Ultra-Broadband Optical Characterization of TiO2 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 (TiO2) at THz frequencies, we report broadband absorbers based on spaced TiO2 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 TiO2 NTs together with their high uniformity and scalable fabrication process open new opportunities for multipurpose low-cost absorbers and compact room-temperature THz devices.