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BaF2具有良好的光学特性,可从可见光到长波红外实现宽谱段光学减反射作用。厚度为0.5 mm的BaF2基底透射率和反射率光谱测试结果如图1所示。
图 1 BaF2基底在1064 nm测试光谱(a)反射率曲线及(b)透射率曲线;BaF2基底在8~12 μm测试光谱(c)反射率曲线及(d)透射率曲线
Figure 1. Spectrum of BaF2 substrate at 1064 nm: (a) Reflectance test curve and (b) transmittance test curve; Spectrum of BaF2 substrate at 8-12 μm: (c) Reflectance test curve and (d) transmittance test curve
分析基底的测试结果可知,BaF2在1064 nm具有93%的透射率,在8~12 μm具有平均92%的透射率。BaF2在10 μm之后的透射率下降,这是因为波长接近BaF2的长波截止限而引起的吸收增大现象。根据红外探测器工作谱段的要求,需要在BaF2基片上沉积1064 nm激光与8~12 μm长波红外双谱段减反射薄膜。具体技术要求如表1所示。
表 1 激光/长波红外双谱段减反射薄膜技术要求
Table 1. Technical requirements of the laser/long-wave infrared dual-band antireflection thin-film
Parameter Technical requirements Substrate material BaF2 Incident angle/(°) 0 Working band/μm 1.064 8-12 Transmittance ≥93% ≥95% 在红外光学薄膜材料中,ZnS与YbF3具有同时满足1064 nm激光与8~12 μm长波红外谱段的光学透明性能,同时二者机械强度较高,在红外区域的吸收也相对较低[17]。因此,根据BaF2材料折射率的匹配性及材料透明区范围,选择ZnS作为高折射率材料,YbF3为低折射率材料。
采用Sub | (0.5 H L 0.5 H )N | Air结构作为初始膜系,参考波长为λ0。其中,H代表高折射率材料ZnS在参考波长处的1/4光学厚度,L代表低折射率材料YbF3在参考波长处的1/4光学厚度。上述膜系结构由N个对称周期组成,每个周期的特征矩阵M可以表示为[18]:
$$ \begin{split} {\boldsymbol{M}} =& \left[ {\begin{array}{*{20}{c}} {\cos \dfrac{{{\delta _H}}}{2}}&{\dfrac{i}{{{n_H}}}\sin \dfrac{{{\delta _H}}}{2}} \\ {i{n_H}\sin \dfrac{{{\delta _H}}}{2}}&{\cos \dfrac{{{\delta _H}}}{2}} \end{array}} \right]\left[ {\begin{array}{*{20}{c}} {\cos {\delta _L}\mathop {}\limits_{}^{} }&{\dfrac{i}{{{n_L}}}\sin {\delta _L}} \\ {i{n_L}\sin {\delta _L}\mathop {}\limits_{}^{} }&{\cos {\delta _L}} \end{array}} \right] \\&\left[ {\begin{array}{*{20}{c}} {\cos \dfrac{{{\delta _H}}}{2}}&{\dfrac{i}{{{n_H}}}\sin \dfrac{{{\delta _H}}}{2}} \\ {i{n_H}\sin \dfrac{{{\delta _H}}}{2}}&{\cos \dfrac{{{\delta _H}}}{2}} \end{array}} \right] \end{split} $$ (1) 式中:nH、δH代表高折射率材料的折射率及位相厚度;nL、δL代表低折射率材料的折射率及位相厚度。位相厚度的计算方法为:
$$ {\delta _H} = \dfrac{{2{\text{π }}}}{\lambda }{n_H}{d_H}\cos \theta $$ (2) $$ \begin{split} \\ {\delta _L} = \dfrac{{2{\text{π }}}}{\lambda }{n_L}{d_L}\cos \theta \end{split}$$ (3) 式中:dH、dL分别表示高折射率膜层与低折射率膜层的物理厚度;θ为光线入射角度。在不考虑薄膜材料色散特性的条件下,对应同一参考波长、同一入射角度下的高低折射率材料位相厚度δH与δL相等。令δH=δL=δ,公式(1)经过矩阵乘法运算并化简,可以求得基本周期的特征矩阵:
$$ \begin{split} & {\boldsymbol{M}} = \left[ {\begin{array}{*{20}{c}} {{M_{11}}\mathop {}\limits_{}^{} }&{{M_{12}}} \\ {{M_{21}}\mathop {}\limits_{}^{} }&{{M_{22}}} \end{array}} \right] =\\ & \left[ {\begin{array}{*{20}{c}} {{{\cos }^2}\delta - \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} + \dfrac{{{n_L}}}{{{n_H}}}} \right){{\sin }^2}\delta }&{\dfrac{i}{{{n_H}}}\left[ {\sin \delta \cos \delta + \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} + \dfrac{{{n_L}}}{{{n_H}}}} \right)\cos \delta \sin \delta + \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} - \dfrac{{{n_L}}}{{{n_H}}}} \right)\sin \delta } \right]} \\ {i{n_H}\left[ {\sin \delta \cos \delta + \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} + \dfrac{{{n_L}}}{{{n_H}}}} \right)\cos \delta \sin \delta - \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} - \dfrac{{{n_L}}}{{{n_H}}}} \right)\sin \delta } \right]}&{{{\cos }^2}\delta - \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} + \dfrac{{{n_L}}}{{{n_H}}}} \right){{\sin }^2}\delta } \end{array}} \right] \end{split} $$ (4) 考虑在正入射的条件下,由于M11=M22,可以用等效单层膜的形式描述基本周期:
$$ {\boldsymbol{M}} = \left[ {\begin{array}{*{20}{c}} {\cos \Delta }&{\dfrac{i}{{H}}\sin \Delta } \\ {i{H}\sin \Delta }&{\cos \Delta } \end{array}} \right] $$ (5) 该基本周期具有等效位相厚度Δ与等效导纳Η:
$$ \Delta = \arccos \left[ {{{\cos }^2}\delta - \dfrac{1}{2}\left( {\dfrac{{{n_H}}}{{{n_L}}} + \dfrac{{{n_L}}}{{{n_H}}}} \right){{\sin }^2}\delta } \right] $$ (6) $$ {H} = {n_H}\left[ {\dfrac{{\cos \delta {{\left( {{n_H} + {n_L}} \right)}^2} - \left( {n_H^2 - n_L^2} \right)}}{{\cos \delta {{\left( {{n_H} + {n_L}} \right)}^2} + \left( {n_H^2 - n_L^2} \right)}}} \right] $$ (7) 对于由N个基本周期组成的整体膜系,其特征矩阵为各个基本周期特征矩阵的累乘:
$$ \begin{split} {{\boldsymbol{M}}^N} = {\left[ {\begin{array}{*{20}{c}} {\cos \Delta }&{\dfrac{i}{{H}}\sin \Delta } \\ {i{H}\sin \Delta }&{\cos \Delta } \end{array}} \right]^N} = \left[ {\begin{array}{*{20}{c}} {\cos N\Delta }&{\dfrac{i}{{H}}\sin N\Delta } \\ {i{H}\sin N\Delta }&{\cos N\Delta } \end{array}} \right] \end{split} $$ (8) 基底与薄膜组合的特征矩阵为:
$$ \left[ {\begin{array}{*{20}{c}} B \\ C \end{array}} \right] = \left[ {\begin{array}{*{20}{c}} {\cos N\Delta }&{\dfrac{i}{{H}}\sin N\Delta } \\ {i{H}\sin N\Delta }&{\cos N\Delta } \end{array}} \right]\left[ {\begin{array}{*{20}{c}} 1 \\ {{n_s}} \end{array}} \right] $$ (9) 式中:B、C为光学薄膜特征运算的过程变量。膜系整体的等效导纳Y为:
$$ Y = \dfrac{C}{B} = \dfrac{{{n_s}\cos N\Delta + i{H}\sin N\Delta }}{{\cos N\Delta + \dfrac{{i{n_s}}}{{H}}\sin N\Delta }} $$ (10) 反射率R可以通过膜系整体等效导纳Y表示:
$$ R = \left( {\dfrac{{1 - Y}}{{1 + Y}}} \right) \cdot {\left( {\dfrac{{1 - Y}}{{1 + Y}}} \right)^ * } $$ (11) 经过上述数值运算,可以求得膜系整体等效导纳及反射率随波长的变化关系。取高折射率材料nH=2.29,低折射率材料nL=1.49,基底折射率ns=1.45,膜层周期数N=8,参考波长λ0=2000 nm。膜层的导纳Y决定了反射率的振幅,周期数N决定了反射率次峰的数量,H与L两种材料的折射率差值决定了反射率次峰的振幅。膜系的导纳及反射率随波长的变化关系如图2所示。
图 2 当λ0=2 000 nm, N=8时(a) 膜系导纳随波长变化曲线及(b) 膜系反射率随波长变化曲线
Figure 2. (a) Curve of admittance of thin-film system with wavelength and (b) curve of reflectance of thin-film system with wavelength when λ0=2 000 nm and N=8
在不考虑基底与薄膜材料色散特性的前提下,对于波长大于参考波长的谱段,其导纳Y很小,因此上述膜系结构具有良好的长波宽带减反射效果。对于波长小于参考波长的谱段,其导纳Y的振动幅度加剧,因此其反射率随波长的变化幅度极为明显。然而,由于短波方向的干涉峰增多,理论上可以通过优化膜系结构的方式调整短波方向干涉峰的位置,实现窄带减反射效果。
进一步调整N与λ0的值,确定最适合激光/长波红外双谱段减反射薄膜的初始膜系结构。接下来构建目标膜系的评价函数。定义评价函数:
$$ {\text{MeritF}} = \sum\limits_{i = 1}^k {\dfrac{{{{\left[ {R\left( {{\lambda _i}} \right) - R\left( {{\lambda _t}} \right)} \right]}^2}}}{k}} $$ (12) 在8~12 μm谱段之内等间隔采样k个数据点,记为λ1, λ2,···, λk, λt为初始膜系在8~12 μm谱段之内的反射率极小值点,R(λ)表示膜系在波长λ处的反射率。使用MATLAB软件编写程序计算膜系的反射光谱极小值点λt的位置,并计算各个波长下的反射率R(λ1), R(λ2),···,R(λk)与R(λt),求解评价函数。评价函数的值越小,则初始膜系的反射率极小值点更接近目标谱段中心。优化过程中取k=100。为了降低制备难度,周期数不宜过多,选择N=8;为了防止膜层总厚度过厚,参考波长不宜过大,选择λ0≤1500 nm。经过拟合求解,得到参考波长λ0=1259 nm。此时膜系的导纳及反射率随波长变化曲线如图3所示,膜系在8~12 μm长波红外谱段具有良好的初始反射率。
图 3 当λ0=1259 nm, N=8时(a) 膜系导纳随波长变化曲线及 (b) 膜系反射率随波长变化曲线
Figure 3. (a) Curve of admittance of thin-film system with wavelength and (b) Curve of reflectance of thin-film system with wavelength when λ0=1259 nm and N=8
随后将薄膜材料的色散特性加入膜系设计中,使用Essential Macleod软件对初始膜系进行优化。考虑到BaF2基底与YbF3之间的结合性更好,故选择在基底上首先沉积一层很薄的YbF3层。对于1064 nm激光,考虑到沉积过程中引起工艺参数不稳定的环境因素,应在膜系设计时考虑一定的工艺容差,在1064 nm波长附近设置不少于80 nm的透射带宽。当膜层沉积厚度过厚时将导致膜层累计应力大,膜层稳定性差,引发脱膜问题;而当膜层沉积厚度过薄时,因膜层初始沉积速率不稳定,将导致膜层沉积厚度不稳定,引发光学性能下降等问题。基于上述考量,在优化过程中对膜层厚度的变化范围加以约束,设置膜层最大厚度为550 nm,最小厚度为30 nm。由于初始膜系具有良好的长波通特性,该初始膜系更容易收敛至目标结果。经优化后,膜系在1064 nm及8~12 μm的反射率及透射率设计曲线如图4所示。
图 4 薄膜在1064 nm理论设计光谱(a)反射率曲线及(b)透射率曲线;薄膜在8~12 μm理论设计光谱(c)反射率曲线及(d)透射率曲线
Figure 4. Spectrum at 1064 nm: (a) Reflectance theoretical curve and (b) transmittance theoretical curve; Spectrum at 8-12 μm: (c) Reflectance theoretical curve and (d) transmittance theoretical curve
对于复合谱段应用背景下的膜系结构,采用文中提出的设计方法所确定的初始膜系开展膜系优化,可快速收敛至目标设计要求,并且没有过厚层与超薄层的存在,降低了制备难度。若采用常规的初始膜系结构直接使用薄膜设计软件开展优化,将导致膜层层数过多、膜层厚度过厚等不良结果,势必会给薄膜制备工艺带来很大的困难;而在设计过程中若对膜层层数与膜层厚度加以限制,薄膜光学性能则难以达到技术要求。因此,文中提出的初始膜系结构对完善复合谱段膜系设计方法具有重要作用。
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采用热蒸发离子束辅助沉积的方法制备薄膜。首先使用乙醇和乙醚的混合液对BaF2基底进行清洗,确保样品表面无污垢、无尘埃粒、无擦痕。将BaF2基底装入真空室中,烘烤温度设定为150 ℃。到达烘烤温度后将真空室抽至1×10−3 Pa,并保温2 h。膜层开始沉积之前,使用霍尔离子源对基底进行二次清洗,可以增强基底与膜层的结合性,也可以起到去除基底表面杂质的作用。为了减小基底后表面引起的反射率损耗,在基板上下表面开展双面镀膜。薄膜沉积过程中的工艺参数如表2所示。
表 2 薄膜沉积工艺参数
Table 2. Film deposition process parameters
Material Substrate temperature/℃ Deposition rate/Å·s−1
(1 Å=10−10 m)Ion beam voltage/V YbF3 150 5 140 ZnS 150 8 120
Design and preparation of laser/long-wave infrared dual-band antireflection thin-film
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摘要: 在氟化钡光学元件上设计并制备多谱段减反射薄膜是提升光电系统探测性能的关键。在氟化钡基底上设计并制备了1064 nm激光/长波红外双谱段减反射薄膜。基于周期对称结构膜系导纳计算方法,以及拟合膜层周期数与参考波长的优化算法,开展了复合谱段减反射薄膜初始膜系的设计方法研究。使用热蒸发离子束辅助沉积方法制备了多层减反射薄膜。测试结果表明,该薄膜在1064 nm处透射率为94.0%,在8~12 μm长波红外谱段平均透射率为96.3%,在8.2 μm处的透射率高达99.4%。该激光/长波红外双谱段减反射薄膜具有良好的光学性能,可以应用于多模复合精确探测光电装备之中,对于提升探测系统的工作性能具有重大意义。
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关键词:
- 光学薄膜 /
- 双谱段减反射 /
- 膜系设计 /
- 热蒸发离子束辅助沉积
Abstract: The design and preparation of multi-band antireflection thin-film on barium fluoride optical elements is the key to improve the detection performance of photoelectric system. 1064 nm laser/long-wave infrared dual-band antireflection thin-film was designed and prepared on barium fluoride substrate. Based on the calculation method of the admittance in periodically symmetric structure thin-film system and the optimization algorithm of fitting periods and reference wavelength, study on the design method of the initial film system of the multi-band antireflection thin-film was carried out. The films were prepared using the thermal evaporation ion-assisted deposition method. The results show that the film has excellent optical properties with a transmittance of 94.0% at 1064 nm, average transmittance of 96.3% in the long-wave infrared spectral band from 8 to 12 μm, and transmittance of 99.4% at 8.2 μm. The laser/long-wave infrared dual-band antireflection thin-film can be applied to dual-mode composite photodetection optoelectronic equipment, which is of great significance to improve the working performance of the photodetection system. -
表 1 激光/长波红外双谱段减反射薄膜技术要求
Table 1. Technical requirements of the laser/long-wave infrared dual-band antireflection thin-film
Parameter Technical requirements Substrate material BaF2 Incident angle/(°) 0 Working band/μm 1.064 8-12 Transmittance ≥93% ≥95% 表 2 薄膜沉积工艺参数
Table 2. Film deposition process parameters
Material Substrate temperature/℃ Deposition rate/Å·s−1
(1 Å=10−10 m)Ion beam voltage/V YbF3 150 5 140 ZnS 150 8 120 -
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