The high-precision, low-damage patterning by laser-induced back side etching techniques is still of interest. particularly for ultra-precision engineering, although the mechanism is not yet clear. The drastically altered optical properties of fused silica etched with LESAL (laser etching at a surface-absorbed layer) give evidence for the etching mechanism. Depth-resolved UV/Vis spectroscopic measurements show that the modified layer is limited to a depth of similar to 60 nm. This is correlated with a very high calculated absorption coefficient of similar to 4 x 10(7) m(-1) of the modified surface layer. With Rutherford backscattering spectrometry (RBS) measurements on LESAL-modified surfaces, it was demonstrated that in dependence on the laser fluence used, an amorphized layer with a thickness of a few nanometers was generated. The RBS measurements show that carbon is incorporated into the LESAL-modified surface. (c) 2012 Elsevier B.V. All rights reserved.