Polarization enhanced two-photon excited fluorescence contrast by shaped laser pulses using a deformable phase plate.

We utilize spatially and temporally tailored laser pulses for polarization enhanced two-photon excited fluorescence contrasts of dyes. The shaped laser pulses are produced by first passing through a temporal pulse shaper and then through a two-dimensional spatial pulse shaper with deformable phase plates. Different spatial beam profiles are presented that demonstrate the potential of the spatial pulse shaper. Particularly, a polarization enhanced fluorescence contrast between two dyes is reported by utilizing specific phase shaping in perpendicular polarization directions. The tailored laser pulses are further modified by the deformable phase plate, and a polarization increased depth-dependent contrast is achieved. This spatial shaping for all polarization directions demonstrates the advantage of deformable phase plate spatial shapers compared to liquid crystals, where only one polarization direction can spatially be modified. The described polarization contrast method allows for three-dimensional scanning of probes and provides perspectives for biophotonic applications.

Temporally shaped vortex phase laser pulses for two-photon excited fluorescence.

We report temporally shaped vortex phase laser pulses for two-photon excited fluorescence of dyes. The particularly tailored pulses are generated by first utilizing a temporal pulse shaper and subsequently a two-dimensional spatial pulse shaper. Various vortex phase shaped structures are demonstrated by combining different two-dimensional phase patterns. Moreover, perpendicular polarization components are used to achieve an enhanced radial two-photon excited fluorescence contrast by applying third order phase functions on the temporal pulse shaper. Particularly, the spatial fluorescence structure is modulated with a combination of Gaussian and vortex phase shaped pulses by modifying only the phase on the temporal modulator. Thereby, interference structures with high spatial resolution arise. The introduced method to generate temporally shaped vortex phase tailored pulses will provide new perspectives for biophotonic applications.

Anionic Hydrogen Cluster Ions as a New Form of Condensed Hydrogen.

We report the first experimental observation of negatively charged hydrogen and deuterium cluster ions, H_{n}^{-} and D_{n}^{-}, where n≥5. These anions are formed by an electron addition to liquid helium nanodroplets doped with molecular hydrogen or deuterium. The ions are stable for at least the lifetime of the experiment, which is several tens of microseconds. Only anions with odd values of n are detected, and some specific ions show anomalously high abundances. The sizes of these "magic number" ions suggest an icosahedral framework of H_{2} (D_{2}) molecules in solvent shells around a central H^{-} (D^{-}) ion. The first three shells, which contain a total of 44 H_{2} or D_{2} molecules, appear to be solidlike, but thereafter a more liquidlike arrangement of the H_{2} (D_{2}) molecules is adopted.

Communication: Dopant-induced solvation of alkalis in liquid helium nanodroplets.

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C60, is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C60. Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C60 is added. The sole exception is atomic Cs, which remains at the surface.

Parametrically shaped femtosecond pulses in the nonlinear regime obtained by reverse propagation in an optical fiber.

We present the experimental realization of a method to generate predetermined, arbitrary pulse shapes after transmission through an optical fiber in the nonlinear regime. The method is based on simulating the reverse propagation of the desired pulse shape in the fiber. First, linear and nonlinear parameters of a single-mode step-index fiber required for the simulation are determined. The calculated pulse shapes are then generated in a pulse shaper.

Parametrically polarization shaped pulses guided via a hollow core photonic crystal fiber for coherent control.

We present ultrafast polarization pulse shaping through a micro structured hollow core photonic crystal fiber. The pulses are shaped in pulse sequences in which the energy, distance, phases, and chirps as well as the state of polarization of each individual sub-pulse can be independently controlled. The application of these pulses for coherent control is demonstrated for feedback loop optimization of the multi-photon ionization of potassium dimers. In a second experiment, this process is investigated by shaper-assisted pump-probe spectroscopy which is likewise performed with pulses that are transmitted through the fiber. Both techniques reveal the excitation pathway including the dynamics in the participating electronic states and expose the relevance of the polarization. These methods will be valuable for endoscopic applications.

Reconstruction of polarization-shaped laser pulses after a hollow-core fiber using backreflection.

We present a method to reconstruct the pulse shape of polarization-shaped femtosecond laser pulses after a hollow-core photonic crystal fiber by reflecting the pulses back through the fiber. First, a procedure is introduced to receive the optical fiber properties and generate parametrically shaped pulses after propagation through the fiber. Changes of the fiber's birefringence by mechanical stress are examined to investigate the correlation between the pulse shapes after one and two passes through the fiber. Finally, we demonstrate the characterization of the pulse after one pass through the fiber by calculating the pulse shape from the measured pulse after two passes.

A stage-scanning two-photon microscope equipped with a temporal and a spatial pulse shaper: Enhance fluorescence signal by phase shaping.

Here, we present a stage-scanning two-photon microscope (2PM) equipped with a temporal pulse shaper and a spatial light modulator enabling full control over spectral and spatial phases of the exciting laser pulse. We demonstrate the capability of correcting wavefronts and temporal pulse distortions without cross-dependencies induced by optical elements at the same time enhancing the fluorescence signal. We implemented phase resolved interferometric spectral modulation for temporal pulse shaping and the iterative feedback adaptive compensation technique for spatial pulse modulation as iterative techniques. Sample distortions were simulated by cover glass plates in the optical path and by chirping the exciting laser pulses. Optimization of the spectral and spatial phases results in a signal increase of 30% and nearly complete recovery of the losses. Applying a measured spatial compensation phase within a real leaf sample shows the enhancement in contrast due to wavefront shaping with local fluorescence increase up to 75%. The setup allows full independent control over spatial and spectral phases keeping or improving the spatial resolution of our microscope and provides the optimal tool for sensitive non-linear and coherent control microscopy.

Amplification of intense light fields by nearly free electrons.

Light can be used to modify and control properties of media, as in the case of electromagnetically induced transparency or, more recently, for the generation of slow light or bright coherent XUV and X-ray radiation. Particularly unusual states of matter can be created by light fields with strengths comparable to the Coulomb field that binds valence electrons in atoms, leading to nearly-free electrons oscillating in the laser field and yet still loosely bound to the core [1,2]. These are known as Kramers-Henneberger states [3], a specific example of laser-dressed states [2]. Here, we demonstrate that these states arise not only in isolated atoms [4,5], but also in rare gases, at and above atmospheric pressure, where they can act as a gain medium during laser filamentation. Using shaped laser pulses, gain in these states is achieved within just a few cycles of the guided field. The corresponding lasing emission is a signature of population inversion in these states and of their stability against ionization. Our work demonstrates that these unusual states of neutral atoms can be exploited to create a general ultrafast gain mechanism during laser filamentation.

Full control over the electric field using four liquid crystal arrays.

We present a femtosecond pulse shaper setup that is capable of shaping the phase, amplitude, and polarization simultaneously and independently. The modulator utilizes four liquid crystal arrays, a pair of half-wave plates, and a polarizer to gain full control of the electrical field. This is done in a common-path common-optic scheme without using interferometry. The functionality of the setup is demonstrated by using systematic parameter scans and example pulses.

Parametric polarization pulse shaping demonstrated for optimal control of NaK.

We present a routine for calculating and producing customized/parametric femtosecond laser pulses for investigating molecular processes involving the polarization. It is applied on the ionization of NaK molecules by feedback-loop optimization using the recently introduced double-pass "serial setup" that is capable of phase, amplitude, and polarization modulation. The temporal subpulse encoding uses the parameters distance, intensity, zero order spectral phase, and polarization state.

Interferometric generation of parametrically shaped polarization pulses.

We demonstrate the capabilities of the recently introduced interferometric parallel pulse shaper setup and present a method for fully tailoring the three-dimensional electrical field of femtosecond laser pulses. The possibility of producing parametric polarization pulses with arbitrary orientations and ellipticities in time is demonstrated with a selection of example pulses.

Phase, amplitude, and polarization shaping with a pulse shaper in a Mach-Zehnder interferometer.

We present a shaper scheme that fully controls the spectral phase, amplitude, and polarization of femtosecond laser pulses. In particular, it enables independent manipulation over the major axis orientation and the axis ratio of the polarization ellipse. This is accomplished by integrating a 4f-shaper setup in both arms of a Mach-Zehnder interferometer and rotating the polarization by 90 degrees in one of the arms before overlaying the beams. The generated pulses are resolved in a simple and intuitive detection scheme.

Isotope selective photoionization of NaK by optimal control: theory and experiment.

We present a joint theoretical and experimental study of the maximization of the isotopomer ratio (23)Na(39)K(23)Na(41)K using tailored phase-only as well as amplitude and phase modulated femtosecond laser fields obtained in the framework of optimal control theory and closed loop learning (CLL) technique. A good agreement between theoretically and experimentally optimized pulse shapes is achieved which allows to assign the optimized processes directly to the pulse shapes obtained by the experimental isotopomer selective CLL approach. By analyzing the dynamics induced by the optimized pulses we show that the mechanism involving the dephasing of the wave packets between the isotopomers (23)Na (39)K and (23)Na (41)K on the first excited state is responsible for high isotope selective ionization. Amplitude and phase modulated pulses, moreover, allow to establish the connection between the spectral components of the pulse and corresponding occupied vibronic states. It will be also shown that the leading features of the theoretically shaped pulses are independent from the initial conditions. Since the underlying processes can be assigned to the individual features of the shaped pulses, we show that optimal control can be used as a tool for analysis.

Femtosecond dynamics of proteoheparan sulfate (HS-PG) after UV excitation--a readout for arteriosclerotic nanoplaque formation?

Ultrashort UV laser pulses were used to excite tryptophan residues of heparan sulfate proteoglycan (HS-PG) in blood substitute Krebs solution. Tryptophan fluorescence is sensitive to the environment, so its shift and decay indicate the conformation and solvation state of the protein. We monitored stimulated emission and excited-state absorption by probing with delayed white-light femtosecond pulses. Comparison with bare tryptophan revealed transient absorption features which are characteristic for HS-PG. Furthermore, the effect of adding calcium salt was investigated. Differences in the spectra from solutions with and without calcium developed during several minutes, which points to changes in protein conformation, but could only be measured in the sub-ps regime. These results provide a first step to a better understanding of the molecular formation of nanoplaques in blood vessels. The goal of this work is to open a way towards biosensing of the initial stages in atherogenesis allowing for a risk assessment in cardiovascular disease.

Optimized isotope-selective ionization of 23Na39K and 23Na41K by applying evolutionary strategies.

We study specific properties of different isotopes by applying optimal control. Selective optimization for 23Na39K and 23Na41K isotopes is reported at two different central wavelengths by employing evolutionary strategies on shaped femtosecond laser pulses. The optimized ionization processes exhibit high enhancements of one isotope compared to the other and reversed. We analyze the pulse spectra for extracting information about the optimally chosen ionization paths and observe vibrational transitions to differing electronic states for the different isotope selections. To get a deeper insight we compare simultaneous phase and amplitude modulation with pure amplitude modulation and as well pure phase modulation. Our approach reveals how the optimization algorithm precisely addresses the vibrational wave functions by coherent interaction with the corresponding pulse components.

Learning from the acquired optimized pulse shapes about the isotope selective ionization of potassium dimers.

Selective optimization of the 39,39K2 and 39,41K2 isotopomers in a three-photon ionization process is presented by applying evolution strategies on shaped fs pulses in a feedback loop. The optimizations at different center wavelengths show considerably large enhancements of one isotope compared to the other and reversed. We compare the acquired optimized pulse shapes for combined phase and amplitude with pure amplitude modulation. Particularly from their spectra we are able to extract information about the optimally chosen differing ionization paths via the involved vibrational states. Furthermore, a comparison of the temporal shape of the optimized pulse forms for combined phase and amplitude with pure phase optimization is given. The presented pulse form analysis demonstrates the potential of restricted optimization to gain insight into the underlying dynamical processes. Our approach reveals how the optimization algorithm precisely addresses the vibrational wave functions both spectrally and temporally.

Photoelectron imaging of helium droplets.

The photoionization and photoelectron spectroscopy of He nanodroplets (10(4) atoms) has been studied by photoelectron imaging with photon energies from 22.5-24.5 eV. Total electron yield measurements reveal broad features, whose onset is approximately 1.5 eV below the ionization potential of atomic He. The photoelectron spectra are dominated by very low energy electrons, with less than 0.6 meV. These results are attributed to the formation and autoionization of highly vibrationally excited He(*)(n) Rydberg states within the cluster, followed by strong final state interactions between the photoelectron and the droplet.

Time-resolved explosion dynamics of H2O droplets induced by femtosecond laser pulses.

Series of time-resolved still images of the explosion dynamics of micrometer-sized water droplets after femtosecond laser-pulse irradiation were obtained for different laser-pulse intensities. Amplified pulses centered around a wavelength of 805 nm with 1-mJ energy and 60-fs duration were focused onto the droplet to initiate the dynamics. Several effects, such as forward and backward plumes, jets, water films, and shock waves, were investigated. Additionally, the influence of different pulse durations produced by chirping the laser pulses was observed.

Isotope selective ionization by optimal control using shaped femtosecond laser pulses.

We report on selective optimization of different isotopes in an ionization process by means of spectrally broad shaped fs-laser pulses. This is demonstrated for (39,39)K2 and (39,41)K2 by applying evolution strategies in a feedback loop, whereby a surprisingly high enhancement of one isotope versus the other and vice versa is achieved (total factor approximately 140). Information about the dynamics on the involved vibrational states is extracted from the optimal pulse shapes, which provides a new spectroscopical approach of yielding distinct frequency pattern on fs-time scales. The method should, in principle, be feasible for all molecules.