Enhancement of laser ablation via interacting spatial double-pulse effect

Nowadays, due to an increasing change from macro fabrication to microfabrication, it is essential to enhance methods and procedures to deal with the difficulties to manufacture smaller components and machines. One important procedure is laser micromaching, which has particular benefits due to non-contact processing with high accuracy, repeatability and flexibility. Laser has become a powerful tool as an excellent alternative to the conventional microprocessing methods. In this paper, a spatial double-pulse laser ablation system is proposed and studied to increase the laser ablation efficiency and quality. Two splitted laser pulses are irradiated on the surface of the material at the same time at a tunable spatial gap between the double laser spots. By the interaction of the splitted laser pulses, the laser energy distribution of substrate surface can be modified, resulting in the change of temperature field to enhance the laser ablation efficiency. The understanding of the spatial double-pulse enhancement effect in the micromachining process significantly broadens the practical applications. The next diagram shows a schematic diagram of the experimental setup for the spatial double-pulse laser ablation, which includes a nanosecond fiber laser, two polarization beam splitters (PBS1 and PBS2), three mirrors (M1-M3), a focal lens (FL) and a linear motion stage.

Schematic diagram of the experimental setup for the spatial double-pulsed laser ablation

    The research group of Dr. Rui Zhou and Prof. Minghui Hong proposed the original idea and supervised this project. In the last decades, pulsed laser ablation of solids has been deeply studied due to its expanding applications such as film deposition, material processing, cluster and nanostructure production, among others. The double pulse laser induced breakdown spectroscopy configuration was suggested with the aim of overcoming the sensitivity shortcomings of the conventional single pulse laser induced breakdown spectroscopy technique. Double pulses and multiple pulses were studied to improve performance, for determination of traces in metallic matrices or to increase the ablation rate in laser material processing. The double pulse approach offers flexibility in the choice of wavelength, pulse width and pulse sequence. 
    Double pulse laser ablation is often used for the enhancement and optimization of many different characteristics of the laser ablation process, emission line intensities, characteristic decay time, ablation rate, ion density, crater size and shape, plume expansion rate, deposition properties, between others. Compared with laser ablation by single laser beam at the same total power, the interacting spatial double-pulse laser ablation shows optimized ablation efficiency in a suitable gap. The main mechanism behind is attributed to the redistribution of temperature field. When two splitted laser beams irradiate on the target with a large gap, there is no interaction between them with independent distributions of temperature fields. Hence, the central temperature between the two splitted laser pulses is insufficient to evaporate the silicon material surface due to relatively low laser energy. However, as the gap decreases to a sufficiently small distance, the integrated distribution of temperature field begins to change by overlapping with each other, resulting in an integrated temperature field. Consequently, the temperature of the area between the two laser pulses can be risen up to the evaporation point.  During the laser ablation, silicon materials can be heated, melted, and eventually evaporated. The evaporated material could be removed quickly from the processing area, resulting in an obvious crater with broader width and larger depth. It means that the efficiency of laser ablation could be enhanced by optimizing the integrated temperature field on substrate surface. Meanwhile, for spatial double-pulse laser ablation, the laser ablated craters are larger and the channels are wider with an optimized setup. Finally, the laser ablation quality and efficiency could be enhanced as the gap distance is optimized at about 80 μm. Experimental analysis shows that the ablation efficiency could be enhanced by 65% with better quality of micromaching by the interacting spatial double-pulse enhancement effect.

About Team
Doctor Rui Zhou is currently an associate professor of engineering in Xiamen University. Funded by the Newton Fund programme, he was selected as Fellow of the Leaders in Innovation Fellowship programme run by the Royal Academy of Engineering (UK) and Chinese Academy of Engineering. His research interests include laser nanofabrication/microprocessing and functional nanomaterials. 
Professor Minghui Hong specializes in optical engineering. He has coauthored 10 book chapters, 26 patents granted, 400+ scientific papers, and 60+ talks in international conferences. Prof. Hong is invited to serve as an Editor of many journals including Light: Science and Applications. Prof. Hong is Fellow of the Academy of Engineering Singapore (SAEng), Fellow of the Optical Society of America (OSA), Fellow of the International Society for Optics and Photonics (SPIE), and Founding Fellow and Vice President of the International Academy of Photonics and Laser Engineering (IAPLE) and Fellow of the Institute of Engineers, Singapore (IES). He is also the Chairman of Phaos Technology Pte. Ltd.

Zhou R, Lin S D, Ding Y, Yang H, Ong Y K K et al. Enhancement of laser ablation via interacting spatial double-pulse effect. Opto-Electronic Advances 1, 180014 (2018).
DOI: 10.29026/oea.2018.180014