Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Nonlinear optical properties of graphene have been discussed for a more than a decade [1]. Experimentally, the most prominent nonlinear effect is high harmonic generation in the THz range [2]. This phenomenon is caused by a reduced conductivity of hot thermalized electrons. In our study, we investigate an effect beyond the response of thermalized hot carriers, which by nature is isotropic. We observe anisotropic THz-induced bleaching related to a change in effective mass of charge carriers under strong THz excitation.
We investigated monolayer and bilayer graphene on SiC with carrier concentrations of 1.0 x 10^13 cm-2 and 6.5 × 10^12 cm−2, respectively. In degenerate pump-probe experiments at 3.4 THz utilizing linearly polarized radiation, the differential transmission was recorded for co- and cross-polarized probe beams. For bilayer graphene, co-polarized probing yields signals that are about two times larger as compared to the cross-polarized case (cf. Fig. 1) [3]. Since the response of thermalized carriers is isotropic, it cannot explain the observed anisotropic bleaching. We describe the physical origin of the anisotropic nonlinear response using a simple semiclassical model: In essence, the change in effective mass in x-direction differs strongly for carriers excited along or perpendicular to the direction of the probe field. For a quantitative comparison, modelling based on the density matrix formalism with a phenomenological scattering time was performed. For a momentum scattering time of 50 fs good agreement with the experimental data is obtained. The monolayer sample shows qualitatively similar behavior, however, the scaling of the induced transmission with the pump electric field is different. In summary, time-resolved THz nonlinear spectroscopy turns out as a powerful method to explore nonlinearities directly related to the bandstructure of Dirac materials.
[1] S. A. Mikhailov and K. Ziegler, J. Phys.: Condens. Matter 20, 384204 (2008).
[2] H. A. Hafez et al., Nature 561, 507 (2018).
[3] A. Seidl et al., Phys. Rev. B. 105, 085404 (2022).