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Picosecond-pulsed laser ablation of zinc: crater morphology and comparison of methods to determine ablation threshold

H. Mustafa, R. Pohl, T. C. Bor, B. Pathiraj, D. T. A. Matthews, and G. R. B. E. Römer

Open Access Open Access

Abstract

Ablation of bulk polycrystalline zinc in air is performed with single and multiple picosecond laser pulses at a wavelength of 1030 nm. The relationships between the characteristics of the ablated craters and the processing parameters are analyzed. Morphological changes of the ablated craters are characterized by means of scanning electron microscopy and confocal laser scanning microscopy. Chemical compositions of both the treated and untreated surfaces are quantified with X-ray photoelectron spectroscopy. A comparative analysis on the determination of the ablation threshold using three methods, based on ablated diameter, depth and volume is presented along with associated incubation coefficients. The single pulse ablation threshold value is found to equal 0.21 J/cm2. Using the calculated incubation coefficients, it is found that both the fluence threshold and energy penetration depth show lesser degree of incubation for multiple laser pulses.

© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Figures (10)
Fig. 1
Fig. 1 Reflection coefficient R, of zinc for corresponding wavelengths calculated from the n and k values from Ref. [32,33] and from ellipsometry measurement on the sample under investigation using Eq. (1). The inset shows measured n and k values of this work.

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Fig. 2
Fig. 2 SEM images (top view) of zinc surface irradiated at different laser pulse energies Ep and corresponding peak fluence F0 levels (rows) and at different number of laser pulses N (columns). Diameter, d and maximum depth, h of the modified surface are derived from CLSM measurements. All images are in same scale. Corresponding crater profile, measured from CLSM measurements, is shown in the top-left image.

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Fig. 3
Fig. 3 SEM micrographs of characteristic surface structures on laser processed zinc surface. (a) jets with spherical endings at N = 1, F0 = 0.98 J/cm2 (tilted 70°), (b) thin membranes surrounding a scratch at N = 1, F0 = 6.87 J/cm2 (tilted 60°), (c) periodic surface structures at N = 1, F0 = 2.7 J/cm2 with a microrim marked with dashed rectangle (tilted 60°), (d) nano-roughness near the edge of the crater at N = 7, F0 = 0.98 J/cm2 marked with dashed rectangle (top view), (e) ablated crater at N = 30, F0 = 3.61 J/cm2 with ‘halo’around the crater, part of which is marked with dashed rectangle (tilted 60°).

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Fig. 4
Fig. 4 Cross-sections (obtained from CLSM measurements) of ablated craters normalized by corresponding number of pulses N at (a) F0 = 9.73 J/cm2 and (b) F0 = 40.75 J/cm2. The dashed circle in graph (b) represents rim around the crater.

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Fig. 5
Fig. 5 Atomic concentration of Zn, Al and O for different number of pulses, N at F0 =12.61, 9.73 and 6.87 J/cm2.

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Fig. 6
Fig. 6 Ar ion sputtering on unprocessed zinc surface. (a) Depth profile at low (left) and high (right) Ar ion energies shown by arrows. Inset shows the ratio of O and Al concentration as a function of sputter depth. Dashed horizontal line shows the O/Al ratio of Al2O3. (b) Schematic representation of native oxide layers on bulk zinc, red line indicates laser beam at 1030 nm. (c) Zinc (Zn2p3/2) spectra and (d) Aluminum (Al2p) spectra at different sputter depths during 1 keV Ar+ sputtering. The dashed and solid lines represent the binding energies of the corresponding metal and its oxide.

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Fig. 7
Fig. 7 (a) Ablated volume per pulse ΔV as a function of peak fluence F0. The solid curves represent the least squared fit according to Eq. (3) in regime I only and dashed curves are extensions of the solid curves in regime II. Inset shows the extrapolated curves to ΔV = 0. (b) Accumulated threshold fluence, N · Fth(N) as a function of laser pulse number N. The solid curve represents a least squared fit according to Eq. (4). (c) Accumulation in energy penetration depth as a function of laser pulse number, N for ω0 = 14.6 μm. The solid line represents least squared fit according to Eq. (5). Note that the error bars are smaller than the data points.

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Fig. 8
Fig. 8 (a) Ablation rates L = h N of Zn in air for different number of pulses N as a function of peak laser fluence F0. The solid curves represent the least squared fit according to Eq. (6) in regime I and dashed curves are extensions of solid curves in regime II. Inset shows the extrapolated curves to h N = 0. (b) Accumulated threshold fluence, N · Fth(N) as a function of laser pulse number N. The solid curve represents least squared fit according to Eq. (4). (c) Accumulation in effective penetration depth N ⋅ δ e L as a function of laser pulse number, N. The solid line represents least squared fit according to Eq. (5).

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Fig. 9
Fig. 9 Average ablation rate L of zinc in air for N = 50 as a function of peak laser fluence. The dashed line represents the least-squares fit according to Eq. (6). The inset shows the dependence of depth h on number of pulses N for F0 = 1.35 J/cm−2. The slope of the fit through the data points corresponds to average ablation rate, L [μm/pulse].

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Fig. 10
Fig. 10 (a) Squared diameter D2 of the ablated crater for different number of pulses as a function of the peak laser fluence (log scale). The solid curves represent the least squared fit according to Eq. (7). The horizontal line at ∼ 2500 μm2 represents halo diameter. (b) Accumulated threshold fluence, N · Fth(N) as a function of laser pulse number N. The solid curve represents least squared fit according to Eq. (4).

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Tables (2)

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Table 1 Ablation Threshold Values of Zn Reported in Literature.

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Table 2 Results Obtained for Single Pulse Ablation Thresholds and the Incubation Coefficients for Polycrystalline Zinc.

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Equations (8)

Equations on this page are rendered with MathJax. Learn more.

R = ( 1 − n ) 2 + k 2 ( 1 + n ) 2 + k 2 .
F ( r , z , ϕ ) = F 0 ⋅ exp ( − 2 ⋅ r 2 ω 0 2 ) ⋅ exp ( − z δ e ) ,
Δ V = 1 4 ⋅ π ⋅ ω 0 2 ⋅ δ e V ⋅ [ ln ( F 0 F th V ) ] 2 ,
N ⋅ F th ( N ) = F th ( 1 ) ⋅ N ζ ,
δ e ( N ) = δ e ( 1 ) ⋅ N ζ δ − 1 ,
L = δ e L ⋅ ln ( F 0 F th L ) .
D 2 = 2 ⋅ ω 0 2 ⋅ ln ( F 0 F th D ) .
F th e = δ . ρ . ( ( T m − T 0 ) ⋅ C p + H m + H v ) A ,
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