Two-photon fluorescence-assisted laser ablation of non-planar metal surfaces: fabrication of optical apertures on tapered fibers for optical neural interfaces

Abstract

We propose a feedback-assisted direct laser writing method to perform laser ablation of fiber optics devices in which their light-collection signal is used to optimize their properties.A femtosecond-pulsed laser beam is used to ablate a metal coating deposited around a tapered optical fiber, employed to show the suitability of the approach to pattern devices with small radius of curvature. During processing, the same pulses generate two-photon fluorescence in the surrounding environment and the signal is monitored to identify different patterning regimes over time through spectral analysis. The employed fs beam mostly interacts with the metal coating, leaving almost intact the underlying silica and enabling fluorescence to couple with a specific subset of guided modes, as verified by far-field analysis. Although the method is described here for tapered optical fibers used to obtain efficient light collection in the field of optical neural interfaces, it can be easily extended to other waveguides-based devices and represents a general approach to support the implementation closed-loop laser ablation system of fiber optics.

Plasmonics on a Neural Implant: Engineering Light–Matter Interactions on the Nonplanar Surface of Tapered Optical Fibers

“…[63] While this drawback can be circumvented using structures with high enhancement factors such as bow-tie antennas, [64,65] ion implantation might diminish the optical transmission of exposed areas of glass, although this has been observed at higher fluencies. [40,66] While traditional methods such as electron beam lithography are hardly suitable to pattern the taper surface, other strategies such as two-photon polymerization lithography, [67] self-assembly, [68,69] or nanostencil lithography [70] might allow obtaining high lateral resolution and relevant field enhancement. In this direction, we will devote our future work to alternative design that can offer the same degree of tunability of cNGs while also providing for a stronger field enhancement.…”

“…This is a peculiar feature with respect to previous observations of light emission from smooth optical apertures of metal-coated tapered optical fibers. [39,40] It arises from the interaction between both forwardand backpropagating light with the grating, i.e., introduced by light confined by the metal layer and back-reflected at the taper tip, [41] as testified by the signs of the impinging wavevector and the vectorial momentum matching relation (1) (see Section S2 in the Supporting Information). The spectral positions of maxima and minima of transmissions were consistent across experiments and simulations, obtained by rigorous coupled wave analysis (RCWA) for a planar grating (see Section S3 and Figures S2-S6, Supporting Information).…”

“…Also in this case, this behavior was not present when light is collected with smooth optical apertures in metal-coated tapered fibers. [39,40,42] The asymmetry in the spectra acquired at +30° and −30° can be attributed to the nontrivial combination of EOT on the nanogratings with the mode-dependent light collection properties of tapered optical fibers (see Section S5 in the Supporting Information). The results summarized in Figure 2 demonstrate that it is possible to excite SPR on tapered fibers' neural implants and that the spectral behavior can be tuned acting on simple geometrical parameters such as the grating periodicity.…”

“…We built a collection map of the curved nanograting using a two-photon scanning system to generate fluorescent point-like spots near the nanostructured fibers immersed in a homogeneous fluorescent solution (Figure 3a). [39,40,42] Then, we used a second detector at the fiber output, synchronized with the scanning beam, to detect the fluorescence collected by the grating. This generated a highresolution collection diagram covering the −90°:90° angular range around the normal to the curved surface.…”

Neurophotonic Tools for Microscopic Measurements and Manipulation: Status Report

“…35(b)]. [652][653][654] Together with the low invasiveness, this technology has contributed to the dissection of basal ganglia neuronal circuitry in the mouse brain: motor-action-related topographical organization of the striatum was probed by depth-selective optogenetic stimulation, 655 while differentiated dopamine transients in dorsal and ventral striatum in reward processing were observed by applying depth-resolved fiber photometry. 651 From the perspective of advanced detection of photons generated in deep brain regions, tapered fibers are compatible with time-correlated single photon counting technologies recently employed to probe the biochemical state of neurons, 656 and can help in bringing this technology to deeper (subcortical) brain structures.…”