Surface-Normal Free-Space Beam Projection via Slow-Light Standing Wave Resonances in Extra-Large Near-Zero Index Grating Couplers

On-chip grating couplers directly connect photonic circuits to free-space light. The commonly used photonic gratings have been specialized for small areas, specific intensity profiles and non-vertical beam projection. This falls short of the precise and flexible wavefront control over large beam areas needed to empower emerging integrated miniaturized optical systems that leverage volumetric light matter interactions, including trapping, cooling, and interrogation of atoms, bio- and chemi- sensing and complex free-space interconnect. The large coupler size challenges general inverse design techniques, and solutions obtained by them are often difficult to physically understand and generalize. Here by posing the problem to a carefully constrained computational inverse design algorithm capable of large area structures, we discover a qualitatively new class of grating couplers. The numerically found solutions can be understood as coupling an incident photonic slab mode to a spatially extended slow-light (near-zero refractive index) region, backed by a Bragg reflector. The structure forms a spectrally broad standing wave resonance at the target wavelength, radiating vertically into free space. A reflection-less adiabatic transition critically couples the incident photonic mode to the resonance, and the numerically optimized lower cladding provides 70 $\%$ overall theoretical conversion efficiency. We have experimentally validated efficient surface normal collimated emission of $\approx$ 90 $\mu$m full width at half maximum Gaussian at the thermally tunable operating wavelength of $\approx$ 780 nm. The variable-mesh-deformation inverse design approach scales to extra-large photonic devices, while directly implementing the fabrication constraints. The deliberate choice of smooth parametrization resulted in a novel type of solution, which is both efficient and physically comprehensible.