目的 超表面具有出色的电磁波调控能力及波段选频性能,在航空航天、光学器件、生物医学界等领域具有广泛应用前景。本研究拟采用不同脉宽及脉冲模式激光对镀铝超表面材料进行对比刻蚀实验与理论探索,从而掌握针对该类材料的优化激光刻蚀方法,并阐明其工作机理。方法 首先,观察评估不同激光参数下超表面金属铝膜的去除程度及形貌变化,对比分析微观表面的成分组成;然后采用COMSOL Multiphysics建立有限元理论模型,对kHz纳秒激光、MHz-burst飞秒激光以及GHz-burst飞秒激光与超表面的相互作用进行仿真分析,揭示三种激光源下温度场和应力场对该类材料的作用过程;最后将实验结果与有限元分析结合,对三种激光源刻蚀镀铝超表面的特性与机理进行深入探究。结果 相比于kHz纳秒激光与MHz-burst飞秒激光,GHz-burst飞秒激光可以实现刻蚀后镀铝层镀铝残留物最低至0.54%（原子数分数）;界面瞬时热应力可达2 600 MPa,获得更为完整的表层金属材料去除效果。结论 由于在镀铝层材料刻蚀的同时,可在介电层界面产生的热应力更大,且应力振幅更明显,采用GHz-burst飞秒激光获得了更优界面分离效果。本研究可为高质量激光刻蚀加工超表面材料提供新的研究思路。
Abstract
Metasurface is a kind of artificially designed and fabricated material, which is typically composed of tiny, periodical patterns on its surface. This feature enables it to flexibly manipulate the polarization, amplitude, phase, and other transmitting properties of the electromagnetic waves irradiating on it. Usually, metasurface has a dual-layer structure: a conductive layer upside and a dielectric layer downside. The downside dielectric layer consists of insulators such as silicon dioxide (SiO2) or polytetrafluoroethylene (PTFE). Meanwhile, the upside conductive layer is generally obtained by sputtering or adding thin metallic film, like copper (Cu), aluminum (Al), or silver (Ag), on top of the dielectric layer. Due to the extraordinary electromagnetic wave manipulation capability, various metasurfaces are widely utilized in many fields like aerospace, optical devices, and biomedicine.
Easy to see, the geometric accuracy of the periodic patterns on the conductive layer and the intactness of the beneath dielectric layer are essential for the metasurface to work properly. Hence, researchers have attempted numerous methods to precisely fabricate them, for instance, electron beam lithography, focused ion beam milling, chemical etching, and laser etching. Electron beam lithography can achieve highly reproducible resonant structures in the manufacturing of metasurfaces with nanoscale feature sizes. Nevertheless, there are still issues in terms of yield and simplicity. Focused ion beams can accurately process samples and patterns on curved surfaces. However, the related processing damage and contamination may lead to a decrease in the optical properties of metasurfaces. Chemical etching can also produce high-quality metasurfaces. Hasegawa et al. have found that gold nanoparticles can be fixed on electrodes through electrochemical deposition, and the advantage of this process is the ability to selectively deposit metals at desired locations. However, the relatively slow reaction time and inevitable surface non-uniformity are its limitations.
Laser etchinging of metasurfaces is also an important approach for metasurface fabrication. Due to the precise focusing characteristic, the accuracy of two-dimensional geometric patterns prepared by this method can reach micrometer level. On the other hand, it may cause potential damage to the beneath dielectric material if not accurately configured. To further explore the applicability of laser sources on metasurface pattern etching, three different laser sources, namely kHz nanosecond laser, MHz burst mode femtosecond laser, and GHz burst mode femtosecond laser, were adopted to etch aluminum-coated metasurface samples. The respective etching results on the samples were characterized and compared. Meantime, mathematical analysis was executed to reveal the different etching mechanisms.
It is found that all the three laser sources can etch the aluminum layer on the metasurface samples. However, the damage of GHz burst mode femtosecond laser on the beneath dielectric layer is smaller than that of kHz nanosecond laser, and its removal effect of the aluminum layer is the most optimal among the three laser sources, evidenced by the minimum aluminum content of 0.54at.%. Both experimental and theoretical analysis indicate that the transient thermal stress generated by GHz burst mode femtosecond laser, i.e. 2 600 MPa, is the greatest among the three laser sources. During the top aluminum layer removal process, both laser ablation and fragmentation caused by transient thermal stress work together to completely remove the top aluminum layer. This study provides new research ideas for high-quality etching and processing of metasurface materials by GHz-burst femtosecond laser.
关键词
超表面 /
激光刻蚀 /
脉宽 /
重频 /
GHz-burst模式
Key words
metasurface /
laser etching /
pulse duration /
repetition rate /
GHz-burst mode
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基金
大学生创新创业训练计划项目（202310500018）
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