Stimulated far-infrared emission from combined cyclotron resonances in germanium

We have measured the spectrum of stimulated far-infrared emission from Be- and Zn-doped germanium single crystals in crossed electric and magnetic fields. The spectrum consists of broad radiation bands in the frequency range of $40--130{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ that are due to intervalence band transitions and new narrow emission bands with their center frequencies rising linearly with magnetic field. We demonstrate that the latter emission is due to cyclotron resonance hole transitions between Landau levels of the light-hole subband accompanied by simultaneous spin flip, ``combined resonances.''