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飞秒、皮秒及纳秒激光对样品的损伤概率随脉冲输出能量变化如图3所示。根据图3可以得到样品800 nm飞秒激光损伤阈值为1.67 J/cm2;波长532 nm和1064 nm皮秒激光输出条件下损伤阈值分别为1.08 J/cm2和1.98 J/cm2;波长532 nm和1064 nm纳秒激光输出条件下损伤阈值分别为9.39 J/cm2和21.57 J/cm2。总结实验结果发现,无论是透射通带内还是通带外,纳秒激光作用下光学薄膜的损伤阈值最高,飞秒激光损伤阈值与皮秒激光损伤阈值相当。纳秒激光损伤阈值要比飞秒激光损伤阈值和皮秒激光损伤阈值高一个量级。
图 3 飞秒、纳秒和皮秒激光对滤光膜损伤几率随激光能量密度的变化
Figure 3. Damage probability distribution of multilayer films irradiated by femtosecond, nanosecond and picosecond laser with different laser fluences
结合滤光膜的透射光谱分布(如图1(b)所示)和不同波长纳秒、皮秒激光作用下的损伤阈值可发现,当激光波长位于滤光膜通带内时的损伤阈值要明显低于激光波长位于截止区时。这是由于当输出激光波长位于滤光膜通带内时,相比于其他波段,更多的能量会透过样品,这些能量被其内部的杂质吸收后更易对样品造成损伤。
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不同脉宽激光对薄膜的损伤阈值差异主要是由于不同的损伤机制。飞秒激光对多层膜的损伤主要是由于体吸收引起的。当电子密度达到特定值时,会造成电离击穿。皮秒、纳秒激光对多层膜的损伤主要是由于薄膜中的杂质缺陷吸收激光能量引起的热效应造成的。为了验证这一结论,使用金相显微镜观察飞秒、皮秒和纳秒激光辐射多层膜的损伤形貌。
图4~图8所示为不同激光脉冲能量下,飞秒、皮秒和纳秒激光照射的多层滤光膜的损伤形貌,可以看出三种不同脉宽激光照射下的损伤形貌有明显的差异。如图4(a)所示,当飞秒激光输出能量接近损伤阈值时,损伤区域较小并表现得相对模糊,其边缘处比较完整且不存在离散的损伤点。这也证明飞秒激光对多层膜的表面损伤是内在的,缺陷对飞秒激光的损伤行为几乎没有影响。随着飞秒激光输出能量增加,样品开始出现自然分层现象,层数越来越多,损伤区域更加清晰与规则,如图4(b)~(d)所示。
图 4 波长为800 nm的飞秒激光输出下滤光膜损伤形貌与对应的激光能量密度
Figure 4. Damage morphology of multilayer film irradiated by 800 nm femtosecond laser for different fluences
对于皮秒激光,输出能量接近损伤阈值时,损伤区域并不规则,其由许多小损伤点组成。在激光照射区内的许多离散的小缺陷对激光的吸收系数高于膜层材料,因此在缺陷及其周围区域最先出现薄膜的损伤,也就是图5(a)和图6(a)中观察到的离散的损伤点。这些损伤点的形状相对规则,分布的比较稀疏,绝大多数的尺寸小于2 μm。随着皮秒激光输出能量的增加,损伤区域出现更多损伤点,分布也更加密集,区域边缘逐渐出现轮廓明显的浅裂缝,此时损伤区域的面积大概为15 μm,如图5(b)、(c)和图6(b)、(c)所示。随着皮秒激光输出能量的继续增加,损伤区域的边缘变得较为清晰和规则,并且是一种往外扩散的趋势。热熔融对表面的烧蚀痕迹更加明显,损伤的中心区域和边缘处有小规模的分层现象,如图5(d)所示,波长为532 nm皮秒激光相对于1 064 nm皮秒激光(图6(d) )对样品的损伤形貌分层现象更加明显,轮廓也更加清晰。
图 5 波长为532 nm的皮秒激光输出下滤光膜损伤形貌与对应的激光能量密度
Figure 5. Damage morphology of multilayer film irradiated by 532 nm picosecond laser for different fluences
图 6 波长为1064 nm的皮秒激光输出下滤光膜损伤形貌与对应的激光能量密度
Figure 6. Damage morphology of multilayer film irradiated by 1064 nm picosecond laser for different fluences
纳秒激光损伤形貌如图7~图8所示。与皮秒激光的损伤规律相似的是:当输出能量接近损伤阈值时,最先出现一些离散的损伤点,随着输出能量增加,损伤点的数量增多,分布更加密集,直至出现成片的损伤区,并伴随着逐渐往外扩散的趋势,并且在内部出现分层现象。其损伤形貌存在几个特点:第一,有熔融物残留以及膜层翘起和裂纹,这也证明纳秒激光对样品造成了热损伤;第二,损伤区域内有一些损伤点,说明损伤的原因为缺陷诱导;第三,观察1064 nm纳秒激光作用下的损伤形貌,发现在膜层界面处发生分层剥落现象,最表层的膜层脱落面积明显大于内层的膜层。出现表层剥落现象的原因可能是样片表面存在一些杂质(比如空气中的灰尘),1064 nm纳秒激光输出的能量密度较高,杂质在高能量密度激光照射下电离出电子,发生光电离现象,这些电子急剧吸收能量,从而造成区域性的膜层剥落。
图 7 波长为532 nm的纳秒激光输出下滤光膜损伤形貌与对应的激光能量密度
Figure 7. Damage morphology of multilayer film irradiated by 532 nm nanosecond laser for different fluences
图 8 波长为1064 nm的纳秒激光输出下滤光膜损伤形貌与对应的激光能量密度
Figure 8. Damage morphology of multilayer film irradiated by 1064 nm nanosecond laser for different fluences
在不同脉宽激光作用下,样品损伤行为的不确定性存在差异,即纳秒激光作用下样品损伤行为不确定性最大,皮秒激光作用下的不确定性较小,飞秒激光作用下几乎不存在损伤行为的不确定性。主要体现在:第一,如图3所示,纳秒激光作用下零概率损伤的能量密度与完全损伤的能量密度跨度最大,皮秒其次,飞秒激光作用下的能量密度跨度最小;第二,如图9所示,在同一能量密度的激光作用下,纳秒激光损伤区域的面积大小存在的差异较大。这是因为皮秒和纳秒激光作用下样品的损伤取决于膜层中的缺陷,其分布和尺寸都会对膜层的损伤造成影响,研究发现膜层中的缺陷对纳秒激光更加敏感,因此相较于皮秒激光,纳秒激光作用下样品损伤行为的不确定性更大。而飞秒激光作用下,多光子电离是造成膜层损伤的主要原因,自由电子的激发是由膜层材料的本征属性决定的,所以其损伤行为最为确定,不确定性最小。
图 9 波长为1064 nm的纳秒激光在24.32 J/cm2能量输出下滤光膜的损伤形貌
Figure 9. Damage morphology of multilayer film irradiated by 1064 nm nanosecond laser when laser fluence is 24.32 J/cm2
观察损伤形貌发现,随着激光能量密度变大,三种脉宽激光作用下滤光膜均会出现分层剥落现象。其中,纳秒和皮秒激光损伤区域中能够观察到热熔融烧蚀和裂纹等热损伤痕迹,而飞秒激光损伤区域完整规则,不存在热损伤痕迹。分析认为,当激光能量密度足够大时,纳秒和皮秒激光作用条件下,一个个损伤点被成区域性的膜层剥落行为所掩盖,众多缺陷可简化成膜层中一个区域性的整体,该区域性的整体相比膜层的其他位置对激光有较高的吸收系数。而对于飞秒激光,能量密度已足够充分电离膜层材料从而造成损伤,决定该损伤过程的是薄膜材料的本征属性,缺陷的作用可以忽略。
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光学薄膜在纳秒、皮秒和飞秒激光照射下具有不同的损伤特性,这也反映出不同脉宽激光对薄膜有不同的损伤机制。针对于薄膜损伤的内部机理,现阶段一般认为当长脉冲激光照射光学薄膜时,薄膜损伤主要源于表面缺陷和内部杂质诱导的热力损伤,短脉冲激光对光学薄膜造成损伤主要是由于雪崩电离以及超短脉冲激光造成的光电离和隧穿电离损伤。
对于飞秒激光来说,其脉冲峰值功率非常高。因此电磁场强度非常高,非线性多光子电离、冲击电离和隧穿电离是造成多层膜损伤的主要原因,杂质和缺陷引起的非内部吸收机制造成的薄膜损伤可以忽略不计。当飞秒激光的脉冲宽度较长时,非线性的多光子电离和碰撞电离一起作用造成损伤;当脉冲宽度较短时,多光子电离会损伤薄膜;当脉冲宽度非常短时,主要损伤机制是隧穿电离。实验中使用的飞秒激光的脉冲宽度为50 fs,属于较短的脉冲宽度,所以针对飞秒损伤机理,只考虑多光子电离对光学薄膜的损伤作用。多光子电离产生初始导带电子,随后导带电子吸收飞秒脉冲能量,当其能量比材料的带隙能量大时,会与价带电子碰撞并产生一个新电子,从而会出现很多新的自由电子,临界电子浓度由等离子体谐振频率与飞秒激光频率相等时来确定:
$$ {n}_{cr}=\frac{{\varepsilon }_{0}{m}_{e}^{*}{\omega }^{2}}{{e}^{2}} $$ (1) 当自由电子的浓度达到1021/cm3时,便会对薄膜造成损伤[18-19]。
实验中使用的皮秒激光的脉冲宽度为50 ps,纳秒激光的脉冲宽度为9 ns,一般认为长脉冲与短脉冲的分界点位于10 ps[20],因此二者都是长脉冲,即造成薄膜损伤的主要原因是热损伤。对于皮秒和纳秒激光,多层膜内部对激光能量的吸收是非常小的,单独的内部吸收不会直接导致多层膜的损坏,其损伤主要由外部吸收激光能量导致,也就是说在皮秒和纳秒激光作用下多层膜的损伤阈值取决于材料中的缺陷。目前被广泛接受的缺陷损伤模型是热损伤模型[21]。缺陷作为损伤源,一部分激光会穿透缺陷表面,聚焦在缺陷内部,使电场增强。由于缺陷本身的吸收特性,使其内部吸收大量的激光能量。缺陷与膜层之间的边界并不连续,缺陷内部的热量流动会受到阻碍,在缺陷和膜层的边界处产生温度梯度,从而形成热应力场。当边界处的温度场达到材料的熔点或应力场达到材料的应力极限时,该缺陷位置附近会首先受到热损伤,从而对薄膜造成损伤。考虑缺陷吸收激光能量的热响应过程,受热区域内的传导方程为:
$$ \mathrm{\kappa }\frac{4\mathrm{\pi }{{r}}_{{m}}^{3}}{3}\frac{\partial T}{\partial t}={{D}}_{{T}}\frac{4\mathrm{\pi }{{r}}^{3}}{3}{\mathrm{\alpha }}_{{T}}{W} $$ (2) 式中:
$ \mathrm{\kappa } $ 为热导率;T为受热区域平均温度;W为激光功率密度;${\mathrm{\alpha }}_{{T}}$ 为杂质热吸收系数。受热区域的半径为:$$ {r}_{m}=r+\sqrt{{D}_{T}{t}_{p}} $$ (3) 式中:r为缺陷半径;DT为热扩散系数;tp为激光脉冲宽度。将公式(2)代入公式(1)中求解,可以得到受热区域的平均温度为:
$$ {T}=\frac{{{r}}^{3}{\mathrm{\alpha }}_{{T}}{W}{{t}}_{{p}}}{\mathrm{\rho }{{c}}_{{m}}{({r}+\sqrt{{{D}}_{{T}}{{t}}_{{p}}})}^{3}} $$ (4) 式中:
$ {\;\rho } $ 为受热区域的密度;${{c}}_{{m}}$ 为受热区域的质量热容。由公式(3)以及根据脉冲激光损伤均匀透明体材料的研究结果,在热损伤机制下,材料表面损伤阈值与激光脉宽的关系服从
$ {t}_{p}^{0.5} $ 规律[20]。假设纳秒和皮秒激光照射下,滤光膜发生损伤的材料属性和光场分布相同,实验中测得滤光膜在532 nm和1064 nm皮秒激光下的阈值分别为1.08 J/cm2和1.98 J/cm2,由$ {t}_{p}^{0.5} $ 规律推得纳秒激光下的阈值为14.49 J/cm2和26.56 J/cm2,实际结果为9.39 J/cm2和21.57 J/cm2。造成实际结果偏小的原因可能是:不同尺寸的杂质对不同脉宽激光的敏感度不同,纳秒激光敏感度高的杂质尺寸较大[20]。由公式(3)可知,受热区域的平均温度与杂质半径成正比例关系,当其他条件相同时,大杂质及其周围区域的温度要高于小杂质及其周围区域,即激光照射下大杂质周围区域更易发生损伤。因此,在考虑到引起损伤的杂质尺寸不同的因素后,实验测得纳秒激光作用下的损伤阈值小于由皮秒激光根据$ {t}_{p}^{0.5} $ 规律推得的损伤阈值是合理的。在目前的实验条件下,飞秒激光对多层膜的损伤机制主要是电离效应,皮秒和纳秒激光对多层膜的损伤机制主要是热效应,这一点也通过获得的损伤形貌得到了验证。
Damage effect of pulsed laser on Ta2O5/SiO2 filter film on quartz substrate
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摘要: 随着高能激光系统的发展,对光学薄膜抵抗激光损伤能力的要求越来越高,而激光脉宽是脉冲激光对薄膜损伤行为的重要影响因素。针对Ta2O5/SiO2多层膜,基于1-on-1测试方法,分析其在飞秒、皮秒、纳秒激光作用下的损伤特性。测得800 nm飞秒激光作用下的损伤阈值为1.67 J/cm2; 532 nm和1 064 nm皮秒激光作用下的损伤阈值分别为1.08 J/cm2和1.98 J/cm2; 532 nm和1 064 nm纳秒激光作用下的损伤阈值分别为9.39 J/cm2和21.57 J/cm2,并使用金相显微镜观察了滤光膜的损伤形貌。实验结果表明:飞秒激光对滤光膜的损伤机理主要是多光子电离效应,而皮秒和纳秒激光对滤光膜的损伤机制主要是热效应。滤光膜在飞秒激光作用下的损伤阈值与皮秒激光作用下的损伤阈值相当,纳秒激光作用下的损伤阈值要高一个数量级,透射通带外损伤阈值约为通带内损伤阈值的2倍。Abstract:
Objective Studying the interaction process and damage mechanism between optical films and laser is of great significance for clarifying the effect of laser on imaging devices when films is damaged or not, improving the design and preparation technology of imaging devices and anti-laser hardening technology, and laying the application foundation for related industries and national defense. Over the past few decades, optical films have been widely used in high-energy laser systems, and their ability to resist laser damage is critical to the operation of the whole laser system. The first step to improve the film damage threshold is to accurately measure the damage threshold of the films. At present, the main factors affecting the damage threshold of optical films include the physical properties of the film material, the processing technology of optical films and the laser output parameters. Among these factors, the preparation method, processing technology and physical properties of the optical films have certain effects on the damage threshold of the films, but the output parameters of the pulsed laser are decisive. As a key optical component in multispectral cameras, multilayer filter is often designed according to the actual needs of the suitable medium multilayer film. The study of the interaction process and damage mechanism between the multilayer film and pulsed laser is of great significance for the improvement of the design and preparation of the multilayer film and the anti-laser hardening technology of multispectral camera. Methods In the experiment, a Ti: sapphire pulse amplification system, EKSPLA picosecond pulse system and Nimma-900 nanosecond pulse system were used to output laser. The laser damage threshold of Ta2O5/SiO2 multilayer films plated on quartz substrate by electron beam evaporation was measured by 1-on-1 test method. The experimental setup diagram for femtosecond, picosecond and nanosecond laser damage to the multilayer films is shown (Fig.2). The metallographic microscopy is used to observe the damage morphology of the film. We aim to analyze the "pulse width effect" of the damage threshold through the morphological method, and to lay a foundation for the subsequent damage mechanism analysis. Results and Discussions The results show that laser-induced damage threshold of the multilayer film by the 800 nm femtosecond laser (1.67 J/cm2), 532 nm/1 064 nm picosecond laser (1.08 J·cm−2/1.98 J·cm−2) and 532 nm/1 064 nm nanosecond laser (9.39 J·cm−2/21.57 J·cm−2). The laser-induced damage threshold of the multilayer film by the femtosecond laser is equivalent to that by the picosecond laser, and the laser-induced damage threshold of the nanosecond laser is one order of magnitude higher. The laser-induced damage threshold outside the transmission passband is about twice that of the laser-induced damage threshold inside the passband. It is verified that the relationship between the material surface damage threshold and the laser pulse width obeys the law of under the thermal damage mechanism. Observing the damage morphology, it is found that with the increase of energy density under femtosecond laser, the film material is ionized, which leads to the obvious delamination spalling phenomenon, and the damage region outline is complete and clear. There are significant differences in the size and density of initial damage points and the thermal damage traces in severe damage although films are damaged by defects under nanosecond and picosecond laser. Conclusions The damage characteristics of the multilayer film induced by femtosecond, picosecond and nanosecond pulsed lasers are studied, and the damage morphology and mechanism of the filters under different pulse widths are mainly discussed. Different pulse width lasers have different mechanisms of damage to the multilayer film, that is, the damage mechanism of femtosecond laser is mainly the ionization effect. In contrast, the damage mechanism of picosecond and nanosecond laser is mainly thermal effect. It is concluded that the difference of laser damage between picosecond and nanosecond laser is caused by the difference of laser sensitivity to different pulse widths. This study has certain reference value for the application of the multilayer film in laser application system and high power laser system. -
Key words:
- damage effect /
- pulsed laser /
- optical thin films /
- damage threshold /
- effect of pulse width
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