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Dittrich S, Barcikowski S, Gökce B. Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease. Opto-Electron Adv 4, 200072 (2021).. doi: 10.29026/oea.2021.200072
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Original Article Open Access
Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease
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Abstract
Understanding shielding cross-effects is a prerequisite for maximal power-specific nanosecond laser ablation in liquids (LAL). However, discrimination between cavitation bubble (CB), nanoparticle (NP), and shielding, e.g., by the plasma or a transient vapor layer, is challenging. Therefore, CB imaging by shadowgraphy is performed to better understand the plasma and laser beam-NP interaction during LAL. By comparing the fluence-dependent CB volume for ablations performed with 1 ns pulses with reports from the literature, we find larger energy-specific CB volumes for 7 ns-ablation. The increased CB for laser ablation with higher ns pulse durations could be a first explanation of the efficiency decrease reported for these laser systems having higher pulse durations. Consequently, 1 ns-LAL shows superior ablation efficiency. Moreover, a CB cascade occurs when the focal plane is shifted into the liquid. This effect is enhanced when NPs are present in the fluid. Even minute amounts of NPs trapped in a stationary layer decrease the laser energy significantly, even under liquid flow. However, this local concentration in the sticking film has so far not been considered. It presents an essential obstacle in high-yield LAL, shielding already the second laser pulse that arrives and presenting a source of satellite bubbles. Hence, measures to lower the NP concentration on the target must be investigated in the future.-
Keywords:
- shadowgraphy /
- power-specific productivity /
- flow dynamics /
- ablation mechanism
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References
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Dittrich S, Barcikowski S, Gökce B. Plasma and nanoparticle shielding during pulsed laser ablation in liquids cause ablation efficiency decrease. Opto-Electron Adv 4, 200072 (2021).. doi: 10.29026/oea.2021.200072
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Figure 1.
Shadowgraphy setup for cavitation bubble imaging during 1 ns, Q-switched laser ablation in water; the solid black lines represent the trigger signals sent by the camera. Also, the signal of the camera and the photodiode are sent to an oscilloscope. In the exemplary oscillogram, the camera and laser pulse signals are shown. The blue-shaded area in this diagram represents the laser's response time to the trigger signal and the gray-shaded area the resulting corrected delay time, which is the time between the laser pulse and the moment of image acquisition.
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Figure 2.
The maximal cavitation bubble volume for different laser intensities are shown for a 1 ns laser (a) and a 7 ns laser (b)67. The productivity m and the ablation efficiency m _spec in (b) are taken from ref.20. For both lasers, the productivity and ablation efficiency are determined at 12 J/cm2. Moreover, the powerspecific maximal CB volume is shown for both lasers (c).
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Figure 3.
The distance between the target surface and the lenses is varied for the 1 ns laser system at maximal laser power, the resulting nominal change in the laser fluence (a), the determined maximal cavitation bubble volume (b), the corresponding productivity (c), and the mass fraction of particles < 10 nm are shown. The gray area in (a) – (d) marks the focal distances where bubble cascades are observed. The exemplary images show bubbles at distances shorter than (orange), within (yellow), and larger (green) than the range where successive bubbles occurred.
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Figure 4.
Energy difference of CBs produced in a pure colloid compared to the respective CB produced in water with different colloid concentrations and with the focal plane shifted into the liquid layer (dark blue squares) as well as the focal plane virtually shifted behind the target (light blue circles) (a). Images show exemplary CBs with the focal plane shifted into the liquid layer (b) and virtually shifted behind the target (c) at a NP concentration of approximately 20 mg/L as marked by the squares in (a).
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Figure 5.
The sketches illustrate the stationary liquid layer above the target and the first cavitation bubble of an image sequence (a). The formed NP are partially trapped in the stationary layer leading to satellite bubble formation, as shown in the sketch, and observed experimentally (b); please note that the sketches in the middle are not depicted on the correct time scale for the sake of the message, the scale bar measures 250 µm.
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Figure 6.
The influence of the NP in the stationary liquid layer is examined under liquid flow conditions by comparing the CB size of consecutive laser pulse with and without a cleaning procedure in between. After 25 laser pulses, the stationary liquid layer is enriched with NP, and a constant cavitation bubble size is reached (a). When the ablation chamber and target are rinsed between two consecutive pulses, a NP-free stationary liquid layer is generated, leading to the formation of an increased cavitation bubble and the absence of satellite bubbles (b), the comparison of the volume increase between the 25th and 26th pulse for (a) and (b) is plotted in (c).
