Optical products are often coated with a sol-gel antireflective film that has a porous structure with a tendency to adsorb organic contamination easily. This reduces their performance in terms of transmittance and laser-induced damaged threshold (LIDT). Hence, organic contamination volatilized from non-metallic materials under a high vacuum environment poses a significant limitation on the output capacity of high-power laser systems. In this study, we investigate the influences of the surface deposition of organic contaminations on the surface morphology and roughness, transmittance and LIDT at 355 nm for 3ω sol-gel antireflective film quantitatively. Based on the experimental results, we discuss the mechanism by which the organic contamination influences the performance of sol-gel antireflective coating. Finally, we propose two measures to reduce organic contamination in high-power laser systems, which serves as a basis for improving the cleanliness and output capacity.
The experimental section was divided into three parts: sample preparation, optical properties characterization, and laser-induced damage test and characterization. First, an experimental vacuum device was set up to simulate the deposition of organic contamination released from non-metallic materials onto optical elements surface in high-power laser systems. The deposition of the organic contamination on the sol-gel antireflective coating was precisely controlled using a heating evaporation chamber. The volatilization of organics was measured using a high precision balance, and the surface mass density of organic deposition was calculated by the principle of angle factor equal division. The second step was to characterize and compare the optical properties of samples before and after contamination. The surface morphology of sample was observed using scanning electron microscope (SEM), the surface roughness was determined using atomic force microscopy (AFM) and the transmittance was measured using LAMBDA 900 UV spectrometer. The last step was the damage test and morphology characterization. Nanosecond laser damage testing platform was used to measure the LIDT at 355 nm. The damage morphology of the coating was characterized using an optical microscope.
We prepared four groups of samples with different surface deposition qualities using the experimental vacuum contamination device. Based on SEM images of samples before and after contamination, we found that a “mist” was formed on the coating after contamination, which reduced the resolution of the surface topography (Fig.4). It proves that organic molecules in the evaporation chamber were volatilized, diffused and deposited on the sol-gel antireflective coating. When the surface deposition quantity was low, such as in Group 1, the surface roughness decreased from 2.785 nm to 2.453 nm. This may be due to the small amount of organic contamination molecules filling in the pores of the sol-gel antireflective film. As the amount of organic contamination deposited on the sample surface increased, the surface roughness also gradually increased. Here, the organic molecules not only filled the pores, but also accumulated on the film surface (Fig.5 and Fig.6). With the increase in deposited mass, the transmittance and LIDT at 355 nm of sol-gel antireflective coating decreased gradually [Fig.7, Fig.8, Fig.9(b)]. When the surface mass density of organic deposition in Group 1 was 1.665 mg/m2, the sample was not damaged. This indicates that the NVR cleanliness class A/10 meets the cleanliness requirements of high-power laser systems [Fig.9(b)]. The damage morphology was observed under an optical microscope. In Group 1, the low-fluence damage morphology was all subsurface-defects induced by substrate damage, leading to film protrusion or rupture that presents as a white-color morphology. The damage morphology of Group 4 samples with the higher surface deposition was significantly different from that of Group 1, indicating that organics played an important role in the damage initiation. When the amount of deposited organic contamination was high, both the film and the substrate of the sample surface were damaged. The morphology of the damage site was irregular and the edge showed thermal ablation contours (Fig.10). With increasing laser fluence and constant deposition quantity on the sample surface, the area of damage sites increased gradually. For Groups 1 to 3, the area of the damage sites of the sample before and after contamination hardly changed, whereas in Group 4 it was significantly larger after contamination. The deposition quantity of organic contamination on the sample surface in Group 4 was high, therefore, the absorption of laser energy by the organics caused strong thermal effect, leading to a larger damage site with smoother contours (Fig.11).
In this study, an organic mixture of C30H62 and C24H38O4 with mass ratio of 17∶3, a common contamination in high-power laser systems, was selected as the contamination source. The influence of different contamination quantities on the properties of sol-gel antireflective coating was studied quantitatively. The interaction between organic molecules and porous structures influences the optical properties, where the size of organic molecules, refractive index and deposition quantity determine the change in properties. The extent of damage was related to the absorption of organic molecules, deposition quantity of organic contamination and surface defects. First, organic contamination deposited on the coating increased the surface roughness of the film and changed the morphology of the film. As organic molecules filled the pores of the film, the equivalent refractive index was increased, which reduced the transmittance and enhanced the intensity of stray light. Second, the organics on the surface of the thin film had stronger absorption characteristics of laser energy than the substrates. Both these factors can easily lead to laser damage. Finally, to reduce the influence of organic contamination in the high-power laser system on the performance of sol-gel antireflective film, we proposed two measures to improve the cleanliness of the high-power laser systems.
