Ultrafast polarization control in κ−(BEDT−TTF)2X

Abstract

We present a theoretical investigation of the dynamics induced by higher-frequency off-resonant light pulse excitation in the metallic phase of $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}X$ [where BEDT-TTF is bis(ethylenedithio)-tetrathiafulvalene and $X$ represents a counteranion] by solving the time-dependent Schr\"odinger equation numerically in the quarter-filled extended Hubbard model for the material. If the magnitude of the applied light pulse exceeds a critical value, the transition to the charge-ordered (CO) state is then induced by the light pulse excitation. In the CO state, one site forming a dimer becomes charge rich, the other site becomes charge poor, and the intradimer electric dipole moments are aligned in a particular direction. The transition is driven by photoinduced reduction of the effective transfer integrals and can therefore be regarded as a dynamical localization-related phenomenon. There are two degenerate A phase and B phase CO states that have opposite polarization directions. The photogenerated CO state is given by the superposition of the A phase and B phase CO states with equal weights, and the polarization of the photogenerated CO state is effectively zero. Transitions to either of the two polarized CO states can be induced selectively, and the polarization can be generated by excitation using light and terahertz pulses. Furthermore, the polarization can be reversed on a timescale of several tens of femtoseconds by excitation using double light and terahertz pulses, which induce the following phase transitions: metal $\ensuremath{\rightarrow}$ polarized CO state of the A (B) phase $\ensuremath{\rightarrow}$ metal $\ensuremath{\rightarrow}$ polarized CO state of the B (A) phase $\ensuremath{\rightarrow}$ metal.