Multi-shot laser damage effects on Ta2O5/SiO2 gradient-index optical thin film
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摘要: 采用离子束溅射（IBS）的方式，制备了1064 nm高反射Ta2O5/SiO2渐变折射率光学薄膜。对其光学性能和在基频多脉冲下抗损伤性能进行了分析。 通过渐变折射率的设计方式，很好地抑制了边带波纹，增加了1064 nm反射率。通过对损伤阈值的分析发现，随着脉冲个数的增加，损伤阈值下降明显；但是在20个脉冲数后，损伤阈值（维持在22 J/cm2左右）几乎保持不变直到100个脉冲数。通过Leica显微镜对损伤形貌的观察，发现损伤诱因是薄膜表面的节瘤缺陷。通过扫描电镜（SEM）以及聚集离子束（FIB）对薄膜表面以及断面的观察，证实了薄膜的损伤起源于薄膜表面的节瘤缺陷。进一步研究得出，渐变折射率薄膜在基频光单脉冲下损伤主要是由初始节瘤缺陷引起的，在后续多脉冲激光辐照下初始节瘤缺陷引起烧蚀坑的面积扩大扫过薄膜上的其他节瘤缺陷，引起了其他节瘤缺陷的喷射使损伤加剧，造成损伤的累积效应。Abstract: Ta2O5/SiO2 gradient-index optical thin films for 1064 nm have been prepared with Ta2O5 and SiO2 by ion beam sputtering (IBS). The optical property and laser-induced damage threshold (LIDT) of the coatings were investigated, respectively. Sidelobe ripples beside the stopband are suppressed by the gradient-index design, meanwhile the design achieves above 99.8% reflectivity at 1064 nm. The multi-shot LIDT decreases as the number of shots increases, but after 20 pulses, it remains almost unchanged all the way to 100 pulses. Typical damage micrographs demonstrate a significant dependence of the initiated damage on the diameter of nodular defect. Scanning electron microscope (SEM) and focused ion beam microscope (FIB) pictures of the nodular defects show that, ablation pits caused by the initiated nodular defect expanded to farther nodular defects under multi-shot pulses. Damage accumulation effects may be attributed to the ejection of farther nodular defects.
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