Nonlinear parameters of Dirac fermions in HgTe structures: trivial and topological

High harmonic generation (HHG) has applications in various fields, including ultrashort pulse measurements, material characterization and imaging microscopy. Strong THz nonlinearity and efficient third harmonic generation (THG) were demonstrated in graphene [1], therefore it is natural to assume the presence of the same effect in other Dirac materials, such as topological insulators (TI) [2,3]. Topological states can be found in HgTe quantum wells with a thickness of more than 6.3 nm [4], and strained 3D Hg1-xCdxTe thin films with cadmium fraction x < 0.16 [5]. We used a series of HgTe samples corresponding to three qualitatively different cases: 2D trivial and topological structures and 3D topological insulators. By using moderate THz fields, the presence of highly efficient THG was measured in these samples at different temperatures and THz powers. This provides insight into physical mechanisms leading to THG in TIs. For in-depth understanding of Dirac fermions dynamics and dominating scattering mechanisms in HgTe TI, we
conducted THz pump-probe experiments that reveal several relaxation time scales.

[1] Hafez, H. A. et al., Nature 561, 507 (2018).
[2] Kovalev, S. et al. Quantum Materials 6.1 (2021): 1-6.
[3] Giorgianni, F. et al. Nature Сommunications 7.1 (2016): 1-6.
[4] Bernevig, B. et al. Science 314, 5806 (2006): 1757-1761.
[5] Brüne, C. et al. Phys. Rev. Lett. 106, 12 (2011): 126803.