  The temporal sidelobes of few-cycle pulses seriously restrict their
applications in ultrafast science. We propose a unique mechanism that enables
the generation of sub-two-cycle pulses with high temporal quality based on
soliton self-cleaning in air-plasma channels. A robust spatiotemporal soliton
could be formed from pulse self-compression by modulating the dispersion of the
air-plasma channel via the adjusted plasma density. Due to ionization, the
blue-shifted soliton with a larger group velocity captures the leading
sidelobes whereas the plasma generated by the soliton defocuses the trailing
sidelobes, which are eventually eliminated after a long-distance propagation.
The self-cleaning of spatiotemporal soliton leads to no sidelobes in the
temporal profile of the sub-two-cycle pulse. The required density of the
preformed plasma for arbitrary central wavelength from near-infrared to
mid-infrared regime is predicted theoretically and confirmed by (3D+1)
simulations.
