Controlling and monitoring the optical drive motion of electrons within a substance is a key challenge in the field of ultra-fast optical wave electronics and laser material processing. Physicists from the Max Planck Institute in Berlin and the University of Rostock have now revealed a hitherto ignored nonlinear optical mechanism that is generated by light-induced tunnelling of electrons in a dielectric. For the intensity close to the material damage threshold, the nonlinear current generated in the tunneling process is the main source of the strong light outbreak, which is the low order harmonic of the incident radiation. These findings, just published in Nature Physics, greatly expand the basic understanding of optical nonlinearity in dielectric materials and its potential applications in information processing and optical-based material processing.