Energy velocity as an optical design indicator for the ultraviolet nonlinear response of ALD-grown TiO 2 thin films

Slow light media enhance light-matter interactions by increasing the residence time of optical energy, a property with particular relevance for nonlinear nanophotonics. Titanium dioxide (TiO 2 ) is a promising dielectric material for ultraviolet (UV) applications, yet its strong dispersion and absorption make the commonly used group velocity unreliable as a measure of slow light. Here, we show that the energy velocity ( v e ) defined as the ratio of the cycle-averaged Poynting flux to the electromagnetic energy density provides a suitable metric for describing optical transport in TiO 2 . We also present the optical and material properties of distinct TiO 2 thin films produced with thermal atomic layer deposition (ALD) with different oxidation agents (H 2 O, H 2 O 2 , O 3 ). Using O 3 at 200 °C , we obtain fully oxidized, hybrid phase (predominantly amorphous with minor anatase crystallites) TiO 2 films with high refractive index and very small extinction coefficient in the UV-A (315 nm to 400 nm ). Ellipsometry combined with analytical estimates and FDTD simulations shows that v e is strongly reduced, reaching values as low as 0.18 to 0.21 c in the UV-A, confirming that these films operate in a slow-light regime. We experimentally observe that the ALD process chemistry strongly affects the THG response of TiO 2 /Si stacks. Because the measurements are performed in reflection on a Si substrate, we interpret the reduced v e as one optical design indicator among several, rather than as a direct measure of the nonlinear response.