JRZ for materials processing with ultrashort pulsed laser sources  

Research Areas 

Research is being conducted on a fundamental understanding of laser-matter interaction in ultrashort laser pulses. The focus is on materials that are difficult to process with conventional laser sources and on materials that are considered promising for the development of microsystems.  

Important here is the control and analysis of the process parameters. The center has extensive analytics for this purpose, from sample preparation to electron microscopy.  

Research findings are applied to some special fields: Processing of dielectric materials and polymeric materials, and selective ablation of microtechnology layer systems.  

Possible application areas for the research results are many: Biochips, sensors, microsystems, microfluidics (behavior of liquids and gases in smallest space).

Surface Functionalization  

One of the main focuses of research at the Josef Ressel Center has been on surface functionalization. Functionalization is understood here as the control of surface wetting behavior by means of laser structuring

The femtosecond laser is an ideal tool to produce almost any surface topographies by a fast direct writing process. In addition, the femtosecond laser has the ability to pattern a wide variety of materials with high precision and with minimal thermal stress. As a result of this work, a fabrication process has been developed to produce surfaces with high wetting contrast, among other things. This process has been patented by project partner Spectra-Physics and marketed as ClearSurface.  

Clear surface^TM 

The manufacturing process of a functional surface based on ClearSurfaceTM takes place in three steps. In the first, a hydrophilic (water-attracting) substrate, such as glass, is structured with a laser. The goal is to produce a so-called hierarchical surface structure, in which geometries in the micrometer range overlap with smaller structures in the nanometer range.

After this first structuring step, the surface behaves superhydrophilic (strongly water-attracting). Subsequent coating with a Teflon-like polymer as a second step makes the same surface superhydrophobic with a wetting behavior similar to lotus bloom (lotus effect).

In order to produce structures with high wetting contrast, a final structuring step follows in which the polymer layer is selectively ablated again with the femtosecond laser. Due to the direct-writing laser process, this can be implemented with any geometries. The high wetting contrasts are demonstrated in the figure. The Spectra-Physics logo made of water is shown. The substrate is flat, and the water is literally piled up just by the very large differences in the wettability of the surface.  

Bionics

An example of how ClearSurfaceTM is used in the field of bionics was demonstrated in a master's thesis. In this, the surface of a "fog-drinking" desert beetle was recreated using laser structuring. Tiny hills on the wings of this beetle exhibit a smooth, water-attracting surface.

The valleys in between, on the other hand, are rough and coated with a wax-like substance so that they repel moisture. As soon as the water droplets hanging from the elevations reach a certain size, they break away and roll down through the channels to the beetle's mouth. This behavior from nature could be replicated with ClearSurface^TM on a glass surface. Measurements show a significant increase in water collection rate compared to an unprocessed sample.

Publication related:

• E. Kostal, S. Stroj, S. Kasemann, V. Matylitskaya and M. Domke: Fabrication of Biomimetic Fog-Collecting Superhydrophilic-Superhydrophobic Surface Micropatterns Using Femtosecond Lasers, Langmuir 34(9):2933-2941 (2018), doi: 10.1021/acs.langmuir.7b03699. 

Manufacture of piezo-actuators from PMN-PT  

Due to its low thermal stress during processing, the femtosecond laser is the ideal tool for processing brittle and temperature-sensitive materials. These include lead magnesium niobate-lead titanate (PMN-PT), for example. This piezoelectric crystal exhibits unusually high dielectric and piezoelectric properties. 

At the Institute of Solid State Physics at the University of Linz, a piezo-actuator has been designed which, by applying a defined voltage, allows the energy of entangled photons emitted by QDs to be tuned without affecting the degree of entanglement of the photon pairs.

This PMN-PT-based actuator has a star-shaped geometry that is very difficult to fabricate using conventional methods. For this reason, an fs laser cutting process was developed at the Ressel Center to manufacture the actuator. The piezoelectric properties of the crystal must not be impaired in the process. Furthermore, certain requirements regarding accuracy and edge quality must be met.

This prevents crack propagation when voltage is applied or enables the manufacture of components with minimal gaps between the actuator arms. 

Publication on this:

• Martín-Sánchez J., Trotta R., Piredda G., Schimpf C., Trevisi G. and Seravalli L. , Frigeri P. , Stroj S. , Lettner T. , Reindl M. , Wildmann J. S. , Edlinger J. , Rastelli A. "Reversible Control of In-Plane Elastic Stress Tensor in Nanomembranes," Advanced Optical Materials, Doi: 10.1002/adom.201500779  
• Trotta R., Martín-Sánchez J., Wildmann J. S., Piredda G., Reindl M., Schimpf C., Zallo E., Stroj S., Edlinger J. and Rastelli A. "Wavelength-tunable sources of entangled photons interfaced with atomic vapours" Nat. Comm. ,15, 10375 (2016). Doi:10.1038/ncomms10375  
• G. Piredda, S. Stroj, D. Ziss, J. Stangl, R. Trotta, J. Martin-Sanchez, and A. Rastelli "Micromachining of PMN-PT crystals with ultrashort laser pulses" Arxiv:1811.12287 (2018)