Out-of-Plane Focusing Grating on Implantable Neural Probes for Spatially Targeted Optogenetic Stimulation

Optogenetics is a valuable tool in dissecting the functions of neural circuits by enabling the optical stimulation of genetically targeted neurons. One key challenge is delivering light at depth beyond the attenuation length in tissue. Implantable silicon nitride (SiN) waveguide-based nanophotonic probes offer complex optoelectronic integration and patterned illumination through grating couplers in a compact form factor, making them a unique tool for interrogating neural circuits [1]. We have previously demonstrated probes with gratings that emit steerable low-divergence beams and planar sheets [2–4]. In those works, the optical intensity decays exponentially from the probe, and neurons closest to the shank are excited. In this work, we present the design and characterization of neural probes with grating couplers that focus the light to a point above the plane of the probe for spatially precise targeting of neurons at potentially lower powers. Fig. 1 (a) and (b) show the neural probes, which were fabricated at Advanced Micro Foundry and had a 120nm-thick PECVD SiN waveguide layer.