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.

Automated summary

This study reports the observation of cyclotron resonance transitions of holes in gallium-doped germanium at low temperatures. Intense, pulsed THz free-electron laser radiation is used to create free holes and induce their cyclotron resonance. The CR peaks of the holes shift to higher (or lower) magnetic field with increasing laser intensity.