Successful PhD defense of Dr. Fu-Der Chen
Fu-Der Chen worked on foundry-fabricated nanophotonic neural probes for simultaneous patterned photostimulation and electrophysiological recording. He is the first PhD student to successfully graduate from the Max Planck – University of Toronto Center for Neural Science and Technology, receiving his PhD degree from the University of Toronto. Congratulations Dr. Fu-Der Chen!
Title: “Foundry-fabricated Nanophotonic Neural Probes for Simultaneous Patterned Photostimulation and Electrophysiological Recording"
Fu-Der Chen enrolled as one of the first students in the Max Planck – University of Toronto Centre in 2021. His thesis about the development and testing of a new microchip-based neurotechnology – foundry-fabricated nanophotonic neural probes with integrated waveguides and microelectrodes – was supervised by Prof. Joyce Poon and Prof. Taufik Valiante. His work spanned the design, characterization, packaging, and in vivo testing of this new tool for neuroscientists. His next steps take him to Taiwan, working at Taiwan Semiconductor Manufacturing Company Limited (TSMC). Congratulations Dr. Fu-Der Chen!
Abstract
Optogenetics enables cell-type-specific control of neural activities using light. However, optical scattering limits the light penetration depth in tissue. Optical implants, specifically nanophotonic neural probes, can deliver light into deep brain regions through on-chip sub-micron waveguides. These probes are mass-manufacturable in commercial foundries and support the integration of optical circuits and other functionalities. Also, by engineering grating emitters, their emission profile can be tailored to illuminate different tissue volumes. Despite the flexibility, most probes only implement uniform grating couplers that emit low-divergence beams for localized stimulation. This thesis aims to develop foundry-fabricated nanophotonic neural probes integrated with recording electrodes and engineered emitters for bidirectional neural interrogation. First, we introduce two types of nanophotonic neural probes integrated with electrodes : (1) a single-shank probe with low-divergence beams for localized stimulation and (2) a four-shank probe with light sheet emission for broad-area stimulation. We developed post-fabrication processes and peripheral systems to enable their multi-emitter photostimulation and electrical recording capabilities. In vivo experiments showed both probe types can selectively stimulate neurons at various depths. Moreover, the light sheet probe induced stronger firing rate fatigue at lower output intensity than the low-divergence probe. Finally, we leveraged the distributed beam emission of the light sheet probe to photostimulate network-wide seizures while maintaining a safe temperature increase <1°C in tissue. Next, we report two passive optical phased array (OPA) designs that control beam emission angle through tuning input wavelength. The first OPA design, which consists of a one-dimensional grating coupler array, achieved a steering range of ± 16° with a beam width < 23 μm in tissue. The second OPA improved the design with an end-fire phased array and a free-propagation region slab to suppress the sidelobes, achieving single-lobe beam steering > 60% of the full steering range with > 4 dB background suppression. Both OPA designs delivered sufficient power for optogenetic stimulation, but improvements in beam width and steering range are required to achieve selective stimulation with beam steering. Nonetheless, these demonstrations lay the foundation for realizing OPAs capable of continuous beam scanning in tissues. Collectively, this thesis shows the applications of nanophotonic neural probes ranging from localized stimulation to population-wide interrogation.
