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.
Key words
metasurface /
laser etching /
pulse duration /
repetition rate /
GHz-burst mode
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Funding
College Students' Innovative Entrepreneurial Training Plan Program (202310500018)
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