Ultrafast laser pulses are used to create features and surface textures in metals, ceramics and other materials for applications including the manufacture of solar cells and biosensors. The lasers pulse at durations of 100 femtoseconds, or quadrillionths of a second, and cause electrons to reach temperatures greater than 60,000 degrees Celsius during the pulse duration. The pulses create precise patterns in a process called “cold ablation,” which turns material into a plasma of charged particles.
Images taken with a high-speed camera show tiny mushroom clouds eerily similar in appearance to those created in a nuclear explosion. The clouds expand outward at speeds of 100 to 1,000 times the speed of sound within less than one nanosecond. However, new findings reveal that an earlier cloud forms immediately before the mushroom cloud, and this early plasma interferes with the laser pulses, hindering performance
This series of high-speed images shows how plasma expands when material is exposed to ultrafast laser pulses. Purdue researchers have discovered details that could help to harness the technology for applications in manufacturing, diagnostics and research. (Yung Shin, Purdue University School of Mechanical Engineering)
Finding a way to eliminate the interference caused by the early plasma could open up new applications in manufacturing, materials and chemical processing, machining and advanced sensors to monitor composition, and chemical and atomic reactions on an unprecedented scale, he said.
In this study, the early-stage plasma evolution generated by an ultrashort laser pulse is investigated through pump-probe shadowgraph measurements and simulations. The measurements are performed to show the evolution of the plasma front, while the simulation model is used to further investigate the evolution process and mechanism. Specifically, the laser pulse propagation in air is simulated using the beam propagation method with the slowly varying envelope approximation. The lattice dynamics, the electron dynamics and the multi-scattering event, and the evolution of charged particles (free electrons and ions), are simulated using a molecular dynamics method, a Monte Carlo method, and a particle-in-cell method, respectively. With this simulation model, the refractive index and plasma evolutions are calculated and compared with measured results to validate the simulation model. Different plasma expansion processes, caused by the air ionization, are found with the focal point slightly above and below the target. Air ionization occurs in both cases, but their primary mechanisms are shown to be different.