Nonlinear terahertz transmission spectroscopy on Ga-doped germanium in high magnetic fields

We report the observation of cyclotron resonance (CR) transitions of holes in the magnetotransmission spectra of gallium-doped germanium at low temperatures, using intense, pulsed THz free-electron laser radiation with a photon energy lower than the ionization energy of the Ga dopants (11 meV). The THz radiation, in the range of 12-89 cm(-1), both creates free holes through photoionization of Ga and induces the CR of these holes. For photon energies above the lowest energy internal Ga transition (55 cm(-1)), intradopant transitions are simultaneously observed with narrow CR peaks. For energies below 55 cm(-1), with increasing THz radiation intensity first the lowest Landau level transitions of all heavy-hole and light-hole subbands appear. This marks the onset of photoionization, which is found to be more efficient for lower laser frequencies, consistent with field-ionization (Keldysh parameter << 1). For the highest laser intensities, the CR peaks of the heavy (light) holes shift to higher (lower) magnetic field, as a result of the increasing population of the higher-energy nonequidistant Landau levels, consistent with the effective-mass theory of the hole subbands in Ge.