High-Index CuI: A Promising Material Platform for Nanoscale Integrated Photonics Waveguides

Abstract The rapid development of nanoscale integrated photonics demands core materials combining high transparency, high refractive index, and low optical loss. In this work, semi-transparent polycrystalline copper iodide (CuI) bulks were fabricated via the vertical Bridgman technique. Structural characterization reveals dense, large-grained microstructures. To reliably evaluate the material’s intrinsic optical parameters, spectroscopic ellipsometry was performed on a nanometer-thick companion CuI thin film, yielding a high refractive index (∼2.2) that ensures strong optical confinement when paired with common low-index claddings and strong subwavelength optical confinement. Critically, to assess the intrinsic optical transport properties before integrating into nanoscale optical waveguides, and exclude the influence of micro−nano fabrication complexities, a polycrystalline bulk waveguide configuration was utilized. Remarkably, despite the presence of grain boundaries, the material exhibits an attenuation as low as 20 dB/cm in the 900−1100 nm spectral range. This performance, achieved in a non-optimized polycrystalline state, decisively demonstrates the excellent intrinsic waveguiding potential of CuI. These findings establish CuI as a highly promising candidate for next-generation high-index-contrast photonic devices and provide a solid foundation for future CuI-based integrated optics.