Hasse, Kore

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.

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.

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.

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.

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.

Ultrafast laser driven, single-cycle THz pulsed sources hold immense potential for scientific and industrial applications; however, their limited average power hinders their widespread application. In particular, applications where high repetition rates in the multi-MHz region and beyond are required are more severely affected, due to the lower pulse energies available for frequency conversion. In this respect, resonant enhancement both in passive and active resonators is a well-known technique for boosting the efficiency of nonlinear frequency conversion; however, this route has remained poorly explored for the generation of broadband THz pulses due to the inadequacy of typically employed nonlinear crystals. Here, we demonstrate that using thin lithium niobate crystals inside multimode diode-pumped mode-locked thin-disk lasers is a promising platform to circumvent these difficulties. Using a 50 µm thin lithium niobate plate intracavity of a compact high-power mode-locked thin-disk laser, we generate milliwatt-level broadband THz pulses with a spectrum extending up to 3 THz at 44.8 MHz repetition rate, driven by 264 W of intracavity average power. This approach opens the door to efficient high-power single-cycle THz generation using affordable nonlinear crystals at very high repetition rates, scalable to kilowatt-level driving power with low cost and complexity.

Here, we report on the fabrication of cm-long microchannels in LiNbO_3 by selective etching of femtosecond laser inscribed tracks using hydrofluoric acid. We achieved a 1 cm long microchannel after 300 h of etching a single track inscribed into the volume along the optical axis of LiNbO_3. Furthermore, we investigated the dependence of the etching behavior on various writing parameters. Highly selective etching with a selectivity up to 10^4 was achieved and a functional relationship between the etched depth and time was found. Thus, our results set the first milestone for future fabrication of 3D-hollow microstructures in the volume of LiNbO_3 combining its outstanding physical properties such as the strong nonlinearity as well as the acousto- and electrooptic properties with both microfluidic and photonic structures in a monolithic setup.