Laser ablation of aqueous solutions with spatially homogeneous and heterogeneous absorption

The ablation efficiency of aqueous solutions with different concentrations and spatially homogeneous (CuCl₂ solution) and heterogeneous (ink solution) absorption was studied as a function of the pulse-energy fluence (Nd:YAG laser, λ=1064 nm, τ_p = 20 ns). The latter was varied over a wide range from 0.15 J/cm² to 8.00 J/cm². The ablation threshold of solutions with heterogeneous absorption was found to be much lower (3 to 4 times) than the ablation threshold of solutions with homogeneous absorption and with the same average absorption coefficient. The ablation efficiency of heterogeneous solutions was higher by more than an order of magnitude. It was found that the ablation efficiency increases drastically for both types of solutions as the pulse energy fluence was raised to exceed the ablation threshold by 2 or 3 times. At such energy fluences, along with small droplets, larger droplets (1.5-2 mm cross section) could be ejected. This points to the ablation of solutions being affected by a hydrodynamic shock formed as a result of the pulsed recoil pressure excerted by the ablation products. The differences between the ablation processes for solutions with homogeneous and heterogeneous absorption as well as the hydrodynamic destruction at high energy fluences are discussed.