Abstract
In this work, we present the possibility of producing multiscale hierarchical micro/nanostructures by the femtosecond laser ablation of transition metals (i.e., Ta and W) in water and investigate their polarization-dependent reflectance. The hierarchical micro/nanostructures are composed of microscale-grooved, mountain-like and pit-rich structures decorated with hybrid laser-induced periodic surface structures (LIPSSs). The hybrid LIPSSs consist of low/high and ultrahigh spatial frequency LIPSSs (LSFLs/HSFLs and UHSFLs). LSFLs/HSFLs of 400–600 nm in a period are typically oriented perpendicular to the direction of the laser polarization, while UHSFLs (widths: 10–20 nm and periods: 30–50 nm) are oriented perpendicular to the curvatures of LSFLs/HSFLs. On the microstructures with height gradients, the orientations of LSFLs/HSFLs are misaligned by 18°. On the ablated W metasurface, two kinds of UHSFLs are observed. UHSFLs become parallel nanowires in the deep troughs of LSFLs/HSFLs but result in being very chaotic in shallow LSFLs, turning into polygonal nanonetworks. In contrast, chaotic USFLs are not found on the ablated Ta metasurfaces. With the help of Fourier transform infrared spectroscopy, it is found that microgrooves show an obvious polarization-dependent reflectance at wavelengths of 15 and 17.5 μm associated with the direction of the groove, and the integration of microstructures with LSFs/HSFLs/UHSFLs is thus beneficial for enhancing the light absorbance and light trapping in the near-to-mid-infrared (NIR-MIR) range.
Highlights
Femtosecond laser ablation is a versatile technique that enables the production of a large variety of surface structures [1,2,3,4,5], and the structures’ diversity can be further enriched in combination with other techniques [6,7]
Our group has shown the necessity to define sub-100 nm [17,18,19,20] periods as a new category of ultrahigh spatial frequency Laser-induced periodic surface structures (LIPSSs) (UHSFLs) because (1) UHSFLs whose periods are as small as 40 nm [17,21] are very difficult to form on semiconductors when compared with normal HSFLs with periods in the range of 100–200 nm [15,22,23]; (2) The periods of UHSFLs prepared on metals are much smaller than normal HSFLs with periods of hundreds of nm [24,25,26]
LIPSSs observed on the microstructures are categorized into the mixture of LSFLs/HSFLs (according to the definition of LSFL (λ/2 ≤ ΛLSFL ≤ λ) and HSFL (ΛHSFL < λ/2), together with the laser wavelength of 1045 nm used for fs-LA in liquids (fs-LAL)), the formation mechanism should be the same due to the small difference in the period
Summary
Femtosecond laser ablation (fs-LA) is a versatile technique that enables the production of a large variety of surface structures [1,2,3,4,5], and the structures’ diversity can be further enriched in combination with other techniques [6,7]. Our group has shown the necessity to define sub-100 nm [17,18,19,20] periods as a new category of ultrahigh spatial frequency LIPSSs (UHSFLs) because (1) UHSFLs whose periods are as small as 40 nm [17,21] are very difficult to form on semiconductors (so far, only two reports on Si) when compared with normal HSFLs with periods in the range of 100–200 nm [15,22,23]; (2) The periods of UHSFLs prepared on metals are much smaller than normal HSFLs with periods of hundreds of nm [24,25,26]. Sedao et al revealed the role of surface melting and resolidification in the formation of UHSFLs of 70–90 nm in a period on Ni [31]
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