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Attosecond Source from Laser-Plasma Interaction

Final Report Summary - ALPINE (Attosecond Source from Laser-Plasma Interaction)

Exploring and understanding the ultrafast processes in matter remains an ongoing multidisciplinary challenge in many scientific fields such as physics, chemistry and biology. Indeed, coherent electronic wavepackets evolve at the ultimate time scale of a few ten attoseconds (1 as=10-18 s) and space scale of angstrom, then triggering and coupling to the nuclear motion. In this context, attosecond science is currently lifting the veil on the most transient processes ever observed. Over the last decade, light bursts of attosecond duration has been used as ultrafast tool to trigger and observe for the first time the motion of electrons in atoms, molecules or solids.
In order to observe extremely brief processes, it is necessary to develop measurement techniques or devices with ever increasing time resolution. So far the most accomplished technique has consisted in inducing nonlinear interaction of an ultrashort laser pulse with a gently ionized gas of atoms or molecules at moderate intensity. This process has enabled to produce XUV bursts of less than 100 as and probe ultrafast electronic phenomena, such as electron tunneling in atoms or electronic motion in condensed matter. If attosecond pulse generation from gas medium represents today the state-of-the-art technique (particularly because it takes full advantage of the most advanced laser technologies, such as sub-cycle control of the light field waveform), the necessary radiative recombination at the root of the attosecond emission prevents to exploit ultrahigh light intensity provided by the actual largest laser facilities.
That is why another process is also under study: the interaction of ultra-intense lasers with plasma mirrors, which could in the near future provide attosecond sources with better characteristics in terms of duration, pulse and photon energy. Indeed, plasma mirrors have the properties, when irradiated by laser fields at relativistic intensities, to radiate attosecond burst of light. As the generating laser field can be increased without limitation, the most intense laser system can be used and therefore generate ultra-bright attosecond pulse sources.
The novelty of the ALPINE project was to generate relativistic high-harmonics with two-cycle laser pulses. The use of few-cycle laser pulses for laser-matter interaction has raised a strong interest in the last decades, because it opens the door to the generation of very few or even only one attosecond pulse, which are ultimate tools for the study of ultrafast processes in matter. However, such extreme regime is extremely challenging to reach, because it requires combining relativistic intensity with two-cycle laser pulses, while ensuring very clean temporal contrast. These are the challenge that the ALPINE project aimed at undertaking.
With the ALPINE project, we have demonstrated for the first time the generation of relativistic high-harmonics using two-cycle laser pulses. This achievement is a major step forward towards a crucial objective in Attosecond science: the generation of bright isolated attosecond pulses. During a large part of the project, numerous necessary improvements have been brought to the existing laser system in order to reach this goal, especially to obtain an excellent laser temporal contrast (1010 in the picosecond range) required for laser-plasma interaction at relativistic intensity. To this end, the pedestal has been reduced by many orders of magnitude by combining a XPW temporal filter and a plasma mirror, and we also eliminated parasitic prepulse generated by one of our temporal shaper devices by redesigning the dispersion control of the laser system. Eventually, we maximized the laser intensity by focusing light down to the micrometer level in order to go much beyond the relativistic limit, which allowed us generating harmonics of high order (> 90 eV).

Our second achievement was to observe strong indication of the dependence of the harmonic spectrum on the laser “carrier-envelop phase” (CEP). This observation is of the utmost importance for the future of relativistic harmonics as a credible source for Attosecond Science. Indeed, it shows that laser-induced relativistic plasmas can be driven by the laser field, which is a necessary condition in order to produce attosecond bursts of lights with sub-cycle stability, and therefore suited for application in ultrafast dynamics.
Our third main result toward the development a well-controlled attosecond source was to study the spectral characteristics of the harmonic signal as a function of the interaction conditions, and to collect information about the temporal structure of the attosecond source. Indeed, the use of two-cycle laser pulses rather than the commonly used multiple-cycle laser sources implies that the laser envelop evolves almost as fast as the electric field itself: as a consequence, the laser pulse is made of only two or three optical cycles with very different amplitude, and led to few attosecond pulses whose characteristics are strongly linked to the dynamic of the generating plasma and whose spectrum contains a signature of this dynamic.