Publikationen

Passively mode-locked semiconductor disk lasers have received tremendous attention from both science and industry. Their relatively inexpensive production combined with excellent pulse performance and great emission-wavelength flexibility make them suitable laser candidates for applications ranging from frequency-comb tomography to spectroscopy. However, due to the interaction of the active medium dynamics and the device geometry, emission instabilities occur at high pump powers and thereby limit their performance potential. Hence, understanding those instabilities becomes critical for an optimal laser design. Using a delay-differential equation model, we are able to detect, understand, and classify three distinct instabilities that limit the maximum achievable pump power for the fundamental mode-locking state and link them to characteristic positive-net-gain windows. We furthermore derive a simple analytic approximation in order to quantitatively describe the stability boundary. Our results enable us to predict the optimal laser-cavity configuration with respect to positive-net-gain instabilities and therefore may be of great relevance for the future development of passively mode-locking semiconductor disk lasers.

As the quantity of waste tires increases, more pyrolysis carbon black (CBp), a type of low value-added carbon black, is being produced. However, the application of CBp has been limited. Therefore, it is necessary to identify and expand applications of CBp. This work focuses on the preparation of activated carbon (AC) from CBp using the physicochemical activation of carbon dioxide (CO2) and potassium hydroxide (KOH). Thereafter, AC is applied to the electrode of the electrical double-layer capacitor (EDLC). The AC prepared by CO2/KOH activation exhibited a hierarchical pore structure. The specific surface area increased from 415 to 733 m^2 g^-1, and in combination with low ash content of 1.51%, ensured abundant ion diffusion channels and active sites to store charge. The EDLC comprising the AC (AC-2) electrode prepared by excitation of CO2 (300 sccm) and KOH had a reasonable gravimetric specific capacitance of 192 F g^-1 at 0.5 A g^-1, and exhibited a good rate capability of 73% at 50 A g^-1 in a three-electrode system. Moreover, the EDLC device comprising the AC-2 electrode delivered excellent cycling stability (capacitance retention of 106% after 10000 cycles at 2 A g^-1 in a two-electrode system). Furthermore, a symmetric supercapacitor based on an AC electrode that exhibits a supreme energy density of 4.7 Wh kg^-1 and a maximum power density of 6362.6 W kg^-1 is demonstrated.

We consider the singular optimal control problem of minimizing the energy supply of linear dissipative port-Hamiltonian descriptor systems. We study the reachability properties of the system and prove that optimal states exhibit a turnpike behavior with respect to the conservative subspace. Further, we derive a input-state turnpike toward a subspace for optimal control of port-Hamiltonian ordinary differential equations with a feed-through term and a turnpike property for the corresponding adjoint states toward zero. In an appendix we characterize the class of dissipative Hamiltonian matrices and pencils.

Recently, data-driven predictive control of linear systems has received wide-spread research attention. It hinges on the fundamental lemma by Willems et al. In a previous paper, we have shown how this framework can be applied to predictive control of linear time-invariant descriptor systems. In the present paper, we present a case study wherein we apply data-driven predictive control to a discrete-time descriptor model obtained by discretization of the power-swing equations for a nine-bus system. Our results show the efficacy of the proposed control scheme and they underpin the prospect of the data-driven framework for control of descriptor systems.

We study the origin and formation of antiphase domains (APDs) and related defects in 7 nm thin, lattice-matched GaP buffer layers deposited by metal-organic chemical vapor deposition (MOCVD) on well-defined, nearly single-domain, double-layer stepped, low-miscut Si(100) substrates obtained by specific treatment with arsenic. Using dark-field imaging modes in low-energy electron microscopy (LEEM), the minority reconstruction domains of Si(100):As and the APDs of the deposited GaP epilayer are identified, quantified, and compared. We show that residual (2x1)-reconstructed terraces of the minority domain on the Si substrate cause the formation of APDs and that the fraction of the minority domain of the substrate (≅0.07) entails a comparable fraction of APDs in thin GaP epilayers. The topographies of APDs are revealed by atomic force microscopy (AFM) and by scanning tunneling microscopy (STM). We observe two very different APD-related defects in the GaP epilayer, both pinned to residual monolayer steps of the substrate. GaP growth on minority domain terraces with widths in the range of 40-100 nm gives rise to APDs of comparable lateral dimensions. Minority domain terraces of the substrate with widths <20 nm cause the formation of 7-20 nm wide trenches in the GaP layer with rampart-like mounds along their rims. Using nanoscale Auger electron spectroscopy (AES), we provide evidence that these trenches extend through the GaP layer down to the exposed, uncovered Si substrate. We conclude that nucleation of GaP on small minority domain terraces is largely inhibited as most Ga and P atoms deposited on these terraces diffuse across the domain boundary and side walls of emerging trenches to adjacent majority domain terraces where they form the observed mounds. Nucleation of GaP does take place on minority domain terraces with widths ≥40 nm and leads to the growth of APDs.