[Show abstract][Hide abstract] ABSTRACT: High-order harmonic generation is a powerful and sensitive tool for probing atomic and molecular structures, combining in the same measurement an unprecedented attosecond temporal resolution with a high spatial resolution of the order of an angstrom. Imaging of the outermost molecular orbital by high-order harmonic generation has been limited for a long time to very simple molecules, like nitrogen. Recently we demonstrated a technique that overcame several of the issues that have prevented the extension of molecular orbital tomography to more complex species, showing that molecular imaging can be applied to a triatomic molecule like carbon dioxide. Here we report on the application of such a technique to nitrous oxide (N2O) and acetylene (C2H2). This result represents a first step towards the imaging of fragile compounds, a category which includes most of the fundamental biological molecules.
[Show abstract][Hide abstract] ABSTRACT: We introduce a scheme for the generation of tunable few-optical-cycle UV pulses based on sum-frequency generation between a broadband visible pulse and a narrowband pulse ranging from the visible to the near-IR. This configuration generates broadband UV pulses tunable from 0.3 to 0.4 μm, with energy up to 1.5 μJ. By exploiting nonlinear phase transfer, transform-limited pulse durations are achieved. Full characterization of the UV pulse spectral phase is obtained by two-dimensional spectral shearing interferometry, which is here extended to the UV spectral range. We demonstrate clean 8.4 fs UV pulses.
[Show abstract][Hide abstract] ABSTRACT: 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
Journal of Physics B Atomic Molecular and Optical Physics 05/2014; 47(20). DOI:10.1088/0953-4075/47/20/204029 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the laser action in a microcavity where both the dielectric mirrors and the active material have a macromolecular nature, resulting in a full plastic laser device. Distributed Bragg reflectors (DBRs) are prepared by spin-coating of polyvinylcarbazole and cellulose acetate orthogonal solutions and the active layer consists of a highly fluorescent conjugated polymer poly(9,9-dioctylfluorenyl-2,7-diyl-co-1,4-benzo-(2,1'-3)-thiadiazole) (F8BT) spun film. A quality factor in the range 80–180 is achieved and the cavity mode is carefully tuned on the peak of the F8BT amplified spontaneous emission spectrum. Under pulsed optical pumping, laser emission is obtained with a surprisingly low lasing threshold (<20 μJ cm−2) for a full plastic DBR optical cavity. This result opens a simple and cheap way to obtain a new class of polymer lasers.
[Show abstract][Hide abstract] ABSTRACT: Extending the dimensionality of high harmonic generation (HHG) measurements has the potential to reconstruct structural features in molecules and resolve multielectron dynamics on attosecond time scales. We demonstrate that structural and dynamical effects in molecules can be unambiguously distinguished using multidimensional HHG techniques.
International Conference on Ultrafast Phenomena; 01/2014
[Show abstract][Hide abstract] ABSTRACT: We demonstrate microJ-level 10-fs pulses in the 315-380 nm spectral range generated by broadband sum-frequency generation. The pulses are characterized using 2D spectral shearing interferometry based on difference-frequency with a visible NOPA.
International Conference on Ultrafast Phenomena; 01/2014
[Show abstract][Hide abstract] ABSTRACT: We studied the two-color HHG emission from xenon in the giant resonance spectral region. We found a substantial departure from the behavior expected for the single-active-electron picture which could be ascribed to electron correlation effects.
International Conference on Ultrafast Phenomena; 01/2014
[Show abstract][Hide abstract] ABSTRACT: We report on coherent synthesis of outputs from two ultra-broadband optical parametric amplifiers; their timing is locked to sub-30-as by a balanced cross-correlator. Synthesised pulses have octave-spanning (500-1000 nm) spectra and nearly single-cycle 3.8-fs duration.
[Show abstract][Hide abstract] ABSTRACT: We study the generation of high order harmonics produced by the
interaction of three intense and ultrafast noncollinear laser pulses.
Harmonic radiation, not overlapped to the direction of the incident
pulses, is observed.
The European Physical Journal Conferences 03/2013; 41:01016-. DOI:10.1051/epjconf/20134101016
[Show abstract][Hide abstract] ABSTRACT: Optical Projection Tomography (OPT) is a three dimensional imaging
technique that is particularly suitable for studying millimeter sized
biological samples and organisms. Similarly to x-ray computed
tomography, OPT is based on the acquisition of a sequence of images
taken through the sample at many angles (projections). Assuming the
linearity of the optical absorption process, the projections are
combined to reconstruct the 3-D volume of the sample, typically using a
filtered back-projection algorithm. OPT has been applied to in-vivo
imaging of zebrafish (Danio rerio). The instrument and the protocol for
in vivo imaging of zebrafish embryos and juvenile specimens are
described. Light scattering remains a challenge for in vivo OPT,
especially when samples at the upper size limit, like zebrafish at the
adult stage, are under study. We describe Time-Gated Optical Projection
Tomography (TGOPT), a technique able to reconstruct adult zebrafish
internal structures by counteracting the scattering effects through a
fast time-gate. The time gating mechanism is based on non-linear optical
upconversion of an infrared ultrashort laser pulse and allows the
detection of quasi-ballistic photons within a 100 fs temporal gate. This
results in a strong improvement in contrast and resolution with respect
to conventional OPT. Artifacts in the reconstructed images are reduced
as well. We show that TGOPT is suited for imaging the skeletal system
and nervous structures of adult zebrafish.
Proceedings of SPIE - The International Society for Optical Engineering 02/2013; DOI:10.1117/12.2004340 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Optical imaging through biological samples is compromised by tissue scattering and currently various approaches aim to overcome this limitation. In this paper we demonstrate that an all optical technique, based on non-linear upconversion of infrared ultrashort laser pulses and on multiple view acquisition, allows the reduction of scattering effects in tomographic imaging. This technique, namely Time-Gated Optical Projection Tomography (TGOPT), is used to reconstruct three dimensionally the internal structure of adult zebrafish without staining or clearing agents. This method extends the use of Optical Projection Tomography to optically diffusive samples yielding reconstructions with reduced artifacts, increased contrast and improved resolution with respect to those obtained with non-gated techniques. The paper shows that TGOPT is particularly suited for imaging the skeletal system and nervous structures of adult zebrafish.
PLoS ONE 11/2012; 7(11):e50744. DOI:10.1371/journal.pone.0050744 · 3.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the imaging of carbon dioxide by high-order harmonic tomography, overcoming some critical obstacles that have prevented this result up to now by exploiting a few-cycle, high-energy, mid-IR parametric source as driving field.
[Show abstract][Hide abstract] ABSTRACT: In this work we present a procedure for the reconstruction of the
Highest Occupied Molecular Orbital in carbon dioxide by exploiting the
generation of high order harmonics in a sample of impulsively aligned
molecules. The procedure allows to extract the single molecule
contribution to the harmonic spectrum in both amplitude and phase as a
function of angular orientation and photon energy and to exploit this
information in the reconstruction of the orbital. The role of
multielectron contributions and of the Coulomb field of the ion on the
reconstruction are discussed.
[Show abstract][Hide abstract] ABSTRACT: High-order harmonics of ultrashort laser pulses appear in the time domain as a sequence of attosecond bursts of coherent extreme ultraviolet radiation separated by half the optical cycle of the laser field. In order to confine this emission to an isolated attosecond pulse, suitable gating techniques have been proposed and investigated. In this work, we review a few gating techniques used in our laboratories, along with the basic tools for the generation and application of attosecond pulses.
Journal of Physics B Atomic Molecular and Optical Physics 04/2012; 45(7). DOI:10.1088/0953-4075/45/7/074002 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Imaging of the outermost molecular orbital by high-order harmonic
generation has been limited so far to very simple molecules like
nitrogen, since several difficulties had to be overcome in order to
extend it to more complex species. In this work we show that molecular
imaging can be applied to a triatomic molecule like carbon dioxide,
hence paving the way to the exploitation of such technique to the
imaging of polyatomic species.
[Show abstract][Hide abstract] ABSTRACT: The production of isolated attosecond pulses is nowadays based on the generation of
high harmonics driven by ultrafast and intense laser sources in gases. The generation of such pulses
requires the confinement of the harmonic emission within a single event, thus obtaining a continuous
XUV spectrum. Here we report on the generation of continuous XUV spectra produced by an ultrabroadband
optical parametric amplifier operating in the mid-IR and generating multi-cycle pulses,
thus avoiding the use of few-cycle Ti:sapphire lasers and, thanks to the favorable cutoff scaling law
for mid-IR drivers, paving the way to the production of isolated attosecond pulses well above the
100-eV photon energy.
[Show abstract][Hide abstract] ABSTRACT: High-order harmonic generation is nowadays a topic of crucial interest
since it allows the production of isolated attosecond pulses. Several
efforts have been made so far for extending the generation of attosecond
bursts beyond the 100 eV spectral region, in order to access to deeper
electronic states in atoms and molecules. Recently, mid-infrared laser
sources have demonstrated to be ideal candidates for pushing the high
harmonic emission up to the soft X-rays. Here we show that it is
possible to obtain continuous harmonic spectra, that are the signature
of single attosecond pulse generation, by properly mixing two laser
pulses, coming from optical parametric amplifiers, with wavelengths of
1.35 and 1.75 μm respectively, with either parallel or perpendicular
polarizations. Our results demonstrate the generation of isolated
attosecond burst driven by mid-infrared laser pulses of a duration
larger than 30 fs.
[Show abstract][Hide abstract] ABSTRACT: High-order harmonic generation is as a powerful tool for the study of
molecular properties. Up to now this investigation tool has been
confined to simple molecules, with a relatively high ionization
potential, since ionization saturation hindered its exploitation to
fragile molecules. In this work we show that such limitation can be
overcome by using mid-IR ultrashort driving pulses; as prototypical
molecules we considered hydrocarbons. Clear signatures of the highest
occupied molecular orbital were found in the harmonic spectra generated
in unsaturated aligned hydrocarbons like acetylene, ethylene, allene and
1,3-butadiene. Our findings demonstrate that high-order harmonic
generation spectroscopy can be extended to complex molecular species.
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate a new technique for
the temporal gating of high-order harmonic generation, based
on the mixing of two mid-infrared laser pulses coming from optical
parametric amplifiers, at wavelengths of 1.35 and 1.75 μm,
respectively. Both parallel and perpendicular configurations of
the polarization directions were investigated. Results obtained
in xenon and argon show continuous spectra with a significant
cutoff extension, up to 160 eV. These outcomes demonstrate
the possibility of extending the two-color gating technique for
the generation of isolated attosecond bursts to a mid-infrared
laser source and are, to our knowledge, the first experimental
demonstration of the gating on a single half-cycle of high-order
harmonics driven by laser pulses longer than 30 fs.