Generation of terahertz radiation in nanometer films:Ge/α-Sn

We observed a strong emission of terahertz radiation, in sandwich nanostructures made of Ge and α-Sn nanometer thick films. Together with the THz emission there arise photocurrent, induced by an irradiation of the structures with ultra-short optical pulses. There a strong generation of photon-drag currents of nonequilibrium Dirac (and/or Weil) electrons has been detected. This result and other discovered novel unique properties of the Ge/Sn multi-layers are indicating on an emergence of a new class of topological materials, where massless electrons have a strong interaction with the irradiated light.