Expanding the Porous Alumina Engineering Toolbox for Nanooptics: Multiscale Morphological Tuning via Square-Wave Pulse Anodizing

Individual control of nanopore diameter, spacing, and hexagonal ordering in porous anodic alumina (PAA) is essential for designing advanced photonic materials and metamaterials, plasmonic surfaces, and SERS substrates. This paper presents a comprehensive investigation of square-wave pulsed direct current (PDC) anodizing for selective morphological optimization across multiple length scales. Conventional potentiostatic (DC) methods inherently couple pore diameter and hexagonal ordering (both dictated by the best self-ordering regimes occurring at specific voltages). In contrast, PDC anodizing decouples these parameters: the DC-to-PDC transition reduces pore size by up to 60%, while pulse frequency governs well-ordered pore array dimensions. Importantly, the PDC approach maintains consistent nonstoichiometric PAA composition, ensuring that visible property changes (e.g., transparency differences) result solely from structural modifications rather than compositional variations. The demonstrated versatility across different electrolyte types and no requirement for complicated aluminum pretreatment make pulse anodizing a promising tool for optimizing PAA-based optical nanostructures.