Dynamics of femtosecond laser absorption of fused silica in the ablation regime

M. Lebugle (1) , N. Sanner (1) , N Varkentina (1) , M. Sentis (1) , O. Uteza (1)

1 LP3 - Laboratoire Lasers, Plasmas et Procédés photoniques

We investigate the ultrafast absorption dynamics of fused silica irradiated by a single 500 fs laser pulse in the context of micromachining applications. A 60-fs-resolution pump-probe experiment that measures the reflectivity and transmissivity of the target under excitation is developed to reveal the evolution of plasma absorption. Above the ablation threshold, an overcritical plasma with highly non-equilibrium conditions is evidenced in a thin layer at the surface. The maximum electron density is reached at a delay of 0:5 ps after the peak of the pump pulse, which is a strong indication of the occurrence of electronic avalanche. The results are further analyzed to determine the actual feedback of the evolution of the optical properties of the material on the pump pulse. We introduce an important new quantity, namely, the duration of absorption of the laser by the created plasma, corresponding to the actual timespan of laser absorption by inverse Bremsstrahlung. Our results indicate an increasing contribution of plasma absorption to the total material absorption upon raising the excitation fluence above the ablation threshold. The role of transient optical properties during the energy deposition stage is characterized and our results emphasize the necessity to take it into account for better understanding and control of femtosecond laser-dielectrics interaction.

Mots clés: Ultrashort; Femtosecond; Pump-probe; plasma dynamics; Laser femtosecond; Laser 3D processing

Domaines: Optique [physics.optics]

M. Lebugle, N. Sanner, N Varkentina, M. Sentis, O. Uteza. Dynamics of femtosecond laser absorption of fused silica in the ablation regime. Journal of Applied Physics, 2014, 116, pp.063105. ⟨10.1063/1.4892158⟩. ⟨hal-01201866⟩