Laserablation

As laser ablation, also known as laser evaporation, the removal of material from a surface is referred to by bombardment with pulsed laser radiation. Here, the use of laser radiation with a place of high power density resulting in the rapid heating and the formation of a plasma on the surface. This crossing of the plasma threshold is differentiating it from the laser desorption, in which no plasma is produced, used.

Physical Basics

The energy of the photon is transmitted to the first electron of the solid. With femtosecond pulses, the electrons can reach a high temperature; within a very short time (a few picoseconds ) are excited with this energy thermal vibrations of atomic nuclei. Until the temperature of the electrons corresponds to the temperature of the atomic vibration, the state of the model of the two temperatures, engl. two- temperature model is described. The high energy electrons can lead to the breaking of chemical bonds in non-metals, it can also come by so- short laser pulses to Coulomb explosion. This means that the electrons leave the solids and a remaining part of the positive ions is spun by Coulomb repulsion of the surface.

With laser pulses in the nanosecond range, the energy of the laser leads to a heating of the surface (in the sense of thermal movements of the atoms) during the laser pulse. Since the heat conduction allows only a slow energy transport into the bulk, the radiated energy is a very thin layer concentrated ( about 1 micron at 10 ns pulse length), therefore, the surface reaches very high temperatures and it comes to sudden evaporation of the material. By ionization (thermal, by electron impact or laser light ) is produced with a high power density of the laser, a plasma of electrons and ions of the material removed; the ions can be accelerated to energies up is about 100 eV.

The minimum power or energy density at which ablation is possible ( for a given wavelength and pulse width ) is referred to ablation. For energy densities above this threshold, the ablation rate increases sharply. With nanosecond pulses and high power densities, the plasma may be so dense that it absorbs a large part of the laser light and the surface to protect against further heating. The ablation rate increases then only approximately linearly with the energy density.

In order to ensure a sufficient absorption of laser light, ultraviolet radiation is often used especially in nanosecond pulses. At these wavelengths, the reflectivity of metals is less than in the visible light; the light absorption of the insulating material is higher in general.

Applications

Material processing

Laser ablation may be used for selectively removing material, for example, instead of a mechanical engraving of hard materials or for drilling of very small holes. Laser ablation can also be used for the removal of thin layers of various impurities; the process is relatively gentle, because only the top ( microns thick ) layer is strongly heated, the work as a whole remains cold.

Coating method

The eroded material can be used to coat another surface. This technique is called Laserstrahlverdampfen (English: Pulsed Laser Deposition) respectively.

Analysis

With the help of a finely focused laser beam smallest sample volumes may be removed. The removed sample particles are rinsed by means of a gas flow ( He, Ar, ...) to a detector (such as ICP-MS) and characterized therein with respect to their element and / or isotopic composition. A subspecies of laser ablation ( LA) is the laser-induced plasma spectroscopy (English laser -induced breakdown spectroscopy, LIBS ), in which not the ablated sample particles is analyzed directly, but which of them by the laser excitation absorbed and subsequently emitted in the form of light energy. By atomic emission this light specific information about the elemental composition of the sample can be obtained here too. Advantages here are the relatively high spatial resolution in the micron range which is lost in a full digestion of the sample and subsequent analysis of the liquid phase.

Medicine

Laser ablation is also used for the removal of tissue in medicine. In contrast to a continuous laser beam can be kept small when using a pulsed laser, the thermal load of the adjacent tissue. For the minimally invasive surgery, the laser beam can also be channeled through light guide into the interior of the body.

Restoration

Characterized in that different materials at different power densities begin to ablate, it is possible to selectively remove impurities, or certain colors, without damaging the underlying layers. This takes place in the restoration application.

• Laser application