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On-chip grating couplers directly connect photonic circuits to free-spacelight. The commonly used photonic gratings have been specialized for smallareas, specific intensity profiles and non-vertical beam projection. This fallsshort of the precise and flexible wavefront control over large beam areasneeded to empower emerging integrated miniaturized optical systems thatleverage volumetric light matter interactions, including trapping, cooling, andinterrogation of atoms, bio- and chemi- sensing and complex free-spaceinterconnect. The large coupler size challenges general inverse designtechniques, and solutions obtained by them are often difficult to physicallyunderstand and generalize. Here by posing the problem to a carefullyconstrained computational inverse design algorithm capable of large areastructures, we discover a qualitatively new class of grating couplers. Thenumerically found solutions can be understood as coupling an incident photonicslab mode to a spatially extended slow-light (near-zero refractive index)region, backed by a Bragg reflector. The structure forms a spectrally broadstanding wave resonance at the target wavelength, radiating vertically intofree space. A reflection-less adiabatic transition critically couples theincident photonic mode to the resonance, and the numerically optimized lowercladding provides 70 $\%$ overall theoretical conversion efficiency. We haveexperimentally validated efficient surface normal collimated emission of$\approx$ 90 $\mu$m full width at half maximum Gaussian at the thermallytunable operating wavelength of $\approx$ 780 nm. The variable-mesh-deformationinverse design approach scales to extra-large photonic devices, while directlyimplementing the fabrication constraints. The deliberate choice of smoothparametrization resulted in a novel type of solution, which is both efficientand physically comprehensible.

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