Surface-Normal Free-Space Beam Projection via Slow-Light Standing-Wave Resonance Photonic Gratings.

Area of Science:

• Photonics and Integrated Optics
• Nanophotonics
• Computational Electromagnetics

Background:

• Traditional grating couplers are limited to small areas and specific beam profiles.
• Emerging optical systems require precise wavefront control over large beam areas for applications like atom trapping, biosensing, and free-space interconnects.
• Existing inverse design methods struggle with large-area structures and yield solutions that are difficult to interpret.

Purpose of the Study:

• To discover a new class of grating couplers capable of large-area, efficient, and precise free-space light coupling.
• To overcome the limitations of current grating couplers and inverse design techniques for large-scale photonic devices.

Main Methods:

• Utilized a constrained computational inverse-design algorithm specifically adapted for large-area structures.
• Employed a variable-mesh-deformation approach to handle fabrication constraints and scale to large photonic devices.
• Focused on smooth parametrization to achieve physically comprehensible and novel solutions.

Main Results:

• Discovered a new class of grating couplers that couple photonic slab modes to slow-light regions.
• Achieved spectrally broad standing wave resonances for efficient vertical emission into free space.
• Demonstrated a theoretical conversion efficiency of 70% with experimental validation of efficient, surface-normal collimated Gaussian emission (≈90 μm FWHM) at ≈780 nm.

Conclusions:

• The developed inverse design approach successfully generates efficient and physically understandable large-area grating couplers.
• These novel grating couplers offer precise wavefront control for advanced integrated optical systems.
• The methodology is scalable to extra-large photonic devices, paving the way for new applications in miniaturized optical systems.