image: (a-b) SEM images showing the side and top views of the cracked structures obtained by fs-LAL of Si at a laser power of 700 mW, a scan interval of 15 μm, and a scan speed of 1 mm/s. (c) Structure of a macropore. (d) Enlarged CLIPSS morphology in (c). (e, f) Twin CLIPSS in a macropore. (g, h) SEM images of the grooves obtained by 50mW-fs-LAL. High contrast applied to (g) is to see macropores in the valleys of grooves more clearly. (i/k, j/l) Enlarged images of green and blue rectangles in (g) and (h), respectively. The direction of laser polarization is shown in (b, l). (i, j, k) and (l) CLIPSS and normal LIPSS, respectively. (m) Velocity vector of a fluid vortex formed in a hole structure upon the impact of high-speed superimposed horizontal and vertical flows of fluids. view more
Credit: OEA
In a new publication from Opto-Electronic Advances; DOI 10.29026/oea.2021.210066, researchers from Shanghai Jiao Tong University, China, Sun Yat-sen University, China and RIKEN Center for Advanced Photonics, Japan, discuss liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS.
Since the discovery of laser-induced periodic surface structures (LIPSS) in 1965, one-step LIPSS formation has been regarded as one of distinct advantages of laser micro/nanomanufacturing over 50 years. LIPSS can be formed in diverse materials including metals, semiconductors, ceramics, dielectrics, and polymers with relevant applications in optics, optoelectronics, electrochemistry, biology, and other fields. LIPSS can be categorized into low spatial frequency LIPSS (LSFL), high spatial frequency LIPSS (HSFL), ultrahigh spatial frequency LIPSS (UHSFL) and suprawavelength periodic surface structure (SWPSS) according to their periods relative to laser wavelength. UHSFL is defined as the periodic structure with a period less than 100 nm; HSFL, ~100 nm to less than 1/2 of laser wavelength; LSFL, larger than 1/2 to the same dimension of laser wavelength, while the period of SWPSS is larger than the laser wavelength. For a linearly polarized laser, the direction of LIPSS is typically perpendicular to the direction of laser polarization. Controlling the orientations of LIPSS presents different rainbow colors at the same viewing angle, enabling color printing to be used for anti-counterfeiting applications. Other kinds of LIPSS including triangular, rhombic, arc and circular LIPSS have also been produced using non-linearly polarized light, such as circular polarized or vortex light.
Article reference: Zhang DS, Li XZ, Fu Y, Yao QH, Li ZG et al. Liquid vortexes and flows induced by femtosecond laser ablation in liquid governing formation of circular and crisscross LIPSS. Opto-Electron Adv 5, 210066 (2022) . doi: 10.29026/oea.2021.210066
Keywords: circular LIPSS / crisscross LIPSS / laser ablation in liquid / femtosecond laser ablation in water / liquid vortex / vortex shedding
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The research groups of Associate Professor Dongshi Zhang, Professor Zhuguo Li from Shanghai Jiao Tong University, Professor Qinghe Yao from Sun Yat-sen University and Professor Koji Sugioka from RIKEN Center for Advanced Photonics demonstrate that while performing ablation with linearly polarized femtosecond laser in water, complex fluid dynamics associated with fluid vortex generation can break the limit of light field to regulate the LIPSS’s orientations, leading to the formation of irregular circular-LIPSS (CLIPPS) and crisscross-LIPSS (CCLIPSS). Theoretical simulation has been performed to verify the fluid scenes, which strongly support the experimental observations. This article not only breaks the traditional cognition but also displays a new path for LIPSS manipulation based on fluids by presenting novel CLIPPS/CCLIPSS structures and proposes the fluid-field-based formation mechanism, which is of great significance for enriching the diversity of LIPSS and accurately interpreting the formation mechanism of LIPSS.
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