Abstract
The absorption of ultraintense, femtosecond laser pulses by a solid unleashes relativistic electrons, thereby creating a regime of relativistic optics. This has enabled exciting applications of relativistic particle beams and coherent X-ray radiation, and fundamental leaps in high energy density science and laboratory astrophysics. Obviously, central to these possibilities lies the basic problem of understanding and if possible, manipulating laser absorption. Surprisingly, the absorption of intense light largely remains an open question, despite the extensive variations in target and laser pulse structures. Moreover, there are only few experimental measurements of laser absorption carried out under very limited parameter ranges. Here we present an extensive investigation of absorption of intense 30 femtosecond laser pulses by solid metal targets. The study, performed under varying laser intensity and contrast ratio over four orders of magnitude, reveals a significant and non-intuitive dependence on these parameters. For contrast ratio of 10−9 and intensity of 2 × 1019 W cm−2, three observations are revealed: preferential acceleration of electrons along the laser axis, a ponderomotive scaling of electron temperature, and red shifting of emitted second-harmonic. These point towards the role of J × B absorption mechanism at relativistic intensity. The experimental results are supported by particle-in-cell simulations.
Highlights
Simulations have proposed novel absorption mechanisms based on the standing wave fields[10]
The preplasma scale length is largely dependent on the intensity contrast ratio of a laser pulse, which in turn can significantly control the absorption processes
We have carried out experimental investigations of the absorption of relativistic femtosecond laser pulses by solids over a wide range of laser intensity and laser contrast ratio
Summary
Simulations have proposed novel absorption mechanisms based on the standing wave fields[10]. A fully relativistic analytical model presented by Haines et al.[11], based on energy and momentum conservation, predicts light absorption as high as 80%–90% for intensity larger than 1019 W cm−2. A very recent theoretical study[13] has created further excitement by placing bounds on the absorption of the 1022 W cm−2 intensity pulses by matter. What is the real illustration of intense, high contrast laser pulse absorption by simple solids like aluminium? Our absorption measurements are sampled over several tens of laser shots at each point in each run and several runs spread over months. We reproduce this full suite of signals in two-dimensional particle-in-cell simulations. We observe that at intermediate levels of contrast (10−7), there is a drop in absorption, which implies that ‘any’ increase in contrast does not necessarily ensure high absorption
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