One of the major achievements of Optics in the last decades has been the generation of light pulses of extremely short duration, down to attoseconds (1 as=10^-18 s). In the femtosecond range (1 fs=10^-15 s), these pulses are now produced with sufficient energies to expose matter to considerable light intensities, exceeding 10^21 W/cm^2. This has opened the development of Ultra-High Intensity (UHI) Physics, a quickly growing research field that investigates the interaction of these incredibly intense lasers with matter.
When such UHI laser beams interacts with a target of any kind, a plasma is instantly created due to the strong ionization of the target. In this regime of very short time scales and extreme intensities, the physics of the subsequent laser-plasma interaction is very different from the one at play in more ‘conventional’ laser-produced plasmas. First, when the laser intensity exceeds typically 10^18 W/cm^2, the laser-induced motion of the electrons in the plasma becomes relativistic. Second, the plasma dynamics is largely dominated by the collective ‘coherent’ motion of large numbers of electrons, directly driven by the light fields E(r,t) and B(r,t). The elementary laws that govern such collective interactions are well known, but from the point of view of fundamental physics the challenge here is to reach a detailed understanding of the key processes hidden behind their apparent complexity. This is a prerequisite to fulfil the ultimate goal of this research field, which is to drive and control collective relativistic motion of matter with light.
Such a control is not only of fundamental interest; it also has potential ground-breaking applications in different fields. Indeed, as a result of the collective relativistic motion of charges, ultrashort bursts of coherent light are emitted by the plasma electrons and beams of high-energy particles (electrons or ions) are expelled from the plasma. Because the laser field is so huge, particles can reach relativistic energies in very short distances before being expelled, and the light they emit extends up to the X-ray range. Such beams of high-energy photons and particles are known to be of high interest for many research fields, as probes of matter on very small scales. They also have societal applications, e.g. for non-destructive testing, or in medicine, for imaging or therapy (e.g. cancer protontherapy).
Until now, advances in Ultra-High Intensity Physics have largely relied on a quest for the highest laser intensities, pursued by pushing the technology to its limits (highest possible laser pulse energy, focused to a diffraction-limited focal spot, with a Fourier-transform limited pulse duration), in order to reach more extreme interaction regimes. In contrast, this project has established a new paradigm in Ultra-High Intensity Physics, by demonstrating the huge potential of using sophisticated ‘structured’ laser beams to drive UHI laser plasma-interactions –in other words, the potential of intentionally ‘distorting’ the laser field in space-time in controlled ways, at the expense of only slight reductions in laser intensities. Based on this general paradigm, we have established two novel concepts, which will impact both the physics of UHI interactions and their foreseen applications:
A. Metrology of the plasma dynamics using structured light beams: we have shown that by measuring the response of the system to structured laser fields, time- and space-resolved information on the collective charge dynamics driven by the laser field in the plasma can be accessed, down to the attosecond (sub-laser cycle) and sub-micrometre temporal and spatial scales on which the laser-driven plasma dynamics occurs. This new approach for the metrology of UHI interactions partially solve one of the main present issues of UHI physics, by bridging the considerable gap that separates the detailed predictions of numerical simulations or models, from the much more limited ‘integrated’ information that can be collected in experiments, due to the insufficient spatial and temporal resolutions of measurements.
B. Control of UHI laser-plasma interactions using structured light beams: we have shown that structured laser fields can be exploited to introduce new physics in UHI experiments, and can provide advanced degrees of freedom to achieve the ultimate goal of UHI physics, i.e. controlling the relativistic motion of charges with light. In other words, we have not only driven collective relativistic motion of charges with light as in previous experiments, but we have also finely controlled this motion by using structured laser fields as an advanced control knob. This improved control will make it possible to investigate new physical effects in UHI interactions, and will be essential for the development of new generations of ultrashort sources of particles and light .
The project, combining experiments with theory and/or simulations, has focused on the two main physical systems being key in UHI physics -plasma mirrors and laser-plasma accelerators. The obtained results have clearly demonstrated the strength of the new paradigm of UHI structured light.
