Laser ablation in liquid (LAL) is a versatile method for the synthesis of nanoparticles (NPs), that enables the production of ligand-free colloids
Opto-Electronic Advances, Volume. 5, Issue 6, 210053(2022)
Time resolved studies reveal the origin of the unparalleled high efficiency of one nanosecond laser ablation in liquids
Laser ablation in liquid is a scalable nanoparticle production method with applications in areas like catalysis and biomedicine. Due to laser-liquid interactions, different energy dissipation channels such as absorption by the liquid and scattering at the ablation plume and cavitation bubble lead to reduced laser energy available for nanoparticle production. Ultrashort pulse durations cause unwanted nonlinear effects in the liquid, and for ns pulses, intra-pulse energy deposition attenuation effects are to be expected. However, intermediate pulse durations ranging from hundreds of picoseconds up to one nanosecond have rarely been studied in particular in single-pulse settings. In this study, we explore the pico- to nanosecond pulse duration regimes to find the pulse duration with the highest ablation efficiency. We find that pulse durations around 1–2 ns enable the most efficient laser ablation in liquid since the laser beam shielding by the ablation plume and cavitation bubble sets in only at longer pulse durations. Furthermore, pump-probe microscopy imaging reveals that the plume dynamics in liquids start to differ from plume dynamics in air at about 2 ns after pulse impact.Laser ablation in liquid is a scalable nanoparticle production method with applications in areas like catalysis and biomedicine. Due to laser-liquid interactions, different energy dissipation channels such as absorption by the liquid and scattering at the ablation plume and cavitation bubble lead to reduced laser energy available for nanoparticle production. Ultrashort pulse durations cause unwanted nonlinear effects in the liquid, and for ns pulses, intra-pulse energy deposition attenuation effects are to be expected. However, intermediate pulse durations ranging from hundreds of picoseconds up to one nanosecond have rarely been studied in particular in single-pulse settings. In this study, we explore the pico- to nanosecond pulse duration regimes to find the pulse duration with the highest ablation efficiency. We find that pulse durations around 1–2 ns enable the most efficient laser ablation in liquid since the laser beam shielding by the ablation plume and cavitation bubble sets in only at longer pulse durations. Furthermore, pump-probe microscopy imaging reveals that the plume dynamics in liquids start to differ from plume dynamics in air at about 2 ns after pulse impact.
Introduction
Laser ablation in liquid (LAL) is a versatile method for the synthesis of nanoparticles (NPs), that enables the production of ligand-free colloids
Although laser-generated particles show advantages over chemically synthesized NPs, only a few commercial distributors offer laser-generated NPs. A possible reason for this might be that LAL only becomes economically more feasible than chemical synthesis for nanoparticle productivities exceeding 550 mg/h (for gold)
Besides scaling up the productivity by increasing the laser power, careful tuning of the laser pulse duration represents another route to optimize the LAL process. Nanosecond LAL (ns-LAL) has been shown to achieve similar power-specific ablation rates as picosecond LAL (ps-LAL)
In contrast to ablation in air, additional mechanisms of energy loss occur during LAL. These mechanisms can be divided into two categories that include (1) losses due to the interaction of the laser pulse with the water layer and (2) losses due to interaction of the laser pulse with the induced ablation dynamics. The extent of these loss mechanisms influence on LAL productivity strongly depends on the laser pulse duration.
Interaction of the laser pulse with the water layer includes linear reflection and absorption, optical breakdown, non-linear absorption, and shielding by NPs immersed in the liquid medium
Owing to the inherent design of LAL experiments, reflectance at the entrance window along with linear absorption of the laser energy by the liquid layer are inevitable. These losses result in the dissipation of up to half of the laser energy, regardless of the laser pulse duration
Optical breakdown within the liquid occurs when a plasma with a certain critical electron density is generated by the laser pulse through multiphoton ionization followed by cascade ionization
NPs that are present in the liquid can significantly lower the threshold fluence for the optical breakdown of ns pulses
LAL performed with ultrashort laser pulses ranging from tens of femtoseconds to a few picoseconds is often accompanied by non-linear effects such as the optical Kerr effect
Independent whether the ablation is performed in gases or in liquids, a reduction of the deposited laser energy is caused by interactions with the ablation dynamics. These interactions include plasma shielding
It was found that the ablation efficiency, which we define as the ablated NP mass per time (productivity) divided by the laser power varies strongly depending on the experimental conditions. This value is typically in the order of 1–10 mg/(hW) for high repetition rate lasers operating in the liquid flow regime
To test the hypothesis of negligible shielding at a pulse duration of ~1 ns, we first determine the losses of laser energy during LAL of gold (Au) for different pulse durations and compare the results to ablation in air to quantify the pulse duration-dependent efficiency loss in water. Afterward, the temporal occurrence of the ablation plume is examined by pump-probe microscopy (PPM) of Au immersed in air and in water. We additionally analyze the response of silver (Ag) and platinum (Pt) to identify whether the onset of ablation plume formation is material-dependent.
Materials and methods
Ag, Au, and Pt bulk samples with a thickness of 1 mm and a purity >99.99% were used throughout the experiments. The samples were first embedded in a resin matrix and subsequently sanded and polished. An arithmetic average surface roughness Ra
Figure 1.(
A glass plate (GP) partially reflected the pump-pulses onto a photodiode (DET10A2, Thorlabs), which served as a trigger source for temporal synchronization. A mechanical shutter (LS6S2TO-NL, Uniblitz Electronics) then selected a single pulse from the pulse train, which was guided through a half-wave-plate (HWP) polarization beam splitter (PBS) combination to adjust the pulse energy. Finally, the single pump-pulses were focused onto the sample surface at an incidence angle of 35° by a plano-convex lens with a focal length of f = 75 mm. The resulting elliptical laser spot on the sample surface with a minor beam waist radius of wmin = (12 ± 1) µm and a major beam waist radius of wmaj = (15 ± 1) µm was characterized by a focal beam profiler (MicroSpotMonitor, Primes) at 1/e2 intensity level. The peak fluences Φ0 were calculated by:
Here, the average laser output power P of the 500 Hz pulse train was measured after the focusing lens using a power meter (PS10Q, Coherent). Note that all peak fluences mentioned throughout this manuscript refer to incident peak fluences in air.
The delay time Δt between the probe-pulse and pump-pulse was realized with a motorized optical delay line, which allowed for the adjustment of Δt between –1 ns and 8.5 ns in steps of 0.1 ns. After passing a quarter-wave-plate (QWP) PBS combination, the probe-pulse imaged the ablation process at normal incidence onto a CCD camera (pco.pixelfly usb, PCO) through a long working distance microscope objective (50x, NA = 0.42; M Plan Apo 20, Mitutoyo) and a tube lens (TL). A bandpass filter (BPF) centered at (532 ± 5) nm was located in front of the camera to suppress undesired pump- and plasma-radiation. The shutter and camera were temporally synchronized to the photodiode trigger signal with a delay-generator (DG645, Stanford Research Systems).
For each Δt, the sample was translated to irradiate a pristine surface and three images were acquired at different times. The reference image (R0) was recorded 5 s before the pump-pulse’s arrival showing the unirradiated surface, which allowed the measurement of the initial target reflectivity. Afterward, an image at the desired delay time Δt (R(Δt)) was taken. This image was used to analyze the temporal change in the surface morphology and reflectivity, i.e. transient reflectivity. Finally, an image (Rinf) was taken 5 s after pump-pulse impact, which was used to determine the final reflectivity of the changed target surface and the resulting target morphology change, i.e. crater size. The transient and final relative surface reflectivity change ΔR/R0 and ΔRinf/R0 were calculated for each pixel by the following equations:
This process is illustrated in
It should be noted that even though the delay time increment was set to 0.1 ns, the temporal resolution of the PPM setup and hence of the reflectivity dynamics is determined by the probe pulse duration of 600 ps. For any given Δt, the measured reflectivity signal presents a convolution of the temporal probe-pulse profile and the actual transient reflectivity change occurring within the probe-pulse duration. In this context, the relative reflectivity change at each Δt does not reflect the reflectivity dynamics occurring at this exact instant of time but rather includes contributions of the dynamic surface reflectivity from a time interval of 600 ps. Therefore, fast transient processes that occur on timescales shorter than the applied probe-pulse duration cannot be resolved with this setup.
Results
Compared to ablation with ns pulses, the water layer has a large influence on the ablation efficiency when ps pulses are applied. Here the maximal ablation efficiency of 40.7 ± 0.8 µg/(W·s) in air drops by approximately 90% to a value of 5.0 ± 0.1 µg/(W·s) in water. For the 1 ns laser, a negligible influence of the water layer on the ablation efficiency is observed. At this pulse duration, comparable maximal ablation efficiencies with respective values of (17.0 ± 0.7) µg/(W·s) and (17.1 ± 0.4) µg/(W·s) are reached in air and water. Ablation with a 7 ns laser results in a 30% decrease of the ablation efficiency from 19.7 ± 5.0 µg/(W·s) in air to 13.7 ± 1.1 µg/(W·s) in water.
Figure 2.Maximal ablation efficiency for the ablation of gold in air (light-colored, solid bars) and water (dark-colored bars) for lasers of 3 ps (blue, ~2 J/cm² and 100 µJ/pulse), 1 ns (green, ~8 J/cm² and 130 µJ/pulse), and 7 ns (orange, ~13 J/cm² and 400 µJ) pulse duration with data from ref.10 where the ablation efficiency is calculated with the incident laser energy (dark-colored, solid bar) and under consideration of the linear energy extinction by the water layer (dark-colored, hatched bar). The error bars represent the statistical error.
The findings of the PPM experiments are presented in
Figure 3.(
For Δt > –0.5 ns, Δ R/R0 stays approximately constant at the zero level. When the delay time increase beyond 0 ns, a pronounced decrease of ΔR/R0 to a minimum of -0.9 and -0.8 at delay times of 1 ns is observed in air and water, respectively. This decrease is accompanied by an expansion ofΔA. After the initial decrease, ΔR/R0 recovers to a value of –0.5 at Δt ≈ 2 ns. During this recovery, ΔA stays approximately constant. When the delay time of 2 ns is exceeded, oscillations of ΔR/R0 are observed over the remaining investigated temporal range. In the case of air ΔR/R0 oscillates between values of –0.6 and –0.2, while in water ΔR/R0 oscillates between values of –0.7 and –0.3. Within this time interval, ΔA stays constant at the final state value for laser ablation in air. However, in the case of ablation in water, this characteristic delay time (blue solid vertical line in
From this characteristic time onwards, it is possible to distinguish between the relative reflectivity change occurring on the sample surface and the relative reflectivity change induced by the outward propagating ablation plume. By averaging over the instantaneous velocities within the temporal interval ranging from the characteristic Δt of 2 ns up to the longest investigated Δt of about 8.5 ns, we calculate a radial expansion velocity of approximately (1700 ± 200) m/s in water. The oscillatory features of ΔA for delay times ranging between 4 ns and 8 ns are attributed to statistical fluctuations of the vapor/cavitation bubble propagation.
To evaluate the material-dependent response to the laser pulse, pump-probe microscopy experiments at a pump-pulse peak fluence of 8 J/cm2 have been performed for Ag and Pt in water.
Figure 4.
For both materials it can be observed that ΔA begins to increase when a delay time of zero is exceeded. At delay times of Δt ≈ 0.7 ns (blue dashed vertical line) and Δt ≈ 1.5 ns (blue solid vertical line) ΔA has exceeded the final state values of (170 ± 40) µm2 and (580 ± 60) µm2 for Ag and Pt, respectively. The rapid increase of ΔA over the remaining investigated delay time range occurs with a radial expansion velocity of (1840 ± 140) m/s in the case of Ag and with approximately (1650 ± 100) m/s in the case of Pt.
Discussion
In order to put the measured ablation dynamics in context, the different mechanisms leading to energy loss during LAL are discussed and compared to our findings. For the ablation efficiencies presented in
The optical breakdown threshold fluences of water for near-infrared laser pulses are about 1–13.5 J/cm2 for 3 ps pulses and 100–300 J/cm2 for nanosecond pulses
Following the observation of Starinskiy et al.
Re-deposition of and shielding by the generated ablation plume has been extensively investigated for ablation in air
In order to support the hypothesis that plume, vapor and cavitation bubble shielding limits the ablation efficiency for 7 ns LAL, while for 1 ns LAL no such limitations are present, the pump-probe microscopy measurements of Au will be discussed in detail. First, it was observed that the surface reflectivity in air and water do not differ significantly for Δt < 1 ns (
For ns-LAL the reflectivity change in water is more pronounced than in air due to supercritical water layer formation and an emerging vapor layer
Next, we observed that for irradiation in air, ΔA expands up to a delay time of 2 ns, where it reaches the final state value of 170 µm². However, for irradiation in water ΔA reaches the final state value of 120 µm² at a delay time of 2 ns and then proceeds to increase continuously up to our maximum observeation time of 8.5 ns (
After reaching the final spot diameter, the spot area remains constant of Au in air, whereas in water it expands with 1700 m/s in the observed time range (Mach number > 1). The shockwave propagation velocity for delay times between 0.2–2 µs after laser pulse impact was measured to be 1700 m/s for Cu (8 ns pulse width, 68 J/cm 2
Later, the cavitation bubble expansion is drastically decreased to, e.g., 50 m/s at a delay time of 0.5–5.0 µs
Since laser absorption is a material-dependent property, we additionally analyzed the material response for Ag and Pt samples within the first 9 ns. For both sample materials, the characteristic delay time, at which ΔA increases beyond its final crater size, is reduced compared to ablation of Au in water. Here characteristic delay times of 0.7, 2.0 and 1.4 ns were observed for Ag, Au, and Pt. This result shows that the optimal LAL pulse duration strongly depends on the materials used. The ΔA change (
Finally, the experimental results are in good agreement with computational predictions. Shih et al. investigated LAL of an Ag target at an absorbed fluence of 0.6 J/cm2 and pulse durations of 400 ps, 1 ns and 2 ns by means of atomistic simulations
Conclusion
Laser ablation synthesis of colloids in liquids is a promising nanomaterial fabrication method but high laser investments costs require the efficient use of laser energy, avoiding pulse attenuation during ablation, and finding the most efficient pulse duration regime. Short pulsed LAL at around 10 ns pulse duration is of similar efficiency compared to ultrashort pulsed LAL, as in both cases, intra-pulse attenuation caused either by the liquid or plume, vapor and cavitation bubble limits efficient energy deposition. It is demonstrated that pump-probe microscopy gives valuable insight into laser ablation mechanisms for targets immersed in air and water. The comparison of the transient laser-modified area with the laser-modified area in the final state allows us to confirm the hypothesis of negligible intra-pulse plume, vapor and cavitation bubble shielding for ablation with 1 ns laser pulses. Furthermore, we are able to determine characteristic shielding times which translate into optimal laser pulse durations. If the optimal pulse durations are met, shielding of the trailing edge of the laser pulse by plume, vapor and cavitation induced by the leading edge of the laser pulse is avoided. Consequently, the reduction of shielding effects during the laser pulse irradiation increases the ablation efficiency observed for 1 ns LAL of Au. The characteristic shielding times are approximately 2 ns, 0.7 ns and 1.4 ns for Au, Ag and Pt, respectively. Hence cavity-length limited, Q-switched nanosecond lasers (such as fiber or microchip lasers) may have high potential in advancing LAL to even higher efficiencies, with the practical and ecological perspective that such lasers are comparable compact and electro-optically energy-efficient.
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Sarah Dittrich, Maximilian Spellauge, Stephan Barcikowski, Heinz P. Huber, Bilal Gökce. Time resolved studies reveal the origin of the unparalleled high efficiency of one nanosecond laser ablation in liquids[J]. Opto-Electronic Advances, 2022, 5(6): 210053
Category: Research Articles
Received: Apr. 29, 2021
Accepted: Sep. 14, 2021
Published Online: Aug. 19, 2022
The Author Email: Huber Heinz P. (;), Gökce Bilal (;)