NCLA - Ultrafast Laser Materials Processing

Initial trials with the workstation centred on establishing the ablation threshold of stainless steel when machining at the focus of the laser beam and when employing the laser beam at 150fs and at a repetition rate of 100 Hz. The resulting ablation curve when processing in air is shown in figure 1. Using the data presented in figure 1 the minimum ablation threshold for the steel was determined to be 0.16 J/cm².

Figure 1: Ablation threshold for stainless steel as a function of laser fluence (ă NCLA)

Percussion drilling of through holes

Having established the ablation curves work commenced on producing percussion drilled holes in 316L stainless steel for a range of applications. A primary aim in this study was the production of percussion drilled holes repeatably and of entrance dimensions in the range 8-30µm in material ranging from 30 to 80µm in thickness. It was found that in order to produce through holes it was necessary to drill above the minimum ablation threshold as through holes could not be produced below a certain threshold fluence which is above the minimum surface ablation threshold, independent of the number of shots to the target.

This is attributed to the changing fluence and fluence distribution with crater evolution. Figure 1. shows a 25µm percussion drilled hole entrance in an 80µm thick stainless steel foil which was drilled with 150fs pulses at 1kHz for 500 shots with a required fluence of 8 J/cm² for breakthrough. The surface roughness around the hole entrance is minimal in this case and very little debris was deposited around the circumference of the hole. Upon breakthrough the hole exit exhibits less circularity. This is due to the linear polarisation of the beam having a preferential absorption in one direction, which leads to the characteristic cross like appearance as shown in figure 2. This phenomenon is exacerbated at the minimum ablation threshold. These effects can be minimised by rotating the polarisation of the beam during the drilling process by means of a high speed rotary half waveplate. This is currently being implemented.

At high fluences (12 J/cm²) clean holes were produced in the central portion of the beam intensity distribution (1/e²) but surface ripples were often evident on the walls of the holes at diameters just outside the 1/e² value of the peak intensity where the fluence was slightly lower, this was especially evident at the hole entrance, as shown in figure 3. The ripples were separated by approximately the wavelength of the laser light.

Working at fluences just inside the second portion of the ablation depth / log fluence graph resulted in breakthrough and it was possible to percussion drill through holes in 50µm thick stainless steel 304 foil with high circularity both at the entrance and the exit without resorting to rotating the polarisation. This was achievable whilst processing in air as well. Figure 4 shows a matrix of holes with 15µm entrance diameters in the left of frame and the cross sections of the same holes are shown in the right of frame with a 10µm exit. The holes were produced at a fluence of 9 J/cm² and 500 shots. The matrix gives an idea of the repeatability and uniformity of the holes. A deliberate taper was engineered into these holes by means of a short focal length objective, the holes could be made parallel by the use of a slightly longer objective where the depth of focus is greater.

Figure 4 (a,b): Through hole drilling in Stainless Steel 304, fluence 9 J/cm², Number of shots 500, hole entrances 15µm, hole exits 10µm, (a) x200, (b) x1000 magnification. ( © NCLA)

converted PNM file

Figure 5: Scanning white light interferometer image of an 8µm entrance hole drilled in a pure aluminium foil 30µm thick. The exit hole is 4µm diameter (© NCLA)

Similar trials were carried out on a 30µm thick pure aluminium foil and repeatable matrices of holes with 8µm entrances and 4µm exits could be produced. Downsizing the holes further requires the use of very short objectives, which lead to very high fluences unless the beam can be attenuated to a very low level, very accurately. Work is ongoing to achieve this. A white light interferogram of a matrix of holes drilled in a pure aluminium foil is shown in figure 5. The holes were produced with 250 shots at a fluence of 4.5 J/cm².

Surface structuring

Surfaces can be readily nano – structured with femtosecond lasers. Much interest lies in the development of nano-patterned surfaces both for industrial applications and for optical devices due to the custom optical absorption characteristics that can be achieved on a patterned surface. The plasmon electron-hole coupling theory reported by Van Vechten gives an explanation for the behaviour. We produced nano-ripples on Si substrates and it was found that the direction of the surface ripples produced were dependent on the polarisation direction of the laser beam as the ripple orientation changed when the polarisation of the laser beam was rotated. The surface ripples were approximately separated by the wavelength of the laser light, i.e. 775nm, as shown in figure 1. Another surface structure was demonstrated whilst processing stainless steel 316L in air, i.e. the formation of columnar spikes. An experiment was performed on the SS316L at a fluence of 0.127 J/cm² just below the reported ablation threshold and only 100 shots were allowed to the target. It was found that whilst processing in air that there was a small plume forming in the interaction zone and air breakdown in the 1/e² region which was influencing the behaviour of the ablation. This led to the formation of spikes after a low number of shots in the central portion of the crater where the irradiance was highest and where the most violent reactions were taking place before any significant material removal. It is thought that this has occurred partly due to the chemical reactions in the interaction zone between stainless steel and air and partly due to the changing optical properties of the substrate as a consequence. The spike formation is shown in figure 2.