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基于相变原理的防护材料可在外界激励作用下发生可逆相变,相变过程中材料的光学性质在极短时间内出现变化,利用相变前后光学性质差异实现激光防护作用。钒氧化物是研究较为广泛的用于激光防护的相变材料,其中二氧化钒(VO2)和五氧化二钒(V2O5)相变前后红外波段透射率变化显著[13]。1959年,贝尔实验室的Morin等[14]发现VO2材料在340 K温度激励下具有金属-绝缘相变(Metal-Insulator Transition,MIT)特性。VO2材料相变前后红外透过率的显著变化,使其成为红外波段重要的激光致盲防护材料。致盲激光攻击成像系统时,VO2材料在激光的激励作用下迅速发生相变转变为金属态,降低致盲激光的透过率,达到保护成像系统的目的。VO2材料在激光致盲防护方面的巨大潜力吸引研究人员围绕其机理探究、制备方法和应用研究开展相关研究。研究人员关注的防护技术指标包括防护带宽、光学密度、响应时间、防护阈值和损伤阈值等。
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诱导VO2材料相变的方式包括光致相变、热致相变和电致相变等,其中光致相变和热致相变与激光防护密切相关。在探究相变机理[15]的相关研究中,相变响应时间定义为激光到达材料与材料开启防护作用的时间间隔,直接影响着VO2材料的激光防护性能,因此,研究人员对VO2材料的相变响应时间开展广泛研究,其中光致相变的响应时间相关研究如表1所示。
表 1 光致相变的响应时间相关研究
Table 1. Studies on response time of photoinduced transition
Sample property Synthesis method Test condition Response time Reference 32 nm-thick
VO2 thin filmLow-temperature process Pulse width: 450-550 fs
Energy density: 3.7 mJ/cm2
Wavelength: 780 nm<500 fs [16] 0.2 μm-thick
VO2 thin filmReaction evaporation deposition and annealing Biasing temperature: 52 ℃
Pulse width: 50 ns
Energy density: 150 mJ/cm2<50 ns [17] 0.2 μm-thick
VO2 thin film- Pulse width: 50 fs
Energy density: 7-25 mJ/cm2
Wavelength: 800 nm50 ps-100 fs [18] VO2-Si3N4 structures Chemically etching Pulse width: 100 fs
Energy density: 50 mJ/cm2
Wavelength: 790 nm<500 fs [19] VO2 microcrystals Vapour transport Pulse width: ~45 fs
Energy density: 3.3 mJ/cm2
Wavelength: 800 nmFemtosecond timescale [20] 25 nm-thick
VO2 thin filmPulsed laser deposition Pulse width: 4.9 fs
Pulse energy: ~100 mJ/cm2
Wavelength: 400-1000 nm(26±6) fs [21] 75 nm-thick
VO2 thin filmPulsed laser deposition and annealing Wavelength: 800 nm 200 fs [22] 1994年,Becker等[16]利用与聚合物基底相兼容的低温工艺制备厚度为32 nm的VO2薄膜,同时,采用波长780 nm的掺钛蓝宝石激光搭建飞秒和皮秒时间尺度的泵浦探针装置,泵浦激光能量密度为3.7 mJ/cm2时测定VO2的光致相变响应时间尺度小于500 fs。该研究证明VO2薄膜具有超快的相变响应时间,有望实现成像系统的激光防护。
1996年,查子忠等[17]利用反应蒸发沉积方法和退火处理镀制VO2薄膜。实验测得偏置温度为52 ℃、脉宽50 ns的TEA CO2激光能量密度为150 mJ/cm2时,VO2薄膜出现相变,响应时间<50 ns且恢复时间约为200 μs。
2001年,Cavalleri等[18]在玻璃基底上制备VO2薄膜,并利用泵浦探测技术测量薄膜的光致相变的响应时间,泵浦脉冲波长800 nm、脉宽50 fs、能量密度25 mJ/cm2情况下,测得薄膜的相变阈值为7 mJ/cm2,进一步通过实验数据的指数拟合发现,能量密度的增加导致相变时间从大于50 ps降低到约100 fs,如图2所示。
图 2 (a)泵浦脉冲的高斯空间轮廓; (b)相变过程中的反射率变化; (c)样品上三个不同位置的反射率的时间分辨演变,对应的能量密度为(c1) 7 mJ/cm2、(c2) 15 mJ/cm2和(c3) 25 mJ/cm2
Figure 2. (a) Gaussian spatial profile of pumping pulse; (b) Reflectivity during the phase transformation; (c) Time resolved evolutions of the reflectivity for three different positions on the sample, corresponding to local fluences of (c1) 7 mJ/cm2, (c2) 15 mJ/cm2, and (c3) 25 mJ/cm2
Cavalleri等[19]在进一步的研究实验中采用硅基底厚度(50±10) nm的VO2薄膜,以厚度(200±10) nm氮化硅(Si3N4)为缓冲层。利用化学刻蚀硅基底制备VO2-Si3N4结构,实验测量波长790 nm、脉宽100 fs、能量密度50 mJ/cm2的脉冲激光作用下VO2-Si3N4结构的反射率和透过率变化情况,可以估算该结构的相变响应时间小于500 fs。并研究泵浦脉宽在1.5 ps~15 fs之间变化时该薄膜的光致相变响应时间变化规律,结果如图3所示,观测到相变时间的80 fs为瓶颈。
图 3 光致相变的响应时间与脉宽的函数关系,脉宽位于1.5 ps~15 fs之间
Figure 3. Response time of photoinduced transition as a function of pulse width, which ranges from 1.5 ps to 15 fs
2015年,O'callahan等[20]采用气相输运方法[23]制备VO2微晶体,并通过频率分辨光学开关技术获取波长800 nm、脉宽~45 fs的激光脉冲,利用泵浦探测技术测量能量密度为3.3 mJ/cm2时相变响应时间与初始温度的关系,结果表明:初始温度升高会导致响应时间下降,且VO2微晶体的光致相变响应时间为飞秒量级。
2017年,Jager等[21]通过脉冲激光沉积法制备厚度25 nm的多晶VO2薄膜,利用泵浦探测技术测得该薄膜的光致相变响应时间为(26±6) fs,实验结果倾向于Mott-Hubbard机制。
2023年,Johnson等[22]采用脉冲激光沉积和退火工艺,在氮化硅膜上制备厚度75 nm的VO2薄膜,采用时间和光谱分辨共振软X射线相干成像技术观测VO2薄膜光致相变的动力学过程,测得相变时间为200 fs。
除光致相变外,激光还可能通过加热诱导VO2薄膜相变。1992年,Rana等[24]发现VO2薄膜在1.06 μm连续激光辐射下,激光加热诱导相变的响应时间为15~10 ms。
2006年,Wang等[25]研究VO2薄膜动态光学限幅性能时,发现在强度255 W/cm2、光斑直径2 mm的近红外连续激光照射下,VO2薄膜透过率相变前为47%,相变后下降到28%,激光加热诱导相变的响应时间为200 ms。
2022年,Wang等[26]利用可变温度Z扫描装置(图4)研究飞秒激光诱导VO2薄膜相变原理时,通过温度变化实验发现激光诱导的相变调制深度与环境温度诱导的相变的调制深度一致,且调制深度随着薄膜厚度线性增加,进一步证实相变是由高重复频率激光的热效应积累引起的。
综上所述,VO2材料的光致相变响应时间约为飞秒量级,热致相变响应时间约为毫秒量级。
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VO2薄膜的制备方法[27-28]多种多样,包括反应蒸发法、脉冲激光沉积法、粒子束溅射法和溶液法等等,研究人员通过优化参数改善VO2薄膜激光致盲防护性能。
2017年,Breckenfeld等[29]研究证明有能力使用简单的市售试剂、相对较低的退火温度和非活性气体(N2)环境,通过聚合物辅助沉积工艺与激光加工相结合,可靠地合成多晶VO2薄膜,如图5所示。最佳条件下制备的薄膜,波长为2500 nm的激光相变前透光率为86%,相变后降至42%。该研究为在相对较低的温度下,潜在的大面积上快速、可靠地合成VO2薄膜的需求提供制备方法。
2019年,Kim等[30]通过脉冲激光沉积技术在二氧化钛(TiO2)基底上沉积带有二氧化钌(RuO2)缓冲层的VO2薄膜。结果显示RuO2的厚度在50~10 nm之间变化时,VO2/RuO2/TiO2结构的对应相变温度范围为59~24 ℃。证明可以通过调整缓冲层的厚度,控制VO2薄膜和RuO2缓冲层之间的外延应变,从而调整VO2薄膜的相变温度。
2020年,Koussi等[31]通过脉冲激光沉积和快速热退火探讨VO2薄膜的制备。研究表明,低至350 ℃的快速热退火温度下,可以获得透射率变化40%左右的薄膜。通过选择基底和修改退火温度,可以控制相变温度和磁滞参数,能够获得低至52 ℃的相变温度和小至3 ℃的磁滞宽度。
2020年,Gurunatha等[32]通过控制热退火工艺中的真空度和退火温度来调节VO2材料的相变特性,结果显示温度引起的应力促进相变温度使其上升至79.5 ℃,而氧空位抑制相变温度使其降低至34.1 ℃,该工作为VO2的选择性相变以及控制相变温度提供可行的途径。
后续研究极有可能围绕VO2材料的可控加工制备展开,通过优化的加工制备参数操控VO2材料的相变温度和响应时间等性质,使得VO2材料更加适用于激光致盲防护。
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VO2材料的相变特性被发现以来,其在激光致盲防护方面的应用研究从未间断,实验数据充分验证其激光致盲防护能力。
1996年,Choi等[33]利用脉冲激光沉积法在蓝宝石基底制备VO2薄膜,并测量60~90 ℃之间VO2薄膜的中红外波段透射率和反射光谱,结果显示随着温度增加,反射率上升而透过率下降,如图6所示。
图 6 加热过程中VO2薄膜在不同温度下中红外波段的反射率和透过率光谱
Figure 6. (a) Reflectance and (b) transmittance spectra of the VO2 film at selected temperatures during the heating process
2004年,Chen等[34]利用反应离子束溅射法在硅基底上制备VO2薄膜,薄膜的厚度约为100 nm。实验测定波长10.6 μm时,相变阈值约为0.85 W/cm2,半导体相VO2的透射率约为54%,金属相的透射率低至3%,证明VO2薄膜保护敏感的红外探测器免受激光致盲的能力。
2007年,陈学荣等[35]利用离子束溅射和退火工艺于K9玻璃基底上制备出氧化钒薄膜,其组成成分为VO2和V2O5,应用波长532 nm、脉宽10 ns的Nd:YAG倍频激光器测得其损伤阈值为21.9 mJ/cm2。
2012年,Huang等[36]通过直流磁控溅射技术和后退火工艺,制备出相变温度为45 ℃的VO2薄膜,并进行红外透射性能实验,实验光源采用波长3.39 μm、最大功率10 mW的氦氖激光器,结果如图7所示,相变前透过率达到63%,相变后透过率为11%,充分说明VO2薄膜是激光防护的潜力材料。
2016年,Vilanova等[37]通过测量增加激光功率时微拉曼光谱来研究强可见光对空气中VO2晶体的影响。激光波长632.8 nm、功率密度超过500 MW/m2时,激光加热诱导相变。激光功率密度超过1300 MW/m2时,激光加热会引起VO2与空气中的氧气的反应,并氧化成V2O5。
同年,王雅琴等[38]采用反应离子束溅射和后退火处理技术在石英玻璃基底上制备VO2薄膜。实验测得该薄膜3 μm波长处相变前透过率达到77%,相变后透过率为18%,且1.08 μm波长处相变阈值为4.35 W/cm2,损伤阈值为404 W/cm2。
2018年,侯典心等[39]利用直流磁控溅射法制备VO2薄膜,并使用泵浦探测技术研究1364 nm波长激光的能量密度对相变特性的影响。实验分析表明:激光能量大于30 mJ/cm2时,单次脉冲即可激发相变,相变响应时间约为14 ns,相变恢复时间与激光能量密度呈现自然指数关系,同时指出可以通过优化VO2基底材料参数来提高薄膜的激光防护能力。
同年,刘志伟等[40]利用分子束外延法制备Al2O3基底VO2薄膜,室温下对中红外波段的透过率达到>80%,其相变温度约为45 ℃,采用波长3459 nm、脉宽50 ns、重频50 kHz、功率密度0.14 W/cm2 的中红外激光实测得到薄膜调制深度达到>60%。同时发现薄膜越厚,相变前对中红外透过率越低,温滞宽度越宽,调制深度相对较低。仿真研究激光功率密度、薄膜基底厚度和薄膜初始温度等因素对相变时间的影响[41]。结果表明:增大功率密度、初始温度或者减小基底厚度可以缩短薄膜的相变时间,并且相变时间和功率密度是类似“指数衰减”关系。实验对比30 nm厚度VO2薄膜近红外和中红外波段透过率调制深度,结果表明:1 064 nm激光的最大透过率调制深度约为13%,而3459 nm激光的透过率调制深度约为62%,因此,VO2薄膜的1064 nm激光防护效果不理想[42]。
复合结构能够利用其他材料来改善和拓展VO2材料的激光致盲防护性能,因此,设计和制备复合结构是利用VO2材料进行光电成像系统激光致盲防护的重要发展方向。
2014年,Zhao等[43]致力于寻找一种可重复的、经济的溶液加工策略来制备VO2-SiO2复合薄膜,利用等效介质理论设计并实际制备出VO2-SiO2复合薄膜,Si/V摩尔比为0.8时,有效地将VO2薄膜可见光波段29.6%的平均透过率提高到48.5%,同时保留VO2薄膜相变前后红外波段透过率的变化特性,结果如图8所示。
图 8 250~2 500 nm波长范围内增加Si/V摩尔比时VO2-SiO2复合膜的透射光谱。实线:30 ℃,虚线:100 ℃
Figure 8. Transmittance spectra of VO2-SiO2 composite films upon increasing Si/V molar ratios in the wavelength ranges from 250 nm to 2500 nm. Solid line: 30 ℃; Dash line: 100 ℃
2020年,Howes等[44]提出一种VO2超表面光限幅器,仿真结果表明,工作波长为1.24 μm时,该光限幅器具有较高的开放态透过率(−4.8 dB)和较大的衰减比率(25.2 dB),实测样品得到开放态透过率与仿真结果一致,限制态透过率为−11.7 dB,计算得到衰减比率为7.7 dB,结果如图9所示。
图 9 (a)仿真和(b)实验的限幅器开启和关闭状态时的透射率。(b)中插图为制造器件的SEM图像
Figure 9. (a) Simulated and (b) experimental transmittance of the limiter in the on and off-states. The inset in (b) is an SEM image of the fabricated device
2021年,Wan等[45]仿真设计基于VO2的金属频率选择表面(FSS)反射光限幅器(FSS-VO2 OL),设计波长为10.6 μm时,参数优化后能够达到≈0.7的开放态透射率、<0.01的限制态透射率、≈0.06的限制态吸收率和FWHM>2 μm的工作带宽。实验测量得到样品10.6 μm波长处的透过率为0.36,样品的峰值透过率偏移到9.8 μm处为0.45,分析造成峰值功率低于仿真数值的原因为忽略衬底背面的影响,造成峰值功率偏移的原因为金属频率选择表面尺寸误差和VO2折射率误差。
纳米尺度图案的制作相对复杂。2023年,Guan等[46]设计并制作具有宽带高开放态透过率的VO2光限幅器,该设计提供>−0.9 dB的开放态透光率和>21.8 dB的衰减比率,并显示出宽带(FWHM >3 μm)工作波长。实验表明该VO2光限幅器具有优异的红外成像性能,且仿真结果表明激光强度90 kW/cm2时,其具有0.23 μs的快速响应时间。该设计和制作方法为VO2光限幅器的近、远红外成像和激光保护提供平台。
Research progress of laser protection technology for optoelectronic imaging system (invited)
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摘要: 随着激光技术的迅速发展,激光武器装备日益增多,广泛应用各个领域的光电成像系统被激光致盲或致眩的概率骤增,信息获取能力急剧下降,因此,光电成像系统激光防护技术研究变得越来越重要。简要介绍了基于线性材料和非线性材料的光电成像系统激光致盲防护技术的机理及局限,重点阐述了以二氧化钒为代表的基于相变材料的激光致盲防护技术的机理、制备方法和应用进展,详细分析了基于计算成像的激光致盲防护技术的机理和初步应用探索,结合激光致盲与致眩的关系补充说明了研究光电成像系统激光致眩防护技术的必要性和可行性,最后总结了光电成像系统各种激光防护技术的优缺点以及未来发展方向。Abstract:
Significance Optoelectronic imaging systems, characterized by their compact size, light weight, high reliability, resolution, and dynamic range, have been extensively employed in various fields, such as medical imaging, media production, security management, high-resolution target reconnaissance, precision guidance, fire control and targeting, and flight assistance. However, with the rapid advancements in laser technology and the widespread use of laser weapon systems, the risk of optoelectronic imaging systems being blinded or dazzled by lasers has significantly increased, resulting in a substantial decrease in information acquisition capabilities. Consequently, investigating laser protection technologies for optoelectronic imaging systems has become increasingly vital. Progress The article initially provides a brief overview of the mechanisms and limitations of laser blinding protection technologies for optoelectronic imaging systems, focusing on linear and nonlinear materials. It then delves into laser blinding protection technologies employing phase-change materials, such as vanadium dioxide, discusses their mechanisms, fabrication methods, and application progress. Subsequently, the article explores the mechanisms and preliminary application studies of laser blinding protection technologies based on computational imaging, highlights the necessity and feasibility of researching laser dazzling protection technologies for optoelectronic imaging systems in relation to laser blinding. Finally, the advantages and disadvantages of various laser protection technologies for optoelectronic imaging systems are summarized, along with potential future development directions. Conclusions and Prospects The application of computational imaging technology for laser protection offers a groundbreaking technical solution, featuring a wide protective spectrum and exceptional adaptability. This approach eliminates the need for prior knowledge of interfering laser locations, wavelengths, or polarization states, as required by linear material protection, as well as considerations of response times and protection thresholds, as demanded by nonlinear or phase-change material protection. Computational imaging technology can defend against common continuous lasers, nanosecond pulse lasers, and emerging ultra-short pulse lasers, such as picosecond or femtosecond pulses. Designing and fabricating high-precision optical field control components and ensuring high-quality image restoration are crucial future development directions for this technology. As lensless imaging technology employing mask modulation, a key research area in computational imaging progressively matures, it may fundamentally resolve the high gain caused by the optical system structure in imaging systems, thereby effectively addressing the issue of laser blinding protection in such systems. Laser dazzling protection technology exhibits broader application scenarios compared to blinding protection technology; However, current research is relatively limited, and no groundbreaking solutions have been proposed. Based on the mechanisms of laser-induced blinding and dazzling in optoelectronic imaging systems, the seperate study on blinding and dazzling technologies is incomplete and unscientific. Future research should focus on integrating laser blinding and dazzling protection for optoelectronic imaging systems, examining protection mechanisms, technical approaches, and cost-effectiveness from multiple perspectives. -
Key words:
- optoelectronic imaging system /
- laser blinding /
- laser dazzling /
- laser protection
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图 2 (a)泵浦脉冲的高斯空间轮廓; (b)相变过程中的反射率变化; (c)样品上三个不同位置的反射率的时间分辨演变,对应的能量密度为(c1) 7 mJ/cm2、(c2) 15 mJ/cm2和(c3) 25 mJ/cm2
Figure 2. (a) Gaussian spatial profile of pumping pulse; (b) Reflectivity during the phase transformation; (c) Time resolved evolutions of the reflectivity for three different positions on the sample, corresponding to local fluences of (c1) 7 mJ/cm2, (c2) 15 mJ/cm2, and (c3) 25 mJ/cm2
表 1 光致相变的响应时间相关研究
Table 1. Studies on response time of photoinduced transition
Sample property Synthesis method Test condition Response time Reference 32 nm-thick
VO2 thin filmLow-temperature process Pulse width: 450-550 fs
Energy density: 3.7 mJ/cm2
Wavelength: 780 nm<500 fs [16] 0.2 μm-thick
VO2 thin filmReaction evaporation deposition and annealing Biasing temperature: 52 ℃
Pulse width: 50 ns
Energy density: 150 mJ/cm2<50 ns [17] 0.2 μm-thick
VO2 thin film- Pulse width: 50 fs
Energy density: 7-25 mJ/cm2
Wavelength: 800 nm50 ps-100 fs [18] VO2-Si3N4 structures Chemically etching Pulse width: 100 fs
Energy density: 50 mJ/cm2
Wavelength: 790 nm<500 fs [19] VO2 microcrystals Vapour transport Pulse width: ~45 fs
Energy density: 3.3 mJ/cm2
Wavelength: 800 nmFemtosecond timescale [20] 25 nm-thick
VO2 thin filmPulsed laser deposition Pulse width: 4.9 fs
Pulse energy: ~100 mJ/cm2
Wavelength: 400-1000 nm(26±6) fs [21] 75 nm-thick
VO2 thin filmPulsed laser deposition and annealing Wavelength: 800 nm 200 fs [22] -
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