Spatiotemporal carrier dynamics in graphene

Doctoral thesis, 2019

Graphene as an atomically thin material exhibits remarkable optical and
electronic properties that suggest its technological application in novel optoelectronic
devices, such as graphene-based photodetectors and lasers. To understand
the properties of such devices on a microscopic level, we study the
interplay of optical excitation, carrier-carrier, carrier-phonon, and carrierphoton
scattering as well as diffusion processes. We apply a microscopic
model based on the density matrix formalism with spatiotemporal graphene
Bloch equations in its core. This approach provides microscopic access to
the temporally, spectrally and spatially resolved carrier dynamics both in
the presence and absence of an electric field, allowing the study of manyparticle
mechanisms behind photodetection and gain in graphene.
The focus of this thesis lies in modelling optics, dynamics and transport
phenomena on consistent microscopic footing. We predict the possibility to
achieve a stable population inversion in graphene, which is the crucial prerequisite
for using graphene as an active material in a nanolaser. Further,
we provide microscopic insights into the impact of an electric field on the
carrier dynamics revealing the appearance of an efficient dark carrier multiplication
that can even enhance the field-induced current. We also provide a
microscopic foundation for the photoconduction and the bolometric effect as
important mechanisms in a graphene based photodetector. Furthermore, we
provide insights into the spatiotemporal dynamics of optically excited carriers,
which create density and temperature gradients resulting in a diffusion
of carriers. The gained insights can be used to study the thermoelectric effect
and dynamics at interfaces of spatial inhomogeneities.