Fig. 4: Probing topological material via vectorial SPRINT.
a, An x-polarized attosecond pulse train in the XUV range (black) and a y-polarized IR pulse (red) interact with a Haldane solid in its topological or trivial phase. The nonlinear interaction induces coherent XUV emission in both polarization components (black and orange). The y-polarized component is heterodyned by its interference with an external reference XUV pulse (magenta). b, Fourier analysis of the orthogonal SPRINT signal, revealing the individual spectral contributions of distinct wave-mixing processes. FWM±, four-wave-mixing processes. c, Reconstructed waveform of the y component of the oscillating electric field, comparing between the topological and trivial cases for the FWM− process. A few cycles of the IR central frequency are plotted as a guide to the eye. d, Same data as c, for the x-polarized component. After ~6 fs, the y component of the two waveforms is shifted by ~π. Such a shift is completely absent in the x-polarized components.
