Integration of photonic chips with millimeter-scale atomic,micromechanical,chemical,and biological systems can advance science and enable new miniaturized hybrid devices and technology.Optical interaction via small evanescent volumes restricts performance in applications such as gas spectroscopy,and a general ability to photonically access optical fields in large free-space volumes is desired.However,conventional inverse tapers and grating couplers do not directly scale to create wide,high-quality collimated beams for low-loss diffraction-free propagation over many millimeters in free space,necessitating additional bulky collimating optics and expensive alignment.Here,we develop an extreme mode converter,which is a compact planar photonic structure that efficiently couples a 300 nm × 250 nm silicon nitride high-index single-mode waveguide to a well-collimated near surface-normal Gaussian beam with an ≈160 μm waist,which corresponds to an increase in the modal area by a factor of ＞105.The beam quality is thoroughly characterized,and propagation over 4 mm in free space and coupling back into a single-mode photonic waveguide with low loss via a separate identical mode converter is demonstrated.To achieve low phase error over a beam area that is ＞100× larger than that of a typical grating coupler,our approach separates the two-dimensional mode expansion into two sequential separately optimized stages,which create a fully expanded and well-collimated Gaussian slab mode before out-coupling it into free space.Developed at 780 nm for integration with chip-scale atomic vapor cell cavities,our design can be adapted for visible,telecommunication,or other wavelengths.The technique can be expanded to more arbitrary phase and intensity control of both large-diameter,free-space optical beams and wide photonic slab modes.