Abstract
This thesis presents an investigation of the Terahertz/Infrared emission and absorption spectroscopy of various Dirac materials in magnetic fields. The studied materials reach from (mono-/multi-layer) graphene over different topological insulators (TIs) to novel Dirac-/Weyl-semimetals.One of the main emphasis is the investigation of magnetically tuneable emission, known as Cyclotron Resonance (CR) or Landau emission. The first ideas of such emission from bulk semiconductors dates back to the late 1950s, and let to the development of the magnetically tuneable p-Ge laser. In recent years, the emergence of new gapless materials, especially graphene, reignited the interest in this technique and the development of related wide tunable THz sources. Generally, CR refers to the resonant absorption of electromagnetic waves by charged particles located in a constant magnetic field. Landau emission is the inverse process, where the charged particles emit electromagnetic waves at a certain magnetic field imposed frequency. A fundamental limitation of this process is the existence of non-radiative Auger processes, which prevail over the radiative recombination channels. Such Auger processes are especially strong in materials with equidistant Landau levels (LLs). Considerable hope was therefore pinned on graphene where highly non-equidistant LLs would not only suppress the self-absorption losses but also allow an increased tunability, covering the THz range at moderately low magnetic fields. However, even if the LLs in graphene are not equidistant in energy, one can always find corresponding subsets which are, again promoting the non-radiative recombination. Consequently, no emission from Landau quantized graphene was reported to date. Even so, there are other materials with massless carries, similar to graphene, where the harmful subsets are broken due to additional contributions. One such example is bulk Hg1-xCdxTe close to the critical concentration, where Landau emission was recently reported and which is further investigated in this work. Another related system is that of HgTe/CdTe quantum wells (QWs) close to the critical thickness or slightly above, where a small bandgap makes them TIs. Here, the experimental results and complementary theore tical calculations show, that the emission, arising in the incipient Landau quantization regime, is favored both by the finite rest-mass and the specific band dispersion of HgTe QWs. The latter breaks the series of equidistantly spaced LL subsets and thus suppresses non-radiative Auger recombination inherent in graphene. At the same time and against all expectations, an emission signal was also observed from some graphene samples. The preliminary results show that the emission may be linked to the semi-classical CR, but deeper analysis is needed to confirm these first results and understand this phenomenon. Finally, some emission has also been obtained from Dirac-/Weyl-semimetals. However, at this point the emission study of these materials is still at the beginning and further investigation is needed.Along with the CR emission, some TIs have also been studied in magneto-absorption. Proven representatives of such 2D TIs, apart from HgTe/CdTe QWs, are InAs/GaSb broken gap QWs. A more recent variety of the latter one are InAs/GaxIn1-xSb/InAs trilayer QWs (TQWs). Here, a major task was the development of devices with semi-transparent gate. Complementary to that, some preliminary magneto-absorption measurements verify the bandstructure calculations of these novel TQWs. On the more common HgTe/CdTe QWs, the magneto-absorption study revealed an additional many-particle contribution, when considering optical transitions close to the magnetic field induced topological phase transition, i.e. the (anti-)crossing of the zero-mode LLs. This constitutes an addition to the established theoretical model so far used to describe such QWs.