Achieving Low Propagation Loss and Small-Index-Contrast Carbonized Porous Silicon Waveguides Using Direct Laser Writing

Abstract

Porous silicon (PS) is often overlooked as a platform for creating large-mode area waveguides suitable for optical sensing applications due to challenges around creating laterally confined (in-plane) open-pore waveguide structures in the PS film. Direct laser writing (DLW) in hydrocarbon atmospheres can selectively increase the refractive index of the PS film; however, this results in large amounts of absorbing pyrolytic carbon in the pores. The efficacy of postprocessing techniques to remove unwanted absorbing carbon species created by this method is investigated through energy-dispersive X-ray and Raman analysis. The results show that oxygen plasma ashing effectively removes carbon from the pores and considerably reduces propagation losses, resulting in the lowest reported laterally confined refractive index contrast in PS waveguides. The low-index-contrast PS waveguides are shown to require adequate insulation from the high-index silicon substrate to further reduce propagation losses. The carbonized PS waveguide mode is modeled and resulting simulations show low alignment tolerance with a SMF-28 fiber mode. Herein, a method is demonstrated for creating carbonized (passivated), open-pore, and low-loss buried waveguide structures in PS films with a low alignment tolerance to SMF-28 fiber using a DLW approach in ethylene and propane atmospheres.