Pierre Lorenz

Crystallization of Ge2Sb2Te5 thin films by nano- and femtosecond single laser pulse irradiation

The amorphous to crystalline phase transformation of Ge2Sb2Te5 (GST) films by UV nanosecond (ns) and femtosecond (fs) single laser pulse irradiation at the same wavelength is compared. Detailed structural information about the phase transformation is collected by x-ray diffraction and high resolution transmission electron microscopy (TEM). The threshold fluences to induce crystallization are determined for both pulse lengths. A large difference between ns and fs pulse irradiation was found regarding the grain size distribution and morphology of the crystallized films. For fs single pulse irradiated GST thin films, columnar grains with a diameter of 20 to 60 nm were obtained as evidenced by cross-sectional TEM analysis. The local atomic arrangement was investigated by high-resolution Cs-corrected scanning TEM. Neither tetrahedral nor off-octahedral positions of Ge-atoms could be observed in the largely defect-free grains. A high optical reflectivity contrast (~25%) between amorphous and completely crystallized GST films was achieved by fs laser irradiation induced at fluences between 13 and 16 mJ/cm2 and by ns laser irradiation induced at fluences between 67 and 130 mJ/cm2. Finally, the fluence dependent increase of the reflectivity is discussed in terms of each photon involved into the crystallization process for ns and fs pulses, respectively.

Nanosecond laser-induced phase transitions in pulsed laser deposition-deposited GeTe films

Phase transformations between amorphous and crystalline states induced by irradiation of pulsed laser deposition grown GeTe thin films with nanosecond laser pulses at 248 nm and pulse duration of 20 ns are studied. Structural and optical properties of the Ge-Te phase-change films were studied by X-ray diffraction and optical reflectivity measurements as a function of the number of laser pulses between 0 and 30 pulses and of the laser fluence up to 195 mJ/cm2. A reversible phase transition by using pulse numbers ≥ 5 at a fluence above the threshold fluence between 11 and 14 mJ/cm2 for crystallization and single pulses at a fluence between 162 and 182 mJ/cm2 for amorphization could be proved. For laser fluences from 36 up to 130 mJ/cm2, a high optical contrast of 14.7% between the amorphous and crystalline state is measured. A simple model is used that allows the discussion on the distribution of temperature in dependency on the laser fluence.

Laser-induced front side etching of fused silica with short and ultra-short laser pulses

The patterning or figuring of fused silica, e.g. for optical components, requires sophisticated methods. The usage of laser radiation enables a fast as well as high-quality machining of transparent materials. In particular, the laser-induced front side etching (LIFE) method has an excellent potential for nm-precision structuring of dielectrics with a high surface quality. At the LIFE process the laser beam interacts with an absorber layer on top of the front side of the dielectric surface to be machined. Here, the LIFE of fused silica is studied by using laser radiation with a wavelength from ultraviolet to infrared with pulse durations from nanosecond to femtosecond. With all investigated laser sources a well-defined, nm-precise etching of fused silica by the LIFE process is possible. A linear dependence of the etching depth on the laser fluence can be found whereas etching depths up to 300 nm can be achieved. The optimal laser fluence ranges as well as the achievable etching depths are dependent on the laser radiation used for the LIFE process.

Ion-exchange molecularly imprinted polymer for the extraction of negatively charged acesulfame from wastewater samples

Acesulfame is a known indicator that is used to identify the introduction of domestic wastewater into water systems. It is negatively charged and highly water-soluble at environmental pH values. In this study, a molecularly imprinted polymer (MIP) was synthesized for negatively charged acesulfame and successfully applied for the selective solid phase extraction (SPE) of acesulfame from influent and effluent wastewater samples. (Vinylbenzyl)trimethylammonium chloride (VBTA) was used as a novel phase transfer reagent, which enhanced the solubility of negatively charged acesulfame in the organic solvent (porogen) and served as a functional monomer in MIP synthesis. Different molecularly imprinted polymers were synthesized to optimize the extraction capability of acesulfame. The different materials were evaluated using equilibrium rebinding experiments, selectivity experiments and scanning electron microscopy (SEM). The most efficient MIP was used in a molecularly imprinted-solid phase extraction (MISPE) protocol to extract acesulfame from wastewater samples. Using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS-MS) analysis, detection and quantification limits were achieved at 0.12μgL(-1) and 0.35μgL(-1), respectively. Certain cross selectivity for the chemical compounds containing negatively charged sulfonamide functional group was observed during selectivity experiments.

Nanosecond laser-induced back side wet etching of fused silica with a copper-based absorber liquid

Cost-efficient machining of dielectric surfaces with high-precision and low-roughness for industrial applications is still challenging if using laser-patterning processes. Laser induced back side wet etching (LIBWE) using UV laser pulses with liquid heavy metals or aromatic hydrocarbons as absorber allows the fabrication of well-defined, nm precise, free-form surfaces with low surface roughness, e.g., needed for optical applications. The copper-sulphatebased absorber CuSO4/K-Na-Tartrate/NaOH/formaldehyde in water is used for laser-induced deposition of copper. If this absorber can also be used as precursor for laser-induced ablation, promising industrial applications combining surface structuring and deposition within the same setup could be possible. The etching results applying a KrF excimer (248 nm, 25 ns) and a Nd:YAG (1064 nm, 20 ns) laser are compared. The topography of the etched surfaces were analyzed by scanning electron microscopy (SEM), white light interferometry (WLI) as well as laser scanning microscopy (LSM). The chemical composition of the irradiated surface was studied by energy-dispersive X-ray spectroscopy (EDX) and Fourier transform infrared spectroscopy (FT-IR). For the discussion of the etching mechanism the laser-induced heating was simulated with finite element method (FEM). The results indicate that the UV and IR radiation allows micro structuring of fused silica with the copper-based absorber where the etching process can be explained by the laser-induced formation of a copper-based absorber layer.

Nanostructuring of sapphire using time-modulated nanosecond laser pulses

The nanostructuring of dielectric surfaces using laser radiation is still a challenge. The IPSM-LIFE (laser-induced front side etching using in-situ pre-structured metal layer) method allows the easy, large area and fast laser nanostructuring of dielectrics. At IPSM-LIFE a metal covered dielectric is irradiated where the structuring is assisted by a self-organized molten metal layer deformation process. The IPSM-LIFE can be divided into two steps: STEP 1: The irradiation of thin metal layers on dielectric surfaces results in a melting and nanostructuring process of the metal layer and partially of the dielectric surface. STEP 2: A subsequent high laser fluence treatment of the metal nanostructures result in a structuring of the dielectric surface. At this study a sapphire substrate Al2O3(1-102) was covered with a 10 nm thin molybdenum layer and irradiated by an infrared laser with an adjustable time-dependent pulse form with a time resolution of 1 ns (wavelength λ = 1064 nm, pulse duration Δtp = 1 – 600 ns, Gaussian beam profile). The laser treatment allows the fabrication of different surface structures into the sapphire surface due to a pattern transfer process. The resultant structures were investigated by scanning electron microscopy (SEM). The process was simulated and the simulation results were compared with experimental results.

Pattern transfer, self-organized surface nanostructuring, and nanodrilling of sapphire using nanosecond laser irradiation

Nanostructures have a widespread field of applications. The structuring of sapphire assisted by a nanosecond laserinduced self-organized molten molybdenum layer deformation process was studied. At low laser fluence the irradiation of a thin metal layer on dielectric surface results in a melting and nanostructuring of the metal layer and partially of the dielectric surface. Furthermore, a subsequent high laser fluence treatment of the metal nanostructures results in different features: (i) pattern transfer, (ii) self-organized surface nanostructuring, and (iii) nanodrilling. (i) Pattern transfer: The irradiation of the pre-structured metal layer with high laser fluences allows the transfer of the lateral geometry of the metal nanostructures into the dielectric surface. (ii) Self-organized surface nanostructuring: The multi-pulse irradiation of the metal layer/dielectric system with moderate laser fluences results in a selforganized nanostructuring of the dielectric surface. (iii) Nanodrilling: The multi-pulse low laser fluence irradiation of the metal layer results in the formation of metal droplets and a further high fluence irradiation of the laser-generated metal droplets results in a stepwise evaporation of the metal and in a partial evaporation of the dielectric and, finally, in the formation of cone-like holes. The resultant structures were investigated by scanning electron microscopy (SEM).

Nanosecond laser-induced nanostructuring of thin metal layers and dielectric surfaces

Nanostructuring of dielectric surfaces has a widespread field of applications. In this work the recently introduced laser method validates this novel concept for complex nanostructuring of dielectric surfaces. This concept combines the mechanism of self-assembly of metal films due to laser irradiation with the concept of laser-assisted transfer of these patterns into the underlying material. The present work focuses on pattern formation in fused silica near the border of the laser spot, where distorted nested ring-like patterns were found in contrast to concentric ring patterns at homogeneous laser irradiation. For the experiments a lateral homogeneous spot of a KrF excimer laser (λ = 248 nm) and a Gaussian beam Yb fiber laser (λ = 1064 nm) was used for irradiation of a thin chromium layer onto fused silica resulting in the formation of different ring structures into the fused silica surface. The obtained structures were analysed by AFM and SEM. It is found that the mechanism comprises laser-induced metal film melting, contraction of the molten metal, and successive transfer of the metal hole geometry to the fused silica. Simulations taking into account the heat and the Navier-Stokes equations were compared with the experimental results. A good agreement of simulation results with experimental data was found. These first results demonstrate that the variation of the laser beam profile allows the local control of the melt dynamics which causes changes of the shape and the size of the ring patterns. Hence, a light-controlled self-assembly is feasible.

Laser-induced processes on the back side of dielectric surfaces using a CuSO4-based absorber liquid

Micro-structured dielectric surfaces in combination with electrode structures are promising in the field of rapid prototyping of micro-sensors. In this work laser-induced back side etching and back side deposition using aqueous copper sulfate in form of a tartrate complex with formaldehyde as absorber liquid has been investigated regarding this aim. Results obtained with different laser systems ranging from UV to Near-IR and with pulse lengths from femtoseconds to nanoseconds will be presented, in order to give a wide-spread overview of the different observable effects. Depending on the specific setup and laser parameters, either well-defined compact Cu deposits, micro- or nanoscaled Cu droplets or ablation of the dielectric substrate was observed. Best quality crystalline and conducting Cu structures were achieved using ns pulses at 532 nm wavelength. Droplet formation with UV excimer laser was observed. Parameters influencing each configuration will be discussed.