Terahertz-Field-Induced Large Macroscopic Polarization and Domain-Wall Dynamics in an Organic Molecular Dielectric

A rapid polarization control in paraelectric materials is important for an ultrafast optical switching useful in the future optical communication. In this study, we applied terahertz-pump second-harmonic-generation-probe and optical-reflectivity-probe spectroscopies to the paraelectric neutral phase of an organic molecular dielectric, tetrathiafulvalene-$p$-chloranil and revealed that a terahertz pulse with the electric-field amplitude of $\sim 400\mathrm{kV}/\mathrm{cm}$ produces in the subpicosecond time scale a large macroscopic polarization whose magnitude reaches $\sim 20\%$ of that in the ferroelectric ionic phase. Such a large polarization generation is attributed to the intermolecular charge transfers and breathing motions of domain walls between microscopic neutral and ionic domains induced by the terahertz electric field.

Figure 1

(a) Molecular structures of TTF and CA. (b),(c) 1D molecular stacks in the neutral phase (b) and the ionic phase (c). $P$ is the spontaneous polarization. (d) Schematic setups of terahertz-pump SHG probe and optical-reflectivity-probe measurements. (e) A waveform of the terahertz electric field $E_{\mathrm{THz}}(t)$. (f) A time evolution of the SH intensity at 90 K normalized by the SH intensity at 65 K ($\mathrm{\Delta }{I}_{\mathrm{SHG}}(t)/I_{\mathrm{SHG}-65\mathrm{K}}$). (g) A time evolution of $\mathrm{\Delta }R(t)/R$ at 90 K. Shaded areas in (f) and (g) show ${[E_{\mathrm{THz}}(t)]}^2$.

Figure 2

(a) Time evolutions of terahertz-field-induced reflectivity changes $\mathrm{\Delta }R(t)/R$ at various temperatures. (b) Temperature dependence of the first and the second peak structures (circles and triangles, respectively). The terahertz electric-field waveform $[E_{\mathrm{THz}}(t)]$ is shown in Fig. 1.

Figure 3

(a) An $I_{+}$ domain with a dipole moment $+\mu$ and an $I_{-}$ domain with a dipole moment $-\mu$, which are generated in the neutral phase. In the right figures, $\rho$ values are expressed as $\pm 1$ for simplicity. (b) Time evolutions of 1D ionic domains. The terahertz electric field increases (decreases) the size of the $I_{+}(I_{-})$ domain via intermolecular CTs (red curved arrows) and changes $\rho$ by $\mathrm{\Delta }\rho (-\mathrm{\Delta }\rho )$ within the ionic domain (yellow curved arrows). Subsequently, coherent oscillations of NIDW pairs occur. (c) The large macroscopic polarization produced by microscopic change of 1D ionic domains.

Figure 4

(a) Normalized time evolutions of $\mathrm{\Delta }R(t)/R$, $\mathrm{\Delta }\rho (t)$ [$\propto E_{\mathrm{THz}}(t)$], and $\mathrm{\Delta }N(t)$ at 140 K. The solid line in the top panel is a fitting curve. (b) Typical simulation results of $\mathrm{\Delta }R(t)/R$ with the fitting curves (solid lines). (c) Temperature dependence of the frequency $\omega$ and the decay time ${\tau }_0$ of the NIDW oscillation.