Integration of photonic systems on deformable substrates has given rise to flexible photonics, a research field that has rapidly emerged in recent years. By adding mechanical flexibility to planar photonic structures, the spectrum of applications gains an incredible expansion. Flexible glassy photonic structures require a careful design and suitable fabrication protocols, in order to keep the optical and spectroscopic properties similar to their traditional rigid counterparts, even under mechanical deformation. Here, a radio frequency (RF) sputtering deposition protocol is developed for fabricating glass-based 1D photonic crystals on ultrathin flexible glass as well as on rigid substrates for comparison. Three different 1D multilayer structures, constituted by SiO2 and HfO2 layers, were first designed and modelled by Transfer Matrix Method to tailor targeted optical features (transmission windows, stopband ranges) and then fabricated by RF-sputtering technique. The structural, morphological, and optical features of the samples were investigated. In particular, the transmission spectra of the glass-based 1D photonic crystals, deposited on both flexible and rigid substrates, were acquired to highlight up to which extent the different nature of the substrates and the mechanical deformations (bending tests on the flexible structures) are not influencing the key spectral properties of the photonic crystals.
The demand for transparent conductive films (TCFs) is dramatically increasing. In this work tungsten oxide (WO3-x) is studied as a possible option additional to the existed TCFs. We introduce WO3-x thin films fabricated by a non-reactive magnetron RF-sputtering process at room temperature, followed by thermal annealing in dry air. Films are characterized morphologically, structurally, electrically, optically, and dielectrically. Amorphous WO3-x thin films are shown to be n-type conductive while the transparency extends to the near-IR. By evaluating a figure of merit for transparent-conductive performance and comparing to some most-widely used TCFs, WO3-x turns out to outperform in the near-IR optical range
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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