Experiments have been undertaken using the VEGA-3 petawatt laser system at the Centro de Láseres Pulsados (CLPU) facility in Salamanca to investigate electron and ion acceleration in under-dense plasma. The respective longitudinal and transverse fields of the ‘bubble’ structure of a laser wakefield accelerator (LWFA) simultaneously accelerates electrons to GeV energies, and ions to 100s keV/u to MeV/u energies. The laser is configured to produce two ultra-intense laser pulses, each with a minimum pulse duration of 30 fs and a variable inter-pulse delay up to 300 fs. The double pulses can superpose or resonantly excite the LWFA bubble to increase the accelerating fields. By focusing the laser beam into a 2.74 mm diameter supersonic jet of He gas, using an F/10.4 parabola, an initial intensity of up to ≈1019 Wcm−2 can be realized at focus. This ionises the gas to produce plasma and the imposes a ponderomotive force that creates the LWFA accelerating structures. For backing pressures of 30 – 60 bar, corresponding to plasma densities of 1–4×1019 cm−3, the fields of the LWFA can exceed 200 MV/m, which is sufficient to accelerate electrons to GeV energies, and ions to 100s keV/u. This study focuses on ion acceleration in the transverse direction. He+1 and He+2 ion spectra have been measured using a Thompson parabola spectrometer and a multi-channel plate detector. He ions with energies up to a few hundred keV/u are observed for both single pulses (5.0 J) and double pulses (5.0 J and 3.6 J, respectively), where the inter-pulse delay is varied between 0 fs and ± 300 fs. The measured spectra are consistent with numerical simulations. Ions are observed to undergo electron exchange in the neutral surrounding gas, which produces different charge states ions and neutral atoms.
A technique to measure the intensity profile of a focused laser pulse at full power is a long-standing desire. High power lasers allow experiments at relativistic intensities (1018 W/cm2 and beyond). At those photon densities all atoms are ionized and therefore it is very difficult to measure directly the peak intensity or the 3D-profile of the focal spot intensity. We would like to present a way to measure it, based on residual atoms in the experimental chamber.
A low-density gas will imply a number of atoms at the laser focal volume. Those atoms will be instantaneously ionized, and the released electrons will move at relativistic speeds driven by the laser field. Plasma effects, at low density, can be neglected, and electrons move independently driven only by the laser field. Nearly 50 years ago, an approach was suggested that is based on relativistic Thomson scattering, which consists of a rich spectrum of Doppler shifted radiation of the laser light, and its harmonics [1]. This reference provides very simple expressions for the scattered Doppler shift vs. intensity. Therefore, such scattered photons give very valuable information about the intensity profile. We propose to measure the Doppler shift of the low order harmonics as an in-situ direct measure of the intensity.
In particular, we will present the first preliminary experimental observation of such a shift of the second harmonic as a non-destructive way to measure the intensity profile of the Salamanca VEGA-2 laser focal profile. The spectrum is consistent with a peak intensity beyond 1018 W/cm2, which correlates well with the expected intensity. This promising result is the theme of this presentation. Details of the experiment, numerical simulations, related experiments and prospects for exploiting relativistic Thomson scattering to develop an in situ intensity profiler will be discussed.
[1] E. S. Sarachik and G. T. Schappert, Phys. Rev. D 1, 2738 (1970).
A few Petawatt lasers are operative now in the world and a few more are under construction in several places. One of
them is under construction at Salamanca, Spain, in the framework of a Consortium between the Central Government of
Spain, the regional Government of Castilla y Leon and the University of Salamanca. The Spanish PW will reach that
extreme power with 30 Joule / 30 femtosecond pulses, using Ti:Sapphire CPA technology and delivering one shot per
second. The Salamanca laser will allow synchronized pump/probe experiments with a unique 200 TW probe sharing the
same front end. The laser is now under construction and full operation is expected in less than two years. That laser will
be a user facility opened to the national and the international scientific community. To understand the meaning of such a
system, a review of the technology and its extreme applications in the foreseen range of powers and intensities is
presented.
We demonstrate here that it is possible to fabricate 1D and 2D diffraction gratings on the (001) surface of RbTiOPO4
(RTP) and KTiOPO4 (KTP) single crystals. We analyzed the linear and nonlinear optical properties of 1D and 2D
nonlinear photonic crystals. We show enhanced second harmonics when the samples were illuminated with a pulsed
Nd:YAG laser, when compared to non-structured surface of the same materials and mainly there exists an asymmetry on
the diffraction patterns of the second harmonic generated light, showing higher intensity in diffraction orders different to
the zero order in the reflection configuration.
KEYWORDS: Random lasers, Silica, Laser damage threshold, Near infrared, Nanoparticles, Absorption, Luminescence, Solids, Solid state lasers, Solid state physics
We report the first observation of two-photon pumped random laser action in the ground powder of a silica gel
containing rhodamine 6G doped silica nanoparticles. When this solid-state dye system is pumped with 800 nm
femtosecond-lasing pulses, random laser-like effects such as spectral narrowing and temporal shortening are observed
with a laser-like emission peak centered around 598 nm. A comparison between the emission features, random laser
behavior and threshold of random laser action, following one- and two-photon excitations is also performed.
In this work we have studied the fragmentation of gold nanoparticles (NPs) after generation by femtosecond laser
ablation of a solid target in deionized water. The fragmentation process was carried out using two different types of
radiation: direct ultra-fast pulses and super-continuum radiation focused in the colloidal solution. In the former case, IR
pulses were applied both in low and high fluences regime, while in the latter, super-continuum was generated by an
external sapphire crystal. In this last case, to assess the effects of the different spectral bands present in the super-continuum
for fragmentation, we have determined different efficiency regions. From the analysis of optical extinction
spectra and Transmission Electron Microscopy (TEM) histograms we can conclude that the main mechanism is linear
absorption in the visible region. Likewise, the super-continuum generated in water during fragmentation resulted more
efficient than that obtained externally by the sapphire crystal. This fact can be attributed to the broadening of the water
continuum band originated due to large intensity used for generation. TEM and Small Angle X-ray Scattering (SAXS)
measurements support the results found from optical extinction spectroscopy.
In this work we design and construct a pulse compressor with volume transmission holographic gratings, to compensate
the second order dispersion in femtosecond laser pulses emitting at 794 nm with a spectral broadband of 10 nm. The
gratings (730 lines/mm) are recorded in PFG-04 dichromated gelatine emulsion with a wavelength of 532 nm, reaching
enough index modulation to use the gratings illuminated with 800 nm light source with high efficiency (around 80% of
efficiency in each grating). This efficiency is expected to be increased with an antireflection coating. We measure the
factor of compression as a function of the grating distance using an autocorrelator, finding a good agreement with
theoretical curve. A dispersed pulse (580 fs) is reduced to the bandwidth limited value of 106 fs with the grating pair
separated by 27 mm.
Non linear propagation of ultra short pulses in air is studied. By preparing an initial field distribution by an amplitude mask we can obtain a Townes soliton[1] (self similar channel of coherent radiation) in air. Experimental observation can be described accurately by the numerical integration of the Non Linear Schroedinger Equation (NLSE) and allow us to explain the origin of the remarkable stability of this soliton as a balance between diffraction and Kerr effect. We further explore on the role of coherence by revisiting the two slit Young's experiment but now in the non linear regime.
In the non-relativistic limit, the dynamics of the interaction of light with matter is described via a Hamiltonian
that does not include spin operators. However, the actual spin configuration of the interacting particles still
plays a fundamental role, via the Pauli's exclusion principle, by forcing a particular symmetry of the spatial
part of the wavefunction. In this paper we analyze the role of symmetry in the process of ionization of two and
three-electron atoms.
We report the observation of self-guided propagation of 120 fs, 0.56 mJ infrared pulse in air for distances greater
than a meter (more than thirty Rayleigh Lengths). The numerical simulations demonstrates the this localized
structure corresponds to a Townes soliton, specially stable under these conditions.
This work deals on the fabrication process of diffraction gratings made on Lithium Niobate substrates by means of focusing femtosecond laser pulses. The main optical features of these photonic structures are presented in this paper. As it was expected the relief gratings showed high diffraction efficiency in accordance to the index modulated profile of the crystal-air border resulting in this kind of diffractive structure. On the other hand, for the inside grating a higher diffraction efficiency for the first orders were found, however the overall diffraction efficiency was found to be similar to that obtained for the ablation structures made in this work; this result suggests that the index increment in the inside grooves should be very important. The thermal stability of these structures is also studied and discussed in this paper. It is found that for the relief gratings the diffraction efficiency is temperature independent up to 400°C degrees, while for the inside gratings a slight decrease on diffraction efficiency was observed after making a thermal annealing at 400°C during two hours. The grating fabrication method presented in this work can be a powerful tool for development of several photonic devices made inside/on Lithium Niobate by using femtosecond laser writing by means of the one step process.
The high order harmonic generation process due to the interaction of a multi-well quantum system with an intense laser field is examined. A plateau extension up to a photon energy of Ip + 8Up and the generation of attosecond pulses are evidenced. The influence of the intermediate ions on the conversion efficiency and on the plateau extension is studied.
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