Femtosecond laser textured porous nanowire structured glass for enhanced thermal imaging
Fig. 1 (a) A schematic diagram of the laser ablation on one side of glass and an optical principle before and after processing (lower left part). (b) 3D topography and section height maps of laser-treated glass.
Fig. 2 (a) IR thermal imaging experimental device. (b) The IR thermal imaging. (c) The surface temperature of LT and UT changes with time. (d) The LT and UT IR thermal imaging when placed on the hand.
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.
图1 （a）激光加工的玻璃示意图和加工前后的光学原理（左下部分）。（b） 加工后玻璃的三维形貌和截面高度图。
图2 （a）红外热成像实验装置。（b） 加工有F字图案玻璃的红外热成像。（c） 加工和未加工玻璃表面温度随时间变化。（d）加工和未加工玻璃放在手上时的红外热成像。
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。