Inverse-Designed Nanophotonic Structures for On-Chip Excitation and Broadband Emission Extraction in Diamond Quantum Spin Sensing.

Diamond nitrogen-vacancy based solid-state quantum spin sensors on a photonic integrated circuit offer exceptional sensitivity, high spatial resolution, and the potential for substantial device miniaturization. Efficient extraction of broadened phonon sideband emission is particularly important for spin sensing, where measurements rely on fluorescence contrast between spin states rather than indistinguishable photons. However, simultaneous high-efficiency delivery of excitation light to the nitrogen-vacancy site and collection of its broad phonon sideband emission remains challenging. This work introduces practical nanophotonic structures using a foundry-compatible material stack and a fabrication-constrained inverse design method. An improved regularization scheme enforces minimum feature sizes in both core and cladding. The optimized sensing unit integrates two compact components on a silicon nitride layer: an emitter and a broadband directional coupler. The optimized emitter enhances green (532 nm) excitation while efficiently extracting and coupling broadband red (620-760 nm) emission into a single-mode waveguide, yielding up to a 3-fold increase in guided-mode power relative to a straight ridge waveguide baseline with the same nanodiamond opening. The optimized directional coupler routes