Hierarchical microstructures with high spatial frequency laser induced periodic surface structures possessing different orientations created by femtosecond laser ablation of silicon in liquids

Abstract

High spatial frequency laser induced periodic surface structures (HSFLs) on silicon substrates are often developed on flat surfaces at low fluences near ablation threshold of 0.1 J/cm 2 , seldom on microstructures or microgrooves at relatively higher fluences above 1 J/cm 2. This work aims to enrich the variety of HSFLs-containing hierarchical microstructures, by femtosecond laser (pulse duration: 457 fs, wavelength: 1045 nm, and repetition rate: 100 kHz) in liquids (water and acetone) at laser fluence of 1.7 J/cm 2. The period of Si-HSFLs in the range of 110-200 nm is independent of the scanning speeds (0.1, 0.5, 1 and 2 mm/s), line intervals (5, 15 and 20 μm) of scanning lines and scanning directions (perpendicular or parallel to light polarization direction). It is interestingly found that besides normal HSFLs whose orientations are perpendicular to the direction of light polarization, both clockwise or anticlockwise randomly tilted HSFLs with a maximal deviation angle of 50° as compared to those of normal HSFLSs are found on the microstructures with height gradients. Raman spectra and SEM characterization jointly clarify that surface melting and nanocapillary waves play important roles in the formation of Si-HSFLs. The fact that no HSFLs are produced by laser ablation in air indicates that moderate melting facilitated with ultrafast liquid cooling is beneficial for the formation of HSFLs by LALs. On the basis of our findings and previous reports, a synergistic formation mechanism for HSFLs at high fluence was proposed and discussed, including thermal melting with the concomitance of ultrafast cooling in liquids, transformation of the molten layers into ripples and nanotips by surface plasmon polaritons (SPP) and second-harmonic generation (SHG), and modulation of Si-HSFLs direction by both nanocapillary waves and the localized electric field coming from the excited large Si particles.

Laser Fragmentation Synthesis of Colloidal Bismuth Ferrite Particles

“…For melting to occur, the laser pulse length needs to be in the range or longer than the electron-phonon relaxation time (τ e−ph ) as for example described by Link et al [30] for the case of nanosecond laser pulses. For femtosecond pulses, Zhang et al reported surface melting of silicon (τ e−ph = 350 fs) when using a 457 fs pulsed laser [31]. In the case of BFO, τ e−ph is reported to be 0.7 ps [32] which is much shorter than the applied pulse length in our experiments.…”

Multiscale Hierarchical Micro/Nanostructures Created by Femtosecond Laser Ablation in Liquids for Polarization-Dependent Broadband Antireflection

“…[18]). This means that liquid is beneficial for achieving narrower periods of LIPSSs, a trend that is also confirmed by the summary of the periods of Si-LIPSSs obtained in different environments [15]. Recently, Kudryashov et al proposed that the factors of the squared optical refraction index (n) of environments can be used to estimate the environment-dependent periods (Λ) of LIPSSs following the trend of Λ ∝ 1/n 2 , which indicated that liquids with a higher refraction index than air normally produced smaller LIPSS periods [48].…”

“…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]. Laser-induced periodic surface structures (LIPSSs) [1,[8][9][10][11][12][13] are the most typical nanoscale structures that are uniquely achievable by fs-LA, whose periods can be manipulated from tens of nm to hundreds of nm by changing the laser properties [14], processing parameters [8] and ablation environments [15]. Generally, LIPSSs are categorized into low and high spatial frequency LIPSSs (LSFL/HSFL) according to the ratio of LIPSS periods (Λ) to the fs laser wavelength (λ) [8].…”

“…The periods of LSFLs and HSFLs are defined as ranging from about the wavelength to half of the laser wavelength (λ/2 ≤ Λ LSFL ≤ λ) and less than half of the wavelength (Λ HSFL < λ/2), respectively [ 8 , 16 ]. Recently, 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 ]. For example, Bosen et al prepared homogeneous UHSFLs (periods: 70–90 nm) on Ti by fs-LA in air (λ = 790 nm, τ = 30–160 fs and υ = 1 kHz) [ 27 , 28 , 29 ] and found that such fine structures can be easily destroyed during the friction tests in two different lubricating oils [ 30 ].…”

Nanostructured Color Filters: A Review of Recent Developments

“…Since their inception, plasmons have been implemented for a number of applications like light emitting diodes (LED) [ 54 , 55 ], biosensors [ 56 , 57 ], laser ablation [ 58 , 59 ] solar cells [ 60 , 61 , 62 , 63 ], nanolasers [ 64 , 65 , 66 ], Surface Enhanced Raman Spectroscopy (SERS) [ 67 , 68 ], waveguides [ 69 , 70 , 71 ] and lithography [ 72 , 73 ].…”