Single-Aperture Dual-Color Ion Addressing with a DUV-Compatible Bilayer Grating

Multi-wavelength optical control is a scaling bottleneck for trapped-ion hardware: separate surface emitters consume trap area, interrupt the electrode plane, and expose charge-sensitive dielectric near the ions. Here, a vertically stacked silicon-nitride bilayer routes the $^{40}\text{Ca}^+$ qubit and repump fields-729.4 and 854.2 nm-through one electrode aperture and focuses them $70~\mu\text{m}$ above the chip. Three-dimensional FDTDX predicts $0.10~\mu\text{m}$ color separation and near-diffraction-limited spots along the ion-chain axis. Multi-level depth-allocation apodization enables this architecture by encoding the coupling envelope in discrete etch levels rather than sub-resolution linewidths. Every feature satisfies a strict $\ge 125\text{ nm}$ deep-UV rule using two etch depths per film. Full-3D Ansys Lumerical simulations independently corroborate directionality, spot size, and repump efficiency. At a common 50 nm reporting grid, the DUV-compatible device matches a 63 nm electron-beam design on the qubit channel (focusing efficiency 0.286 vs 0.288; crosstalk -24.0 vs -24.3 dB). Vertical integration therefore converts wavelength scaling from a lateral-footprint penalty into a layer-allocation problem, providing a pathway toward compact multi-color photonic interfaces for trapped ions and other chip-addressed quantum emitters.