Coherent Control of Charge and Lattice Dynamics in a Photoinduced Neutral-to-Ionic Transition of a Charge-Transfer Compound

Coherent control of a charge-lattice coupled oscillation associated with the neutral ($N$)-ionic ($I$) transition was successfully demonstrated for a charge-transfer (CT) compound, tetrathiafulvalene–$p$-chloranil, with use of double femtosecond laser pulses. By changing the interval of the two laser pulses, amplitude of the oscillation in the reflectivity for the intramolecular transition band sensitive to the degree of CT was periodically changed. With increase of the pump intensity, the oscillation amplitude was dramatically enhanced. Such an enhancement is discussed in terms of the cooperative nature of the $N\mathrm{\text{−}}I$ transition.

Figure 1

(a) Polarized-reflectivity spectra of TTF-CA in the $I$ phase at 77 K ($R_I$) and in the $N$ phase at 90 K ($R_N$). (b),(c) Photoinduced reflectivity changes ($\Delta R/R$) at 90 K for the IMT band (b) and the CT band (c) for the 1.55 eV excitation with $\mathbf{E}\parallel \mathbf{a}$ ($0.6\times {10}^{16}\mathrm{\text{photons}}/{\mathrm{cm}}^2$). Upper panels of (b),(c) show the differential spectra [$(R_I-R_N)/R_N$].

Figure 2

(a),(b) Time characteristic of $\Delta R/R$ (a) and its oscillatory component (b) at 2.25 eV in the IMT band. (c),(d) Time characteristic of $\Delta R/R$ (c) and its oscillatory component (d) at 1.1 eV in the CT band. The excitation energy and density are 1.55 eV ($\mathbf{E}\parallel \mathbf{a}$) and $0.6\times {10}^{16}\mathrm{\text{photons}}/{\mathrm{cm}}^2$, respectively.

Figure 3

(a) Time evolutions of $\Delta R/R$ at 2.25 eV induced by the single-pulse excitations (the top panel) and by the double-pulse excitation with the different interval $\Delta t$ (the middle and lower panels). The pump pulses are indicated by the red lines. $I_s$ and $I_d$ indicate the excitation density of each pulse for the single-pulse and the double-pulse excitations, respectively, $I_0=1\times {10}^{16}\mathrm{\text{photons}}/{\mathrm{cm}}^2$). (b) The peak-to-peak amplitudes of the oscillations ($\Delta R/R$) as a function of $\Delta t$. (c) Time profiles of $\Delta R/R$ with $\Delta t=0.62\mathrm{ps}(=T)$ for various $I_d$. (d) $\mathrm{Max}\Delta R/R$ (open circles) and the oscillation amplitude (closed circles) as a function of $I_d/I_0$.

Figure 4

(a),(c) Contour maps obtained by continuous wavelet transformation of the oscillations in the IMT band [Fig. 2b] and the CT band [Fig. 2d]. (b),(d) Time-integrated Fourier power spectra of these oscillations. The IR absorption spectrum of the $I$ phase at 75 K is presented by the gray line in (d).