The advancement of high-power lasers poses challenges to the damage resistance of coatings. Currently, studies on coating damage largely rely on offline end-state characterization to understand and infer associated damage processes and mechanisms. However, given the thickness and intricate composition of these coatings, coupled with the exceedingly brief damage process during pulsed laser exposure, establishing a link between the origin of the damage and its ultimate morphology becomes challenging. Anti-reflection films commonly apply to windows or lenses and are susceptible to damage due to their transmitted electric field. Depending on specific application needs, anti-reflection films position either on the laser entry or exit surface. In this study, the HfO2/SiO2 anti-reflection coating, operating under both conditions, undergoes exposure to a 1064-nm nanosecond laser. By integrating offline end-state characterization with online dynamic process monitoring, the analysis reveals damage traits and mechanisms. This insight aids in refining anti-reflection coating fabrication techniques and their practical use.
The output pump beam of an Nd∶YAG laser (wavelength of 1064 nm, pulse width of ~10 ns) is vertically focused on the surface of an anti-reflection coating. A continuous probe beam with a wavelength of 532 nm, perpendicular to the pump beam, sweeps across the surface of the anti-reflection coating. An intensified charge-coupled device (ICCD) is combined with an imaging system to detect dynamic damage processes. By adjusting the delay between the ICCD shutter and trigger signal of the pump laser, damage images at different moments are captured, and the entire dynamic damage process is documented. The optical microscope (OM), scanning electron microscope (SEM), and focused ion beam (FIB) are used to characterize the final damage morphologies. Under irradiation with the same laser fluence, the anti-reflection film is located either on the laser incident or exit surface. The damage characteristics of the two irradiation methods are analyzed and contrasted.
In this study, combining offline end-state characterization with online dynamic process detection, the damage to the HfO2/SiO2 anti-reflection coating under the mentioned two working conditions is investigated. Findings show that under identical irradiation conditions, regardless of the anti-reflection film location on the laser incidence (forward process) or exit (reverse process) surface, two types of damages occur: pits with and without layer peeling-off. However, the central pits in the forward and reverse processes exhibit significant differences. Morphologically, the bottom center of the pits in the forward process displays a smooth and consistent melted region, while the melting characteristics in the reverse process are not pronounced and show signs of stress fragmentation. Moreover, the size of the damaged area, whether considering the lateral diameter or the longitudinal depth of the pit, is larger in the reverse process than in the forward process (Figs.4 and 5). Finite element analysis indicates that the electric field intensity (EFI) at the substrate-coating interface for both processes is comparable (Fig.11). A noticeable large-sized plasma flash appears in the damage process with the peeling-off layer, whereas this phenomenon remains unobservable without layer peeling-off (Figs.7, 8, 9, and 10). The plasma expands in the direction opposite to the laser incidence. In the forward process, the plasma hinders subsequent laser energy transfer to the coating surface, leading to comparatively minor damage. Conversely, a large amount of laser energy absorbed by the plasma in the reverse process is deposited inside the material, intensifying the damage (Fig.12). The more potent shockwave energy in the reverse process further validates this damage process. Regardless of the coating position on the laser incident or exit surface, the energy absorbed by the damage with layer peeling exceeds that without layer peeling (Fig.13 and Table 1).
Upon irradiation of the anti-reflection coating positioned on the laser incidence or exit surface with a 1064-nm nanosecond laser, damage morphologies are characterized, and the dynamic damage processes are analyzed using ICCD. The study investigates the dynamic processes of plasma, shock wave, and material ejection corresponding to various types of damage. The conclusions are as follows:
1) Two damage morphologies are identified for the coating positioned on the laser incidence (forward process) and exit (reverse process) surfaces. Under OM, the less severe damage reveals a central pit surrounded by a discolored area, indicative of nanoscale holes resulting from plasma ablation. In pits with greater damage, the SiO2 surface layer peels away. The emergence of this peeling layer is associated with larger plasma flashes.
2) The damaged areas in the reverse process are larger and deeper than those in the forward process. Electric field simulations for the forward and reverse processes exhibit a similar electric field strength at the film-substrate interface, which is insufficient to form these morphological differences. The energy of the shock wave in the dynamic process of laser damage is calculated using the propagation speed of the shock wave, and the ratio of shock wave energy in the reverse process to that in the forward process is as high as 23.93.
3) Due to the plasma propagating in the direction opposite to the laser incidence, in the forward process, the plasma evolves from the coating toward the air, inhibiting subsequent laser pulses from interacting with the material. Conversely, in the reverse process, the plasma moves from the coating toward the substrate, leading to deposition of laser energy within the substrate, which in turn results in enhanced plasma and material ejection. Hence, the notable morphological differences between the forward and reverse processes stem from the varied energy absorptions dictated by the plasma development direction under laser support.
