Investigations of the ultrafast laser induced melt dynamics by means of transient quantitative phase microscopy (TQPm)

摘要

Modifications of bulk aluminum irradiated well above ablation threshold (F < 300 J·cm~(-2)) have been investigated in situ by means of shadowgraphy and transient quantitative phase microscopy (TQPm) using ultrafast laser radiation (t_p=80 f_s, λ=800 nm). This novel pump-probe technique enables quantitative time-resolved measurements of object's properties, e.g. dimensions of melt droplets and layer thickness or transient refractive index changes. A series of time-resolved phase images of vaporized material and/or melt, which are induced by n=1..8 pulses on an aluminum target, are obtained using TQP_m. Dynamics and characteristics of melting, dependence of the ablated material volume on process parameters and thereby induced structural modifications have been studied. An increase of material ejection rate is observed at delay time of approximately T=300 ns and >800 ns after the incident pulse.Transient refractive index modifications have been investigated in technical glass (Schott D263) by means of TQPm. By using high-repetition rate ultra-short pulsed laser radiation (t_p=400 f_s, λ=1045 nm, f_(rep)=l MHz) focused by a microscope objective (ω_0≈4 μm) heat accumulation and thereby glass melting as well as welding is enabled. Transient optical phase variation has been measured up to τ=2.1 μs after the incident pulse and can be attributed to the generation of free charge carriers and compression forces inside glass.