The interaction of intense femtosecond laser with matter produces a plasma of high temperature (several tens of thousand °C). The plasma consists on highly ionized ions and electrons through which the laser transfers energy to the matter. Driven by electrostatic forces, the plasmas of electrons and ions oscillate relative to each other at their natural frequencies. Typical studies in laser-matter interaction include the analysis of laser energy deposition inside the plasma, the analysis of plasma properties (atomic physics of hot plasmas, plasma dynamics), and analysis of the radiation or particles emitted during the interaction.
The physical processes that drive the interaction differ significantly depending on the density of the target (a gas or a solid), on the laser pulse duration and on the laser intensity. The threshold to produce a plasma is close to 1011 W/cm2 at the surface of a solid target, and about 1013 W/cm2 in a gas. Beyond 1018 W/cm2, the physics of the interaction becomes relativistic because electrons, being lighter than the ions, oscillate in the laser field and acquire a speed close to c, the speed of light. Beyond 1024 W/cm2, the protons interact directly with the oscillating electromagnetic field and become relativistic. This regime can not be studied up to now because the maximum available laser intensities on target are of the order of 1020 - 1021 W/cm2. At still higher intensities of the order of 1034 W/cm2, quantum electrodynamics (creation of electron-hole pairs, etc ...) could be addressed with lasers.
The principle of a laser-matter interaction experiment is as follows. A laser beam between 1012 watts to 1015watts is focused onto a gas or solid target with focal spots of several micrometers. A high temperature plasma is produced. The main laser beam can be divided into multiple beams to probe the plasma used for experiments or applications.
Several processes drive the physics of interaction between an intense femtosecond laser and a gas. Below the ionization threshold intensity, the nonlinear physics can significantly extend the spectrum of radiation through the production of high harmonics of the fundamental laser wavelength. Beyond this threshold, plasma ionization dynamics can generate population inversions in the different energy levels of ions and produce lasers in the XUV spectral range. By focusing in air, the laser beam can self-focus and thus propagate over very long distances in the atmosphere. In a more dense plasma compared to the density of a gaz at atmospheric pressure (eg 1018 electrons/cm3), the beam can propagate over a few centimeters. In its wake, a cavity free of electron can be created leading to the generation of tremendous electrostatic fields (1010 V / m) . LOA research activities address all these issues, and the teams play a major role in the understanding of plasma physics with ultrafast intense lasers.
For laser-matter interaction with solid targets (electron density equal to 1023 electrons/cm3), nonlinear optics and cross-polarization techniques below the ionization threshold are used for the development of temporal filtering of laser beam profiles. At higher laser intensities above the ablation threshold, we take advantage of non-thermal energy deposition thanks to the femtosecond laser pulse duration (the energy is deposited during the period the laser pulse, which is shorter than the time required for the energy to be spread and heat in the material) to remove organic tissue with high precision. At further higher intensities (from 1016 W/cm2) the ultrashort laser interacts with a dense plasma close to the initial density of the solid because, despite its high temperature, the plasma has no time to expand during the laser pulse. Furthermore, the resonance properties between the laser oscillating electric field and plasma oscillations lead to the generation of particles (electrons and protons) with high peak currents as well as harmonics of the laser light in the spectral range up to X-rays.
During the interaction of an intense femtosecond laser beam with the surface of a solid target, a beam of energetic electrons (blue) can be produced. These electrons propagate through the target, and if the latter is ultrathin (a few tens of nanometers,1 nm = 10-9 m), and are ejected from the rear surface. Under the influence of electrostatic forces, the ions (in red) of the target are then accelerated, which produces as well a beam of ions like protons.
All of these different regimes of femtosecond laser-matter interaction cover a wide range of physics. They give access, with relatively compact systems, up to high-energy physics studies which was exclusively restricted up to now for large particle accelerators. It is also remarkable and unique to have the opportunity to study in a laboratory the matter under extreme conditions: temperatures of hundreds of millions degrees, magnetic fields of several hundred tesla, electron current of several billion ampere, accelerated particles of several billion electron volts, pressures of several million bar and electric fields exceeding one trillion Volt / m!