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What is laser ablation?

 
The term laser ablation or laser machining is most usually associated with the use of excimer lasers. The energies carried by single excimer photons are in the range 3.5 - 7.9 eV. Each photon contains sufficient energy to break apart many of the organic materials commonly found in nature and synthesised by man. Figure 1, shows the binding strength of some of the most common chemical bonds that hold atoms together to form molecules, compared to the photon energies available from laser sources. Illuminating many substances (in solid, liquid or gaseous form) with excimer laser photons can cause a change to either their chemical nature or structural form. Unlike many applications of lasers, ablative excimer techniques make use of several of the unique properties of this light form. The process relies on short, intense bursts of light to create a rapid rise in pressure to break bonds at the material surface. In a confined volume, the bond breaking increases the local particle number density (i.e. pressure). The corresponding rapid rise in pressure is released as a shock wave that ejects material fragments as gases and particulates at high speed. The process takes place with little excess heat transferred to the surrounding material, and as a result can be used to great effect in materials such plastics, paper, ceramics, glasses, crystals, composite and biological tissue.
Fig.1 Strengths of some molecular chemical bonds compared with laser photon energies
Fig.1 Strengths of some molecular chemical bonds compared with laser photon energies

High beam intensities are required to interact with many molecules simultaneously, and the narrow wavelength spectrum of the excimer lasers offer the possibility of selective breaking of some bonds whilst leaving others intact. The coherence of the laser source allows the beam to be used to relay images of intricate relief patterns without incurring appreciable loss in intensity, as well as providing small focused spots when necessary.

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