Time-resolved observation of gas-dynamic discontinuities arising during excimer laser ablation and their interpretation

Abstract

Ablation of materials (Cu is presented in this report) in air at an ambient gas pressure of 1 bar with a KrF excimer laser (3-47 J cm-2) leads to gasdynamic processes above the target surface which affect the processing result, the efficiency of the treatment and the debris in the environment of the irradiated area. These laser-induced processes have been diagnosed using fast schlieren photography and shadowgraphy. Five discontinuities have been discerned and their propagation mechanisms have been detected. A physical interpretation of the discontinuities is given along the lines of existing theories and plausible reasoning. The locally most advanced discontinuity can be explained by the classical Sedov-Taylor blast wave theory, and conclusions on the energy content in the shock wave, the pressure distribution and the surface pressure evolution will be presented. The results show that, at high energy densities (3-47 J cm-2), about 80% of the available laser pulse energy is deposited in the shock wave. A reduction in the energy density leads to a decrease in the fraction of the energy that is deposited in the shock wave. Close behind the first discontinuity follows a second one that is interpreted as the ionization front. The contact front, which separates shocked air and ablated material vapour, has been observed within the laser pulse duration. The complex structure of the contact front is interpreted in terms of gas flow phenomena inside the two outer discontinuities.

Imaging femtosecond laser-induced electronic excitation in glass

“…These include pulsed laser deposition of high criticaltemperature superconductor films 1,2 , femtosecond laser micro-machining and structural modification of dielectrics 3,4 , ultrafast laser-assisted material and chemical analysis 5,6 , and ultrashort X-ray pulse generation 7,8 . However, high power, short pulse laser ablation is still largely unexplored at the fundamental level.Mechanisms underlying laser ablation processes are quite complex, most previous investigations have focused on laser ablation at relatively long (e.g., nanosecond and picosecond) time scales 9,10 . Since the femtosecond time scale is much shorter than the electron-lattice energy relaxation time, thermal equilibrium cannot be established during the time the laser interacts with a solid material.…”

“…Mechanisms underlying laser ablation processes are quite complex, most previous investigations have focused on laser ablation at relatively long (e.g., nanosecond and picosecond) time scales 9,10 . Since the femtosecond time scale is much shorter than the electron-lattice energy relaxation time, thermal equilibrium cannot be established during the time the laser interacts with a solid material.…”

Application of self-mixing interferometry for depth monitoring in the ablation of TiN coatings

“…32 On the other hand, Callies et al have calculated surface pressure during the ablation of Cu target with 60 ns pulses. 33 The authors reported that surface pressure returns to ambient conditions soon after the end of the pulse duration, at around 100 ns. Therefore, the used gas pressure can suffice for divert the ablation plume.…”

Investigation of pulse shape characteristics on the laser ablation dynamics of TiN coatings in the ns regime

“…These works report spherical expanding shock waves when high energy pulses in mJ range with ns-level durations are used. Callies et al estimated the fraction of pulse energy deposited in the shockwave assuming a point explosion on metallic targets and pulse energies varying between 1.4-12 mJ and 60 ns pulses [20]. Amer et al compared the energy component in a spherical expansion to the laser energy to estimate the energy conversion efficiency, using 15 mJ and 12 ns pulses on polycrystalline boron nitride (PCBN) [24].…”

“…On the other hand, several techniques for monitoring ablation dynamics can be employed for a direct observation of the ablation plume and shock wave. The monitoring methods are mainly based on optical approaches such as probe beam deflection [12,13], shadowography [14][15][16][17][18][19], schlieren photography [20], interferometry [21,22] and digital holography [23][24][25][26]. The previous works in the literature have provided a better understanding of the ablation phenomenon in general.…”