Ultrashort Pulse Laser Ablation
The energy of a focused laser beam can be used to remove material by so called ablation mechanisms. The laser light excites electrons in a material which rapidly relax to lower energy states by transferring the energy to the lattice of the material. Thereby the material can melt and evaporate which can be used to remove material in a relatively controlled way.
With ultrashort laser pulses the energy is deposited in a time that is shorter than the relaxation time between the electrons and the lattice. This creates very high energy densities and allows ablation to take place before heat conduction to the bulk material becomes important. As a result ultrashort laser pulses can be used to process materials without thermally affecting the surrounding bulk material. This can be used to machine materials more precisely with much less heat-related negative effects.
Typical pulse durations used in ultrashort pulse laser processing range from tens of femtoseconds to several tens of picoseconds. Lasers with a pulse length of about 10 picoseconds have become a standard in industrial micromachining with ultrashort pulses and have proven to be suitable for 24/7 production. These pulse durations are about a thousand times shorter compared to the pulses in conventional short pulse laser machining systems and thereby avoid several detrimental effects like a heat effected zone, burrs, and material cracks.
A second advantage of ultrashort pulse processing is that practically any material can be processed. This is caused by the extremely high energy-densities that allow for multi-photon absorption. Materials that are transparent to the laser wavelength – like glass for 1064 nm laser radiation – can still be processed. Multi-photon absorption can also be used for local modification inside transparent materials.
A single ultrashort laser pulse influences an area of approximately the size of the laser spot with a typical diameter between 10 and 30 micrometer. Depending on the material and pulse energy a single pulse removes between a few nanometers up to a micrometer of depth. To process larger volumes and specific structures laser scanning techniques are used to rapidly move the focused laser beam over the workpiece.
To machine larger / curved parts the surface is divided into tiles. A multi-axis motion system is used to position the laser scanner over a tile which then is machined using the scanner. After that the machine steps to the next tile. This step-and-scan laser micromachining is fast and accurate: the use of the scanner allows fast manipulation of the laser spot; and because the larger motion system does not need to move during the actual processing it can maintain its position very accurately.
Laser surface texturing
Ultrashort pulse lasers offer many new possibilities to tailor surface textures and control their properties in a way that cannot be matched by other techniques. Using this technique very high definition textures can be created with features on multiple scales, starting in the nanometer range and extending up to the millimeter range. The technology can be applied on any material and can be used for texturing 3D curved surfaces.
Ultrashort pulse laser texturing enables new applications of engineered / functional surfaces, where specific surface textures can be used to influence the functional properties, e.g., the wetting, tribologic, or optical properties.
In laser micro-milling material is removed layer-by-layer to produce complex 3D shaped structures. The accuracy that can be achieved is typically in the micrometer range.
The technique can be used to machine practically all materials. The ability to accurately and reproducibly machine complex shapes in very hard materials can be used to machine cutting tools or stamps.
Laser cutting using ultrashort pulse lasers can create highly accurate cuts with exceptional quality and a very low amount of damage to the substrate material.
Typical advantages of ultrashort pulse laser cutting are the absence of a heat affected zone, melt, cracks and burrs. No mechanical forces are applied on the substrate, making this cutting method ideal for delicate materials or thin foils. This also makes it possible to machine very thin walls. The cut width needed to cut through a substrate can be as small as the focused laser spot, which is typically 10–30 μm. Hard and brittle materials like glass or ceramics can be cut without cracks or chipping. Due to the very low heat input, even composites with large differences in thermal properties can be machined.
Laser drilling using ultrashort pulse lasers allows small holes to be machined with very smooth side walls and without burrs or cracks. Typical substrate thicknesses range from a couple of micrometers up to a millimeter. Percussion drilling, i.e., drilling using a static spot position, is used for the smallest holes. The hole diameter depends on the laser spot size, resulting in typical diameters ranging from several micrometers to some tens of micrometers. Using a scan system larger holes can be machined, e.g., by scanning in a circular pattern (trepanning).
Laser drilled holes typically show a tapered cross section. This taper can be reduced by a careful selection of the process settings, but it cannot be fully avoided without using special drilling optics.