Abstract
Charge acceleration during the application of an intense light field to solids attracts much attention as elementary
processes in high-harmonic generation and photoelectron emission. For manipulating such attosecond
dynamics of charge, carrier-envelope-phase (CEP: relative phase between carrier oscillation of
light field and its envelope function) control has been employed in insulators, nanometal and graphene.
In superconducting materials, charge motion is expected to be controlled by taking advantage of the
strongly coherent nature of quasi-particles.
Here, in a layered organic superconductor, a non-linear petahertz current driven by a single-cycle 6 femtosecond
near infrared field shows up as second harmonic generation (SHG), which is in contrast to the
common belief that even harmonics are forbidden in the centrosymmetric system. The SHG represents a
CEP sensitive nature and an enhancement near the superconducting temperature. The result and its
quantum many-body analysis indicate that a polarized current is induced by non-linear acceleration of
charge, which is amplified by superconducting fluctuations.
