In this study, we performed two-dimensional terahertz(2D-THz) spectroscopy in La-based
high-T c cuprate superconductors, La 2-x Sr x CuO 4 (LSCO) and La 1.6-x Nd 0.4 Sr x CuO 4 (LNSCO) to
investigate the interplay between the superconductivity and the stripe order. In both
systems, nonlinear THz signals with the electric fields polarized along c-axis were clearly
observed in the superconducting phase. In LSCO, the nonlinear signal appeared only
below the critical temperature of bulk superconductivity (T c 3D ), indicating the development of
c-axis superconducting coherence only below T c 3D . The temperature dependence of the
nonlinear signal manifests its origin as the Josephson plasma resonance along c-axis. By
contrast, in LNSCO with x=0.15 in which the charge and spin stripe order appears, the
nonlinear THz signal persisted at temperatures well above T c 3D , suggesting the existence of
superconducting coherence in the stripe phase. The experimental result, i.e. the
appearance of nonlinear THz response above T c 3D , despite the lack of the Josephson
plasma resonance in the reflectivity spectrum, was well reproduced by assuming a pair
density wave (PDW) order in the stripe phase. In the presentation, we will report the details
of 2D-THz spectroscopy measurements and the analysis of nonlinear Josephson current
based on the PDW order.