Implantable silicon neural probes with nanophotonic phased arrays for single-lobe beam steering

Introduction

Optogenetics with patterned photostimulation combines spatially targeted optical actuation of neurons with cell-type specificity to enable the investigation of synaptic connectivity in neuronal circuits at cellular resolutions. Discrete optical beam scanners, such as galvanometers and acousto-optic deflectors^{1, 2–3}, spatial light modulators^4,5, and digital micromirror devices⁶ are commonly used to impart patterns on an optical beam for spatially targeted photostimulation. However, these components form illumination patterns in free space, and the penetration depth of visible to near-infrared light (400–1100 nm) is limited to only about 1 mm from the brain surface in rodents⁷. To bring light into deep brain regions, implantable optical devices are being investigated, including optical fibers^{8, 9–10}, miniature gradient index (GRIN) lenses^{11, 12, 13–14}, and silicon (Si) neural probes with micro light-emitting diodes (μLEDs)^{15, 16, 17–18}, organic LEDs¹⁹, or integrated nanophotonic waveguides^{20, 21, 22, 23, 24–25}.

Among these options, nanophotonic waveguide-based Si neural probes are promising for delivering light with spatial precision. A rich variety of light emission patterns can be achieved using waveguide-based grating emitters on the implant without the need for any lenses or discrete optics. In contrast, light-emitting implants, such as single-core optical fibers without wavefront compensation and LED probes, emit diffracting Gaussian and Lambertian beam profiles, respectively. Furthermore, nanophotonic waveguide probes have small form factors with widths and thicknesses ≤100 μm and sharp tips, which ease surgical implantation and displace less tissue compared to fiber bundles and GRIN lenses, which have typical diameters exceeding 300 μm. Through the design of grating emitters, we have demonstrated a wide range of beam patterns for optogenetic applications in the visible spectrum, including low-divergence beams^24,26, light sheets^26,27, focused²⁸, and steerable beams²⁹.

Steerable beams provide dynamically patterned illumination by serially scanning a beam across a continuous region in tissue samples. This approach is valuable for localized spatial mapping of neuronal connections within a circuit^1,2. Optical phased arrays (OPAs) are common photonic circuits to realize steerable optical beams without mechanically moving parts^{29, 30, 31, 32–33}. Notably, ref. ²⁹ has demonstrated passive OPAs where the beam emission angle can be controlled by tuning the input wavelength,...