation in
comparison to the protonated one. In the case of singly (doubly) Cs+
substituted angiotensin II, the PSD mass spectrum shows only fragments with
one (two) Cs cation(s). These results are interpreted in terms of additional
interactions of the caesium cation(s) with the peptide. In order to
investigate this suggestion, the molecular structures were calculated with
semi-empirical molecular dynamic (MD) simulations and further optimized at the
quantum chemical level (BP86, SVP) of theory. On the one hand, secondary
structures of Cs+ substituted angiotensin II are more compact than the
structure of protonated angiotensin II, indicating electrostatic interactions
of the Cs cations and the heterocyclic structures. Moreover, oxyphilic
interactions of the cations with the oxygen atoms of the peptide backbone also
contribute as further van-der-Waals interactions of the Cs+ substituted
angiotensin II. These interactions are able to explain its higher stability
due to reduced dissociation in comparison to the protonated angiotensin II. On
the other hand, most MD simulations of doubly and triply Cs+ substituted
angiotensin II show a formation of a [2 Cs] cluster, surrounded by the peptide
molecule. The formation of this cluster would explain the lack of singly Cs+
substituted fragments in the PSD mass spectrum of doubly Cs+ substituted
angiotensin II
Photo-oxidation by laser pulse induced desorption of phthalocyanines
Photo-oxidation of iron(II)-phthalocyanine (PcFe) has been observed in matrix
assisted laser desorption/ionization (MALDI) and laser desorption/ionization
(LDI) and is interpreted by theoretical molecular dynamics simulations. The
two ionization methods show different amounts of μ-oxo-bridged PcFe-dimer and
deliver evidence that MALDI produces less mechanical stress on the analyte.
The typical proton-transfer in the MALDI-process does not occur which leads to
the assumption of a released electron of the delocalized π-system
Combined temporal and spatial laser pulse shaping for two-photon excited fluorescence contrast improvement
We report on combined simultaneous temporal and spatial laser pulse shaping by utilizing light polarization properties. Thereto, a setup comprising a temporal pulse shaper, a waveplate, and a spatial shaper was developed and characterized by comparison with simulations. This enables to simultaneously shape one polarization component temporally and spatially while the perpendicular polarization component is modified temporally. The spatially and temporally modulated light fields were recorded and visualized by suitable contour plots, which was particularly demonstrated for cylindrically symmetric pulse profiles. Moreover, temporally and spatially shaped pulses were applied for two-photon excited fluorescence of dyes. These measurements were conducted by scanning third order phase functions for specific spatial pulse components which yields an enhanced contrast difference between fluorescing dyes. The presented temporal and spatial shaping method of ultrashort laser pulses has a high potential for biophotonic applications
Photoassociation and coherent transient dynamics in the interaction of ultracold rubidium atoms with shaped femtosecond pulses - I. Experiment
We experimentally investigate various processes present in the
photoassociative interaction of an ultracold atomic sample with shaped
femtosecond laser pulses. We demonstrate the photoassociation of pairs of
rubidium atoms into electronically excited, bound molecular states using
spectrally cut femtosecond laser pulses tuned below the rubidium D1 or D2
asymptote. Time-resolved pump-probe spectra reveal coherent oscillations of the
molecular formation rate, which are due to coherent transient dynamics in the
electronic excitation. The oscillation frequency corresponds to the detun-ing
of the spectral cut position to the asymptotic transition frequency of the
rubidium D1 or D2 lines, respectively. Measurements of the molecular
photoassociation signal as a function of the pulse energy reveal a non-linear
dependence and indicate a non-perturbative excitation process. Chirping the
association laser pulse allowed us to change the phase of the coherent
transients. Furthermore, a signature for molecules in the electronic ground
state is found, which is attributed to molecule formation by femtosecond
photoassociation followed by spontaneous decay. In a subsequent article [A.
Merli et al., submitted] quantum mechanical calculations are presented, which
compare well with the experimental data and reveal further details about the
observed coherent transient dynamics
The adsorption of helium atoms on coronene cations
We report the first experimental study of the attachment of multiple foreign
atoms to a cationic polycyclic aromatic hydrocarbon (PAH). The chosen PAH was
coronene, C24H12, which was added to liquid helium nanodroplets and then
subjected to electron bombardment. Using mass spectrometry, coronene cations
decorated with helium atoms were clearly seen and the spectrum shows peaks
with anomalously high intensities (“magic number” peaks), which represent ion-
helium complexes with added stability. The data suggest the formation of a
rigid helium layer consisting of 38 helium atoms that completely cover both
faces of the coronene ion. Additional magic numbers can be seen for the
further addition of 3 and 6 helium atoms, which are thought to attach to the
edge of the coronene. The observation of magic numbers for the addition of 38
and 44 helium atoms is in good agreement with a recent path integral Monte
Carlo prediction for helium atoms on neutral coronene. An understanding of how
atoms and molecules attach to PAH ions is important for a number of reasons
including the potential role such complexes might play in the chemistry of the
interstellar medium
Coherent control with shaped femtosecond laser pulses applied to ultracold molecules
We report on coherent control of excitation processes of translationally
ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond
laser pulses. Evolution strategies are applied in a feedback loop in order to
optimize the photoexcitation of the Rb2 molecules, which subsequently undergo
ionization or fragmentation. A superior performance of the resulting pulses
compared to unshaped pulses of the same pulse energy is obtained by
distributing the energy among specific spectral components. The demonstration
of coherent control to ultracold ensembles opens a path to actively influence
fundamental photo-induced processes in molecular quantum gases
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