Dual-Spectral Design of Porous Anodic Alumina for Tunable Colored Passive Radiative Cooling

Passive radiative cooling requires materials that combine high mid-infrared (MIR) emissivity within the atmospheric transparency window (8-13 µm) with minimal solar absorption, while maintaining scalability and long-term stability. Porous anodic alumina (PAA) is an attractive dielectric platform due to its intrinsic phonon-polariton infrared emission and electrochemically tunable nanostructure. However, achieving independent control of visible photonic properties and MIR emissivity within a single architecture remains challenging. Here, we demonstrate dual-spectral control of PAA photonic structures integrated with aluminum (Al) substrates using charge density-controlled sinusoidal pulse anodization. Periodic voltage modulation generates well-defined photonic stop bands (PSBs) in the visible range, enabling tunable structural coloration while preserving high MIR emissivity. Systematic pore widening produces a progressive blue shift of the PSB accompanied by non-monotonic reflectance changes, revealing distinct mechanisms governing spectral position and optical coherence. In contrast, MIR emissivity remains robust against variations in the visible photonic response, as confirmed by a constant-voltage reference sample. The PAA architecture provides intrinsic optical impedance matching through a gradual refractive-index transition, while the Al substrate acts independently as a back reflector that suppresses transmission losses. These findings establish PAA as a versatile platform for color-designed radiative cooling surfaces with independently engineered optical and thermal functionalities.