Fig. 4

Optical spectra of anatase TiO₂ single crystals. Imaginary part of the dielectric function at 20 K with the electric field polarised along a the a-axis (E ⊥ c) and b the c-axis (E ǁ c). The experimental data measured by SE on a pristine (n ~ 0 cm⁻³) anatase TiO₂ single crystal are reported in blue, while those obtained on a highly n-doped single crystal (n = 2 × 10¹⁹ cm⁻³) in red. The quasiparticle indirect gap E _ind = 3.47 eV and direct gap E _dir = 3.97 eV, as estimated by ARPES, are indicated by dashed grey and black vertical lines, respectively