Ultrafast quantum optics with solid state nanosystems
Semiconductor quantum dots
Semiconductor quantum dots are one of the most promising candidates for implementing robust qubits in quantum information processing. The possibility to optically initialize, manipulate and read out the charge and spin state on ultrafast timescales makes them very attractive. We use femtosecond pump-probe experiments to study the ultrafast dynamics in single semiconductor quantum dots .
To reach the limit of only a single photon manipulating a single electron in a quantum dot the coupling of light with a wavelength of a few hundred nanometers into the object of nanometer dimensions has to be optimized. We mainly study two nanophotonic elements to reach this goal: dielectric microcavities [2, 3] and metallic nanoantennas [4, 5].
Color centers in diamond
Alternatively, we investigate color centers in diamond. Since their first optical characterization on a single emitter level, they have been used as robust single-photon sources. We optically study these nanoscopic light emitters, which operate at room-temperature [6, 7]. Photonic nanostructures are designed and fabricated to increase light coupling.
Atomically thin semiconductors
Graphene is an exceptional two-dimensional material, but a zero band gap semiconductor. In contrast, a monolayer of MoS2 has a band gap and emits photons in the visible regime. We have shown that MoSe2 and WSe2 also shine bright in monolayer form .
- Quantum optics
We found bright and stable single-photon emitters in monolayer WSe2  and could position them on the nanoscale . We showed that the van der Waals semiconductor GaSe also hosts single-photon emitters  and coupled the non-classical light to a dielectric waveguide on an optical chip . We also investigated the emitter-phonon coupling in hBN .
One of the outstanding properties of atomically thin materials is their ability to withstand high strain levels of about 10% without breaking. Strain strongly modifies the electronic band structure and the fundamental optical transitions. We tuned the exciton energy in monolayer WSe2 with uniaxial tensile strain . The change of the electronic band structure is accompanied with a change of the electron-phonon coupling. We demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain . As a consequence, also the Strokes shift - the energy difference of optical absorption and emission - can be deliberately changed. We find that this effect becomes large in TMDC bilayers .We also showed that CVD-grown MoS2 monolayers are well-suited for strain experiments on a wafer scale .
- Ultrafast optics
We elucidated the ultrafast valley dynamics for atomically thin WS2 , which are governed by various couplings [19-22].
- Nonlinear optics
The prototypical nonlinear process second harmonic generation (SHG) is a powerful tool to gain insight into nanoscale materials, because of its dependence on crystal symmetry. We performed ultra-broadband SHG spectroscopy of atomically thin semiconductors by using few-cycle femtosecond infrared laser pulses . Our experimental technique provides the calibrated frequency-dependent nonlinear susceptibility of atomically thin materials.
We measured the g-factor of excitons in monolayer MoTe2  and explained the magnetic-field-induced rotation of polarized light emission from monolayer WS2 .
- Optical nanostructures
We coupled a metal nanoantenna to atomically thin WS2 and enhanced the photoluminescence by one order of magnitude .
- Excitons and trions
Excitons are quasiparticles made of electrons and holes. In atomically thin semiconductors, they have very high binding energies. Hence, they exist even at room temperature in these materials. Based on their spin, they can be either optically bright or dark. We have shown that dark excitons play a crucial role in understanding the light absorption and emission of atomically thin TMDC semiconductors . Excitons with an extra electron or hole are called trions. They are found in doped semiconductors, because they require an extra charge. We found the first excited state (2s) of trions . Another line of research is excitons, which are delocalized between multiple layers. We detected these so-called "interlayer excitons" in bulk crystals [29,30] and investigated their optical response under strain in a MoS2 bilayer . We found the first excited state (2s) of trions .
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