Ultra‐Broadband Optical Characterization of TiO ₂ Nanotube Arrays Revealing Annealing‐Tunable THz Absorption

Conventional terahertz (THz) absorbers often suffer from narrow bandwidths, bulky designs, and complex, multi‐step fabrication processes. By exploiting the extremely high phonon‐induced permittivity of titanium dioxide (TiO 2 ) at THz frequencies, we report broadband absorbers based on spaced TiO 2 nanotubes (NTs) fabricated by electrochemical anodization and thermal annealing featuring ∼ 80% absorptance from ∼10 THz up to 300 THz. Furthermore, the structures displayed a broad (∼1 THz wide) and tunable absorption peak in the 1.4–3.6 THz range by simply controlling the annealing time. To map the millimeter‐wave‐to‐ultraviolet spectrum and to uncover the dielectric response of the NTs, a multi‐instrument spectroscopy characterization strategy was combined with full‐wave electromagnetic simulations. The tunable and broadband absorption of spaced TiO 2 NTs together with their high uniformity and scalable fabrication process open new opportunities for multipurpose low‐cost absorbers and compact room‐temperature THz devices.