High-efficiency grating fiber-chip couplers in wide bandgap III-nitride material platforms

Wideband III-nitrides, aluminum nitride (AlN), and gallium nitride (GaN) possess excellent material properties that could cater to a broad range of chip-scale photonic applications in telecommunications, microwave photonics, quantum information processing, nonlinear optics, and optical neural networks, with the freedom to extend the platform to lower wavelengths in the visible and UV owing to their bandgap. While both materials inherently hold huge potential for photonic integrated circuit (PIC) applications, the development of epitaxial thin films (< 1 μ m thickness) with low defects and superior crystal quality is of utmost importance. To this end, we develop 500 nm thick AlN and GaN films on sapphire substrate, with an XRD (002) ω -scan FWHM of 120 arcsec for AlN and 618 arcsec for GaN, and (102) ω -scan FWHM of 980 arcsec and 1020 arcsec, with a surface roughness of 0.2 nm and 0.4 nm for AlN and GaN, respectively. The crystal quality achieved for the thin films of AlN and GaN is comparable with some of the best reported in literature at these thicknesses. Using this material, we demonstrate overlay grating couplers on both AlN and GaN material platforms, achieving a peak coupling efficiency of −3.7 dB/coupler at 1545 nm in AlN and −4.6 dB/coupler at 1575 nm in GaN. The bandwidths are 48 nm for AlN and 29 nm for GaN. The coupling efficiencies are the best reported so far for both material platforms.