Terahertz Technology and Physics

Terahertz waves: a tool for new physics

Terahertz spectroscopy makes it possible to probe matter and study the physical phenomena within it that are of great interest to fundamental science.

In our research platform, we apply this spectroscopy to the study of materials with unusual properties that hold great potential for a wide range of applications, such as graphene, “topological insulators,” and “Dirac semimetals”… The materials studied include, for example, HgCdTe, InAs/GaSb superlattices, and semiconductor nanowires.

Terahertz waves consist of very low-energy photons. This energy range corresponds to a wide variety of excitations in solids, such as phonons, plasmons, and magnons, as well as spin transitions and transitions between Landau levels. Terahertz spectroscopy therefore allows for the effective probing of matter and the study of these physical phenomena, which are of great interest to fundamental science.

When a quantizing magnetic field is applied to a crystal, it splits the valence and conduction bands into Landau levels. The energy of these levels is proportional to the applied magnetic field when the system has a parabolic band structure, such as in semiconductors. Conversely, the energy of the Landau levels is proportional to the square root of the applied magnetic field when the system has no band gap, as in graphene, the surfaces of topological insulators, or in three-dimensional Dirac semimetals. Magneto-absorption spectroscopy experiments in the THz range therefore allow for the precise measurement of the band gap or the electron mass in these materials. These experiments are of great interest for the study of topological phase transitions in materials such as HgCdTe or InAs/GaSb. Indeed, for example, we have recently been able to continuously control the electron mass within a single crystal, even to the point of eliminating it, by changing its temperature. This discovery allows us to probe the vicinity of a topological phase transition, during which electrons behave like relativistic elementary particles with a constant and universal velocity.

These parameters were measured using a unique experimental magneto-absorption spectroscopy setup, which enables the measurement of very low-energy optical transitions in the THz frequency range as a function of temperature and magnetic field. These results point to potential applications in THz optoelectronics as well as new avenues for studying topological insulators.

Suppressed Auger scattering and tunable light emission from Landau-quantized massless Kane electrons

Cyclotron emission spectra measured on a gapless HgCdTe sample kept in liquid helium at selected magnetic field values		Experimentally determined emission maxima compared with theoretically predicted cyclotron- modes. The LL spectrum, with schematically depicted emission lines (red arrows), is shown in the inset.

We have demonstrated cyclotron emission from massless electrons. This emission was observed in gapless HgCdTe—a system hosting 3D massless Kane electrons. The presence of significant cyclotron emission is directly related to the specific LL spectrum, which consists solely of non-equidistantly spaced levels. Systems hosting massless Kane electrons are thus promising candidates for the active medium of a low-level laser, which, in this particular case, would operate in the THz and infrared spectral ranges and would be widely tunable by very low magnetic fields.

D. B. But, M. Mittendorff, C. Consejo, F. Teppe, N. N. Mikhailov, S. A. Dvoretskii, C. Faugeras, S. Winnerl, M. Helm, W. Knap, M. Potemski, and M. Orlita
Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons, Nature Photonics, vol. 13, pp. 783–787 (2019)