Emission and Absorption of Light by Hot Electrons in Multivalley   Semiconductors (TERAHERTZ Range)
  The angular dependences of the spontaneous emission by hot electrons in multivalley semiconductors are studied theoretically and experimentally using $n$-Ge as an example. We demonstrate that the change in the scattering mechanism caused by the growth of electron temperature can affect the angular scattering dependence. In the case when the heating field is applied along the symmetry axis of the crystal [for $n$-Ge it is the axis (1,0,0,)], the angular dependence of the emission was observed experimentally for the first time, and the corresponding theory is proposed. When electrons have identical concentration and temperature in every valley, the angular dependence of emission is shown to be related to the violation of symmetry of the energy distribution of electrons (from the theoretical viewpoint, this effect means going beyond the scope of the traditional diffusion approximation). 
P.M. Tomchuk, V.M. Bondar