Fig. 5: Reversal of the ferroelectric polarization in the bulk.
a, Fluence dependence of the A1 modes for τ = 0.08 ps. A vertical offset was applied to each spectrum for clarity. The FSRS peaks reverse sign at the highest fluence of 185 mJ cm−2 (30 MV cm−1 field strength). b, First-principles calculations of the normalized Raman tensors for QIR and QP as a function of the ferroelectric polarization P/P0, where P0 is the value of the static polarization at room temperature and equilibrium. c, Amplitudes of the Raman peaks associated with QIR and QP. Horizontal error bars indicate the systematic uncertainty in the fluence measurements. At fluences above approximately 60 mJ cm−2 (17 MV cm−1 field strength), the reductions of the peak amplitudes begin to exhibit a saturation-like behaviour, possibly indicating a renormalization of the frequency of the driven mode due to a QIR4 term in the crystal potential. d,e, The peak amplitudes as a function of τ at the highest excitation fluence of 185 mJ cm−2 for QIR (d) and QP (e). The calculations were performed in the same manner as those of Fig. 4, except that here it is assumed that a transient sign reversal of the Raman tensor propagates throughout the medium, rather than a simple reduction. The results of b were used to model the corresponding Raman tensors for QIR and QP. The peak amplitudes shown in c–e were extracted via Lorentzian fits to the FSRS spectra. Vertical error bars indicate the uncertainty of the extracted amplitudes as determined from the fit residuals.
