THz and Physics
THz waves: a tool for new physics
Terahertz spectroscopy makes it possible to probe matter and study physical phenomena that are of great interest to fundamental science.
On the platform, we apply this spectroscopy to the study of materials with unusual properties that have numerous potential applications, such as graphene, topological insulators, Dirac semimetals, and more. The materials studied include HgCdTe, InAs/GaSb superlattices, and semiconductor nanowires.
Terahertz waves are composed of very low-energy photons. This energy range corresponds to a large number of excitations in solids, such as phonons, plasmons, magnons, as well as spin transitions or transitions between Landau levels. Terahertz spectroscopy therefore makes it possible to effectively probe matter and study these physical phenomena, which are of great interest to fundamental science.
When a quantizing magnetic field is applied to a crystal, it separates 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, as in semiconductors. However, the energy of the Landau levels is proportional to the square root of the applied magnetic field when the system has no gap, as in graphene, the surface of topological insulators, or in three-dimensional Dirac semimetals. Magneto-absorption spectroscopy experiments in the THz range therefore make it possible to accurately measure the gap or electron mass of these materials. These experiments are of great interest in the study of topological phase transitions in materials such as HgCdTe or InAs/GaSb. For example, we have recently been able to continuously control the mass of electrons in a single crystal, to the point of canceling it out, by changing its temperature. This discovery makes it possible 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 device, which allows the measurement of very low-energy optical transitions in the THz frequency range as a function of temperature and magnetic field. These results suggest both applications in THz optoelectronics and new studies of topological insulators.
Suppressed Auger scattering and tunable light emission of Landau-quantized massless Kane electrons
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| Cyclotron emission spectra measured on a gapless HgCdTe sample kept in liquid helium at selected values of the magnetic field | Experimentally deduced maxima of emission compared with theoretically expected cyclotron- modes. The LL spectrum with schematically depicted emission lines (red arrows) is shown in the inset. |
We have demonstrated cyclotron emission of massless electrons. This emission was observed in gapless HgCdTe – a system hosting 3D massless Kane electrons. The existence of sizeable cyclotron emission is directly related to the particular LL spectrum, which comprises only non-equidistantly spaced levels. Systems hosting massless Kane electrons are thus promising candidates for the active medium of a LL laser, which would, in this particular case, 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 volume 13, pages 783–787(2019)