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

Abstract

The fields of photonics and neuroscience are enjoying a virtuous circle, where breakthroughs in one field spur novel research endeavors in the other. In particular, the rise of optogenetic techniques in neurobiology ignited the growth of a vibrant neurophotonic community [1] that, in turn, developed a technological platform centered on radiative optoelectronic interfaces to stimulate and monitor neural activity. These interfaces include photonic probes integrated with waveguides, micro-LEDs (light-emitting diodes), and recording electrodes alongside implantable, multifunctional optical fibers. [2,[3][4][5][6][7][8][9][10][11][12][13][14][15] After a decade of continuous innovation, [16] there is growing agreement that next-generation probes should provide access to neural dynamics in the electrical, optical, biochemical, and mechanical domains. [17] Recently, the field started to explore optically confined systems at Optical methods are driving a revolution in neuroscience. Ignited by optogenetic techniques, a set of strategies has emerged to control and monitor neural activity in deep brain regions using implantable photonic probes. A yet unexplored technological leap is exploiting nanoscale light-matter interactions for enhanced bio-sensing, beam-manipulation and opto-thermal heat delivery in the brain. To bridge this gap, we got inspired by the brain cells' scale to propose a nano-patterned tapered-fiber neural implant featuring highly-curved plasmonic structures (30 μm radius of curvature, sub-50 nm gaps). We describe the nanofabrication process of the probes and characterize their optical properties. We suggest a theoretical framework using the interaction between the guided modes and plasmonic structures to engineer the electric field enhancement at arbitrary depths along the implant, in the visible/near-infrared range. We show that our probes can control the spectral and angular patterns of optical transmission, enhancing the angular emission and collection range beyond the reach of existing optical neural interfaces. Finally, we evaluate the application as fluorescence and Raman probes, with wave-vector selectivity, for multimodal neural applications. We believe our work represents a first step towards a new class of versatile nano-optical neural implants for brain research in health and disease.

Neurophotonic Tools for Microscopic Measurements and Manipulation: Status Report

“…The photonic properties of tapered fibers also offer the opportunity to control nanoscale lightmatter interactions on the fiber surface 659 providing a suitable platform to translate in vivo the many options for enhanced biomolecular interfacing using plasmonic resonances that have been proposed in vitro. 660 These include Surface Enhanced Raman Scattering (SERS), refractive index sensing, metal-enhanced fluorescence, and highly localized, ultrafast heat delivery in combination with the modulation of the radiative field through sub-wavelength apertures for light delivery and collection in highly scattering tissue.…”

Deep brain cancer metastasis detection with wide-volume Raman spectroscopy through a single tapered fiber

“… a , Scanning electron Microscopy (SEM) image of a tapered fiber covered by a thin gold layer (150 nm deposited by thermal evaporation with an optical window fabricated by locally removing the gold layer with low-current Focused Ion Beam milling ( 31 ); the scale bar is 100 μm. b , Raman spectra acquired in the center of the metastasis (red) and in the contralateral emisphere (blue) through the microstructured window the FP range; the thicker line show the average of 5 measurements c , as in b for the HW range.…”

Toward Plasmonic Neural Probes: SERS Detection of Neurotransmitters through Gold‐Nanoislands‐Decorated Tapered Optical Fibers with Sub‐10 nm Gaps

“…Possible fabrication strategies to achieve this aim include top‐down and bottom‐up approaches, and our group recently demonstrated the integration of cellular‐sized curved plasmonic structures on a TF to engineer the spectral and angular patterns of optical emission and collection, [ 39 ] using focused ion beam milling at low fluences. [ 40 ] However, scaling this top‐down approach will intrinsically result in unmanageable fabrication times when large surface area sensitivity is required.…”