Experimental masses

Ablated masses are obtained from experiment by analysing the craters produced on the target after ablation. Experiments were set up in order to replicate conditions in POLLUX as closely as possible, most importantly the fact that POLLUX cannot model a vacuum and instead uses an inert background of a set pressure; in experiments this pressure is set to 20 Pa. Plus, due to simulations having no chemical reactions between the ablated material and the background, the gas used during the ablation of craters was argon. This also has the benefit of not having any chemical interactions with the target which could affect the craters formation. As POLLUX can only model a single laser ”shot”, whereas experimentally a single shot created practically no visible crater (at least not one that can be reliably measured), multiple shots were used to ablate a larger volume. From this the ablated mass from a single shot was determined from the appropriate fraction of the total volume, and the known

density of the target. Volumes of each crater were measured by a Filmetrics Ltd. [200] Profilm 3D White Light Profilometery tool, which is able to create a 3-D map of the crater with a depth resolution of 0.05 µm. Other techniques such as Atomic Force Microscopy can result in higher depth resolution, however these techniques are limited to very small areas, not suitable for the 1 mm diameter craters produced here.

A histogram is created corresponding to the height profile of the crater, with the ablated volume determined from the area of this histogram, with any material above the crater discarded with this highlighted on the plots in dark red. Interesting physics can also be learnt from the structure of the crater itself, therefore Figures 4.1 & 4.2 shows both the crater and the associated histogram for each material. Line profiles of cross sections from each crater can also be seen in Figure 4.3.

Figure 4.1: Cu (left) and Cu2O (right) craters after approximately 6000 laser shots, and

Figure 4.2: Zn (left) and ZnO (right) craters after approximately 6000 laser shots, and associated histograms of hight profiles.

The first clear difference seen in Figures 4.1 & 4.2 between the metals and oxides is the structure of the crater, with both metal Zn and Cu targets showing a rippling effect, whereas both Cu2O and ZnO targets are smoother, which is also visible within Figure 4.3.

The rippling effect is a result of the long pulse length of the laser, as when the pulse length of the laser is longer than the interaction time of phonon’s within the material (10−10 to 10−12 seconds), then classical phase change transitions occur within the target. This involves the melting of target material into a liquid which is then evaporated by the laser into a gas. Once the laser is no longer incident on the target and no further energy is imparted into the material, it is able to cool and re-solidify into this ripple pattern, an affect seen previously during the exploration of different pulse length lasers on metal targets [184]. However although one would think that this would also occur within the oxides due to being

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -80 -70 -60 -50 -40 -30 -20 -10 0 10 D e p t h ( m m )

Position across crater (mm) 1 2 3 Cu 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -700 -600 -500 -400 -300 -200 -100 0 100 D e p t h ( m m )

Position across crater (mm) 1 2 3 Cu 2 O 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -500 -400 -300 -200 -100 0 100 200 D e p t h ( m m )

Position across crater (mm) 1 2 3 Zn 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -800 -700 -600 -500 -400 -300 -200 -100 0 100 D e p t h ( m m )

Position across crater (mm) 1

2

3 ZnO

Figure 4.3: Line profiles of craters after laser ablation. Top left: Cu, Top Right: Cu2O.

Bottom Left: Zn. Bottom right: ZnO. Numbers in legend correspond to lines on craters as seen in Figs. 4.1 & 4.2

exposed to identical laser conditions, the underlying properties of the target material itself are very different; when looking at all of the relevant properties of the material, see the EoS table created for use in POLLUX (Table 4.1), various thermodynamic properties of the oxides important for classical phase changes are significantly different for the metals, such as the much higher melting point and lower thermal conductivities.

It is in fact that these targets are a compound of various elements which needs to be considered, as unlike the pure metal where one can assume the target will behave homo- geneously, the compound targets cannot behave in the same ways due to various different internal properties within the material due to two key aspects. Firstly the different bond strengths between the metals atoms and oxygen atoms , i.e. Cu − Cu, Cu − O, Zn − Zn and Zn − O, are quite different with both metal-metal bonds being quite weaker than the metal-

Table 4.2: Physical size of each ablated creater for metal and metal oxide tragets

Target Void volume (mm3) Number of shots

Copper 2.6407×10−2 5992 2.8433×10−2 6018 2.6832×10−2 6018 2.8441×10−2 6024 Copper Oxide 0.30651 6037 0.30842 6354 0.2955 6028 0.31121 6052 Zinc 5.9766×10−2 6020 6.715×10−2 6043 6.1694×10−2 6070 Zinc Oxide 0.4553 6203 0.42887 6045 0.41707 6034 0.4411 6040

oxygen bonds [201]. One also needs to consider that the metal atoms are approximately 4 times heavier than the oxygen atoms, meaning that, although bonds between metals are easier to break within the target, oxygen atoms require far less energy to ablate from the surface.

This process of phase change is referred to as decomposition, an effect that can occur within compound materials when heated to their melting temperature. When this temper- ature is reached bonds begin to break within the target, with lighter elements (e.g. oxygen) effectively sublimating from the target, and accelerated easier than the metals due to their lighter mass. This leaves behind a target solely of heavier atoms (e.g. Cu, Zn), which either are ejected from their lattice sites via sublimation, due to the many broken bonds within their lattice, or to either melt and undergo classical phase changes if there are sufficient bonds between these heavier atoms [4, 63]. This volatile nature of oxygen and resulting de- composition within the film corresponds to the deeper craters observed, and can also explain the films created by S. Rajendrian who deposited films from the ablation of a copper oxide target [2]; These films were observed to be amorphous, and consisting of large grains of very inconstant sizes, implying that the target ablation was a much more irregular process.

However so far only the structure of the craters has been commented on. When looking at the histograms in Figures 4.1 & 4.2, and the associated volumes and masses in Tables

4.2 & 4.3, it can be seen that the ablated masses are larger for the oxides. Copper oxide ablates 7.6 times as much as pure copper, and zinc oxide 5.4 times as much as pure zinc, due to these metal-oxide target decomposing, an affect that overall requires less energy to be imparted from the laser and, therefore, allowing for a greater mass to be ablated. This affect is quantified with POLLUX in the following section. To note, each crater was ablated for 10 minutes, corresponding to roughly 6000 shots due to the 10 Hz rep. rate of the laser. Calculated masses per shot were determined from the crater volume, appropriate density of the material, and number of shots, with the uncertainty quoted being the standard deviation from the average of three or four craters. These masses and observed trends are what will be used to compare to POLLUX simulations in the following section.

Table 4.3: Calculated ablated mass per each laser shot, of each metal and metal-oxide target.

Target Mass ablated per shot (kg) Error

Copper 4.10×10−11 0.2×10−11

Copper Oxide 3.15×10−10 0.1×10−10

Zinc 7.43×10−11 0.4×10−11

Zinc Oxide 4.02×10−10 0.1×10−10