  Charge acceleration during an intense light field application to solids
attracts much attention as elementary processes in high-harmonic generation and
photoelectron emission [1-7]. 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 [8-10]. In superconducting materials,
collective control of charge motion is expected because of its strongly
coherent nature of quasi-particles.
  Here, in a layered organic superconductor, second harmonic generation (SHG)
is observed by using a single-cycle 6 femtosecond near infrared pulse, which is
in contrast to the common belief that even harmonics are forbidden in the
centrosymmetric system. The SHG shows 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-dissipative
acceleration of charge, which is amplified by superconducting fluctuations.
This will lead to petahertz functions of superconductors and of strongly
correlated systems.
