  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.
