Table of Contents:
This thesis presents investigations on the ultrafast dynamics of excited electrons and
photocurrents in the topologically protected Dirac-cone-like surface state of threedimensional
topological insulators. The focus is on the study of the prototype materials
Sb2Te3 and Bi2Te3. Probably most important for possible future applications
is the discovery, that THz radiation can drive electric currents in the Dirac cone of
these materials. These currents consist of spin-polarized electrons which travel ballistically
without losses over several hundreds of nanometers. Based on this, it might
be possible to realize new light-wave driven electronics in the future, combining low
power consumption and clock rates that exceed those of conventional semiconductor
devices by at least three orders of magnitude.
The experiments use time- and angle-resolved photoelectron spectroscopy (tr-
ARPES) in combination with newly developed schemes for the transient direct excitation
of electrons and photocurrents. Ultrashort mid infrared (MIR) and THz pump
pulses are combined with femtosecond ultraviolet probe pulses for photoemission.
Fundamentally new results could be obtained on three different topics.
Two-photon photoemission experiments with visible pump pulses unambiguously
show, for the first time, the existence of the linear dispersing Dirac cone in the bulk
band gap of Sb2Te3 and Sb2Te2Se. Both materials are intrinsically p-doped, such that
the Dirac cone was previously not accessible by conventional ARPES. Time-resolved
experiments reveal that the decay of indirectly excited electrons in the Dirac-cone is
dominated by scattering with electrons in the partially unoccupied valence band, as
well as by transport into the bulk. Electron-phonon scattering is shown to play only
a minor role.
Mid infrared (MIR) pump pulses with photon energies in the range of the bulk
band gap enable a direct optical transition from the occupied into the unoccupied
part of the Dirac cone in Sb2Te3. This new excitation mechanism allows for the direct
generation of an asymmetric electron distribution in momentum space. The timeresolved
investigation of the redistribution of this asymmetry makes it possible to gain
quantitative information about elastic momentum scattering, which is the limiting
mechanism for charge transport. Elastic scattering times as long as 2.5 ps confirm
theoretical predictions about considerable restrictions on momentum scattering in
the surface state, due to its special spin texture. The results further show that the
asymmetry depends on the crystal orientation and can be controlled with circularly
polarized light of opposite helicity.
Finally, a novel time- and angle-resolved photoemission experiment based on excitation
in the THz range was developed and successfully demonstrated. For this
purpose, the apparatus for photoelectron spectroscopy of our group in Marburg was
combined with a laser setup of the group of Rubert Huber at the university of Regensburg
which can produce intensive THz pump pulses. Femtosecond UV probe
pulses allow for time-resolved photoemission with subcycle resolution. The new experiment
makes it possible to drive electron currents in the topological surface state of
Bi2Te3 with an electric field strength of up to 2,8 kV/cm at frequencies around 1 THz.
The acceleration of the electrons in the sample was directly observed in momentum
space with ARPES. The displacement of the Fermi circle of almost 10% of the Fermi
wavevector leads to a spin-polarized current in the surface state with a current density
of up to 2 A/cm.
An experimentally challenging aspect of the THz excitation originates from the
interaction of the electric field of the pump pulses with the photoemitted electrons in
the vacuum, which can no longer be neglected as in previous tr-ARPES experiments.
Methods were developed to correct the photoelectron spectra from additional shifts
in kinetic energy and parallel momentum that arise from the acceleration and deceleration
of the photoelectrons. At the same time, this so-called energy and momentum
streaking makes it possible to sample the electric field at the surface in situ with high
The precise knowledge of the electric field transient makes it possible to extract
characteristic times for elastic and inelastic scattering for the experiment. For this
purpose, the time-resolved observation of the electron distribution in the surface state
is compared with semi-classical calculations based on the Boltzmann-equation. In
Bi2Te3, scattering times of 1 ps or higher could be derived. Thus, the revealed scattering
times in this genuine time-resolved transport measurement are at least two orders
of magnitude longer than in conventional materials. Combined with the inertia-free
acceleration in the quasi-relativistic Dirac cone, this results in a ballistic mean free
path of several hundred nanometers. Together, these results considerably improve
our understanding of electron dynamics and strong-field interaction in novel solids.
At the same time, they open the way towards all-coherent lightwave-driven electronic