Loss-free shaping of few-cycle terawatt laser pulses.

We demonstrate loss-free generation of 3 mJ, 1 kHz, few-cycle (5 fs at 750 nm central wavelength) double pulses with a pulse peak separation from 10 to 100 fs, using a helium-filled hollow core fiber (HCF) and chirped mirror compressor. Crucial to our scheme are simulation-based modifications to the spectral phase and amplitude of the oscillator seed pulse to eliminate the deleterious effects of self-focusing and nonlinear phase pickup in the chirped pulse amplifier. The shortest pulse separations are enabled by tunable nonlinear pulse splitting in the HCF compressor.

Polarity reversal of wakefields driven by ultrashort pulse laser.

Using an analytical model and computer simulation, we show that the wakefield driven by an ultrashort laser pulse in high-density plasma periodically reverses its polarity due to the carrier-envelope phase shift of the driver. The wakefield polarity reversal occurs on spatial scales shorter than the typical length considered for electron acceleration with the laser-wakefield mechanism. Consequently, the energies of accelerated electrons are significantly affected. The results obtained are important for the laser-wakefield acceleration under the conditions relevant to present-day high-repetition-rate laser systems.