33 research outputs found
Strong-field physics with mid-IR fields
Strong-field physics is currently experiencing a shift towards the use of
mid-IR driving wavelengths. This is because they permit conducting experiments
unambiguously in the quasi-static regime and enable exploiting the effects
related to ponderomotive scaling of electron recollisions. Initial measurements
taken in the mid-IR immediately led to a deeper understanding of
photo-ionization and allowed a discrimination amongst different theoretical
models. Ponderomotive scaling of rescattering has enabled new avenues towards
time resolved probing of molecular structure. Essential for this paradigm shift
was the convergence of two experimental tools: 1) intense mid-IR sources that
can create high energy photons and electrons while operating within the
quasi-static regime, and 2) detection systems that can detect the generated
high energy particles and image the entire momentum space of the interaction in
full coincidence. Here we present a unique combination of these two essential
ingredients, namely a 160\~kHz mid-IR source and a reaction microscope
detection system, to present an experimental methodology that provides an
unprecedented three-dimensional view of strong-field interactions. The system
is capable of generating and detecting electron energies that span a six order
of magnitude dynamic range. We demonstrate the versatility of the system by
investigating electron recollisions, the core process that drives strong-field
phenomena, at both low (meV) and high (hundreds of eV) energies. The low energy
region is used to investigate recently discovered low-energy structures, while
the high energy electrons are used to probe atomic structure via laser-induced
electron diffraction. Moreover we present, for the first time, the correlated
momentum distribution of electrons from non-sequential double-ionization driven
by mid-IR pulses.Comment: 17 pages, 11 figure
Ultrafast nonlinear optical response of Dirac fermions in graphene
The speed of solid-state electronic devices, determined by the temporal dynamics of charge
carriers, could potentially reach unprecedented petahertz frequencies through direct
manipulation by optical fields, consisting in a million-fold increase from state-of-the-art
technology. In graphene, charge carrier manipulation is facilitated by exceptionally strong
coupling to optical fields, from which stems an important back-action of photoexcited carriers.
Here we investigate the instantaneous response of graphene to ultrafast optical fields,
elucidating the role of hot carriers on sub-100 fs timescales. The measured nonlinear
response and its dependence on interaction time and field polarization reveal the back-action
of hot carriers over timescales commensurate with the optical field. An intuitive picture is
given for the carrier trajectories in response to the optical-field polarization state. We note
that the peculiar interplay between optical fields and charge carriers in graphene may also
apply to surface states in topological insulators with similar Dirac cone dispersion relations.Peer ReviewedPostprint (published version
Vortex stability and permanent flow in nonequilibrium polariton condensates
The following article appeared in Journal of Applied Physics 109.10 (2011): 102406 and may be found at http://scitation.aip.org/content/aip/journal/jap/109/10/10.1063/1.3576151We study the effects of imprinting a single-quantized vortex on the steady state of a microcavity exciton-polariton condensate generated via parametric scattering. Interestingly we observe two distinct regimes: In the first case, at low polariton densities, the effect of the pulsed probe, containing the vortex state, is to generate a gain response in the condensate lasting for tens of picoseconds during which no dissipation of the circulating currents is detected. In the second regime, at higher densities, the gain lasts much less and the circulation is imprinted directly into the steady state, which acquires permanent rotation for as long as the vortex remains within the condensate. We use two different ways of measuring the circulation of the condensate and demonstrate that in both cases, polariton condensation in the parametric scattering regime can sustain permanent supercurrentsThis work was partially supported by the Spanish MEC (MAT2008-01555 and QOIT-CSD2006-00019), the CAM (S2009/ESP-1503) and FP7 ITNs “Clermont4” (235114). D.S. acknowledges financial support from the Ramón y Cajal program. G.T. is grateful for the FPI scholarship from the Ministerio de Ciencia e Innovació
Kinematically complete measurements of strong eld ionisation with mid-IR pulses
Recent observations of three unique peaks near 1Â eV, 100 meV and 1 meV in the electron spectra generated by ionization using intense mid-IR pulses have challenged the current understanding of strong-field (SF) ionization. The results came as a surprise as they could not be reproduced by the standard version of the commonly used SF approximation. We present results showing the simultaneous measurement of all three low energy ranges at high resolution. This capability is possible due to a unique experimental combination of a high repetition rate mid-IR source, which allows probing deep in the quasi-static regime at high data rates, with a reaction microscope, which allows high resolution three dimensional imaging of the electron momentum distribution.Peer ReviewedPostprint (author's final draft
Imaging an aligned polyatomic molecule with laser-induced electron diffraction
Laser-induced electron diffraction is an evolving tabletop method, which aims
to image ultrafast structural changes in gas-phase polyatomic molecules with
sub-{\AA}ngstr\"om spatial and femtosecond temporal resolution. Here, we
provide the general foundation for the retrieval of multiple bond lengths from
a polyatomic molecule by simultaneously measuring the C-C and C-H bond lengths
in aligned acetylene. Our approach takes the method beyond the hitherto
achieved imaging of simple diatomic molecules and is based upon the combination
of a 160 kHz mid-IR few-cycle laser source with full three-dimensional
electron-ion coincidence detection. Our technique provides an accessible and
robust route towards imaging ultrafast processes in complex gas phase molecules
with atto- to femto-second temporal resolution.Comment: 16 pages, 4 figure
Petahertz optical response in graphene
The temporal dynamics of charge carriers determines the speed with which
electronics can be realized in condensed matter, and their direct manipulation
with optical fields promises electronic processing at unprecedented petahertz
frequencies, consisting in a million-fold increase from state of the art
technology. Graphene is of particular interest for the implementation of
petahertz optoelectronics due to its unique transport properties, such as high
carrier mobility with near-ballistic transport and exceptionally strong
coupling to optical fields. The back action of carriers in response to an
optical field is therefore of key importance towards applications. Here we
investigate the instantaneous response of graphene to petahertz optical fields
and elucidate the role of hot carriers on a sub-100 fs timescale. Measurements
of the nonlinear response and its dependence on interaction time and field
polarization allow us to identify the back action of hot carriers over
timescales that are commensurate with the optical field. An intuitive picture
is given for the carrier trajectories in response to the optical-field
polarization state. We note that the peculiar interplay between optical fields
and charge carriers in graphene may also apply to surface states in topological
insulators with similar Dirac cone dispersion relations.Comment: 6 pages, 4 figure
Polyatomic Molecular Structure Retrieval using Laser-Induced Electron Diffraction
Laser-induced electron diffraction is a developing dynamical imaging technique that is already able to probe molecular dynamics at few-femtosecond temporal resolutions and has the potential to reach the sub-femtosecond level. Here we provide the recipe for the extension of the technique to polyatomic molecules and we demonstrate the method by extracting the structure of aligned and anti-aligned acetylene (Câ‚‚Hâ‚‚). We show that multiple bond lengths can be simultaneously imaged at high accuracy including elusive hydrogen containing bonds. Our results open the door to the investigation of larger complex molecules and the realization of a true molecular movie
Polyatomic Molecular Structure Retrieval using Laser-Induced Electron Diffraction
Laser-induced electron diffraction is a developing dynamical imaging technique that is already able to probe molecular dynamics at few-femtosecond temporal resolutions and has the potential to reach the sub-femtosecond level. Here we provide the recipe for the extension of the technique to polyatomic molecules and we demonstrate the method by extracting the structure of aligned and anti-aligned acetylene (Câ‚‚Hâ‚‚). We show that multiple bond lengths can be simultaneously imaged at high accuracy including elusive hydrogen containing bonds. Our results open the door to the investigation of larger complex molecules and the realization of a true molecular movie