Direct laser writing of graded-index SiGe waveguides via phase segregation

We report direct laser writing of graded-index optical waveguides via phase segregation in initially homogenous silicongermanium (SiGe) thin films epitaxially-grown on silicon substrates. We used a continuous wave (CW) laser operating at a wavelength of 532 nm. The laser beam was focused to a 5 μm diameter spot on the surface of SiGe films with a thickness of 575 nm and a Ge concentration of %50. Compositional separation of a SiGe film was induced by melting the surface, and the composition profile was tailored by controlling the scan speed of the laser-induced molten zone in a range of 0.1-200 mm/s. At high scan speeds, scanning the laser beam produces a travelling Ge-rich molten zone, where a build-up of Ge content occurs at the trailing edge because of insufficient diffusion-limited Ge transport. Material characterizations have revealed that the laser-processed SiGe microstripes consist of Ge-rich strip cores (> 70% Ge) surrounded by Si-rich under-claddings (<30% Ge). Scan-speed dependent phase segregation allows for fabrication of graded-index SiGe waveguides with tunable compositional profiles, which were characterized by optical transmission measurements, and modal analysis using simulations. Our method could also be applied to pseudo-binary alloys of ternary semiconductors (AlGaAs), which have equilibrium phase diagrams similar to that of SiGe alloys.