Plasmon resonance and anomalous dispersion of the dielectric permittivity and refractive index of the porous laser-structured surface of anodized titanium

   In recent years, laser-structured titanium dioxide (TiO₂) surfaces have attracted considerable attention due to their combination of high specific surface area, biocompatibility, and unique optical properties, offering promising opportunities for photonics, sensing, and energy applications. Of particular interest is the study of the optical manifestations of porous Ti/TiO₂ films fabricated via laser structuring, with potential evidence of plasmonic resonances and anomalous dispersion. The samples were prepared from titanium foil subjected to anodization in potassium hydroxide solution, followed by nanosecond laser structuring at the wavelength of 1064 nm and an energy density of (3.2 ± 0.2)∙10³ J/cm². Surface morphology was analyzed using scanning electron microscopy and optical profilometry, while optical characteristics were investigated by spectrophotometry and ellipsometry. To interpret the spectral data, a modified Adachi-Forouhi model within the dipole approximation was applied, enabling quantitative description of the contributions of interband transitions and plasmonic modes. The surfaces produced by laser structuring exhibited pronounced porosity (pore sizes of 300–1100 nm, depth ~200 nm), submicron cracks, and nanoparticles of the laser-structured material. Reflection spectra revealed minima corresponding to the excitation of surface plasmons and interference modes. Dielectric permittivity spectra displayed a region of anomalous dispersion and field localization at a wavelength of 625 nm. Calculated parameters included the skin layer thickness, Purcell factor for a nanopore, damping length of plasmon oscillations on the surface, propagation length of surface plasmons, and the critical value of polarizability enhancement in the plasmon resonance localization region. Modeling indicated a narrowing of the bandgap to 1.016 eV. Contributions to the dielectric permittivity of the semiconductor component from interband absorption saturation, changes in band structure, and free carriers were determined. While the bandgap narrowing played a decisive role, the dominant contribution to the experimentally observed dielectric behavior arose from the generation of resonant plasmonic modes. It was established that the key mechanism of the optical response is the resonant localization of the electromagnetic field within the nanopores, confirming the manifestation of hyperbolic metamaterial behavior. The material obtained exhibited significant bandgap narrowing due to the nanosecond laser treatment. The results highlight the potential of porous laser-structured anodized titanium surfaces for photonic and sensing devices, as well as for use in waveguiding structures.

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