Resonant material processing using (ultra-)short laser pulses
The invention relates to the usage of a tunable, resonant electromagnetic field for both, the targeted fabrication of structures with dimensions smaller than the used beam diameter and the targeted fabrication of ultra small particles. Therefore, the electromagnetic field that is created by an ultra short laser pulse on the surface of an object is superposed with an external field to achieve a resonance rise specific to the processed material.
The conventional fabrication of micro- or nano-structured surfaces is of great interest for a huge amount of industrial or R&D applications. Using (ultra) short laser pulses, so called LISOS ("laser induced self organizing structures") can be produced quite easily. Since the process is taking place in close vicinity to the ablation threshold, small laser intensities suffice to fabricate these LISOS. However, to process larger areas the average laser power needs to be increased to several kilowatts. Taking today's state of the art, this results in high acquisition and maintenance costs for the needed laser systems consequently raising the inhibition threshold in industry.
When an electromagnetic field (e.g. that of a laser) hits a surface, it propagates within the irradiated material and, depending on the laser geometry and the properties of the material, modes develop. If the material is excited resonantly (i.e. with a material-specific frequency) these modes create standing waves leading to local increase or decrease of the electromagnetic field on the surface. Hills are represented by nodes, valleys by antinodes. The emerging structures have dimensions which are in the order of the wavelength of the laser making them considerably smaller than the actual beam diameter. The goal is now to find these resonance conditions for a variety of materials and to reliably adjust them. This task can be immensely facilitated by applying an external electric or magnetic field that superposes the electromagnetic field of the laser. This can also be achieved by radiating the sample with microwaves. Now the shape of the resonant modes can be directly controlled by modulating the applied electromagnetic field. Easy control of the resonant modes can be used to shape the structural surface parameters like the relative pattern distance or the average height distribution.
Setup for resonant surface treatment using a laser and an external field. Via two electrodes which are located on the work piece (left- and right-hand side) of the area to be treated, a tunable electrical field can be applied. This field superposes the electro-magnetic field of the laser making it much easier to reach the material dependent resonance condition. (Source: V. Schütz)
Only due to the resonant material processing technique a diversity of industrial applications become economically feasible or at least much more cost effective.
Some application areas of the resonant material processing technique are mentioned below:
Laser processed (400 pulses per point, laser fluence 880 mJ/cm²) steel surface without (left) and with (right) an additional electromagnetic field. (Source: V. Schütz)
The inventors have reproducibly structured surfaces of several materials. Using a 7W laser system, large areas of silicon substrates have been machined with a speed of almost 16 mm2/s. The used crystals where multi-crystalline substrates that are much cheaper than mono-crystalline substrates but cannot be machined using etching technologies. The increase in effectiveness with respect to results that have been obtained by simple etching was about 0.21% (absolute). A theoretical ansatz exists which allows to calculate the laser fluence and the external field for any final surface structure. In addtion, a laser machining setup combined with microwave generated electromagnetic fields was developed and successfully tested for different materials.
German patent application: DE102012025294 (A1)