Silicon nitride (SiN) has a high refractive index compared to silicon dioxide (SiO2), which leads to tight confinement in SiO2-clad SiN waveguides; SiN is also compatible with standard complementary-metal-oxide semiconductor (CMOS) technology. These facts, and others, make SiN promising as a replacement for silicon in integrated photonics for some uses. Researchers at Aachen University (Aachen, Germany) and Illumina (Hayward, CA) have solved three problems that inhibited use of SiN for integrated photonics at visible
wavelengths.
First, they created waveguides with an ultrathin (100 nm) film layer that boosts the interaction of the light field with the top cladding (important for sensing applications). Second, they created a free-space coupler for visible wavelengths (660 nm in the prototype) with an insertion efficiency in the range 38% to 44%. Third, they developed a multimode interference coupler (MMI) for splitting light within the circuit; the 57.3-μm-long MMI split one beam into four with an efficiency higher than 93% and an imbalance less than 10%. Waveguide losses were below 0.71 dB/cm and 0.51 dB/cm with water cladding and SiO2 cladding, respectively, and bend losses for various radii were low—for example, 0.02 dB per 90° bend for a 55 μm radius. Potential uses include biosensing in the visible and near-IR regions. Contact Sebastian Romero-García at[email protected].
