Implantable Photonic Neural Probes with Out-of-Plane Focusing Grating Emitters

TL;DR

This is the first report of focused spots with the size scale of a neuron soma in brain tissue formed from implantable neural probes with nanophotonic grating emitters, using the holographic principle.

Abstract

We have designed, fabricated, and characterized implantable silicon neural probes with nanophotonic grating emitters that focus the emitted light at a specified distance above the surface of the probe for spatially precise optogenetic targeting of neurons. Using the holographic principle, we designed gratings for wavelengths of 488 and 594 nm, targeting the excitation spectra of the optogenetic actuators Channelrhodopsin-2 and Chrimson, respectively. The measured optical emission pattern of these emitters in non-scattering medium and tissue matched well with simulations. To our knowledge, this is the first report of focused spots with the size scale of a neuron soma in brain tissue formed from implantable neural probes.

Figures (6)

• Figure 1: Grating emitter design. (a) Schematic overview of an out-of-plane focusing grating emitter with focal point located at coordinates ($x_0,y_0,z_0$). (b) Phase map generated for $\lambda$ = 488 nm. The contour lines dictated by the phase matching condition are overlaid in green. (c) The finalized layout design of the grating emitter for $\lambda$ = 488 nm. Emitted beam profile on the $y=0$ plane simulated in 3D FDTD for (d): blue emitter ($\lambda$=488 nm), and (e): red ($\lambda$=594 nm) emitter.
• Figure 2: The Si neural probe with out-of-plane focusing grating emitters. (a) (top): Annotated micrograph of a neural probe. (bottom): Optical micrograph of an out-of-plane focusing grating emitter (brightness and contrast adjusted). (b) A photograph of a neural probe attached to the input multicore fiber.
• Figure 3: Side-view profile measurements by immersing the probe in a bath of fluorescent dye solution. (a) Diagram of measurement setup. Captured side profile for (b) $\lambda = 488$ nm, and (c) $\lambda = 594$ nm.
• Figure 4: Fluorescent coverslip measurement to obtain the 3D volumetric emission pattern of the grating emitter. (a) Diagram of the measurement setup. (b) Captured cross-section profiles ($x-y$ plane) at various heights above the grating for $\lambda = 488$ nm. (c) Beam intensity on the $y=0$ plane obtained by stitching together the captured cross-sections for $\lambda = 488$ nm. (d) FWHM measurement of the interpolated beam waist for $\lambda = 488$ nm. (e, f, g): The corresponding images to (b, c, d) for $\lambda = 594$ nm.
• Figure 5: Verification of light focusing in fixed tissue. (a) Diagram of the experimental setup. (b) Photograph of implanted neural probe. Captured fluorescent side profiles with background subtracted ($x-z$ plane) of the emitted beam and linecut at the beam waist from (c) red emitter implanted in fixed tissue with jRGECO1a expression, and (d) blue emitter implanted in fixed tissue with GCaMP6s expression.