Lorenz Maximilian Schneider

Quality Control of Sugar Beet Seeds With THz Time-Domain Spectroscopy

Sugar beet seeds have a comparably high rate of defective seeds of about 35%-40% which are discarded before dissemination in the field. The separation of proper and defective seeds is a complex and labor-intensive process. We show how THz time-domain spectroscopy can be used for the examination of the seeds including an algorithm for automated classification of the seeds.

Gate Tuning of Förster Resonance Energy Transfer in a Graphene - Quantum Dot FET Photo-Detector.

Graphene photo-detectors functionalized by colloidal quantum dots (cQDs) have been demonstrated to show effective photo-detection. Although the transfer of charge carriers or energy from the cQDs to graphene is not sufficiently understood, it is clear that the mechanism and efficiency of the transfer depends on the morphology of the interface between cQDs and graphene, which is determined by the shell of the cQDs in combination with its ligands. Here, we present a study of a graphene field-effect transistor (FET), which is functionalized by long-ligand CdSe/ZnS core/shell cQDs. Time-resolved photo-luminescence from the cQDs as a function of the applied gate voltage has been investigated in order to probe transfer dynamics in this system. Thereby, a clear modification of the photo-luminescence lifetime has been observed, indicating a change of the decay channels. Furthermore, we provide responsivities under a Förster-like energy transfer model as a function of the gate voltage in support of our findings. The model shows that by applying a back-gate voltage to the photo-detector, the absorption can be tuned with respect to the photo-luminescence of the cQDs. This leads to a tunable energy transfer rate across the interface of the photo-detector, which offers an opportunity to optimize the photo-detection.

The Impact of the Substrate Material on the Optical Properties of 2D WSe2 Monolayers

Abstract2D-materials, especially transition metal dichalcogenides (TMDs) have drawn a lot of attention due to their remarkable characteristics rendering them a promising candidate for optical applications. While the basic properties are understood up to now, the influence of the environment has not been studied in detail, yet. Here we highlight a systematic comparison of the optical properties of tungsten diselenide monolayers on different substrates. Subtle changes in the emission spectrum and Raman signature have been found as well as surprisingly pronounced differences in the pump-power-dependent and time-resolved output at higher excitation densities. For all samples, exciton–exciton annihilation can be obtained. Nevertheless an analysis of different pump-dependent decay rates suggests substrate-dependent changes in the diffusion constant as well as exciton Bohr radius.

Broadband antireflection coating for optimized terahertz beam splitters

We investigate the potential of anti-ferromagnetic nanofilms as broadband antireflection coatings in the terahertz frequency range. The anti-ferromagnetic layer is modeled by an analytic wave-impedance matching approach. The experimental results of the transmission and reflection measurements demonstrate the effectiveness of our antireflection coatings. Furthermore, we use anti-ferromagnetic nanofilms as antireflection coating for a terahertz beam splitter. Compared with conventional terahertz beam splitters consisting of an uncoated thick silicon wafer, the coated silicon beam splitter has two advantages: elimination of multiple reflections and improvement of the signal-to-noise ratio for terahertz time-domain spectroscopy in reflection geometry.

Density-dependent excitonic properties and dynamics in 2D heterostructures consisting of boron nitride and monolayer or few-layer tungsten diselenide

The understanding of two-dimensional (2D) materials has grown tremendously, especially for isolated monolayers. Recently, complex structures formed by stacking 2D materials have attracted considerable attention. This is in part due to the fact that the properties of monolayers are known to be influenced by their surroundings. Consequently, monolayer properties are predicted to be affected by “heterostructuring”. A study involving high-charge-carrier-density effects and dynamics is presented here for monolayers of WSe2 on different substrates and heterostructures comprised of 2D h-BN and WSe2. The influence of h-BN as well as the bilayer stacking order on the spectral and dynamical properties of WSe2- monolayer emission is discussed for the low-density regime and evidenced for high-density effects such as exciton-exciton annihilation and the Mott transition.