Implantable Neural Probe with Thermo-Optic Switches Based on Multimode Interference (MMI) in Thermogenetic Application

This paper presents the design of an implantable neural probe with 8 optical switches that can route the beams in the different brain regions which are sensitive to near-infrared (1550 nm) for thermogenetic applications. The presented probe, which is fabricated in a standard SOI technology, can enable robust activation of neurons using thermosensitive transient receptor potential (TRP) cation channels with a higher spatial and temporal resolution, thanks to the utilization of optical switches and grating couplers with precise spot stimulation. Each optical switch is linked to a waveguide and a grating coupler, enabling simultaneous stimulation of different sites within the brain. The activation or deactivation of optical switches allows controlling passage of light through the desired waveguide to effectively providing optical stimulation to specific neurons or neural circuits. The optical switch works based on the thermo-optics effect that consists of a silicon Multimode Interference (MMI) as an optical splitter and a Titanium nitride (TiN) metal heater that is used for changing the refractive index for π-phase shifting. The measured π-phase shifting power was 19.8 mW with an extinction ratio of 23 dB at 1550 nm. Also, we used a TiN metal as a temperature sensor for controlling the heater. The difference in the temperature sensor impedance was 0.7 Ω when the power was applied to the heater.