High spatiotemporal resolution deep-brain optical excitation for optogenetics would enable activation of specific neural populations and in-depth study of neural circuits. Conventionally, a single fiber is used to flood light into a large area of the brain with limited resolution. The scalability of silicon photonics could enable neural excitation over large areas with single-cell resolution similar to electrical probes. However, active control of these optical circuits has yet to be demonstrated for optogenetics.
Here we demonstrate the first active integrated optical switch for neural excitation at 473 nm, enabling control of multiple beams for deep-brain neural stimulation. Using a silicon nitride waveguide platform, we develop a cascaded Mach-Zehnder interferometer (MZI) network located outside the brain to direct light to 8 different grating emitters located at the tip of the neural probe. We use integrated platinum microheaters to induce a local thermo-optic phase shift in the MZI to control the switch output. We measure an ON/OFF extinction ratio of >8dB for a single switch and a switching speed of 20 microseconds. We characterize the optical output of the switch by imaging its excitation of fluorescent dye.
Finally, we demonstrate in vivo single-neuron optical activation from different grating emitters using a fully packaged device inserted into a mouse brain. Directly activated neurons showed robust spike firing activities with low first-spike latency and small jitter. Active switching on a nanophotonic platform is necessary for eventually controlling highly-multiplexed reconfigurable optical circuits, enabling high-resolution optical stimulation in deep-brain regions.
An integrated optical interleaver has been demonstrated in SOI platform using large cross section single mode rib
waveguide structures. The 2×2 device structure is in fact an unbalanced Mach Zehnder Interferometer formed
by cascading two identical directional couplers. The device was designed to separate alternate ITU channels
operating at λ ~ 1550nm. The fabricated devices have been characterized in terms of insertion loss, polarization
and wavelength dependencies, channel extinction etc. The first prototype device operating at ~ 100 GHz ITU
channel spacing has been observed to be slightly polarization dependent and a channel extinction of ~ 8 dB was
recorded for TE polarization.