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The research interest for tungsten oxide (WO3) as a photonic material is prompted by its extraordinarily broad range of applications, deriving from its polymorphism and variety of substoichiometric WO3-x phases. WO3 contributes compact and smooth electrochromic layers for smart windows, UV optical detectors and nanoporous photo-electro-catalytic membranes. Full-optical chemical sensors can also be provided, by optically read-out of its chemically modified dielectric response. Radio-Frequency (RF) sputtering is a versatile method to synthesize WO3-x. It provides films of different stoichiometric ratios, either glassy or crystalline, either porous or compact. In this work we report about an ongoing experimental activity on optical-grade WO3-x thin films, fabricated by non-reactive magnetron RF-sputtering in Ar atmosphere. Stoichiometry and structure are tailored by post-growth thermal annealing in dry air. Annealed films are quite transparent in the near infrared NIR and short-wave infrared SWIR optical range. Their quality was assessed by morphological, structural and compositional characterizations. Dielectric properties were evaluated by optical spectroscopy and ellipsometry, also rating the amount of optical anisotropy of thin films in its crystalline phase. To boost the sensitivity of RF-sputtered WO3-x layers as spectroscopic opto-chemical sensors, their direct integration at top of 1D Photonic bandgap structures can be conceived. We summarize the results on the design of a photonic chip structure, formed by a silica-titania multilayer and capped with a 450 nm-thick WO3-x film, that effectively confines near-IR optical field in the topmost layer, as suitable for sensing of environmental contaminants in gas and vapor phase.
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