121 research outputs found
Laser induced strong-field ionization gas jet tomography
We introduce a novel in-situ strong field ionization tomography approach for
characterizing the spatial density distribution of gas jets. We show that for
typical intensities in high harmonic generation experiments, the strong field
ionization mechanism used in our approach provides an improvement in the
resolution close to factor of 2 (resolving about 8 times smaller voxel volume),
when compared to linear/single-photon imaging modalities.
We find, that while the depth of scan in linear tomography is limited by
resolution loss due to the divergence of the driving laser beam, in the
proposed approach the depth of focus is localized due to the inherent physical
nature of strong-field interaction and discuss implications of these findings.
We explore key aspects of the proposed method and compare it with commonly used
single- and multi-photon imaging mechanisms. The proposed method will be
particularly useful for strong field and attosecond science experiments.Comment: 8 pages, 3 figure
Studying the universality of field induced tunnel ionization times via high-order harmonic spectroscopy
High-harmonics generation spectroscopy is a promising tool for resolving
electron dynamics and structure in atomic and molecular systems. This scheme,
commonly described by the strong field approximation, requires a deep insight
into the basic mechanism that leads to the harmonics generation. Recently, we
have demonstrated the ability to resolve the first stage of the process --
field induced tunnel ionization -- by adding a weak perturbation to the strong
fundamental field. Here we generalize this approach and show that the
assumptions behind the strong field approximation are valid over a wide range
of tunnel ionization conditions. Performing a systematic study -- modifying the
fundamental wavelength, intensity and atomic system -- we observed a good
agreement with quantum path analysis over a range of Keldysh parameters. The
generality of this scheme opens new perspectives in high harmonics
spectroscopy, holding the potential of probing large, complex molecular
systems.Comment: 11 pages, 5 figure
High-order Harmonic Generation in Thermotropic Liquid Crystals
Thermotropic liquid crystals are versatile optical materials that exhibit a
state of matter intermediate between liquids and solids. Their properties can
change significantly with temperature, pressure, or other external factors,
leading to different phases. The transport properties within these materials in
different phases are still largely unexplored and their understanding would
enable exciting prospects for innovative technological advancements. High-order
harmonic spectroscopy proved to be a powerful spectroscopic tool for
investigating the electronic and nuclear dynamics in matter. Here we report the
first experimental observation of high-order harmonic generation in
thermotropic liquid crystals in two different phase states, nematic and
isotropic. We found the harmonic emission in the nematic phase to be strongly
dependent on the relative orientation of the driving field polarization with
respect to the liquid crystal alignment. Specifically, the harmonic yield has a
maximum when the molecules are aligned perpendicularly to the polarization of
the incoming radiation. Our results establish the first step for applying
high-order harmonic spectroscopy as a tool for resolving ultrafast electron
dynamics in liquid crystals with unprecedented temporal and spatial resolution.Comment: The paper is composed by 12 pages including references. Five images
are presen
Ultrafast THz probe of photo-induced polarons in lead-halide perovskites
We study the nature of photo-excited charge carriers in CsPbBr3 nanocrystal
thin films by ultrafast optical pump - THz probe spectroscopy. We observe a
deviation from a pure Drude dispersion of the THz dielectric response that is
ascribed to the polaronic nature of carriers; a transient blueshift of observed
phonon frequencies is indicative of the coupling between photogenerated charges
and stretching-bending modes of the deformed inorganic sublattice, as confirmed
by DFT calculations
Generation of ultrashort pulses by four wave mixing in a gas-filled hollow core fiber
The four wave mixing (FWM) process is widely exploited for the generation of tunable ultrashort light pulses. Usually this process is driven in bulk materials, which are however prone to optical damage at high pump laser intensities. A tunable source of ultrashort 10 mu J level pulses in the visible spectral region is described here. In particular, we report on the implementation of FWM driven by a two-color ultrafast laser pulse inside a gas-filled hollow core fiber (HCF). Due to the high-damage threshold and the long interaction distance, the HCF-based FWM configuration proves to be suitable for high-energy applications. Moreover, this technique can be potentially used for ultrashort pulses generation within a wide range of spectral regions; a discussion on the possibility to extend our scheme to the generation of few-cycle mid-IR pulse is provided
Fast stabilization of a high-energy ultrafast OPA with adaptive lenses
The use of fast closed-loop adaptive optics has improved the performance of optical systems since its first application. Here we demonstrate the amplitude and carrier-envelope phase stabilization of a high energy IR optical parametric amplifier devoted to Attosecond Science exploiting two high speed adaptive optical systems for the correction of static and dynamic instabilities. The exploitation of multi actuator adaptive lenses allowed for a minimal impact on the optical setup
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Multidimensional high harmonic spectroscopy of polyatomic molecules: detecting sub-cycle laser-driven hole dynamics upon ionization in strong mid-IR laser fields
High harmonic generation (HHG) spectroscopy has opened up a new frontier in ultrafast science, where electronic dynamics can be measured on an attosecond time scale. The strong laser field that triggers the high harmonic response also opens multiple quantum pathways for multielectron dynamics in molecules, resulting in a complex process of multielectron rearrangement during ionization. Using combined experimental and theoretical approaches, we show how multi-dimensional HHG spectroscopy can be used to detect and follow electronic dynamics of core rearrangement on sub-laser cycle time scales. We detect the signatures of laser-driven hole dynamics upon ionization and reconstruct the relative phases and amplitudes for relevant ionization channels in a CO2 molecule on a sub-cycle time scale. Reconstruction of channel-resolved complex ionization amplitudes on attosecond time scales has been a long-standing goal of high harmonic spectroscopy. Our study brings us one step closer to fulfilling this initial promise and developing robust schemes for sub-femtosecond imaging of multielectron rearrangement in complex molecular systems
Carbon and Nitrogen K-Edge NEXAFS Spectra of Indole, 2,3-Dihydro-7-azaindole, and 3-Formylindole
The near-edge X-ray absorption fine structure (NEXAFS) spectra of indole, 2,3-dihydro-7-azaindole, and 3-formylindole in the gas phase have been measured at the carbon and nitrogen K-edges. The spectral features have been interpreted based on density functional theory (DFT) calculations within the transition potential (TP) scheme, which is accurate enough for a general description of the measured C 1s NEXAFS spectra as well as for the assignment of the most relevant features. For the nitrogen K-edge, the agreement between experimental data and theoretical spectra calculated with TP-DFT was not quite satisfactory. This discrepancy was mainly attributed to the many-body effects associated with the excitation of the core electron, which are better described using the time-dependent density functional theory (TDDFT) with the range-separated hybrid functional CAM-B3LYP. An assignment of the measured N 1s NEXAFS spectral features has been proposed together with a complete description of the observed resonances. Intense transitions from core levels to unoccupied antibonding π* states as well as several transitions with mixed-valence/Rydberg or pure Rydberg character have been observed in the C and N K-edge spectra of all investigated indoles
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Application of Matched-Filter Concepts to Unbiased Selection of Data in Pump-Probe Experiments with Free Electron Lasers
Pump-probe experiments are commonly used at Free Electron Lasers (FEL) to elucidate the femtosecond dynamics of atoms, molecules, clusters, liquids and solids. Maximizing the signal-to-noise ratio of the measurements is often a primary need of the experiment, and the aggregation of repeated, rapid, scans of the pump-probe delay is preferable to a single long-lasting scan. The limited availability of beamtime makes it impractical to repeat measurements indiscriminately, and the large, rapid flow of single-shot data that need to be processed and aggregated into a dataset, makes it difficult to assess the quality of a measurement in real time. In post-analysis it is then necessary to devise unbiased criteria to select or reject datasets, and to assign the weight with which they enter the analysis. One such case was the measurement of the lifetime of Intermolecular Coulombic Decay in the weakly-bound neon dimer. We report on the method we used to accomplish this goal for the pump-probe delay scans that constitute the core of the measurement; namely we report on the use of simple auto- and cross-correlation techniques based on the general concept of “matched filter”. We are able to unambiguously assess the signal-to-noise ratio (SNR) of each scan, which then becomes the weight with which a scan enters the average of multiple scans. We also observe a clear gap in the values of SNR, and we discard all the scans below a SNR of 0.45. We are able to generate an average delay scan profile, suitable for further analysis: in our previous work we used it for comparison with theory. Here we argue that the method is sufficiently simple and devoid of human action to be applicable not only in post-analysis, but also for the real-time assessment of the quality of a dataset
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