Ultrahigh intensity laser – plasma physics is normally driven by lasers with few-10 fs or longer pulse duration. Certain applications involving the generation of isolated attosecond electron and x-ray pulses, however, require much shorter pulses that is not available from lasers. We report on the optical parametric synthesis of quasi-single cycle waveforms which can reach ultra-relativistic intensities up to 10^21 W/cm^2. A pulse duration below 4.5 fs is achieved by amplifying the spectrum between 580 – 1020 nm in two separate spectral regions in two consecutive optical parametric chirped pulse amplifiers. One stage pumped by 355 nm is optimized below 700 nm, while another pumped by 532 nm is optimized above 700 nm. This combination of amplifiers is called optical parametric synthesizer (OPS), which serially synthesizes the spectrum (full spectrum propagates through all amplifiers). Three such OPS double stages provide 440-500 mJ energy in the short light pulse corresponding to 100 TW peak power. Typical applications will be shortly introduced, such as electron acceleration from relativistic laser-plasmas and nonlinear attosecond x-ray interaction.
Vacuum laser acceleration (VLA) of electrons has been an intense field of research for a long time due to the extremely high (>1 TV/m) accelerating fields. However, it is very challenging to realize and only a few promising experiments have been performed which have demonstrated the principle. Here, we report on the interaction of relativistic intensity (1020 Wcm-2) sub-two optical cycle (<5 fs) laser pulses with nanotips to realize and optimize VLA. Various properties of accelerated electrons (angular distribution, charge, and electron spectrum) are measured with different intensities and carrier envelope phases of the laser pulse. Among others, waveform dependence of the electron propagation direction is observed. Furthermore, comparable or even higher electron energies beyond 10 MeV are detected with lower laser intensity, i.e., longer focusing, than with high intensity. These surprising results are reproduced using particle-in-cell simulations, which indicate a nanophotonics electron emission from the nanotip followed by VLA. In fact, the unexpected observations are a direct proof of the VLA process and provide a way to optimize it towards higher energy, isolated, attosecond electron bunch generation.
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