Space-time metrology and control of high-power femtosecond lasers

One of the major achievements of optics in recent decades has been the generation of light pulses of extremely short duration, down to a few femtoseconds in the 1990’s and tens of attoseconds in the past few years. Throughout the development of these sources, the ability to accurately measure the temporal properties of ultrashort light pulses has been crucial. Several well-established techniques are now available to measure their electric field E(t) from the femtosecond down to the attosecond range. However, within any ultrashort beam, the temporal properties of the light pulse can vary spatially and vice versa. Such correlations between the spatial and temporal properties are called spatiotemporal couplings (STCs) and prevent the decomposition of the laser field E(t,r) as E(t,r) = f(t) g(r). STCs are ubiquitous in ultrafast optics. In the femtosecond range, chirped-pulse amplification (CPA), the key technology of amplified ultrashort pulses, relies on the use of massive STCs induced at different locations in laser systems, which should all eventually perfectly cancel out at the laser output. Residual STCs, for example resulting from imperfect compensation, decrease the peak intensity at focus by increasing both the focal spot size and the pulse duration. This is particularly detrimental for ultrahigh-intensity (UHI) lasers, which aim for the highestpossible peak intensities. On the other hand, the ability to control STCs could provide a wealth of new possibilities to shape light beams and thus control laser-matter interactions. Accurately measuring STCs is thus essential in ultrafast optics. In this talk, I will present two simple spatiotemporal measurement techniques, suited to high-power laser beams, or more generally to any ultrashort laser system with a repetition rate higher than 1 Hz. One technique, TERMITES [1], naturally applies to collimated beams, while the other, INSIGHT [2], can only work around a focal point (typically the focus used in experiments). I will present the results of measurement campaigns performed with these techniques on different lasers, including the UHI100 100 TW-25 fs laser at CEA Saclay (France) [1], and the BELLA 1.2 PW-30 fs laser at Lawrence Berkeley National Laboratory (USA) [3]. Finally, as an illustration of the potential of STC, I will explain how they enable the generation of light pulses of arbitrary and programmable group velocity in vacuum or in a medium [4]. Such beams could be highly beneficial in applications such as laser-driven particle acceleration in plasmas.