Flexible glass photonics is a cutting-edge technological and scientific research field that, thanks to a very broad spectrum of applications, has tremendously grown during the last decade and is now a strategic topic. Here, we present the results of the spectral transmittance and reflectance of a 10-layer SiO2/TiO2 1D photonic crystal deposited on a flexible polymeric substrate under different bending conditions, obtained with a home-made adjustable sample holder.

Thin-film optics is a key technology for the fabrication of miniaturized photonic devices, spanning from optical waveguides and photonic-integrated-circuits for optical signal processing, to multi-layered resonant structures and cavities for the confinement and spectral selection of the optical field. Active optical waveguides and photonic crystals are among the most versatile examples. One further step to add versatility to thin-film photonic structures involves the use of flexible materials. In fact, by adding mechanical flexibility to the rigid photonic systems, the range of applications greatly expands. However, passing from rigid to flexible substrates requires the development of suitable fabrication protocols, to preserve the optical and spectroscopic properties of the systems under mechanical deformation. We present the RF-sputtering fabrication of 1D photonic crystals and active Er3+ planar waveguides deposited on polymers and ultrathin flexible glass substrates. The structures deposited on ultrathin flexible glass show interesting results in terms of both optical and mechanical properties, making RF-sputtering a promising and scalable technique to fabricate flexible photonic devices [1,2]. Moreover, we report on the spectroscopic study of a 1D photonic crystal, fabricated via RF-sputtering on a flexible thermosetting polymer, in different bending conditions. Acknowledgements: This research is supported by the projects: FESR-PON 2014-2020 BEST4U ARS01_00519; CNR-PAS "Flexible Photonics" (2020-2022); NAWA PPN/IWA/2018/1/00104; MIUR-'Departments of Excellence' L 232/2016; ERC-H2020 PAIDEIA GA 816313; "nuovi Concetti, mAteriali e tecnologie per l'iNtegrazione del fotoVoltAico negli edifici in uno scenario di generazione diffuSa" CANVAS and NAWA-MAECI Canaletto (2022-2023). References: [1] A.Carlotto, et.al., Proc. SPIE 12142, pp. 1214206 (2022); doi:10.1117/12.2621281 [2] A.Carlotto, et.al., Ceramics International, (2023); doi:10.1016/j.ceramint.2023.03.012

Polymorph tungsten oxide and its sub-stoichiometric phases (WO3-x) have an extraordinarily broad range of possible photonic applications, from electrochromic layers in smart windows, to UV optical detectors, full-optical chemical sensors and photo-electro-catalytic membranes.We focus on Radio-Frequency (RF) magnetron sputtering deposition as a broadly versatile method to synthesize WO3-xin different stoichiometries and nanostructures, easily up-scalable to industrial processes. We report on WO3-xfilms fabricated under different recipes, which deeply differ in terms of possible applications. Optical-grade compact WO3- x films can be fabricated by non-reactive magnetron RF-sputtering deposition from a WO3 target at room temperature, followed by thermal annealing to tailor stoichiometry and structure. Films transparent in the NIR and electrically conductive can be obtained, that perform well as transparent contacts [1], but also insulating films, with the possibility to embed them into multi-layered IR resonant photonic structures for opto-chemical sensing [2]. Nanoporous WO3-xphotoelectrodes for green H2production are obtained from metallic W target, by RF-plasma sputtering process in a diode configuration in a reactive 40% O2 /Ar atmosphere. By changing the total gas pressure in the growth chamber, at same partial O2pressure, a bilayered diode-like structure is formed, which improves photoelectron transfer and decreases the interfacial resistance (Rct), leading up to about a 30% increase in photo-electro-catalytic (PEC) performance compared to monolayers coatings and to a 93% faradaic efficiency, which is among the highest reported so far for WO3 photoanodes [3].

In honor of the International Year of Glass 2022 (IYoG'22), this article highlights important aspects of glass and light, focusing on more advanced photonic applications that drive a great many current and future commercially and societally beneficial products and services. Topics include a brief history of photonic glasses, an overview of light guiding, the subsequent use in planar waveguide devices, and their ubiquity as carriers of information and sensors in the form of optical fibers. These sections highlight the strong interconnections on the understanding of the glass composition, glass structure and optical properties. For next developments in glass photonics, new opportunities will be offered by machine learning, artificial intelligence, 3D printing and additive manufacturing. Additionally, the future of glass photonics may also depend on our ability to meet current challenges such as climate change, i.e., the development of an environmentally-friendly manufacturing process, considering environmental impact and possible material shortages.

Polyethylene terephthalate (PET) is one of the most widely used polymeric substrates for flexible photonic applications. Like all polymers, its usage is limited by the modest environmental and temperature resistance. In this work, the optical properties deterioration of PET substrate, in the form of 175 ?m thick film, is discussed and the strategy to overcome this problem, by application of the sol-gel coating, is presented. The investigations of physicochemical changes occurring in PET after the thermal treatment at temperatures of 160 °C, 190 °C, and 210 °C were performed by various characterization techniques, such as thermal analysis (DSC - differential scanning calorimetry and TG - thermal gravimetry), structural and chemical composition analysis (XRD - X-ray diffraction, Raman spectroscopy, and XPS - X-ray photoelectron spectroscopy) and microscopic analysis (SEM-EDS - scanning electron microscopy with chemical composition analysis, AFM - atomic force microscopy). The 25-layer organically modified SiO2-TiO2 coating obtained by the sol-gel method is demonstrated to be an effective protection against the optical properties deterioration of PET. The protective effect is demonstrated by the comparison of surface morphology, and optical properties of PET, with and without the protective coating, after thermal treatment at different temperatures. The refractive index of materials and thickness of coatings have also been studied with M-line and white light reflectance spectroscopy (WLRS).

Thin films have a crucial role in integrated optics. The development of passive and active devices such as splitters, waveguides, multiplexers and optical amplifiers is by now established on rigid substrates. Therefore, an important further step to improve this kind of technologies and open the route to new applications is to extend these functionalities to mechanically flexible devices. One fundamental brick to obtain this features extension is the fabrication of low loss inorganic active planar waveguides on flexible glass substrate. Here, we present the preliminary results of a novel top-down fabrication of an active SiO-HfO:Er all-glass flexible planar waveguide, via radio frequency sputtering. The waveguiding system, deposited on ultrathin flexible glass substrate, showed an attenuation coefficient lower than 0.2 dB/cm at 1.53 ?m and exhibits emission in the NIR region.

n this review, we present a short overview of the development of sol-gel glasses for application in the field of photonics, with a focus on some of the most interesting results obtained by our group and collaborators in that area. Our main attention is devoted to silicate glasses of different compositions, which are characterized by specific optical and spectroscopic properties for various applications, ranging from luminescent systems to light-confining structures and memristors. In particular, the roles of rare-earth doping, matrix composition, the densification process and the fabrication protocol on the structural, optical and spectroscopic properties of the developed photonic systems are discussed through appropriate examples. Some achievements in the fabrication of oxide sol-gel optical waveguides and of micro- and nanostructures for the confinement of light are also briefly discussed.

Since the invention and further development of lasers in the 1960s, photonics has grown into a field that permeates virtually every aspect of modern life. As photonics deals with the generation, transmission, modulation, amplification, conversion, and detection of light, glasses have played crucial and central roles in a multitude of applications given glasses' diverse range of compositions, properties, and forms. They are also low-cost materials, recyclable and generally easy to fabricate. In homage to the United Nations International Year of Glass 2022, this paper reviews the past, present, and future of photonic glasses.

While conventional photonic devices are fabricated on rigid substrates, integration of glass systems on deformable substrates has given birth to flexible photonics, a research field which has rapidly emerged in recent years. However, a careful design of the structures, selecting of the materials and the development of a suitable fabrication protocol are required. RF-sputtering deposition protocols are here developed for the fabrication of SiO2/HfO2 glass-based 1D photonic crystals and planar waveguides on flexible polymeric and glass substrates. 1D multilayer structures, made up on SiO2 and HfO2 films, were first modelled by Transfer Matrix Methods and then fabricated by RF-sputtering technique on different substrates. The optical features of the samples were investigated to highlight as the different nature of the substrates and the mechanical deformations of the samples do not influence the transmittance of the samples.

The benefits obtained in terms of costs and applicability by the development of flexible and stretchable electronics, compared to their rigid counterparts, have fostered the birth of the idea of the photonics analogue. By adding mechanical flexibility to photonic structures, the fields of application expand incredibly. In particular, we are interested in 1D photonic crystals that, due to their versatility, are exploited in several applications from sensors to dichroic mirrors in photovoltaic cells. Here, a radio frequency (RF) sputtering deposition protocol is developed for fabricating dichroic mirrors on ultrathin flexible glass as well as on rigid substrates for comparison. The 1D multilayer structures, constituted by silica and hafnia 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 optical features of the samples, on both flexible and rigid substrates, were studied to highlight up to which extent the different nature of the substrates and the mechanical deformations are not influencing the key spectral properties of the photonic crystals.

Photonic glass-ceramics are two-phase materials constituted by nanometer sized crystals, passive or activated by luminescent species, embedded in a glass matrix. Among the different techniques used to fabricate transparent glass ceramics the sol-gel route is highly appreciated because it allows to explore different compositions and develop novel functionalities. By engineering glass-ceramics chemistry, nature, or volume fractions of crystalline and amorphous phases, several interesting properties can be achieved and tailored so that the sol-gel technique appears one of the most versatile processes for the fabrication of photonic systems. The aim of the present paper is to give a brief state of the art and some significant examples of systems developed, even in our research network, during the last ten years of activity, in order to highlights the reliability and versatility of sol-gel -derived glass-ceramics for photonics application. With this aim, the main topics discussed here deal with: (i) transparent rare-earth-activated SiO2-SnO2 glass-ceramics; (ii) mechanical properties of glass ceramics coatings.

In this work, we present preliminary results of the fabrication and characterization of 1D Fabry-Perot microcavity realized on Yb3+ activated SiO2-SnO2 glass-ceramic (SiO2-SnO2:Yb3+). A radiofrequency-sputtering/sol-gel hybrid deposition process was developed for the microcavity fabrication. The fabrication included (i) radiofrequency-sputtering (rf-sputtering) of SiO2/HfO2 Bragg reflectors and (ii) sol-gel deposition of the active SiO2-SnO2:Yb3+ defect layer. A good control and enhancement of the spontaneous emission for Yb3+ luminescence sensitized by SnO2 nanocrystals was achieved exploiting microcavity properties. Such results are valuable for development of low-threshold rare-earth-based coherent light sources, pumped by broadband UV diodes.

For the development of flexible photonic devices, polymeric substrates are of capital importance due to their low cost and mechanical flexibility. However, they are sensitive to the environment's influence on their durability. To overcome problems of polymeric substrates' stability, we developed organically modified hybrid silica-titania coated PET (polyethylene terephthalate) films. The focus was put on the determination of the optical properties' changes in dependence on the synthesis and materials processing parameters. Thanks to the application of the coatings, a protection effect on substrate and improvement in the transparency in comparison to the uncoated heat-treated polymeric material, was obtained.

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 comparative analysis of two pressing methods such as conventional uniaxial pressing and the collector one was carried out to manufacture optically transparent oxide ceramics of various compositions. The finite element modeling of the consolidation processes was carried out for various kinematic schemes of movement of mold shaping members taking into account the simulation of thermal treatment at sintering. The uniaxial pressing with following free sintering and Spark Plasma Sintering (SPS) techniques were considered for the modeling. The simulation results showed that the SPS technique usage with a uniaxial pressing method allows to decrease the nonuniformity of density distribution over the volume of ceramics. In case of SPS usage with a collector pressing method the density distribution is almost uniform. The influence of the pre-pressing method at SPS on the mechanical and optical properties of the oxide transparent ceramics has been studied experimentally. The efficiency of using the collector pressing in the SPS process has been confirmed for YAG:Ce3+, ZrO2 -Y2O3 (YSZ) and MgAl2O4 ceramics.

The unique properties of the Eu3+ ion make it a powerful spectroscopic tool to investigate structure or follow processes and mechanisms in several high-tech application areas such as biology and health, structural engineering, environment monitoring systems and quantum technology, mainly concerning photonics. The traditional method is to exploit the unique photoluminescent properties of Eu3+ ions to understand complex dynamical processes and obtain information useful to develop materials with specific characteristics. The objective of this review is to focus on the use of Eu3+ optical spectroscopy in some condensed matter issues. After a short presentation of the more significant properties of the Eu3+ ion, some examples regarding its use as a probe of the local structure in sol-gel systems are presented. Another section is devoted to dynamical processes such as the important technological role of nanocrystals as rare-earth sensitizers. The appealing effect of the site-selection memory, observed when exciting different sites into the 5D1 state, which the 5D0 -> 7F0 emission band reflects following the sites' distribution, is also mentioned. Finally, a section is devoted to the use of Eu3+ in the development of a rare-earth-based platform for quantum technologies.

The structural and optical properties of thin layers based on 70%SiO 2 -30%HfO 2 doped with different concentra- tion of rare earth ions (terbium and ytterbium) have been studied with a view to integrating them in a photovoltaic cell as a spectral conversion layer in order to improve its efficiency, by using down-conversion process. These thin films were synthesized by using sol gel technique and deposited on the pure silica substrate by dip-coating method. The DC layer can be placed on the front side of a solar cell and can enhance the current by converting ultraviolet (UV) photons into a large number of visible photons. In present study two series of samples are compared, the first series corresponds to samples treated at 900 °C (glass- S) while the second series concerns samples treated at 1000 °C (glass-ceramic- SC). These series are based on 70SiO 2 -30HfO 2 activated by different molar concentrations of rare earths [Tb + Yb]/[Si + Hf] = 7%, 9%, 12%, 15%, 17%, 19% and 21%. Photoluminescence results of reference samples (without Yb 3 + ) showed an emission from 5 D 4 to 7 F J ( J = 3, 4, 5, 6) level characteristic transitions of Tb 3 + , with a maximum peak in the green centered at 543.5 nm cor- responding to the 5 D 4 ->7 F 5 transition. For the co-doped samples a clear NIR PL emission around 980 nm was detected, due to the 2 F 5/2 ->2 F 7/2 transition of Yb 3 + ions. From luminescence decay curves of Tb 3 + maximum emission peak ( 7 F 5 ->5 D 4 transition at 543.5 nm) we have identified the energy transfer efficiency. The quantum efficiency increases by increasing the total [Tb + Yb] concentration. The most significant yield was achieved with [Tb + Yb] = 19%, the maximum quantum transfer efficiency obtained was 190% for glass-ceramic samples and 161% for glassy one.

Flexible photonics is undoubtedly the next technological platform, capable to revolutionize current light-based technologies, thanks to their spatial freedom characteristics. Beyond polymer-based flexible photonics, the recent advent of ultrathin glasses with their mechanical flexibility has opened a new avenue for developing all- inorganic flexible photonic structures and devices. However, deposition and processing of functional coatings on such very thin glasses is an emerging challenge, in particular to obtain very good adhesion. Furthermore, to find proper management for maintaining the mechanical flexibility, investigation of impacts induced by pro-cessing and application of coatings on the ultrathin glasses is necessary. This work reports progress in obtaining rare-earth activated SnO2 photoluminescent thin films (SnO2:RE3+ with RE3+: Er3+ or Yb3+) on ultrathin AS 87 eco Schott glass (175 ?m), valuable for development of flexible inorganic systems with efficient RE3+ lumi-nescence. In such flexible photoluminescent thin films, SnO2 nanocrystals act as effective rare earth host- sensitizers, enhancing near-infrared emission of Er3+ or Yb3+. A sol-gel derived synthesis and fabrication pro-tocol of SnO2:RE3+ coatings on the ultrathin glass was realized. A heat-treatment process at 500 oC for 4 h was defined to be suitable for obtaining crystallization of the coatings while avoiding thermally induced cracking of the AS 87 eco glass. The obtained SnO2:RE3+ thin films exhibit a wide transparency window with a transmittance of about 80% covering the 400 nm-3200 nm range. Three point bending test was carried out on the ultrathin glass and coated samples. Both thermal treatment and application of SnO2:RE3+ coatings connected with the thermal treatment (used for coating fabrication), resulted in a decrease of possible maximum elastic deformation of the final flexible structures.

In the article, we present and discuss the features of the structure and photoluminescence properties of Yb-doped As-S films synthesized by PECVD employing solid-state initial precursors. The doping was carried out during the deposition process. By controlling the temperature of the precursor sources and the composition of the lowtemperature nonequilibrium plasma, we synthesized amorphous As-S:Yb films with Yb content ranging from 0.6 to 8.4 at. %. The change in the ratio of structural elements and surface morphology as a function of the elemental composition is shown. The increase of the Yb content leads to a redshift of the short-wavelength absorption edge. The dependence of the shape and intensity of 2F5/2 -> 2F7/2 photoluminescence, observed in the range 930-1030 nm on the excitation wavelength (632.8 nm and 785 nm) and Yb concentration was evaluated. The relation between composition and atomic structure of the amorphous matrix of arsenic sulfide on the photoluminescent properties of Yb3+ ions is discussed.

A variety of studies have been performed on WGM microresonators made of different materials and exploiting very high quality factors. This feature has allowed their use in a broad variety of applications including micro-lasers, optical filtering and switching, frequency conversion through non-linear effects, RF photonics and sensing. The easiest way to shape a glass spheroid with high quality factor is to use the surface tension during the thermal reflow of a highly transparent amorphous dielectric. Depending on the application, alternatives shapes like micro-bubbles or micro-bottles can be implemented in order to obtain specific performances. This manuscript reports the results we obtained on micro-laser sources in erbium doped microspheres, parametric frequency conversion in silica microspheres, stimulated Brillouin scattering in silica microbubbles, and non-linear effects in polymer coated or fluorophore filled glass microcavities.