The 1064 nm, Spol LIDT results for glass → coating incidence, like their 532 nm counterparts, are characterized by a rapid rise in the cumulative number of NP damage sites as the fluence increases, in this case from 8 to . This behavior is quite linear between 19 and , indicating that a linear interpolation of the threshold fluence for 25 or more NP damage sites is reliable. This linear interpolation, shown by the black vertical line in Fig. 9, specifies an LIDT of . The question again is why so much NP damage occurs in the glass → coating case at fluences significantly below the threshold for propagating damage in the air → coating case. A comparison of the coating layers and E-fields for the glass → coating and air → coating cases at 1054 nm provides some insight. In the former case [Fig. 7(b)], the very high intensity peak, at 400% of incident intensity, is at a depth of within the Beilby layer of the optically polished substrate surface. Then the next highest intensity peak, at of incident intensity, is in the innermost layer, the layer deposited directly on the optical substrate surface, and near that layer’s interface with the first layer. That is a -over- interface. This is different from the peak at of incident intensity in the outermost layer in the case of air → coating incidence [Fig. 7(a)], because that peak is near the interface of the outermost layer with the next to outermost layer, which is a -over- interface. In our coating process, and layers deposit at 3 and , respectively. This means that there is more relaxation time in the formation of -over- than -over- interfaces, and this could correlate with higher microstructural stability for -over- compared to -over- interfaces. One study24 found that delamination of e-beam deposited mirror coatings due to catastrophic laser damage occurs preferentially at -over- interfaces, and another study25 found similar delamination behavior for narrow band-pass filter coatings. This indicates that the bonding force of layers to high index layers on which they are deposited is weaker than that of high index layers to layers on which they are deposited. The latter study25 attributes higher defect densities to the -over- interfaces because the filter coating of that study exhibited initiation of laser damage at those interfaces. That conclusion may, however, be fortuitous because the E-field intensities for that filter coating peaked only at the -over- interfaces and were at minima of near zero intensity at the -over- interfaces. This means the -over- interfaces of that study may also have had as high or even higher defect densities which did not initiate laser damage simply because of the near zero E-field intensity at those interfaces.25 We are, therefore, not convinced that our -over- interfaces have more defects or are less microstructurally stable than our -over- interfaces. Higher microstructural stability for -over- interfaces correlates not only with their stronger mechanical bond24 but also with the increase of NP damage at 1064 nm for glass → coating compared to air → coating incidence, which is what we observe (Fig. 9).