Kip, Detlef

By post-etching annealing, the roughness of microchannels fabricated by fs laser assisted selective etching could be reduced to values of 2 nm. The influence of inscription parameters and annealing conditions on the microchannels’ roughness and the evolution of their shape with annealing temperature and time have been investigated. A functional dependence enabling the estimation of roughness values resulting from certain processing parameters was determined. The low surface roughness achieved enables the transport of fluids with very low friction factors and optical-grade surfaces for the combination of microfluidic channels with optical waveguides, allowing the acousto-optical, electro-optical, and (nonlinear) optical properties of lithium niobate to be utilized for future monolithic optofluidic devices.

Optical damage resistant ridge waveguides for blue and near UV wavelengths have been fabricated using high-temperature Zr and Zn diffusion doping and vapor transport equilibration (VTE) of congruent LiTaO_3 crystals. For both dopants high optical damage thresholds >10 MW/cm^2 for 532 nm light were demonstrated at room temperature, which can be increased by a factor ~3 when heating the samples to ~150°C. Ridge waveguides with low optical losses of ~0.4 dB/cm were fabricated using diamond-blade dicing. First-order periodic poling with grating periods of ~3 μm can be used for efficient nonlinear frequency conversion for both SHG (800 nm pump) or SPDC (400 nm pump) processes.

3D-hollow microstructures with few tens of micrometer in diameter with an optical-quality surface roughness (R_a ≤ 1 nm) have been fabricated in the volume of lithium niobate by selective etching of fs-laser written structures and post-etching annealing. The fs-laser writing parameters and the annealing process have been refined to reduce the average surface roughness and the shape change. Systematically investigating the annealing process, an empirical functional description of the temporal evolution of the surface roughness was found completing the data set of processing parameters for selective etching of fs-laser written structures, allowing to control the fabrication process of the hollow microstructures concerning both shape and surface roughness. Thus, our results represent another milestone within the research towards monolithic micro-(opto)fluidic applications inside the multifunctional crystal lithium niobate.

We report on the study of high-temperature zirconium in-diffusion in congruent z-cut lithium tantalate crystals. Zirconium (Zr) concentration profiles in samples prepared at different diffusion times and temperatures were investigated using secondary neutral mass spectrometry. From the diffusion profiles specific diffusion constant and activation energy were obtained. Zr-induced near-surface refractive index changes were extracted by means of prism-coupler measurements, and their correlation with Zr concentration was studied. Subsequently, low-loss optical ridge waveguides were diced and their resistance to photorefractive optical damage in the blue (405 nm) and green (532 nm) spectral ranges was measured, demonstrating damage thresholds in the multi-MW/cm^2 range.

We study second harmonic generation in LiNbO₃ nanowaveguides, operated with sub-pJ pulses at 1424 nm in the picosecond regime generating 0.5 μW at 712 nm. Spectra recorded at both wavelengths provide evidence for enhanced χ(2) cascading effects.

Innovative terahertz waveguides are in high demand to serve as a versatile platform for transporting and manipulating terahertz signals for the full deployment of future six-generation (6G) communication systems. Metal-wire waveguides have emerged as promising candidates, offering the crucial advantage of sustaining low-loss and low-dispersion propagation of broadband terahertz pulses. Recent advances have opened up new avenues for implementing signal-processing functionalities within metal-wire waveguides by directly engraving grooves along the wire surfaces. However, the challenge remains to design novel groove structures to unlock unprecedented signal-processing functionalities. In this study, we report a plasmonic signal processor by engineering topological interface states within a terahertz two-wire waveguide. We construct the interface by connecting two multiscale groove structures with distinct topological invariants, i.e., featuring a π-shift difference in the Zak phases. The existence of this topological interface within the waveguide is experimentally validated by investigating the transmission spectrum, revealing a prominent transmission peak in the center of the topological bandgap. Remarkably, we show that this resonance is highly robust against structural disorders, and its quality factor can be flexibly controlled. This unique feature not only facilitates essential functions such as band filtering and isolating but also promises to serve as a linear differential equation solver. Our approach paves the way for the development of new-generation all-optical analog signal processors tailored for future terahertz networks, featuring remarkable structural simplicity, ultrafast processing speeds, as well as highly reliable performance.

The coupling of electronic and nuclear motion in polyatomic molecules is at the heart of attochemistry. The molecular properties, transient structures, and reaction mechanism of these many-body quantum objects are defined on the level of electrons and ions by molecular wave functions and their coherent superposition, respectively. In the present contribution, we monitor nonadiabatic quantum wave packet dynamics during molecular charge motion by reconstructing both the oscillatory charge density distribution and the characteristic time-dependent nuclear configuration coordinate from time-resolved Auger electron spectroscopic data recorded in previous studies on glycine molecules [Schwickert et al. Sci. Adv. 2022, 8, eabn6848]. The electronic and nuclear motion on the femtosecond time scale was induced and probed in kinematically complete soft X-ray experiments at the FLASH free-electron laser facility. The detailed analysis of amplitude, instantaneous phase, and instantaneous frequency of the propagating many-body wave packet during its lifecycle provides unprecedented insight into dynamical processes beyond the Born–Oppenheimer approximation. We are confident that the refined experimental data evaluation helps to develop new theoretical tools to describe time-dependent molecular wave functions in complicated but ubiquitous non-Born–Oppenheimer photochemical conditions.

Highly photorefractive optical damage resistant ridge waveguides for near UV and short-wavelength visible ranges have been fabricated using high-temperature diffusion doping with different metal ions and vapor transport equilibration method of commercially available congruently melting LiTaO_3 crystals.

Femtosecond laser written and selectively etched 3D-micro-optofluidic devices have been fabricated with high selectivity (~2900) and low roughness (~23 nm), combining a waveguide with a microchannel in the volume of a monolithic lithium niobate chip.

We demonstrate a terahertz plasmonic signal processor by introducing defects into a two-wire waveguide. The defects are achieved by connecting two multiscale structures with inverted unit cells, leading to the occurrence of a transmission peak in the forbidden band.