节瘤缺陷平坦化提高高反射膜的激光损伤阈值

探究了节瘤缺陷平坦化技术中平坦化层(刻蚀层)厚度和种子源尺寸之间的刻蚀规律,同时解释了平坦化技术提高节瘤缺陷的损伤阈值的机制。在双离子束溅射系统中,使用SiO₂微球模拟真实的种子源置于基板上,镀制1064 nm HfO₂/SiO₂高反膜,制备人工节瘤缺陷。对类似于实际种子源的SiO₂微球一系列不同刻蚀程度的实验得出了节瘤缺陷平坦化技术的刻蚀规律:只要平坦化层(刻蚀层)的厚度稍大于节瘤缺陷的种子源粒径,就可以将种子源完全平坦化。使用时域有限差分法(FDTD)模拟不同平坦化程度的节瘤缺陷内电场增强的结果与节瘤缺陷的损伤形貌进行对比实验,将损伤形貌和损伤阈值与电场强度分布之间建立联系,表明平坦化技术可以改变节瘤缺陷原有的几何结构,有效抑制节瘤缺陷的电场增强效应。最后,通过对未经平坦化和经过平坦化处理后的节瘤缺陷进行损伤阈值测试,对比结果直接验证了节瘤缺陷平坦化技术可以实现对节瘤缺陷的调控,大幅度提高了节瘤缺陷的损伤阈值。     

45°高反膜中节瘤缺陷的电场增强效应及损伤特性

研究设计和制备了中心波长为1064 nm的45°多层膜反射镜,通过数值仿真结合实验,对薄膜中节瘤缺陷引起的电场增强效应及其对薄膜抗激光损伤性能的影响进行了研究。结果表明:当1064 nm激光从右至左45°斜入射时,电场增强效应主要出现在节瘤缺陷的表层及其左侧轮廓中部,电场增强效应随节瘤缺陷尺寸增大而增强。实验上,在清洁的基板表面喷布单分散SiO2微球作为人工节瘤种子,采用电子束蒸发制备法完成多层全反膜的制备,采用R-on-1方式对薄膜样品进行激光损伤测试。结果表明,薄膜的损伤阈值随着节瘤缺陷尺寸增加而减小。通过综合分析电场增强效应、薄膜损伤测试结果及损伤形貌特征得出,薄膜损伤阈值降低是由于节瘤缺陷和薄膜中微缺陷共同作用的结果。     

渐变折射率Ta_2O_5/SiO_2薄膜多脉冲激光损伤特性

采用离子束溅射(IBS)的方式,制备了1064nm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1064nm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22J/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"。     

节瘤缺陷对中红外高反射膜电场增强影响的数值分析

运用时域有限差分(FDTD)方法建立了薄膜中节瘤缺陷在高斯激光照射下电磁场响应模型，分析了节瘤的深度、起始颗粒大小和入射光角度对薄膜中电磁场的影响。结果表明：节瘤缺馅对薄膜中电场强度有显著的加强作用，其内部峰值场强是入射光的6倍，大而浅节瘤缺陷对倾斜入射p偏振态的激光具有最高的加强效应.     

HfO_2/SiO_2薄膜的激光预处理作用研究北大核心CSCD

激光预处理是提高激光薄膜抗激光损伤阈值的重要手段。对电子束蒸发方式镀制的HfO2/SiO2反射膜采用大口径激光进行了辐照,并采用激光量热计测量了激光辐射前后的弱吸收值。采用聚焦离子束(FIB)技术分析了激光辐照后薄膜的损伤形态并探究了损伤原因,首次采用扫描电镜拍摄到了节瘤部分喷发时的形貌图,并对其进行了FIB分析,为进一步了解节瘤的损伤过程提供了依据。实验发现,激光辐照过后的激光薄膜弱吸收明显降低,激光预处理有效减少了引起薄膜吸收的缺陷,存在明显的清洗效应;在本实验采用的HfO2/SiO2反射膜中,激光预处理技术对于祛除位于基底上种子形成的节瘤是有效的,原因是激光辐射过后该节瘤进行了预喷发并不会对后续激光产生影响;而激光预处理技术对位于膜层中间的可能是镀膜过程中材料飞溅引起的缺陷是无效的,需要通过其他手段对该类节瘤进行祛除。     

HfO_2/SiO_2高反射膜的缺陷及其激光损伤

用原子力、Normaski和扫描电子显微镜等分析仪器 ,对高损伤阈值薄膜常采用的 Hf O2 /Si O2 薄膜进行了表面显微图象研究 ,分析了薄膜常见的表面缺陷 ,如节瘤 ,孔洞和划痕等。薄膜表面缺陷的激光损伤实验表明 ,不同缺陷的抗激光损伤能力大不相同 ,节瘤缺陷最低 ,约为 1 5 J/ cm2 ,薄膜的损伤阈值主要由其决定 ,孔洞的激光损伤能力与节瘤相比较高 ,约为节瘤的 2～ 3倍。节瘤缺陷在低能量密度的激光损伤所形成的孔洞 ,与镀制过程中形成的孔洞形貌相似 ,激光再损伤能力也相似。低能量密度的激光把节瘤缺陷变为孔洞缺陷是激光预处理提高薄膜损伤阈值的原因之一     

HfO2／SiO2高反射膜的缺陷及其激光损伤

用原子力、Normaski和扫描电子显微镜等分析仪器,对高损伤阈值薄膜常采用HfO2/SiO2薄膜进行了表面显微图象研究,分析了薄膜常见的表面缺陷,如节瘤,孔洞和划痕等。薄膜表面缺陷的激光损伤实验表明,不同缺陷的抗激光损伤能力不大相同。节瘤缺陷最低,约为15J/cm^2,薄膜的损伤阈值主要由其决定,孔洞的激光损伤能力与节瘤相比较高,约为节瘤的2－3倍。节瘤缺陷在低能量密度的激光损伤所形成的孔洞,与镀制过程中形成的孔洞形貌相似,激光再损伤能力也相似。低能量密度的激光把瘤缺陷变为孔洞缺陷是激光预处理提高薄膜损伤阈值的原因之一。     

SiO2/HfO2高反射膜的研制

主要讨论了电子束蒸发SiO2/HfO2薄膜的面形控制和损伤性能。研究了电子束蒸发工艺参数对薄膜应力以及面形的影响;分析了制备工艺对薄膜吸收、节瘤缺陷密度的影响,测量了制备薄膜的损伤阈值。研究结果表明:调整SiO2蒸发时的氧分压可以有效地将薄膜的应力控制在-250~-50 MPa。同时采用金属Hf蒸发可以显著地将节瘤缺陷密度从12.6 mm-2降低至2.7 mm-2,同时将损伤阈值从30 J/cm2提高至55 J/cm2。     

HfO2/SiO2高反射膜的缺陷及其激光损伤

用原子力、Normaski和扫描电子显微镜等分析仪器,对高损伤阈值薄膜常采用的HfO2光损伤所形成的孔洞,与镀制过程中形成的孔洞形貌相似,激光再损伤能力也相似。低能量密度的激光把节瘤缺陷变为孔洞缺陷是激光预处理提高薄膜损伤阈值的原因之一。     

横向电场下的类金刚石薄膜激光损伤

类金刚石(DLC)薄膜在红外区有很高的透过率,但激光损伤阈值低,严重限制了其应用领域。采用直接在DLC薄膜上沉积Ti电极,基于激光损伤阈值(LIDT)测试平台,用1-on-1零几率损伤法,研究了在不同偏置电场下DLC薄膜损伤阈值及损伤形貌的变化。发现电场强度从0增加到700V/cm,损伤阈值明显增大;进一步增大偏置电场,损伤阈值相对不变。分析认为偏置电场改变了激光辐照DLC薄膜区域的光生载流子漂移速度,减小了DLC薄膜的局部热累积,减缓了薄膜的石墨化进程,提高了DLC薄膜的抗激光损伤阈值。     

Study on high-reflective coatings of different designs at 532 nm

Conventional HfO2/SiO2 and Al2O3/HfO2/SiO2 double stack high reflective （HR） coatings at 532 nm are deposited by electron beam evaporation onto BK7 substrates. The laser-induced damage threshold （LIDT） of two kinds of HR coatings is tested, showing that the laser damage resistance of the double stack HR coatings （16 J/cm2） is better than that of the conventional HR coatings （12.8 J/cm2）. Besides, the optical properties, surface conditions, and damage morphologies of each group samples are characterized. The results show that laser damage resistance of conventional HR coatings is determined by absorptive defect, while nodular defect is responsible for the LIDT of double stack HR coatings.     

Accumulation effect of SiO_2 protective layer on multi-shot laser-induced damage in high-reflectivity HfO_2/SiO_2 coatings

The accumulation effects in high-reflectivity(HR) HfO2/SiO2 coatings under laser irradiation are investigated.The HR HfO2/SiO2 coatings are prepared by electron beam evaporation at 1 064 nm.The laser-induced damage threshold(LIDT) are measured at 1 064 nm and at a pulse duration of 12 ns,in 1-on-1 and S-on-1 modes.Multi-shot LIDT is lower than single-shot LIDT.The laser-induced and native defects play an important role in the multi-shot mode.A correlative theory model based on critical conduction band electron density is constructed to elucidate the experimental phenomena.     

超声清洗与激光预处理对减反膜阈值的影响

 研究了超声清洗和激光预处理两种后处理手段对减反膜的损伤特性的影响。采用电子束蒸发技术制备了1 064 nm减反膜，利用超声清洗及激光预处理的方法分别对样品进行处理，并对处理前后的样品分别进行激光损伤阈值测试及破斑深度测量。结果表明：处理后减反膜的损伤阈值均有所提升，但激光预处理的阈值增强效果更加明显；超声清洗前后的破斑深度没有大的变化，而激光预处理后的破斑深度比处理前浅得多；原因在于超声清洗只能去除表面杂质，激光预处理可减少和抑制膜层内较深处的缺陷。     

Postprocessing treatments to improve the laser damage resistance of fused silica optical surfaces and SiO2 coatings

The combination of deep wet etching and a magneto-rheological finishing(MRF) process is investigated to simultaneously improve laser damage resistance of a fused-silica surface at 355 nm. The subsequently deposited SiO2 coatings are researched to clarify the impact of substrate finishing technology on the coatings. It is revealed that a deep removal proceeding from the single side or double side had a significant impact on the laser-induced damage threshold(LIDT) of the fused silica, especially for the rear surface. After the deep etching, the MRF process that followed does not actually increase the LIDT, but it does ameliorate the surface qualities without additional LIDT degradation. The combination guarantee both the integrity of the surface’s finish and the laser damage resistance of the fused silica and subsequent SiO2 coatings.     

Laser-induced damage of Ta2O5/SiO2 narrow-band interference filters under different 1064 nm Nd:YAG laser modes

The laser-induced damage (LID) behavior of narrow-band interference filters was investigated with a Nd:YAG laser at 1064 nm under single-pulse mode and free-running mode. The absorption measurement of such coatings was performed with surface thermal lensing (STL) technique. The damage morphologies under the two different laser modes were also studied in detail. It was found that all the filters exhibited a pass-band-center-dependent absorption and laser-induced damage threshold (LIDT) behavior, but the damage morphologies were diverse. The explanation was given with the analysis of the electric field distribution and the operational behavior of the irradiation laser.     

Ultraviolet laser-induced damage on fused silica substrate and its sol-gel coating

A comparative study of the laser-induced damage thresholds (LIDTs) of fused silica substrates and their sol-gel silica coatings was carried out with 355 nm laser irradiation. Chemical etching and superpolishing were employed in different ways to improve the substrate. The laser damage tests showed that the coated substrate was no more susceptible to laser damage than the bare substrate, showing that the substrate quality was the dominant factor limiting the LIDT for UV irradiation. In addition, it was found that high value of substrate microroughness was more harmful to the LIDT of the coated than the bare substrate, and that a proper combination of etching and superpolishing can optimize the LIDT.     

Damage on HfO2/SiO2 high-reflecting coatings under single and multiple Nd:YAG laser pulse irradiation

The single- and multi-shot damage behaviors of HfO2/SiO2 high-reflecting (HR) coatings under Nd:YAG laser exposure were investigated. Fundamental aspects of multi-shot laser damage, such as the instability due to pulse-to-pulse accumulation of absorption defect and structural defect effect, and the mechanism of laser induced defect generation, are considered. It was found in multi-shot damage, the main factors influencing laser-induced damage threshold (LIDT) are accumulation of irreversible changes of structural defects and thermal stress that induced by thermal density fluctuations.