Figure 3. Nonlinear JPWs in LBCO_9.5.

a1, Normalized spectrally integrated pump-probe response $\frac{\text{Δ}{E}_{\text{probe}}}{E_{\text{probe}}}\left(t,{\tau }_0=2\text{ ps}\right)$. Experimental data are shown along with calculations based on the sine-Gordon equation in nonlinear regime (displayed with a vertical offset). Dashed lines indicate an average reduction that accompanies the oscillations (see model). The reduction was subtracted through Fourier filtering (>0.2 THz) to obtain the oscillations shown in a2. The signal buildup region affected by perturbed free induction decay is shaded in grey. b, Fourier transform of the extracted oscillations, showing a peak at ~1 THz. c, Phase θ_i,i+1(x,t) and (d) corresponding oscillator strength f induced by a strong THz pump field, as determined by numerically solving the sine-Gordon equation in nonlinear regime. Horizontal dotted lines indicate the spatial coordinate x at which the line cuts are displayed (lower panels).