(a) 4 μm wide Si waveguide on SiO2 with rough sidewalls was smoothed. (b) After exposure to 20 laser pulses of 900 mJ/cm−2 at a repetition rate of 1 Hz. (c) Roughness profile along the length direction of a 4 μm wide Si waveguide on SiO2 before and after. (d) Self-perfection (SPEL). Figures are taken from [15].