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Model independent expansion history from supernovae: Cosmology versus systematics
We examine the Pantheon supernovae distance data compilation in a model independent analysis to test the validity of cosmic history reconstructions beyond the concordance CDM cosmology. Strong deviations are allowed by the data at z 1 in the reconstructed Hubble parameter, Om diagnostic, and dark energy equation of state. We explore three interpretations: 1) possibility of the true cosmology being far from CDM, 2) supernovae property evolution, and 3) survey selection effects. The strong (and theoretically problematic) deviations at z 1 vanish and good consistency with CDM is found with a simple Malmquist-like linear correction. The adjusted data is robust against the model independent iterative smoothing reconstruction. However, we caution that while by eye the original deviation from CDM is striking, χ2 tests do not show the extra linear correction parameter is statistically significant, and a model-independent Gaussian Process regression does not find significant evidence for the need for correction at high-redshifts
High-order Harmonic Generation and Dynamic Localization in a driven two-level system, a non-perturbative solution using the Floquet-Green formalism
We apply the Floquet-Green operator formalism to the case of a
harmonically-driven two-level system. We derive exact expressions for the
quasi-energies and the components of the Floquet eigenstates with the use of
continued fractions. We study the avoided crossings structure of the
quasi-energies as a function of the strength of the driving field and give an
interpretation in terms of resonant multi-photon processes. From the Floquet
eigenstates we obtain the time-evolution operator. Using this operator we study
Dynamic Localization and High-order Harmonic Generation in the non-perturbative
regime
Theory of high-order harmonic generation by an elliptically polarized laser field
We generalize a recently formulated theory of high-order harmonic generation by low-frequency laser fields [Anne L'Huillier et al., Phys. Rev. A 48, R3433 (1993)] to the case of an elliptically polarized light. Our theoretical description includes both the single-atom response and propagation. Phase matching significantly modifies the results obtained in the single-atom response. The results of our calculations, including propagation for both the intensity and polarization properties of harmonics as a function of laser ellipticity, compare very well with recent experimental observations
Two-photon double ionization of neon using an intense attosecond pulse train
We present the first demonstration of two-photon double ionization of neon
using an intense extreme ultraviolet (XUV) attosecond pulse train (APT) in a
photon energy regime where both direct and sequential mechanisms are allowed.
For an APT generated through high-order harmonic generation (HHG) in argon we
achieve a total pulse energy close to 1 J, a central energy of 35 eV and a
total bandwidth of eV. The APT is focused by broadband optics in a
neon gas target to an intensity of Wcm. By tuning
the photon energy across the threshold for the sequential process the double
ionization signal can be turned on and off, indicating that the two-photon
double ionization predominantly occurs through a sequential process. The
demonstrated performance opens up possibilities for future XUV-XUV pump-probe
experiments with attosecond temporal resolution in a photon energy range where
it is possible to unravel the dynamics behind direct vs. sequential double
ionization and the associated electron correlation effects
Attosecond pulse trains generated using two color laser fields
We investigate the spectral and temporal structure of high harmonic emission from argon exposed to an infrared laser field and its second harmonic. For a wide range of generating conditions, trains of attosecond pulses with only one pulse per infrared cycle are generated. The synchronization necessary for producing such trains ensures that they have a stable pulse-to-pulse carrier envelope phase, unlike trains generated from one color fields, which have two pulses per cycle and a pi phase shift between consecutive pulses. Our experiment extends the generation of phase stabilized few cycle pulses to the extreme ultraviolet regime
Spatio-temporal coupling of attosecond pulses
The shortest light pulses produced to date are of the order of a few tens of
attoseconds, with central frequencies in the extreme ultraviolet range and
bandwidths exceeding tens of eV. They are often produced as a train of pulses
separated by half the driving laser period, leading in the frequency domain to
a spectrum of high, odd-order harmonics. As light pulses become shorter and
more spectrally wide, the widely-used approximation consisting in writing the
optical waveform as a product of temporal and spatial amplitudes does not apply
anymore. Here, we investigate the interplay of temporal and spatial properties
of attosecond pulses. We show that the divergence and focus position of the
generated harmonics often strongly depend on their frequency, leading to strong
chromatic aberrations of the broadband attosecond pulses. Our argumentation
uses a simple analytical model based on Gaussian optics, numerical propagation
calculations and experimental harmonic divergence measurements. This effect
needs to be considered for future applications requiring high quality focusing
while retaining the broadband/ultrashort characteristics of the radiation
Ionization and fragmentation of C-60 via multiphoton-multiplasmon excitation
We study the intensity dependence of ionization and fragmentation of buckminsterfullerene (C-60) in strong laser fields. Our data provide strong evidence that at intensities less than or similar to 10(14) W/cm(2) these processes occur predominantly via multiphoton excitation of the 20 eV plasmon resonance of C-60 At least two plasmons have to be created to initiate fragmentation or multiple ionization
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