Threefold Efficiency Enhancement and Narrowed Nanoparticle Size Distribution in Laser Ablation of Gold in Water by GHz-Burst Irradiation

Technical Summary: Threefold Efficiency Enhancement and Narrowed Nanoparticle Size Distribution in Laser Ablation of Gold in Water by GHz-Burst Irradiation

Problem Statement
Laser ablation in liquids (LAL) is a established method for synthesizing surfactant-free nanoparticles, offering advantages for biomedicine, catalysis, and additive manufacturing. However, its scalability and productivity are fundamentally constrained by the liquid environment. Key limitations include:

• Nonlinear optical losses: Kerr-induced self-focusing and optical breakdown in the liquid lead to plasma formation, which absorbs and scatters incident radiation before it reaches the target.
• Material redeposition: Ablated material can redeposit onto the target surface on sub-nanosecond timescales due to liquid inertia.
• Cavitation bubble shielding: Vapor cavities form approximately 1–2 ns after pulse impact and persist for tens to hundreds of microseconds. These bubbles shield subsequent pulses, reducing energy delivery to the target.
  Current productivity rates (e.g., 4 g/h for gold) remain significantly below theoretical potential, necessitating strategies to mitigate these intrinsic physical constraints without compromising nanoparticle quality.

Methodology
The study investigates the use of GHz-burst irradiation to temporally shape the laser pulse intensity. Instead of a single ultrashort pulse, the total energy is distributed into a sequence of picosecond sub-pulses (2–25 sub-pulses) separated by a 205 ps inter-pulse delay (4.88 GHz).

• System: A 1030 nm, 10 ps ultrafast laser source was used to ablate polycrystalline gold (99.99% purity) in distilled water.
• Experimental Design: Ablation was performed in both air and water to isolate liquid-specific effects. The incident fluence was adjusted, and the number of sub-pulses ($P$) was varied.
• Characterization:
  • Transmittance: Measured to determine the threshold for nonlinear losses ($\Phi_{NL}$) in the water layer.
  • Ablation Efficiency: Calculated as the ablated volume per unit incident energy ($\eta = V_{abl}/E_I$) using confocal microscopy.
  • Cavitation Dynamics: Pump-probe imaging (532 nm, 500 ps) captured the temporal evolution of cavitation bubble radius and lifetime.
  • Nanoparticle Analysis: Nanoparticles were collected, stabilized with NaCl, and analyzed via Transmission Electron Microscopy (TEM) to determine size distribution.

Key Contributions and Results

1. Suppression of Nonlinear Losses:
   By distributing the pulse energy across multiple sub-pulses, the peak intensity in the liquid is reduced. This increases the threshold fluence for nonlinear losses ($\Phi_{NL}$) from 9 J/cm² (single pulse) to 18 J/cm² for $P=3$, and effectively eliminates nonlinear losses up to 22 J/cm² for $P \ge 4$. This allows higher total energy to be delivered to the target without triggering optical breakdown in the liquid.

2. Threefold Efficiency Enhancement:
   In water, GHz-burst irradiation significantly improves ablation efficiency compared to single-pulse ablation.

   • At an incident fluence of 18 J/cm², the maximum efficiency for single-pulse ablation was ~0.8 µm³/µJ.
   • For GHz-burst ablation ($P=5$ to $10$), efficiency peaked at 2.3 µm³/µJ, representing a threefold increase.
   • This efficiency gain brings the specific energy required for ablation down to ~440 J/mm³, a reduction of nearly a factor of 8 compared to ultrafast LAL benchmarks and a factor of 2.5 compared to 1 ns nanosecond benchmarks.
   • The enhancement is attributed to the delivery of energy before the onset of cavitation bubble shielding and a reduction in material redeposition. Notably, this efficiency in water under burst mode matches the efficiency of single-pulse ablation in air, suggesting the mitigation of liquid-confinement losses.

3. Optimized Cavitation Bubble Dynamics:
   Crucially, the efficiency gain does not come at the cost of larger or longer-lived cavitation bubbles.

   • The maximum bubble radius decreased from ~225 µm (single pulse) to ~200 µm ($P=5$).
   • The primary bubble collapse occurred ~5 µs earlier for the burst mode.
   • The energy converted into the cavitation bubble was reduced by approximately 30%, indicating that less energy is wasted on liquid heating and plasma formation in the bulk liquid.
   • The study identifies a temporal window of approximately 2 ns (the bubble onset time) as the critical limit; burst durations exceeding this (e.g., $P > 10$) result in reduced efficiency as later sub-pulses encounter the forming bubble.

4. Narrowed Nanoparticle Size Distribution:
   Burst irradiation improves the quality of the synthesized nanoparticles.

   • The size distribution became twofold narrower, with the interquartile range decreasing from 5 nm (single pulse) to 2 nm ($P=5$).
   • The fraction of nanoparticles smaller than 10 nm increased from 83% (single pulse) to 96% (burst).
   • This is attributed to the interaction of subsequent sub-pulses with the molten layer and secondary nanoparticles within the sub-nanosecond formation window, potentially suppressing the formation of large secondary particles or fragmenting them.

5. Robustness Under Repeated Irradiation:
   Under repeated irradiation (10 events per spot with 10 s delay), both single-pulse and burst modes showed efficiency increases due to surface modification (incubation). However, the relative advantage of the burst mode was maintained and slightly amplified (efficiency ratio increased from ~3 to ~3.2), confirming that GHz-burst processing does not introduce detrimental incubation effects.

Significance
The paper demonstrates that GHz-burst ablation is a viable strategy to simultaneously enhance productivity and improve nanoparticle quality in LAL. By temporally shaping the pulse to deliver energy within the narrow window preceding cavitation bubble formation, the method effectively suppresses nonlinear losses and material redeposition without increasing bubble shielding. This approach addresses the fundamental physical constraints of LAL, offering a pathway toward scalable, high-efficiency synthesis of surfactant-free nanoparticles with narrow size distributions. The findings suggest that optimizing the temporal intensity distribution is a critical lever for advancing laser-based nanoparticle manufacturing.