  Advancing ultrafast high-repetition-rate lasers to shortest pulse durations
comprising only a few optical cycles while pushing their energy into the
multi-millijoule regime opens a route towards terawatt-class peak powers at
unprecedented average power. We explore this route via efficient
post-compression of high-energy 1.2 ps pulses from an Ytterbium InnoSlab laser
to 9.6 fs duration using gas-filled multi-pass cells (MPCs) at a repetition
rate of 1 kHz. Employing dual-stage compression with a second MPC stage
supporting a close-to-octave-spanning bandwidth enabled by dispersion-matched
dielectric mirrors, a record compression factor of 125 is reached at 70%
overall efficiency, delivering 6.7 mJ pulses with a peak power of about 0.3 TW.
Moreover, we show that post-compression can improve the temporal contrast at
picosecond delay by at least one order of magnitude. Our results demonstrate
efficient conversion of multi-millijoule picosecond lasers to high-peak-power
few-cycle sources, opening up new parameter regimes for laser plasma physics,
high energy physics, biomedicine and attosecond science.
