Three-Dimensional Magneto-Optical Trap Beam Delivery with Scalable Wafer-Level Optics

Magneto-optical traps (MOTs) are essential for preparing and capturing cold atoms in precision atomic applications. Conventional free-space beam delivery systems are bulky, expensive, and difficult to align. Current photonic integrated circuit (PIC) MOT approaches, although scalable, still require manual alignment of external quarter-wave plates (QWPs) and mirrors, limiting miniaturization and reduction of complexity. Here, we present a two-wafer beam delivery architecture for a PIC-based MOT using an inverse-designed multifunctional grating coupler and integrated metasurface retroreflectors, eliminating the need for all external optics. Both components are designed via inverse optimization to deliver circularly polarized light for ⁸⁷Rb at 780 nm. We experimentally demonstrate a retroreflector with 74% efficiency and 0.85 ellipticity, meeting performance requirements for MOT operation. The grating coupler is characterized through full-wave electromagnetic simulations, showing 20% out-coupling efficiency and 0.85 polarization ellipticity. This two-wafer architecture proposed here significantly simplifies alignment and maintains a compact, scalable, foundry-compatible footprint for all beam-delivery components.