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Non-linear ionization is the physical excitation mechanism by which an ultrashort laser locally transforms a dielectric from an initial insulating to a highly conductive state with considerable changes on its local electrical and optical properties. When controlled, this local excitation of the material yields well-calibrated macroscopic post-mortem transformation on which every optimized process of laser micromachining builds on. The work presented here is part of this perspective to get control of such laser-induced transformation, aiming to propose an accurate experimental evaluation of laser energy deposition at the surface of a dielectric material exposed to ultrashort pulses, and in particular of its produced free-electron – hole plasma density. For that objective, we conceive appropriate single-shot energy balance experiments with specifically designed dielectric targets consisting of wedged and plane-parallel samples of different thickness (below and above the Rayleigh of the focused laser) in order to observe the free-electron hole plasma produced at the dielectric front surface and also to act on the ratio between photo-ionization and total beam losses. Experiments consists in measuring the incident, reflected and transmitted energy on a wide range of incident fluence (<< Fth to ~ Fth, where Fth indicates the laser-induced ablation threshold) by means of calibrated and identical photodiodes in order to establish a precise energy balance of the interaction. We choose fused silica material because of its popularity for micromachining and photonics applications and we also use ultrashort femtosecond pulses ( 15 - 100 fs, 800 nm) to vary the photo-ionization rate and to well decorrelate ionization process from hydrodynamics and any energy transfer and recombination processes. The information retrieved from these experiments and further confronted to Drude-Lorentz theoretical framework helps us understanding ionization processes (photo-ionization, impact ionization) and their respective importance for dielectric macroscopic transformation in the ablation regime.
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