Dawn of nonlinear optics in the soft X-ray region

This article explores the generation of intense soft X-ray pulses and its application to nonlinear multiphoton processes. Using such nonlinear processes, the temporal width of the 42 eV soft X-ray pulse was measured directly by an autocorrelation technique. A train of attosecond frequency-pulses was also characterized by frequency-resolved autocorrelation. Intense high harmonics produced by the phase-matching technique enable the observation of these nonlinear optical processes