Femtosecond laser textured porous nanowire structured glass for enhanced thermal imaging

As is well-known, every hot object with a temperature above absolute zero, whether artificial or natural, will emit thermal radiation. In nature, many organisms achieve better survival by adjusting their own infrared (IR) radiation. In particular, rattlesnakes can detect prey at night rely on their IR-sensitive pit organs. To threaten rattlesnakes, ground squirrels send out distinct and deceptive infrared tail flagging signals by increasing blood flow to their tails. The total thermal energy radiated from an object is related to its emissivity and temperature, which is based on the Stefan-Boltzmann's law. Consequently, the thermal radiation intensity can be manipulated by adjusting the emissivity or changing the surface temperature to meet different needs. Inspired by natural creatures, the regulation of thermal radiation has been widely used in different fields, including personal thermal management, smart windows, IR camouflage, IR imaging, and so on.

Since emissivity is a material-dependent parameter, emissivity engineering has been considered as one of the most effective ways to develop IR camouflage technologies. In general, the IR emissivity of an object can be tuned by constructing unique surface micro/nanostructures, such as photonic crystals, plasmonic metasurfaces, and optical gratings. Nevertheless, most studies focus on reducing the IR emissivity to achieve IR camouflage. There are few reports on increasing IR emissivity for IR image enhancement. In addition, many IR emissivity regulation procedures are complex, difficult to operate and environmentally unfriendly. Therefore, it is necessary to explore a simple, fast, and environmentally friendly method to strengthen IR emissivity.

The research group led by Assistant Prof. Kai Yin from the China University of Central South researched the glass textured by femtosecond laser with porous nanowire structures for enhanced thermal imaging in the infrared wavelength (2.5-25 μm). The research results are published in Chinese Optics Letters, Volume 20, No. 3, 2022 (T. N. Wu, et al., Femtosecond laser textured porous nanowire structured glass for enhanced thermal imaging).

The porous nanowire structure glass introduced in this work is produced by femtosecond laser direct writing technology. Figure 1 shows the schematic illustration of the process of the femtosecond laser ablating glass, three-dimensional and cross-sectional profiles, and the optical principle of the untreated (UT) and laser-ablated (LT) glass. The size of nano cavity is about 200-500 nm and the diameter of nanowire is about several nanometers. The absorption of visible light and emissivity of IR light get rise due to the production of the micro/nanostructures. The LT glass exhibits a lower transmittance of 16%-51% accounts for the enhancement of scattering sunlight and higher absorption of 8%-16.4% in the visible wavelength than the UT glass. In the IR wavelength range, it can also be observed that the emissivity of the LT glass is significantly increased, which leads to an improvement in the outward radiation heat.

In addition, the researchers experimentally demonstrate the IR image temperature of the LT glass is always closer to the actual background than the UT sample wherever they were placed. The experimental device as shown in Figure 2a. The surface of the heater was partly covered with two pieces of insulating cotton, which were used to prevent the heater and the sample from contacting directly. When the constant temperature was set at 150 ℃，for the UT and LT area, the temperature raised quickly in the first 60 s, and then stabilized after about 110 s. After stabilization, the temperature of the UT and LT area was recorded as about 81.2 ℃ and 72.6 ℃, respectively. During the whole procedure, the temperature of the LT glass surface is always higher than that of the UT surface, which is attributed to the higher emissivity of the LT region. The UT and the LT glass, with entirely the same size of 2 × 2 cm², were placed on the hand respectively. The area covered with the LT glass becomes nearly invisible under the IR camera as its surface temperature is too close to the hand. On the contrary, the UT glass image is obvious. All the data shown above illustrate that the glass ablated by femtosecond has better IR thermal imaging, as shown in Figure 2.

This work presented the excellent potential of the porous nanowires-structured glass in IR emissivity modulation, which is processed by the femtosecond laser direct writing technology, and its application in IR imaging. The researchers believe that this simple, fast and environmentally friendly processing method is also suitable for the manufacture of micro/nanostructures on various materials. It broadens the vision for the regulation of infrared emissivity of material surface.

飞秒激光直写玻璃微纳结构增强热成像

众所周知，温度在绝对零度以上的物体都会向外发出热辐射，在自然界中，许多生物通过调节自身的红外辐射得以安全生存。比如，响尾蛇在黑暗中凭借对红外敏感的优势搜寻猎物，地松鼠通过增强尾部的血液流量进而发出强烈而虚假的红外信号，达到震慑响尾蛇的目的。根据Stefan-Boltzmann定理，物体表面辐射的总热能同自身表面的发射率以及温度有关，因此可以通过调节发射率或改变表面温度来调控热辐射。受自然界的启发，对热辐射的调控已被广泛应用于个人热管理、智能窗户、红外伪装、红外成像等领域。

其中，调控发射率被认为是调节红外辐射最有效的途径之一。一般来说，可以通过构造独特的表面微纳结构来调节物体的红外发射率，例如光子晶体、等离子体表面和光学光栅等。较为普遍的方法是在目标物体上覆上红外伪装涂料和涂层、红外伪装网和遮障。然而，目前大多数研究几乎都集中于降低红外发射率进而实现热伪装方面，关于提高发射率进而改善红外成像效果的报道却很少。此外，许多红外发射率调控界面的制造工程复杂、操作困难且污染环境。因此，有必要探索一种简单、快速且环保的方法来增强界面的红外发射率。

来自中南大学银恺副教授课题组发表在Chinese Optics Letters，2022年第20卷第3期上(T. N. Wu, et al.，Femtosecond laser textured porous nanowire structured glass for enhanced thermal imaging)的研究成果，介绍了飞秒激光加工的多孔纳米线结构玻璃在红外波长(2.5~25 μm)下具有增强红外发射率的效果，并被选为当期封面。

此项工作介绍的多孔纳米线结构玻璃是由飞秒激光直写加工技术作用于玻璃表面制备产生的，飞秒激光加工玻璃的示意图、光学原理及形貌如图1所示。其中多孔结构的尺寸约为0.2~1.0 μm，纳米线直径＜100 nm，微纳结构的产生使得可见光的吸收率和红外光的发射率都得到提高。较未处理的玻璃其可见光透过率降低16%~51%，吸收率增加了8%~16.4%，这是由于加工表面光散射能力的增强。同时，在红外波长范围内，其表面发射率也明显增强。

该实验装置与测试结果如图2所示，在该实验中加热台表面覆盖有两块绝缘棉，用于防止加热器和样品直接接触。当恒温设定为150℃时，未加工和加工区域均在在前60 s 迅速升温，在大约110 s后趋于稳定，未加工和加工区域的温度分别稳定在81.2 ℃和72.6 ℃。其中，加工玻璃表面的温度始终高于未加工玻璃表面的温度，这是由于加工区域的发射率较高。将尺寸完全相同（2×2 cm²）的未加工和加工的玻璃同时放在手掌时，相比于未加工玻璃，加工样品的红外测试温度与手表面温度比较接近，这表明经激光加工的玻璃红外温度较未处理玻璃的红外温度更接近实际背景温度，可以有效提高红外热成像效果。

该项工作展示了飞秒激光直写技术在玻璃表面烧蚀产生的多孔纳米线结构在发射率调制方面的潜力及其在红外热成像中的应用，并且该加工方法简单、快速、环保，适用于多种透明介质材料表面的微纳结构制造，为材料表面红外辐射率的调控提供了可能。