## 4 Discussion
### Machining Thresholds
Figure 7:1  
Figure 7:2  
where \(I_{\text{ref}}^{\prime}\) is the intensity reflected from the lower surface and \(I_{\text{trans}}\) is the intensity transmitted through the lower surface. In contrast to the upper surface, the refractive index change at the lower surface is from high to low. Thus, no phase change occurs and constructive interference takes place. The two intensities \(I_{\text{in}}\) and \(I_{\text{ref}}^{\prime}\) are superposed and the intensity at the lower surface \(I_{\text{lower}}\) increases. Figure 7:3  
Moreover, for machining to only take place on the lower surface, the following condition must be met. Figure 7:4where \(I_{\rm thresh}\) is the intrinsic damage threshold intensity of the material [22]. Next, as the values of transmission, \(I_{\rm trans}/I_{0}\), have been measured, it is possible to calculate \(R\) values, as shown in the following equation, and consequently \(n_{\rm m}\) values by rearranging Equation (6). Figure 7:5  
For the HPHT sample, we obtain \(n_{\rm m}\) = 2.13 and \(R\) = 0.13. This n value is in fair agreement with theoretical values; the refractive index of high-quality HPHT diamond is 2.41\(\sim\)2.42 [23; 24]. Therefore, the intensity becomes  
Figure 7:6  
Figure 7:7  
Intensity is higher at the lower surface, thus machining of the lower surface occurred even at low fluences when no machining occurred on the upper surface. The boundary fluence was 10.3 J/cm\({}^{2}\) for upper surface machining and 7.8 J/cm\({}^{2}\) for lower surface machining. Taking into account the fact that incident laser fluence is directly proportional to the incident intensity of the laser, \(I_{0}\), these values resulted in effective threshold values of  
Figure 7:8  
Figure 7:9  
where \(P_{\rm upper}\) is the machining threshold obtained from the upper surface and \(P_{\rm lower}\) is the threshold obtained from the lower surface. As these values are the effective machining threshold of the HPHT sample, they should be equal. However, the lower surface has a lower threshold possibly due to multiple reflections within the sample. The reflections would not result in a phase change and thus constructive interference may occur. This would lead to a greater effective intensity. Additionally, with regards to the machining rate, a deeper groove is machined the lower surface, as shown in Figure 5, possibly due to the fact that there is no plasma shielding. On the upper surface, a plasma would form above the diamond, absorbing the laser and preventing it from reaching the diamond surface. However, on the lower surface, this plasma does not form between the laser and the surface. Thus, no shielding occurs and all energy can be applied to machine the diamond. In contrast, it must be highlighted that there were no grooves found on the lower surface of CVD diamond. There are several conceivable reasons as to why reverse irradiation did not take place in this case. First, the CVD diamond is not of high quality, as can be seen from the Raman spectrum of its unirradiated surface shown in \[\alpha\ =1-r-t \tag{1}\]0 It is dominated by a peak representing crystal defects, indicated by the peak at 1424 cm\({}^{-1}\). As CVD diamond possesses a greater abundance of crystal defects than HPHT diamond, less energy is required to break the diamond structure and the ablation threshold for the upper surface may be greatly lowered. Thus machining may be possible even if destructive interference occurs. Moreover, due to the abundance of crystal defects, which may be non-transparent with regards to the wavelength used, it is possible that the absorption cannot be ignored. In this case, at low fluences, laser energy would be absorbed by near-surface crystal defects and be unable to reach the lower surface. From the above results and the knowledge that HPHT diamond generally contains more nitrogen while CVD diamond contains more crystal defects, it can be tentatively suggested that these defects contribute more with regards to lowering the ablation threshold.