The workshop is aimed at presenting STELLA_school ongoing research activity to the broad community of young researchers working at ICFO as well as at other neighboring institutions, and to start open discussions in the addressed research topics in view of future collaborative works.
Invited speakers:
A. Couairon, Centre de Physique Theorique, Ecole Polytechnique, F-91128 Palaiseau
The filamentation dynamics of a femtosecond laser probe pulse can be controlled by means of the quantum revivals of pre-aligned molecules prepared through impulsive rotational Raman excitation with an advancing ultrashort pump pulse. I will present numerical experiments made in collaboration with H. Zeng's group from East China Normal University, demonstrating that several features of the filamentation process including supercontinuum generation, the length of the plasma channel generated in the wake of the filament, and the associated secondary radiations are all easily modified by tuning the cross-phase modulation induced by the field-free revivals of molecular alignment, through the delay between the pump and the probe pulses. I will also show that molecular alignment can be used to generate conical shocked X-waves with extremely short intensity spikes and to further tune the frequency of a few-cycle laser pulse in the wake of a filament.
Cord Arnold, Laboratoire d'Optique Appliquee, ENSTA-Ecole Polytechnique, CNRS, F-91761 Palaiseau, France
I will present recent numerical investigations on laser pulse propagation and damage in transparent materials under tight focusing conditions. Understanding nonlinear propagation in these conditions is required for applications in cell surgery based on ultrashort laser pulses and noninvasive intra cellular dissection at sub-diffraction resolution within vital cells, in order to avoid hazardous effects to adjacent cell organelles. With microscope objectives of high numerical aperture (NA), plasma formation and thus material manipulation is limited to the very focus. Nonetheless additional unwanted nonlinear effects like self-focusing and filamentation may accompany plasma formation. Using water as a model substance for biological soft tissue and cellular constituents, I will present a detailed analysis of nonlinear plasma formation at high NA that includes accurate modeling of nonparaxial and vectorial effects as well as a modern description of plasma generation based on multiple rate equations. I will show that parasitic effects become stronger and increasingly distortive for NA <0.9, limiting the achievable precision and reproducibility of applications.
Sergio Cacciatori, department of Physics and Mathematics, University of Insubria, Como, Italy.
The world we live in is governed by four fundamental interactions: the strong and the weak interaction, electromagnetism and gravitation. While the first three forces may be described by quantum field theories, gravitation is described by a purely classical theory, i.e. general relativity. One of the most interesting predictions is the so-called Hawking radiation, i.e. a tunnelling effect by which particle-antiparticle pairs form from quantum fluctuations close to the black-hole event horizon and one of these tunnels outwards. Seen from the outside the black-hole appears to emit energy with a black-body distribution. However this prediction suffers some difficulties and to date, Hawking radiation has never actually been observed. It is in this context that Unruh first proposed investigating systems that bear a close analogy to the semiclassical gravitation model. So-called "dielectric" or "optical" analogies, characterized by an electromagnetic field in a superluminally moving dielectric, have been proposed. It was only very recently that Philbin et al. proposed an extremely interesting scenario in which Hawking radiation may have a real possibility to be detected: superluminal soliton propagation in one-dimensional optical fibers. Here we will explain how a 3-dimensioanl spicetime geometrical description can provide a simple deduction of the frequency-angle dispersion relations for light scattered by a (eventually superluminal) 3-dimensional soliton moving in a nonlinear medium.
Jean-Claude Diels, Department of Physics and Astronomy, University of New Mexico, USA
Ultraviolet pulses of 200 ps duration are focused in vacuum, and launched into the atmosphere through an aerodynamic window. Instead of diffracting, above a threshold of 100 mJ, the beam propagates in a self-induced waveguide in air. The peak electric field and beam profile are consistent with the eigenfunction of a modified nonlinear Schrödinger equation. The evolution of the beam waist with distance is calculated by a combination of the nonlinear Schrödinger equation and an equation for the nonlinear losses.
Filaments are always described as shyly enrobing themselves in a blanket of "energy reservoir". We present evidence of "naked" filaments, stripped from their universally acclaimed mantel. Ultraviolet or infrared filaments are investigated, prepared either from initial conditions in vacuum, or by the conventional self-collapse of a macroscopic beam. There is a striking difference in properties between filaments launched from a beam waist in vacuum, or produced by propagation of a large beam in air. The different properties suggest that different mechanisms are dominant in either case.
