Coherent dynamics of photoinduced phase formation in a strongly correlated organic crystal

The photoinduced phase formation in a strongly correlated crystal $(\mathrm{EDO-TTF}{)}_2{\mathrm{PF}}_6$ (EDO-TTF: 4,5-ethylenedioxytetrathiafulvalene) is investigated using a 12 fs laser pulse. The formation time is determined as 40 fs with observation of coherence of electron-phonon coupled excited states prior to formation. The temperature-independent dephasing time is determined as ∼22 fs up to 180 K and the frequency of phonon oscillation is ∼38 THz, corresponding to the intramolecular vibrations in EDO-TTF. The phase formation is coherently controlled by relative-phase-controlled two-pulse excitation.

Figure 1

(a) Schematic diagram of charge distributions and EDO-TTF molecule deformation in the low-temperature phase and photoinduced phase of (EDO-TTF)₂PF${}_6$. (b) Optical conductivity spectrum for the low-temperature phase of (EDO-TTF)₂PF${}_6$ at 10 K (solid line) and the laser pulse spectrum (shaded area with solid line). Regions A (red), B (blue), and C (green) represent the three different detection regions of the probe pulse. (c) Temporal profiles for ΔR/R probed at regions A (red), B (blue), and C (green). The gray shaded area represents the cross-correlation profile of the pump and probe pulses. (d) Calculated spectrum considering the coupling between the CT2 excited state and intermolecular vibrations (see Supplementary Material for details [31]). The black and red lines show spectra with broadening factors of ε/$t_0$ = 0.5 and 0.1, respectively.

Figure 2

(a) Schematic illustration of the changes in charge distribution for the initial process of PIPT in (EDO-TTF)₂PF${}_6$. (b) and (c) Experimental (circles) and simulated (solid lines) temporal profiles of ΔR/R probed at regions (b) A and (c) B. The dotted and dashed lines represent the first and second components in the fitting function, respectively.

Figure 3

(a) Schematic diagram of the perturbed free induction decay in the negative delay time and the transient reflectivity change in the positive delay time. GS and ES notations correspond to the ground and excited states. NL indicates the light diffracted by the third-order nonlinear process. (b) Magnified view of the temporal profiles in the negative delay time probed in region C at 25 K (green circles) and at 180 K (pink circles). The solid lines indicate the fitting functions.

Figure 4

(a) and (b) Schematic illustrations of the pump and probe pulse sequences in the coherent control experiments. (c) The black circles and solid line represent profiles of ΔR/R measured using the sequence (a). The red dashed lines represent linear autocorrelation calculated from the second harmonic autocorrelation. (d) Temporal profiles of ΔR/R measured using the sequence (b). The red and black circles represent constructive and destructive excitations, respectively. (e) Calculated profiles of the photoinduced phase population assuming the sequence (a). The black solid and the red dashed lines represent the profiles assuming $T_2$ = 26 and 1 fs, respectively. (f) The calculated profiles of the photoinduced phase population assuming the sequence (b). The red and black solid lines represent the population of the photoinduced phase excited by constructive and destructive two-pulse excitations, respectively. The black dashed line represents the population excited by a single pulse.