Experimentalphysik und Materialwissenschaften

A monolithic refractive index sensor fabricated by selective etching and fs laser inscription in lithium niobate and characterization with sucrose solutions.

Selective etching of material areas modified by femtosecond laser pulses in the volume of lithium tantalate has been applied to produce hollow microchannels. In a fully monolithic approach, microchannels up to 2.5 mm long with cross sections of 2.5 µm × 20 µm were etched into the crystal volume at a depth of 430 µm. The influence of the laser repetition rate, the pulse energy and the writing speed on the etching time and the etching selectivity was investigated as part of a systematic study. Characteristic process parameters, i.e. selectivity and diffusion coefficient were determined by fitting the etch depth versus time using a superdiffusion model. The obtained parameters are suitable for predicting the results of selective etching for certain process parameters, and thus enable the process to be controlled. A similar study was carried out in sapphire for comparison purposes.

Planar optical waveguides were fabricated in Pr:YLF crystals by ion implantation. In a further step, ridge waveguides were fabricated using precision diamond dicing. These enable strong light confinement and have propagation losses as low as 0.4 dB/cm. To study the influence of ion implantation on the spectroscopic properties, fluorescence and lifetime measurements were conducted in the ridge waveguides. Under blue pumping, small-signal optical gains of 6.5 dB/cm and 5 dB/cm were demonstrated at wavelengths of 607 nm and 639 nm, respectively. These results make ion-implanted ridge waveguides in Pr:YLF promising candidates for compact integrated lasers in the visible spectral region with high output powers in the watt range.

Single-cycle, high-power, high-repetition-rate THz pulse sources are becoming the cornerstone of several scientific and industrial applications. A promising and versatile method for high-power THz generation is optical rectification in nonlinear crystals pumped by powerful near-infrared ultrafast laser systems. In this context, ytterbium-based laser sources are particularly advantageous in terms of power scalability and technology establishment. However, as the repetition rate increases toward hundreds of MHz, the conversion efficiency typically decreases, as most laser systems do not reach sufficiently high average powers to correspondingly enhance the peak power to drive the nonlinear conversion process efficiently. An alternative approach to achieving a sufficiently high average power at a high repetition rate is based on passive enhancement cavities, which boost the pulse energy of standard watt-level ytterbium lasers by orders of magnitude. We present the first demonstration of optical rectification in a passive enhancement cavity at multi-kW levels, achieved by a 240-fold power enhancement. By irradiating a 50-μm thin lithium niobate plate with 1.9-kW average power inside the enhancement cavity, we generate milliwatt-level THz pulses with 2-THz bandwidth and 93-MHz repetition rate, mostly limited by the driving pulse duration. To the best of our knowledge, this represents the highest driving average power used for optical rectification. This methodology represents a promising new step toward high-repetition-rate and high average power single-cycle THz sources using widely available multi-watt level Yb lasers.

We report on the efficient second harmonic generation at a near-UV wavelength of 399.7 nm in periodically poled Zr-doped LiTaO3 waveguides. High-temperature in-diffusion of ZrO2 layers into congruent LiTaO3 substrates with simultaneous improvement of crystal stoichiometry by vapor transport equilibration in a Li-rich atmosphere was performed for the fabrication of planar waveguides. This new method enables very high photorefractive damage thresholds and allows us to avoid the known disadvantages of, e.g., proton exchange or titanium in-diffusion waveguide fabrication methods, which lower resistance to photorefractive damage. Low-loss ridge waveguides were then fabricated using a diamond blade saw. At a maximum available coupled power of 540 mW of the near-infrared pump laser, 49 mW of second harmonic light was generated in a 22 mm long sample with a normalized conversion efficiency of 3.5%·W-1·cm-2, exceeding the previously reported powers for LiTaO3 waveguides in this wavelength range. Furthermore, by heating the sample, a temperature tuning coefficient of 32 pm/°C was measured for the generated second harmonic wave.

Gegenstand dieser Arbeit ist die hochpräzise Herstellung von Silizium-Lamellengitterspiegeln für die reflektierende Split-and-Delay-Unit (SDU) im extremen Ultraviolett- und weichen Röntgenbereich (Photonenenergien von 10 eV bis ~1 keV). Um die gute Reflektivität der Spiegel zu gewährleisten, war es notwendig, eine gute Oberflächenqualität zu erhalten, um die geringe Rauheit zu bewahren und die auftretenden Verformungen zu minimieren. Es wurden zwei verschiedene Hochpräzisions-Herstellungstechniken angewandt: a) nasschemisches Ätzen und b) Diamantsägeblattschneiden. Das nasschemische Ätzen in KOH führte zu ultratiefen (1 mm und tiefer) Durchätzgittern, jedoch wurde die Siliziumoberfläche während des Ätzvorgangs durch die Ätzmaske aus SiNx-Multilayer auf SiO2 ständig angegriffen, was zu zahlreichen Defekten führte, was diese Technik für die Herstellung von Lamellengittern ungeeignet machte. Neue, qualitativ hochwertige Lamellengitterspiegel wurden mit Hilfe eines Diamantscheiben-Dicing-Verfahrens hergestellt. Die im Rahmen dieses Projekts neu entwickelten Dicing-Protokolle sorgen für eine größere Stabilität der Diamantscheibe während des Schneidevorgangs, wodurch die Verschiebung der Gitterstäbe aufgrund von Verformungen und Materialabplatzungen am Rand der Schnittfuge verringert wird. Die Oberflächenrauheit der neu hergestellten lamellaren Gitter wurde von den zuvor erreichten ~4 nm auf ~2 nm erheblich verbessert. Dank der verbesserten Probenstabilität während der Herstellung konnte auch die effektive Länge der hergestellten Lamellengitterspiegel mehr als verdoppelt werden, nämlich von 6 mm auf 13 mm. Dadurch kann die neu hergestellte SDU bei der Hälfte des Einfallswinkels der früheren Aufbauten betrieben werden. Dies verbessert das Reflexionsvermögen des Geräts erheblich, erhöht seine Präzision und erweitert die Kurzwellenlängengrenze der reflektierten Strahlung auf mindestens 600 eV, was ein großer Vorteil für weitere Untersuchungen der ultraschnellen Elektronendynamik ist, die die Elementspezifität des weichen Röntgenbereichs nutzen.

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.