Investigation of directional THz spin currents in topological surface states

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Topological insulators (TIs) exhibit topologically protected spin-polarized surface states that offer a high application potential for spin electronic devices, preferably working at frequencies reaching the elusive terahertz (THz) window. In our project, we have studied the coupling between surface/bulk electrons, spins and phonons by ultrafast excitation and probing of model 3D TIs such as Bi2Se3. We have predominantly used optical laser pulses (~1.5 eV photon energy) of suitable polarization to selectively excite surface electrons. To probe the resulting spin, charge and transport dynamics, we have developed novel measurement schemes including magneto-optic spin and terahertz (THz) conductivity and current probes. We were able to reveal important features of the ultrafast relaxation of optically spin-polarized electrons as well as novel types of ultrafast surface photocurrents.While the first funding period was focused on optical excitation and single-material samples, we want move to (i) substantially more tailored and selective excitation and (ii) hybrid TI structures in the second period. Goal (i) is achieved by using lower pump photon energies (~1 to 400 meV) in the THz to mid-infrared spectral range. In 3D model TIs such as Bi2Se3, these energies are perfectly suited to directly and selectively drive interband transitions between Dirac states. As these surface states are located in the TI bulk band gap, we expect substantially larger spin-polarization lifetimes, surface current amplitudes and exclusive insights into the intraband surface state dynamics. The dynamics of spins, charges and transport are monitored by the probes developed in the first funding period. Experiments will be accompanied by ab initio electronic-structure and transport theory. Additional control of and insight into the very early ultrafast spin dynamics is achieved by laser and THz pulses of tunable and stabilized carrier-envelope phase. Concerning goal (ii), we will study ferromagnet/TI hybrid structures in terms of the ultrafast dynamics of their excited states. We will reveal the influence of the topological surface state on dynamics of the ferromagnetic order (e.g via spin-transfer torque and optical spin torque) and the reverse effect, the ultrafast spin injection from the ferromagnet into the TI surface and bulk. Using our newly developed spin, conductivity and transport probes, we will study the dynamics of these processes in real time. We expect to gain new insights into the impact of the ferromagnet onto the TI dynamics as well as an estimate of the TI spin Hall angle. Importantly, first applications will emerge: the generation of spin torque in the adjacent ferromagnet induced by TI spin currents and spin-to-charge conversion in the TI surface/bulk states, thereby directly leading to new and efficient emitters of broadband THz radiation.