https://www.sciencedaily.com/releases/2020/06/200630111507.htm

Our current understanding of non-linear optics at moderate light intensities is based on the so-called Kerr non-linearity, which describes the non-linear displacement of tightly bound electrons under the influence of an incident optical light field. This picture changes dramatically when the intensity of this light field is sufficiently high to eject bound electrons from their ground state. At long wavelengths of the incident light field, this scenario is associated with the phenomenon of tunneling, a quantum process where an electron performs a classically forbidden transit through a barrier formed by the combined action of the light force and the atomic potential.

Since the 1990's and pioneered by studies from the Canadian scientist François Brunel, the motion of electrons that have emerged at the "end of the tunnel," which happens with maximal probability at the crest of the light wave, has been considered as an important source for optical non-linearity. This picture has now changed fundamentally. "In the new experiment on glass, we could show that the current associated with the quantum mechanical tunneling process itself creates an optical non-linearity that surpasses the traditional Brunel mechanism," explains Dr. Alexandre Mermillod-Blondin from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy, who supervised the experiment. In the experiment, two ultrashort light pulses with different wavelengths and slightly different propagation directions were focused onto a thin slab of glass, and a time- and frequency-resolved analysis of the emerging light emission was performed.