Abstract: In a particular embodiment, a relatively high-energy thulium fiber laser operating at the wavelength ?=2 ?m may be used to selectively modify a front and/or a back surface of silicon and gallium arsenide wafers. The processing regime was studied in terms of the process parameters variation, and the corresponding modification fluence thresholds were determined. The results revealed considerable differences in morphology between front and back surface modifications, and that the back surface modification threshold of Si is significantly higher than at the front surface. Basic analytic modeling and z-scan measurements were performed to study the absorption mechanisms. In a broader embodiment the processing regime is not specifically limited to a thulium fiber laser.