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Table 1. Comparison between simulated and measured losses for different PIC components for TE- and TM-polarized modes at 760 nm.

differences may be attributed to fabrication tolerances, such as slight variations in the etch depth of the grating and the duty cycle of the non-uniform grating design. They may also be attributed to deviations from the targeted layer stack, which arose from the omission of the upper metallic and dielectric layers in the devices that were characterized experimentally. The more pronounced deviation observed for the TM mode is consistent with the increased sensitivity of grating couplers emitting at larger angles, where small angular variations within the SiO 2 cladding translate into larger deviations in free space. Additional contributions may arise from the finite numerical aperture of the collection optics, minor phase-front distortions, and aberrations in the imaging system. Overall, the beam morphology, emission direction, and polarization-dependent divergence remain in good agreement, confirming that the fabricated grating accurately reproduces the simulated optical performance.

The GC efficiencies also align well with simulated predictions( see Tab. 1). Small deviations may originate from small fabrication-induced variations. The lower coupling efficiency observed for the TM grating coupler is attributed to its weaker grating coupling strength. This behavior of TM GCs may be beneficial in the design of highly focusing GCs or GCs that emit large beams, where precise control over the outcoupling strength is essential. The observed 8 – 10 ° angular separation between the TE and TM beams enables two distinct operation regimes:( i) individual addressing of ions or emitters at spatially separated locations when the beam waist is small, or( ii) polarization-state discrimination by monitoring the relative output intensity at the TE- and TM-specific emissionangles. Theangular separation corresponds to a spacing of roughly 14 – 18 lm at an ion height of 100 lm. This separation can be tuned through the waveguide geometry and grating duty cycle, which set the offset in effective index between the modes and thus control the resulting beam spacing. Accordingly, this effect is more pronounced in high-index platforms. This effect is fundamentally limited by the requirement that both modes satisfy the Bragg condition; consequently, at large backward emission angles, the other polarization may not yet be present.

5 Conclusion

We designed key Si 3 N 4 PIC components and experimentally characterized their polarization-dependent performance for ion trap applications. The fabricated GCs support outcoupling for both polarizations and exhibit distinct emission angles.

This dual-polarization capability expands the accessible optical design space and may be used for multi-ion addressing through angular multiplexing, enabling more compact and versatile on-chip architectures while reducing the number of required GCs in integrated ion trap platforms. At the same time, the ability to excite both modes highlights the importance of robust polarization management, as inadvertent TM excitation can generate parasitic beams and increase stray light in the trapping region. Robust solutions such as TE – TM splitters or polarization converters can help to ensure a well-defined input state [ 53, 54 ]. The weaker coupling strength of the TM mode is promising for the design of highly focusing GCs for single-ion addressing in trapped-ion quantum processors or GCs that emit large beams for multi-ion addressing in multi-ion clocks.

Furthermore, since TM modes exhibit stronger sensitivity to substrate leakage, transitioning to fused-silica substrates [ 52 ] or employing thicker SiO 2 BOX cladding layers may be advantageous in future implementations. Taken together, TE – TM operation introduces a useful form of polarization diversity that complements spatial-multiplexing strategies explored in multimode photonics [ 55 ] and provides a promising pathway toward densely integrated, flexible, and scalable photonic interfaces for nextgeneration multi-zone trapped-ion quantum technologies.

Acknowledgments

WethankMarkusKromreyfortakingimagesofthechipsand Fatemeh Salahshoori for preparation of the chips. Funding

The authors gratefully acknowledge the support by the Quantum Valley Lower Saxony, the BMFTR-project ATIQ( FKZ: 13N16116, 13N16126, 13N16130), Braunschweig International Graduate School of Metrology B-IGSM, the cluster of Excellence Quantum Frontiers( EXC-2123-390837967), and the Cluster of Excellence PhoenixD( EXC-2122).

Conflicts of interest

The authors declare that they have no competing interests to report.

Data availability statement

Data supporting the findings of this study are available from the corresponding author upon reasonable request.