  function of the CEP, we observe a transition from a modulated to a continuous
SHHG spectrum, indicating the transition from double to high-contrast isolated
attosecond pulse emission. Single shot-acquisitions of XUV spectral continua
support isolated attosecond pulses with isolation degrees between 10 and 50 for
the majority of the driving pulse CEPs, which represents a significant
improvement in terms of isolation degree as well as repetition rate over
previous results obtained with 2-cycle drivers at 10-Hz. 2D Particle-in-cell
simulations corroborate this interpretation and predict percent-level
efficiencies for the generation of an isolated attosecond pulse even without
any spectral filtering.
