Multiplexing on a Nanophotonics Platform for Quantum Optics and Neural Interfaces

Abstract

The integrated silicon photonics, or nanophotonics, platform was originally developed for optical interconnects for high bandwidth data transfer. The need to dramatically scale this platform for communications applications has led to active research in switching and multiplexing signals using different degrees-of-freedom of light such as path, wavelength, and the transverse spatial mode. In recent years, extending the platform to use new materials has allowed for a new range of applications such as quantum optics, biosensing, and spectroscopy that span the visible to mid-infrared wavelength range. Fully utilizing this platform allows an unparalleled level of control over a large set of optical channels in waveguides with reconfiguration capability, low power consumption, and more stability which can be applied to this broad range of applications. This allows large table-top optical setups for quantum optics and microscopy to be miniaturized to the compact footprint of a nanophotonics chip. Here in this dissertation, we extend this multiplexing nanophotonic platform into the visible wavelength regime in order to address two areas that require high density optical channels: quantum optics and neural interfaces for optogenetics. We develop a platform based on low-loss silicon nitride waveguides that can be thermally tuned using integrated microheaters to introduce reconfigurability in the visible wavelength range. We show how nanophotonic optical channels can be used for both quantum interference as well as neuron activation within the brain. To scale up the integrated quantum optics platform, we developed nanophotonics building blocks that utilize the transverse spatial modes within a single multimode waveguide for key quantum interference experiments that form the basis of more complex transformations within a smaller footprint. We used the transverse spatial degree-of-freedom because it can be controlled by simple geometric design, relieves the burden on the light source, and does not require non-standard materials, long delay lines, and high speed electronics, which is the case for time and frequency encoding. The high confinement of the silicon nitride allows for well separated modes that can be accessed by using geometry controlled selective phase matching. We demonstrated quantum interference using the higher order modes within a single multimode waveguide. We demonstrated both passive and active tuning of the interference between different modes while maintaining high visibility interference in the quantum regime. To address the reconfiguration and resolution limitations of current optogenetic neural studies, we developed an implantable neural probe with an embedded reconfigurable nanophotonic switching network for creating high resolution spatiotemporal optical patterns. We use the same silicon nitride platform redesigned for the blue wavelength range. Using full phase control within a path interferometer network, we demonstrated a fully reconfigurable 1x8 switch that can control an array of microbeams to excite single neurons on a 20 microsecond timescale, much faster than neuron response times. We developed packaging techniques for the implantable nanophotonic chip for in vivo studies. We show for the first time precisely defined and repeatable neural spike patterns in vivo with unprecedented spatiotemporal resolution. Finally, we discuss how this platform can be extended for large-scale neural studies.