  Intense light-field application to solids produces enormous/ultrafast
non-linear phenomena such as high-harmonic generations 1, 2 and attosecond
charge dynamics 3, 4. They are distinct from conventional photonics. However,
main targets have been limited to insulators and semiconductors, although
theoretical approaches have been made also for correlated metals and
superconductors 5. Here, in a layered organic superconductor, a non-linear
charge oscillation driven by a nearly single-cycle strong electric field of >10
megavolts /cm is observed as a stimulated emission. The charge oscillation is
different from a linear response and ascribed to a polar charge oscillation
with a period of 6 fs. This non-linear polar charge oscillation is enhanced by
critical fluctuations near a superconducting transition temperature and a
critical end point of first order Mott transitions. Its observation on an
ultrafast timescale of 10 fs clarifies that the Coulomb repulsion plays an
essential role in superconductivity of organic superconductors.
