Ultrafast IR and THz spectroscopy of photo-induced insulator to metal transition in highly correlated organic system

Photo-induced insulator to metal transitions (PIMT) in quarter filled layered organic conductors ET bis(ethylenedithio)tetrathiafulvalene]-based salts α-(ET)₂I₃, θ-(ET)₂RbZn(SCN)₄, and κ-(d-ET)₂CuN(CN)₂Br] were investigated using ultrafast spectroscopy in the near, mid-infrared and terahertz (THz) regions. In charge ordered salts α-(ET)₂I₃ and θ-(ET)₂RbZn(SCN)₄, an immediate (<30 fs) generation of a microscopic metallic state is driven by the electronic process. Subsequently, condensation of the microscopic metallic domain to the macroscopic scale is accompanied by a small molecular rearrangement in α-(ET)₂I₃. However, in θ-(ET)₂RbZn(SCN)₄, a large structural difference between the insulator and metallic phases prevents stabilization of the macroscopic metallic state. In a dimer Mott insulator κ-(d-ET)₂CuN(CN)₂Br], photo-generation of the metallic state shows a finite rise time of ca. 1 ps, which is attributable to the photo-induced change of on-site Coulomb energy on each dimer (U_dim) through dimeric molecular rearrangement. Thus, the ultrafast dynamics of PIMT depend strongly on the molecular arrangement in the layer of ET salts.