Highly Reflective Periodic Nanostructure Based on Thermal Evaporated Tungsten Oxide and Calcium Fluoride for Advanced Photonic Applications

In this paper, we report the fabrication and characterization of high-quality distributed Bragg reflectors (DBRs) deposited by low-energetic thermal evaporation. This technique allows deposition of high-quality thin films with an accurate control of thickness at the nanoscale. We investigated, for the first time, the use of tungsten oxide (WO₃) and calcium fluoride (CaF₂) as high (2.15) and low (1.4) refractive index materials, respectively, for DBR fabrication. They consist of nine pairs of WO₃/CaF₂ layers, with a central wavelength λ_c tuned at 640 nm and a maximum reflectance of 99.6%. A passive microcavity consisting of a λ/2 thick spacer of CaF₂ sandwiched between 8.5 pairs of WO₃/CaF₂ has also been fabricated. We investigated the optical behavior of this microresonator after post-deposition thermal treatment at different temperatures, showing a blue shift of the resonant mode and an increase in its intensity. Indeed, heat treatment induced a compaction of the DBR multilayer, leading to more defined and clearer interfaces, as demonstrated by scanning electron microscopy cross section measurements. This behavior results in an improvement of the microcavity quality factor (up to 400) as the temperature increases, which is currently the highest value obtained for microcavities obtained by thermal evaporation processes, so far. These results represent a further step forward toward the development of high-reflectivity mirrors and high-quality microresonators fabricated by the low-energy deposition technique. Such a structure could find extensive application in nano-photonics, optoelectronics, and sensors, even based on soft organic materials.