Anomalous photo-induced response by double-pulse excitation in the organic conductor (EDO-TTF)₂PF₆

We measured ultrafast reflectivity changes induced by double-pulse excitation in the organic conductor (EDO-TTF)₂PF₆. Using double-pulse excitation with a relatively high intensity, the sign of reflectivity change became reversed at around 0.8 ps and subsequently the reflectivity change reverted to that of the normal photo-induced state after about 1 ps. Using a optically phase-locked double-pulse with low intensity, we found that the temporal profile excited by an in-phase double-pulse differs from that by an out-of-phase double-pulse despite the time difference between the double-pulses being only 1.31 fs. This was true even when there is almost no overlap between each pulse in the double-pulse. These results indicate that the photo-response in this material to double-pulse excitation differs greatly from the linear sum of the responses to single pulses.