Researchers Achieve Record Efficiency For Ultrafast Laser Ablation

Improved ablation efficiency with increased pulse rate could enhance precision material removal for applications such as micromachining and semiconductor manufacturing

Researchers have achieved an unprecedented level of efficiency for ultrafast laser material ablation with a technique that uses a higher frequency of pulses. This advance makes it possible to use less energy and remove more material in applications such as micromachining and semiconductor manufacturing.

Paul Repgen from Bilkent Üniversitesi in Turkey and Ruhr-Universität Bochum in Germany will present this research at the Optica Laser Congress and Exhibition held 20 – 24 October 2024 in Osaka, Japan.

“The ablation-cooled material removal combines the high precision of femtosecond processing with the high efficiency of nanosecond processing,” said Repgen. “This is very interesting in applications like dental surgery – our laser reaches efficiencies similar to mechanical drills, but can still be selective in the material removal and induce less pain as vibrations can be almost omitted.”

Ultrafast laser ablation provides precise removal of almost any material by using extremely short pulses to vaporize the material instantly while minimizing heat damage to the surrounding area. However, this ultrafast single-pulse material ablation typically suffers from low process efficiency because limited light absorption requires extremely high laser power to achieve significant depth.

To improve efficiency, the researchers increased the pulse repetition rate of a ytterbium-doped fiber laser system to more than 50 GHz while at the same time reducing pulse energy. This allows the process to take place in the ablation-cooled ultrafast removal regime where each additional pulse removes a small amount of material and ablation depth increases linearly with the number of pulses.

Starting with seed pulses with a repetition rate of 100 MHz from a standard fiber oscillator, the researchers used a series of nine asymmetric fiber-based Mach-Zehnder interferometers to delay a copy of the pulses at each step. This created a continuous train of 50-GHz pulses from which an acousto-optic modulator picked out groups, or bursts, of pulses that were subsequently amplified and compressed. In the end, this made available pulses with less than 100 fs duration and 20 nJ energy, which is several orders of magnitude lower than considered necessary for laser material removal.

Using the 100-fs pulses and an 80-ns burst duration with 84 μJ burst energy and 20-nJ individual pulse energy, the researchers ablated silicon with record efficiencies of up to 5.2 ± 0.8 mm³/(W·min). This is 10 times more effective than conventional single-pulse processing and six times more efficient than previously reported results using a 3.4 GHz repetition rate.

“Along with the high removal efficiency, we already reach material removal speeds that exceed the speed of sound in air, “said Repgen. “In the future, we want to go further into the supersonic regime and investigate if there are physical limitations to the speed of material removal. Right now, we expect the limitation at the speed of sound in the material, and it would be exciting to achieve this mark.”

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