In the presence of a high-intensity optical field, electrons are released from atoms on an attosecond time scale. Moreover,
in the tunnelling regime, this process displays a strong sensitivity to the carrier-envelope phase (CEP) of a few-cycle
light pulse. Tunnelling ionization - a fascinating quantum mechanical phenomenon - leads to a quasi-stepwise increase
of free electron density and, as a consequence, of the refractive index of the medium. These steps of the refractive index,
corresponding to half-cycles of the driving optical field, impose a transient attosecond phase mask. By scattering probe
light off this mask we detect quasi-periodic higher-order harmonics, the spectrum of which, unlike that of the harmonics
originating from intrinsic nonlinearity or driven by electron re-collisions, do not depend on the probe intensity and recollision
dynamics. The implemented noncollinear pump-probe experimental technique allows optical harmonics
generated due to a tunnelling-ionization-induced modulation of the electric current to be spatially separated from the
harmonics originating from atomic and ionic nonlinear susceptibilities, enabling background-free time-resolved detection
of electron-tunnelling-controlled harmonic spectra and offering an attractive solution for attosecond optical metrology of
gases and bulk solids.
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