Thermal regime of laser ablation of metals by ultrashort pulses of low fluence

Abstract

Thermal ablation of a metal surface by low-energy ultrashort laser pulses is considered theoretically. The temporal dynamics of the surface electron and lattice temperatures is studied within the framework of the two-temperature model and for different temperature dependences of the characteristics of the metal (electron-relaxation time, heat capacity, thermal conductivity). The approximation of evaporation into a vacuum is used to determine the ablation depth. Analytical expressions for the ablation-threshold fluence Fth as well as the threshold values of the lattice temperature Tth(Fth) and the characteristic time of lattice temperature decay td(Fth) are obtained. This analytical description is in satisfactory agreement with particular numerical calculations.

Hydrodynamic regime of laser ablation of metals by ultrashort pulses of low fluence

“…5 experimental data for ablation of Cu [12] by 150-fs laser pulses are compared with the results of our calculations (Z = 1 and Tcr --0.465 eV). The experimental value of the threshold fluence F~: u) = 200 mJ/cm 2 is also close to the results of both computer simulation on the basis of the model under consideration and analytical model [3].…”

“…The results are in good agreement and correspond to the threshold fluence F (Au) -~ 100 mJ/cm 2. It should be noted that this value is close to the analytical value of 90 mJ/cm 2 obtained within the framework of the surface evaporation model [3], i.e., the predictions of both models do not contradict experimental data.…”

“…According to [3] the maximum ion temperature 0.41 eV corresponding to the ablation threshold Fth ---~ 90 mJ/cm 2 in Au can be comparable to the critical temperature. Certainly, the surface ion temperature becomes critical for F _> Fth.…”

“…The definition of the threshold fluences in the problem of laser ablation of metals by pico-and femptosecond pulses [1,2] assumes that at low laser fluences F, close to the ablation threshold Fth, the thermal mode is the dominant ablation mechanism [3]. This means that the ablation is determined by the heating of the electron gas due to thermal conductivity followed by electron-ion relaxation providing the heating of the lattice.…”

“…The heated region inside the metal is considered as motionless, and the ablation rate (the ablation depth per pulse d) depends only on the time evolution of the surface ion temperature. The value d is connected with evaporation in vacuum [5] and can be determined by the integral [3] d ~ n (x) exp -T--~ dr, (2n) 1/2 t mi J 0 where mi is the lattice ion mass, Ti(t) is the surface lattice (ion) temperature (here and below the temperature is written in energy units), U0 is the bond energy, and b ..~ 103 is a dimensionless parameter. Such a problem was solved in [3], where analytical expressions for the threshold laser fluences Fth and the surface ion temperature Ti(Fth, t) --Tth(t) were obtained as functions of typical metal parameters.…”