Femtosecond laser processing of micro- or nano surfaces: controlling tailored physical or chemical properties of materials

Photonic processes, particularly laser based material processing methods allow for a variety of possibilities to provide materials with novel characteristics. Especially laser beam sources, emitting ultra short laser pulses with durations in the femtosecond regime (10-15 s), enable the realization of such novel material properties. On this time scale the light – materials interaction is too short for an efficient heat transfer into the material. Subsequently the processed material does not melt and micro- or nanostructures build up within the area of the Laser focus. Depending on the Laser pulse parameters (pulse energy, pulse duration, number of pulses, shape of pulse, wavelength of laser) and the surrounding process gas atmosphere a variety of structures can be adjusted precisely. Moreover tailored laser pulse can be formed on a time scale individually by adaptive dispersive optical elements. A feedback loop is implemented for monitoring the emerging structure of surface during laser process. This enables a set up of a closed process loop with a self learning algorithm which is used to optimize the shape of femtosecond laser pulses (figure 1). By this materials with specific tailored properties can be designed within one processing step:

Hydrophobia: Hydrophobic surfaces can be functionalized by nano- and microstructures, so that they exhibit dirt-repellent (figure 2).

Heat sink: The characteristic surface appearance of micro structured metal surfaces is a spiked-structure at micrometer level (figure 3). Aluminum plates with micro structured surfaces have a significant enhanced heat radiation and can be used for cooling tasks (figure 4). In comparison to conventional heat sinks the weight and volume of micro structured Aluminum plates are strongly reduced which makes them attractive for cooling elements of LEDs or for space technologies for satellites.

Optics: The reflectivity of the laser structured surfaces can be adjusted precisely with the choice of tailored laser pulses. This can be used to fabricate reflection standards for optics or diffuse scattering screens for lightning applications with LEDs (figure 5).  

Photovoltaic: Femtosecond laser structuring of Silicon in Sulfur Hexafluoride atmosphere enables the expansion of light absorption into the mid infrared which makes such materials attractive for solar industry or as elements for NIR detectors.

Rechargeable Batteries: Nano structured surfaces of electrodes are promising candidates for enhanced production of hydrogen or for rechargeable energy storage in novel rechargeable zinc-air batteries (see Project Zisak).