Investigation of crystallization in nanolayered TiO2-based superlattices

Abstract

In the realm of high precision optical applications, such as gravitational wave detectors, the quest for optical coatings with exceptional characteristics, such as high reflectivity, high optical density, and low thermal noise, requires meticulous attention. Achieving the requested optical quality demands a comprehensive approach encompassing design, production, and characterization. A recent study by I.M. Pinto et al. explored the potential of nanolayer-based superlattices as a valuable alternative to the high refractive index component of dielectric Bragg-like mirrors [https://dcc.ligo.org/LIGO-G1301061, https://dcc.ligo.org/LIGO-G1902307]. Following the proposed approach, we have produced and investigated binary nanolayers, employing a combination of TiO2 with other oxides, such as SiO2, Ta2O5, Al2O3, and ZrO2. Within our superlattice structures, each layer's thickness ranges from a few to a few tens of nanometers. It is tailored, together with the number of nanolayers composing the structure, to cater to optical applications involving a light wavelength of 1064 nm (as for the laser of gravitational wave detectors). Our findings indicate that the superlattice structures fulfill the morphological and structural requirements for application in high precision optics when TiO2 is as thin as 2 nm. In this case, the surface attains a level of flatness on the nanometer scale – making spurious light scattering negligible – and its crystallization temperature Tc, a key parameter for reaching high optical quality, exceeds 500 °C. Finally, we demonstrate that further tuning of Tc is possible by varying the interface energy, thus changing the material coupled with TiO2 within the superlattice. This discovery paves the way for even greater control and optimization of these proposed metamaterials.