Optoelectronic Phenomena Induced by Terahertz/Infrared Laser Radiation in Topological Insulators and Graphene

Within this thesis it is demonstrated that the excitation of systems, which are characterized by a linear dispersion, with polarized terahertz radiation results in the generation of optoelectronic phenomena. In three dimensional topological insulators several nonlinear effects are observed, among these are the linear photogalvanic and photon drag effect, the circular photogalvanic effect, and the trigonal photon drag effect. In high mobility graphene, edge photogalvanic effects are verified in two different regimes, the semi-classical and the quantum Hall one. All experimental results and their discussion are shown in two parts. In the first part results on Bi2Te3- and Sb2Te3-based three dimensional topological insulators, studied at room temperature and for a wide range of excitation frequencies, are presented. Due to symmetry filtration between the centrosymmetric bulk and the non-centrosymmetric surface, a photogalvanic current is excited in the surface states. It is verified, that the role of a competing photon drag current, being allowed in the bulk states too, is excluded by measurements using front and back illumination. It is shown, that the photocurrents give insight in the high frequency transport properties of the materials and allow an estimation of scattering times and mobilities. Moreover, it is stated that the photogalvanic effect can be applied to characterize the materials and provides information on the local domain orientation and uniformity of the high frequency transport. Furthermore, photocurrents are generated in two excitation regimes: In the lower THz range, where Drude-like free carrier absorption dominates and the current formation is treated semi-classically, and in the higher THz range, where direct optical transitions, possibly from the Dirac cone into bulk bands, dominate. For the second case, a helicity sensitive contribution to the current is observed. Changing the experimental geometry allowed the observation of the trigonal photon drag effect, which is caused by the additional transfer of the in-plane photon momentum. All experimental findings are supported by models and microscopic theories, which describe all characteristics features. In the second part results on edge photocurrents in high mobility graphene are presented. It is shown that the current at zero magnetic field stems from scattering in the vicinity of the edge and is formed in a narrow strip given by the mean free path. The discussion reveals that in this case the edge photocurrent direction is determined by the polarization and the carrier type. Strikingly and in contrast to this, the results further revealed that for graphene being in the quantum Hall regime, the observed edge currents are determined solely by the sign of the magnetic field. The study demonstrates that they are generated by optical transitions within the chiral edge channels. All these findings are supported by a microscopic theory

Infrared/terahertz spectra of the photogalvanic effect in (Bi,Sb)Te based three-dimensional topological insulators

We report on the systematic study of infrared/terahertz spectra of photocurrents in (Bi, Sb) Te based three-dimensional topological insulators. We demonstrate that in a wide range of frequencies, ranging from fractions up to tens of terahertz, the photocurrent is caused by the linear photogalvanic effect (LPGE) excited in the surface states. The photocurrent spectra reveal that at low frequencies the LPGE emerges due to free carrier Drude-like absorption. The spectra allow us to determine the room temperature carrier mobilities in the surface states despite the presence of thermally activated residual impurities in the material bulk. In a number of samples we observed an enhancement of the linear photogalvanic effect at frequencies between 30 and 60 THz, which is attributed to the excitation of electrons from helical surface to bulk conduction band states. Under this condition and applying oblique incidence we also observed the circular photogalvanic effect driven by the radiation helicity

Edge photocurrent driven by terahertz electric field in bilayer graphene

We report on the observation of edge electric currents excited in bilayer graphene by terahertz laser radiation. We show that the current generation belongs to the class of second order in electric field phenomena and is controlled by the orientation of the THz electric field polarization plane. Additionally, applying a small magnetic field normal to the graphene plane leads to a phase shift in the polarization dependence. With increasing the magnetic field strength, the current starts to exhibit 1/B-magneto-oscillations with a period consistent with that of the Shubnikov-de Haas effect and amplitude by an order of magnitude larger as compared to the current at zero magnetic field measured under the same conditions. The microscopic theory developed shows that the current is formed in the edge's vicinity limited by the mean-free path of carriers and the screening length of the high-frequency electric field. The current originates from the alignment of the free carrier momenta and dynamic accumulation of charge at the edges, where the P-symmetry is naturally broken. The observed magneto-oscillations of the photocurrent are attributed to the formation of Landau levels

Photon drag effect in (Bi1−xSbx)2Te3 three-dimensional topological insulators

We report on the observation of a terahertz radiation-induced photon drag effect in epitaxially grown nand p-type (Bi1-xSbx)(2)Te-3 three-dimensional topological insulators with different antimony concentrations x varying from 0 to 1. We demonstrate that the excitation with polarized terahertz radiation results in a dc electric photocurrent. While at normal incidence a current arises due to the photogalvanic effect in the surface states, at oblique incidence it is outweighed by the trigonal photon drag effect. The developed microscopic model and theory show that the photon drag photocurrent can be generated in surface states. It arises due to the dynamical momentum alignment by time-and space-dependent radiation electric field and implies the radiation-induced asymmetric scattering in the electron momentum space. We show that the photon drag current may also be generated in the bulk. Both surface states and bulk photon drag currents behave identically upon variation of such macroscopic parameters as radiation polarization and photocurrent direction with respect to the radiation propagation. This fact complicates the assignment of the trigonal photon drag effect to a specific electronic system

Production and differentiation of myeloid cells driven by proinflammatory cytokines in response to acute pneumovirus infection in mice

Respiratory virus infections are often pathogenic, driving severe inflammatory responses. Most research has focused on localized effects of virus infection and inflammation. However, infection can induce broad-reaching, systemic changes that are only beginning to be characterized. In this study, we assessed the impact of acute pneumovirus infection in C57BL/6 mice on bone marrow hematopoiesis. We hypothesized that inflammatory cytokine production in the lung upregulates myeloid cell production in response to infection. We demonstrate a dramatic increase in the percentages of circulating myeloid cells, which is associated with pronounced elevations in inflammatory cytokines in serum (IFN-γ, IL-6, CCL2), bone (TNF-α), and lung tissue (TNF-α, IFN-γ, IL-6, CCL2, CCL3, G-CSF, osteopontin). Increased hematopoietic stem/progenitor cell percentages (Lineage¯Sca-I⁺c-kit⁺) were also detected in the bone marrow. This increase was accompanied by an increase in the proportions of committed myeloid progenitors, as determined by colony-forming unit assays. However, no functional changes in hematopoietic stem cells occurred, as assessed by competitive bone marrow reconstitution. Systemic administration of neutralizing Abs to either TNF-α or IFN-γ blocked expansion of myeloid progenitors in the bone marrow and also limited virus clearance from the lung. These findings suggest that acute inflammatory cytokines drive production and differentiation of myeloid cells in the bone marrow by inducing differentiation of committed myeloid progenitors. Our findings provide insight into the mechanisms via which innate immune responses regulate myeloid cell progenitor numbers in response to acute respiratory virus infection

Edge currents driven by terahertz radiation in graphene in quantum Hall regime

We observe that the illumination of unbiased graphene in the quantum Hall regime with polarized terahertz laser radiation results in a direct edge current. This photocurrent is caused by an imbalance of persistent edge currents, which are driven out of thermal equilibrium by indirect transitions within the chiral edge channel. The direction of the edge photocurrent is determined by the polarity of the external magnetic field, while its magnitude depends on the radiation polarization. The microscopic theory developed in this paper describes well the experimental data