Ultrafast and selective reduction of sidewall roughness in silicon waveguides using self-perfection by liquefaction

We use a novel technique, self-perfection by liquefaction (SPEL), to smooth the rough sidewalls of Si waveguides. An XeCl excimer laser with 308 nm wavelength and 20 ns pulse duration is used to selectively melt the surface layer of the waveguide. This molten layer flows under surface tension and this results in smooth sidewalls upon resolidification. Our experimental results show that this technique reduces the average sidewall roughness (1σ) from 13 to 3 nm. Our calculations show that the waveguide transmission loss due to sidewall roughness in these waveguides would be reduced from 53 to 3 dB cm−1, an improvement with light transmission five orders of magnitude greater. Due to a low viscosity of molten Si (below water), SPEL can be achieved on a Si surface within ∼100 ns. This short time, together with SPEL's material selectivity, makes it possible to repair defective components on a chip without damaging surrounding components and materials, making SPEL a promising candidate for defect repair in integrated optics and nanophotonics.