Ultra-high numerical aperture waveguide-integrated meta beam shaper

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The integration of metasurfaces with guided mode sources like waveguides has opened new frontiers for on-chip optical integration. However, the state-of-the-art in the field has targeted applications where long focal distances over thousands of light wavelengths are needed. This regime where the paraxial approximation holds enables inverse design of metasurfaces with weakly confining elements that are typically thicker than the wavelength in the material. For short focal length applications at distances less than 100λ, where the paraxial approximation fails and high numerical apertures (NAs) are necessary, a different approach is required. Here, we designed and experimentally demonstrated single-mode waveguide-integrated meta beam shapers capable of redirecting the confined light into the free space and focusing it at focal distances less than 100λ above the chip surface into a tightly focused spot. Focal spot characteristics measured at 460 nm operating wavelength approach diffraction-limited focusing across a range of focal lengths, device footprints, and numerical apertures, demonstrating the robustness of the approach. Focal volumes smaller than 1µm3 are demonstrated for a range of focal distances below 50 µm (100λ). For a device with NA of 0.95 that is one of the highest amongst integrated metasurfaces, the measured focal volume is as small as just 0.06µm3 at a focal distance of 13µm (28λ). These on-chip integrated ultra-high NA meta beam shapers have the potential to unlock new applications in quantum optical computing with trapped ions, localized optogenetic neurostimulation, and high resolution in situ microscopy.