Peter Christianen, HFML Nijmegen.

The development of pulsed, high-intensity terahertz free-electron lasers (FELs) has enabled the investigation of nonlinear phenomena in molecules and solids in the terahertz regime. To expand their potential, recently THz FELs were combined with high magnetic fields in some laboratories around the world. At HFML-FELIX, researchers used FLARE, a terahertz FEL, in combination with a 33 tesla DC magnet to investigate photoionization and cyclotron resonance in the galliumdoped semiconductor germanium over a wide range of THz intensities.
Nonlinear processes in hole-doped germanium (p-Ge) are relevant for the development of THz lasers and THz technology. Despite the fact that p-Ge has been studied before, also in high magnetic fields, several outstanding questions still remain, which are mostly related to the non-parabolic band structure of the valence band, the appearance of non-equidistant Landau levels, and the lack of a clear understanding of the light-induced ionization of the dopant atoms. In this study, the use of a wide range of photon energies (1.5 – 11 meV or 12 – 89 cm-1) enables the investigation of the laser-frequency dependence on the onset of the THz photoionization and to distinguish different regimes.
At high photon energies (> 6.8 meV or 55 cm-1), we observed both internal transitions within the Ga dopants and cyclotron resonance (CR) transitions of free holes, where the latter dominate at higher laser intensities (see figure). For energies below the lowest internal Ga transition (< 6.8 meV), multiple CR transitions are seen, corresponding to the four lowest-energy Landau-level transitions of the heavy hole and light hole subbands. The appearance of these lowest energy CR peaks marks the onset of photoionization, which is found to be more efficient for lower FEL frequencies. With increasing FEL intensity, complex behavior of the CR signals is observed, where the heavy- (light-) hole CR peaks mostly shift to higher (lower) magnetic field with increasing radiation intensity. This is a result of the saturation of the CR transitions and an increasing occupation of the higherenergy, non-equidistant Landau levels with increasing FEL radiation intensity. This study, thus, provides new insights about the actual photoionization mechanism, the saturation of the CR transitions, and the excitation of higher-energy Landau levels. The spectroscopic technique used here offers an exciting perspective for researching nonlinear magneto-optical processes in solid-state materials.

Figure: The combination of intense, pulsed THz free-electron laser radiation with high static magnetic fields provides new exciting perspectives for the investigation of nonlinear magnetooptical processes in solid-state materials.

Nonlinear terahertz transmission spectroscopy on Ga-doped germanium in high magnetic fields, B. Bernáth, P. Gogoi, A. Marchese, D. Kamenskyi, H. Engelkamp, D. Arslanov, B. Redlich, P. C. M. Christianen, and J. C. Maan, Phys. Rev. B 105, 205204 (2022).

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.205204

Contact: peter.christianen@ru.nl