Power-absorption mechanism and coupling effects in dual-wavelength laser-sustained plasma

Dual-wavelength laser-sustained plasma (LSP), which absorbs energy from lasers with two different wavelengths, is promising as a high-performance light source for wafer optical inspection; however, the mechanisms of power absorption from two lasers are still not well understood. Here a fully coupled laser-thermal-hydrodynamic fluid model is established to investigate the formation characteristics of dual-wavelength LSP. The model is validated by our comparing the laser power absorption results from the simulation and those from experiments. The simulation results show that the near-infrared (915-nm) laser heats a relatively large area of the LSP. In contrast, the visible (435-nm) laser can penetrate the LSP bulk and deposit energy mainly at the focal point, resulting in a smaller, hotter, and higher-radiance LSP core. The time-dependent evolution of the dual-wavelength LSP and the energy components of laser power absorption are presented; moreover, the coupling effects of the two lasers are observed. The near-infrared laser can sustain a large-area background plasma at a relatively low temperature (approximately 15 000 K), which increases the visible laser power absorption at the LSP core, reaching the highest temperature of approximately 30 000 K. The results from this work indicate that dual-wavelength LSP can be a competitive candidate for developing high-performance broadband and high-intensity radiative light sources.