Low Loss CMOS-Compatible PECVD Silicon Nitride Waveguides and Grating Couplers for Blue Light Optogenetic Applications

This paper presents silicon nitride (SixNy) photonic integrated circuits (PICs) with high performance at a wavelength of 450 nm, which, therefore, is suitable for neuronal stimulation with optogenetics. These PICs consist of straight and bent waveguides, and grating couplers that are fabricated in a complementary metal-oxide-semiconductor (CMOS)-compatible plasma enhanced chemical vapor deposition SixNy platform. Their characterization shows propagation losses of 0.96 $\pm$ 0.4 dB/cm on average for straight waveguides that are 1-5 $\mu$m wide and bend insertion losses as low as 0.2 dB/90 $^\circ$ for 1 $\mu$m wide waveguides with a radius of 100 $\mu$m. Additionally, the grating coupler characterization shows that they can deliver about 10 $\mu$W of light in an area of 5 $\times$ 9 $\mu$m
²
(240 mW/mm
²
), which is captured from an uncollimated laser diode (70 mW). Besides delivering sufficient power for optogenetic applications, the gratings have dimensions that are comparable to the size of a neuron, which would allow single cell interaction. These results demonstrate that, with this SixNy platform, high-density and large-scale implantable neural devices can be fabricated and readily integrated into existing CMOS-compatible neuro-electronic platforms.