Abstracts Category : Other

Terahertz Laser Spectroscopy of Two- and Three-Dimensional Topological Insulators based on HgTe Nanostructures

by Kathrin-Maria Dantscher

In the framework of this thesis, two- and three-dimensional topological insulators (TI) are investigated with opto-electronic effects in the terahertz range. It is demonstrated that in both cases, photogalvanic effects can be utilized for the detection and the analysis of Dirac fermion states.In the three-dimensional case, a cyclotron-resonance-enhanced photocurrent is observed by exciting 80 nm strained HgTe films with terahertz radiation. The arising resonances could be clearly attributed to the topologically protected top and bottom surface states of the system. Studying these resonant responses in a gated sample, the behaviour of the photocurrent is observed upon a modification of the Fermi energy, which allows the extraction of the cyclotron masses of the top and bottom surface states as a function of the Fermi energy. For larger gate voltages, an additional cyclotron resonance is detected, which is caused by bulk carriers and corresponds to a mass about two times larger than that obtained for the surface states. These results are in full agreement with the band structure of the strained HgTe films, calculated by the kp method. The cyclotron masses of the surface states estimated from this calculation fit well with the experimental ones. Based on the experimental results, a microscopic theory of the resonant photocurrent is developed. It is shown that an orbital effect emerges due to an asymmetric scattering of non-equilibrium surface electrons, resulting in a fully spin-polarized direct electric current, which exhibits a resonant behaviour due to the cyclotron-resonance-enhanced absorbance. Photogalvanic spectroscopy is also used for the investigation of two-dimensional topological insulators. There spin-polarized photogalvanic currents flowing mainly in the edges of the sample are observed by exciting 2D HgTe QWs with circularly polarized terahertz radiation. These currents exhibit a strong helicity-dependence, meaning that the reversal of the radiation helicity reverses the direction of the photocurrent. Moreover, the sign of the photosignal for a fixed helicity is opposite for the opposite edge, which demonstrates that the current indeed flows in the edges of the sample. Such chiral edge photocurrents are detected in a wide range of applied gate voltages, in which they change their sign twice, which allows a division into three different regions. The mechanisms responsible for the generation of the photocurrent strongly depend on the location of the Fermi level within the band structure. For a Fermi level in the conduction band, the helicity-sensitive photocurrent flowing in the bulk can be attributed to the circular photogalvanic effect, whereas the part generated in the edges is attributed to diffusive scattering at the boundaries of the sample together with the action of the electric field of the circularly polarized radiation on the carrier. In contrast to this, in the TI-regime, the photocurrent is caused by a photoionization of helical edge electrons into the conduction band. For this effect, aAdvisors/Committee Members: Ganichev, Sergey D. (advisor).