Master thesis defense by Alba Molina Sardina

Design of photonic gratings for cold-atom experiments

In the context of cold-atom experiments, integrated photonics offer a promising route to replace bulk optical components, which currently limit miniaturization, scalability and reproducibility of systems such as the three-dimensional magneto-optical trap (3D-MOT), used for laser cooling atoms. This thesis focuses on the design of optimized photonic grating outcouplers for on-chip 3D-MOTs targeting strontium and ytterbium atoms.

A systematic design methodology is established to identify grating geometries that enable efficient atom cooling while reproducing the performance typically achieved with bulk optical systems. The methodology involves uniform grating parameter sweeps, spatial signal mode analysis, and geometry optimization based on the adiabatic approximation to identify grating configurations that improve optical power delivery, enable Gaussian-like beam shaping, and reduce far-field divergence. The design process is applied to the relevant cooling transitions at 461 nm for strontium and 556 nm for ytterbium.

The optimized structures show that longer grating lengths reduce far-field divergence while maintaining stable emission angles close to 46 ° for the Sr-design and 48 ° for Yb. Additionally, an engineered linear variation in the grating decay constant enables Gaussian-like intensity shaping. For the ytterbium design, the emission efficiency is found to be strongly dependent on the SiO2 buried oxide layer beneath the gratings, indicating interference effects within the multilayer stack. Overall, the results demonstrate the feasibility of replicating the optical functionality of bulk components using integrated photonic devices in a systematic design framework.