Light-Confining Nanoporous Anodic Alumina Microcavities by Apodized Stepwise Pulse Anodization

This study presents an innovative approach to fabricate nanoporous anodic alumina optical microcavities (NAA-μCVs) with enhanced quality factor and versatile optical properties. An apodization strategy using a logarithmic negative function is applied to a stepwise pulse anodization process in order to engineer the effective medium of NAA so that it confines light efficiently. The architecture of these light-trapping photonic crystals is composed of two highly reflecting mirrors with an asymmetrically apodized effective medium. Various anodization parameters such as the anodization time, anodization period, current density offset, and pore-widening time are systematically modified to assess their effect on the optical properties of NAA-μCVs in terms of the quality factor and position of the resonance band. We demonstrate that this fabrication approach enables the generation of NAA-μCVs with a high quality factor (∼113) and well-resolved and tunable resonance bands across the spectral regions, from UV to near-IR, through manipulation of the anodization parameters. These results represent a comprehensive rationale for the development of high-quality NAA-μCVs with enhanced light-confining capabilities, providing new opportunities for further fundamental and applied research across a broad range of fields and disciplines such as photonics and optical sensing.