Laser ablation
Laser ablation or photoablation (also called laser blasting[1][2][3]) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimates. At high laser flux, the material is typically converted to a plasma. Usually, laser ablation refers to removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough. While relatively long laser pulses (e.g. nanosecond pulses) can heat and thermally alter or damage the processed material, ultrashort laser pulses (e.g. femtoseconds) cause only minimal material damage during processing due to the ultrashort light-matter interaction and are therefore also suitable for micromaterial processing.[4] Excimer lasers of deep ultra-violet light are mainly used in photoablation; the wavelength of laser used in photoablation is approximately 200 nm.
For the medical technique, see Laser-induced thermotherapy.Fundamentals[edit]
The depth over which the laser energy is absorbed, and thus the amount of material removed by a single laser pulse, depends on the material's optical properties and the laser wavelength and pulse length. The total mass ablated from the target per laser pulse is usually referred to as ablation rate. Such features of laser radiation as laser beam scanning velocity and the covering of scanning lines can significantly influence the ablation process.[5]
Laser pulses can vary over a very wide range of duration (milliseconds to femtoseconds) and fluxes, and can be precisely controlled. This makes laser ablation very valuable for both research and industrial applications.
Mechanism[edit]
Material dynamics[edit]
A well-established framework for laser ablation is called the two-temperature model by Kaganov and Anisimov.[26] In it, the energy from the laser pulse is absorbed by the solid material, directly stimulating the motion of the electrons and transferring heat to the lattice, which underlies the crystalline structure of the solid. Thus, the two variables are: the electron temperature itself and the lattice temperature . Their differential equations, as a function of the depth , are given by
Here, and are the specific heat of the electrons and the lattice respectively, is the electron thermal conductivity, is the thermal coupling between the electron and (lattice) phonon systems, and is the laser pulse energy absorbed by the bulk, usually characterized by the fluence. Some approximations can be made depending on the laser parameters and their relation to the time scales of the thermal processes in the target, which vary between the target being metallic or a dielectric.
One of the most important experimental parameters for characterization of a target is the ablation threshold, which is the minimum fluence at which a particular atom or molecule is observed in the ablation plume. This threshold depends on the wavelength of the laser, and can be simulated assuming the Lennard-Jones potential between the atoms in the lattice, and only during a particular time of the temperature evolution called the hydrodynamic stage. Typically, however, this value is experimentally determined.
The two-temperature model can be extended on a case-by-case basis. One notable extension involves the generation of plasma. For ultra-short pulses (which suggest a large fluence) it has been proposed that Coulomb explosion also plays a role [26] because the laser energy is high enough to generate ions in the ablation plume. A value for the electric field has been determined for the Coulomb-explosion threshold, and is given by
where is the sublimation energy per atom, is the atomic lattice density and is the dielectric permittivity.
Plume dynamics[edit]
Some applications of pulsed laser ablation focus on the machining and the finish of the ablated material, but other applications are interested in the material ejected from the target. In this case, the characteristics of the ablation plume are more important to model.
Anisimov's theory considered an elliptical gas cloud growing in vacuum. In this model, thermal expansion dominates the initial dynamics, with little influence from the kinetic energy,[26] but the mathematical expression is subject to assumptions and conditions in the experimental setup. Parameters such as surface finish, preconditioning of a spot on the target, or the angle of the laser beam with respect to the normal of the target surface are factors to take into account when observing the angle of divergence of the plume dynamics or its yield.