LAUSR

We report an observation of stimulated emission induced by a nearly single-cycle 6 fs near infrared electric field of 10 MV cm-1 in an organic superconductor (κ-(h-ET)2Cu[N(CN)2]Br). The stimulated emission is attributed to a non-linear synchronized coherent charge oscillation. We also report that, in the same organic superconductor, a light-induced current before the scattering time shows up as carrier-envelope phase (CEP)-sensitive second harmonic generation (SHG). This unconventional SHG in the centrosymmetric compound is regarded as a light induced spatial symmetry breaking. These ultrafast optical non-linearities induced by petahertz charge oscillations show anomalous enhancements around the superconducting transition temperature (11.6 K). These results indicate that the microscopic mechanism of superconducting fluctuations is closely related to the Coulomb repulsive interaction in this compound.