Table of Contents:
In the past years Germanium has gained much attention due to its good optical properties.
This lead to a constant improvement of the material quality archived with these material system.
Today there are high quality bulk and quantum well samples available. Also the desired strain of these structures can be tailored.
Using one of these samples different optical phenomena were observed. The physical properties of Germanium lead to several unique behaviors of this material, compared to direct gap semiconductors.
Despite the indirect band gap, Germanium has a local conduction band minimum at the Gamma point, which is the reason for its often direct semiconductor like behavior.
The spectroscopic analysis of a (GaIn)As sample under the presence of strong THz pulses reveal an interexcitonic Autler-Townes effect. This is one of the first times this effect is has ever been observed. By applying a microscopic many body theory, the exact coherent dynamic is determined. This reveals that the typically used two level systems are not able to correctly describe the dynamics. The full microscopic calculations show that the excition population is reaching far into the continuum states and is not confined to a limited number of states as implied by using a 2 level system. This result is also reproduced for Germanium samples, despite of the much shorter dephasing time in this material.
In this thesis also the dynamical Stark-effect is studied in a Germanium quantum well sample. For the first time this effect is measured as a function of the energetic detuning of the exciting light pulse in a range of several 100meV. Using a driving electrical field, which is almost resonant to the lowest excitonic resonance, a combination of several different effects is observed. Beside an energetic blue shift of the resonances, as known from the dynamical Stark effect, also the formation of a Mollow-Triplet, a modulation signal in the continuum states and optical amplification without carrier inversion are observed.
In the incoherent time regime the optical properties of Germanium are mainly influenced by the electron scattering into the L-valleys and the cooling dynamic of the hole system. The electronic scattering time to the L-valleys is measured in dependence of the sample temperature. It varies from 270fs at a temperature of 100K to 192fs at 250K. This fast scattering of the electrons allows to easily determine the scattering rate between the different hole spin states. When creating a spin polarized hole population at a sample temperature of 7K a spin scattering time of 2,5ps is observed. At increasing sample temperature this decay time decreases to 1,1ps at 250K.
In the following time regime, the observed dynamic is attributed to the cooling of the hole system. The time scale of this process is in the order of several hundreds of ps. Using a theoretical model, it is shown that that even under resonant excitation of the quantum well samples, the initial temperature of the hole carrier distribution is over 1000K. The energy necessary for this high temperature comes from an efficient coupling to the electronic system. The dynamic of the carrier temperature shows a reduction of the cooling rate around a temperature of 200K. This observation shows the transition to a regime, in which the cooling is dominated by optical phonons to a cooling only by acoustical phonons.
In the end the optical gain dynamic in strained Germanium layers is discussed. A transient gain is observed in a n-doped Germanium sample, lasting for approximately 1ns. By etching of a resonator structure on this sample, the formation of an emission line in the photoluminescence spectrum is observed. This line is created by stimulated spontaneous emission in the sample.
In conclusion, the potential of Germanium for the silicon photonics is yet unclear. The demonstration of optically pumped lasers based on this material gives hope for the construction of electrically pumped lasers. The realization of such a device could be the important step towards the realization of integrated opto-electronical circuits. However it turns out to be, the availability of high quality germanium nanostructures definitely opens a door to a deeper understanding of the excitation dynamics in this material.