Low-NA two-photon lithography patterning of metal/dielectric tapered optical fibers for depth-selective, volumetric optical neural interfaces.

Optical neural implants allow neuroscientists to access deep brain regions, enabling to decipher complex patterns of neural activity. In this field, the use of optical fibers is rapidly increasing, and the ability to generate high-quality metal patterns on their non-planar surface would further extend their application. Here, we propose to use alternating metal shielding and dielectric confinement to engineer the mode-division properties of tapered optical fiber neural implants. This is accomplished through an unconventional application of two-photon lithography (TPL), which employs a low-numerical aperture objective to pattern extensive waveguide sections at both low and high curvature radii. The low-NA TPL is used to polymerize a mask of photoresist, while the rest of the taper undergoes wet metal etching. This implies no direct destructive interaction between the laser beam and the metal to be removed, preserving the optical properties of the dielectric waveguide and of the metal coating. The advantages provided by the presented fabrication method, combined with the intrinsic modal properties of the dielectric waveguide, enable the engineering of the light guiding mechanisms, achieving depth-selective light delivery with a high extinction ratio. The device's light emission and collection properties were investigated in quasi-transparent media and highly scattering brain slices, finding that our proposed method facilitates 360° symmetric light collection around the dielectric-confined section with depth resolution. This opens a perspective for the realization of optical neural implants that can interface the implant axis all-around, with low-NA TPL that can also be applied on other types of non-planar surfaces.