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; Polish National Agency for Academic Exchange (NAWA) grant no. 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

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

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.

The eruption of the Hunga-Tonga volcano in the South Pacific Ocean on January 15, 2022, at about 4:15 UTC, generated a violent explosion, which created atmospheric pressure disturbances in the form of Rayleigh-Lamb waves detected all over the globe. Here we discuss the observation of the Hunga-Tonga shock-wave performed at the Ny-Ålesund Research Station on the Spitsbergen island, by the detectors of the PolarquEEEst experiment and their ancillary sensors. Online pressure data as well as the results of dedicated offline analysis are presented and discussed in details. Results include wave arrival times, wave amplitude measurements and wave velocity calculation. We observed five passages of the shock wave with a significance larger than 3 ? and an amplitude up to 1 hPa. The average propagation velocity resulted to be (308 ± 0.6) m/s. Possible effects of the atmospheric pressure variation associated with the shock-wave multiple passages on the cosmic-ray rate at ground level are also investigated. We did not find any significant evidence of this effect.

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.

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.

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.

This work presents state of the art rare-earth activated SnO2 nanocrystals - based transparent glass-ceramics. With combined enhancements in both photorefractivity and rare-earth photoluminescence, the glass-ceramic has unique benefits as a lasing material. It exhibits high photorefractivity with UV induced refractive index modifications in the order of 10-3. Exploiting its high photorefractivity, optical gratings are fabricated on the glass-ceramic under an energy-efficient direct UV writing process. Furthermore, SnO2 semiconductor nanocrystals are also employed as efficient rare-earth sensitizers enhancing drastically the rare-earth photoluminescence.

Various studies report that Tb/Yb co-doped materials can split one UV or 488 nm (visible) photon in two near infrared (NIR) photons at 980 nm by an energy-transfer process involving one Tb and two Yb ions. Additionally, it was demonstrated that Ag multimers can provide an efficient optical sensitizing effect for rare earth ions (RE ions), resulting in a broadband enhanced excitation, which could have a significant technological impact, overcoming their limited spectral absorptions and small excitation cross sections. However, a systematic and detailed investigation of the down-conversion process enhanced by Ag nanoaggregates is still lacking, which is the focus of this paper. Specifically, a step by step analysis of the energy-transfer quantum-cutting chain in Ag-exchanged Tb/Yb co-doped glasses and glass-ceramics is presented. Moreover, the direct Ag-Yb energy-transfer is also considered. Results of structural, compositional, and optical characterizations are given, providing quantitative data for the efficient broadband Ag-sensitization of Tb/Yb quantum cutting. A deeper understanding of the physical processes beneath the optical properties of the developed materials will allow a wiser realization of more efficient energy-related devices, such as spectral converters for silicon solar cells and light-emitting devices (LEDs) in the visible and NIR spectral regions.

Down conversion (DC) in rare-earth-doped optical materials is a process of great interest for the possibility of a substantial increase of the efficiency of silicon solar cells. Here we report the structural and photoluminescence properties of co-doped CaF phosphors obtained by hydrothermal synthesis. In particular, the DC photoluminescence characteristics for UV (353 nm) excitation of Nd/Yb co-doped CaF phosphors are discussed, underlining the effects due to the co-doping with Li. The photoluminescence emission is dominated by the near-infrared (NIR) Yb emission peaked at 975 nm, although the excitation spectrum corresponds to the characteristic peaks associated with Nd excitation at both the UV and visible wavelength ranges. The Nd to Yb energy transfer mechanisms were determined from a detailed analysis of the excitation spectra characteristics of Nd and Nd/Yb/Li doped CaF phosphors. DC photoluminescence for UV excitation was confirmed by both the analysis of DC quantum yield efficiency and effective quantum yield measurements. In the first case, an efficiency up to 150% was found, while the effective quantum yield measurements, carried out for the UV (DC) and visible (downshift) excitation and NIR photoluminescence emission, give values of 81 ± 10% and 37 ± 5% for excitation with 353 nm and 577 nm light, respectively.

We introduce an easily implementable optomechanical device for pressure and vibration sensing using a multilayer structure on a flexible substrate. We present the design, fabrication and evaluation steps for a proof-of-concept device as well as optical glass components. The design steps include optical, mechanical, and optomechanical correlation simulations using the transfer matrix method, finite element analysis, geometric optics and analytical calculations. The fabrication part focuses on the deposition of multilayers on polymeric flexible substrates using the radio frequency sputtering technique. To investigate the quality of the glass coatings on polymeric substrates, atomic force microscopy and optical microscopy are also performed. Optical measurements reveal that, even after bending, there are no differences between multilayer samples deposited on the polymeric and SiO2 substrates. The performance assessment of the proof-of-concept device shows that the sensor resonance frequency is around 515 Hz and the sensor static response is capable of sensing from 50 Pa to 235 Pa.

In this chapter, we highlight some basic aspects and applications of doped glasses in integrated optics. First, we will briefly summarize the role of rare-earth (RE) ions in integrated optics, reporting some consolidated results presented in the literature as well as some new results concerning the photon management exploited for the luminescence-enhancement mechanisms. Few lines are also assigned to recall the use of the Judd-Ofelt theory in the determination of the spectroscopic parameters, crucial for the design of more novel and efficient photonic devices based on rare earth-doped (RED) glasses. The second section is devoted to the novel and exciting topic of transparent glass ceramics. First, we introduce this kind of materials and we focus the attention of the reader on their important optical, structural, andspectroscopic properties. Of particular interest is the role of tin-dioxide-based glass ceramics as effective sensitizers of the RE ions luminescence. Novel spectroscopic results regarding the Er3+-activated SiO2-SnO2 system are also presented with a discussion about the different possible structures of integrated optics. The different kinds of nanoceramics operating in luminescence quantum yield enhancement are shortly discussed. The differences between the top-down and bottom-up fabrication techniques are presented, as well as some well-consolidated results about the transparent glass ceramics doped with RE ions. In this regard, it is worthy to note the results obtained in a hybrid glass-ceramic fabricated employing an amorphous matrix SiO2-HfO2 loaded by nanoparticles of lithium lanthanum tetraphosphates doped with different concentration of Eu3+ ions. Finally, we conclude the chapter with the perspectives of glass materials for integrated optics.

As already done in electronics, passive and active photonic devices demand integration on flexible substrates for a broad spectrum of application ranging from optical interconnection to sensors for civil infrastructure and environments, to coherent and uncoherent light sources and functionalized coatings for integration on biological tissue. In this communication we will present some recent results concerning the fabrication of novel flexible optical layers by sol-gel and radio frequency sputtering deposition techniques. The perspective is to give a technological way to transform intrinsically rigid or brittle materials into a highly mechanically flexible and optically functional systems

For integrated photonics, waveguide structures based on rare-earth-activated glasses are potential candidates for implementing compact integrated light sources and amplifiers. However, rare-earth ions (REs) possess low absorption cross-section, and this limits the light emission and amplification efficiency. As long as the REs are involved, there are other phenomena detrimental to their luminescence quantum yield including ion-ion interactions and non-radiative relaxation processes. To solve such problems, photonic glass-ceramics can be strategic solutions. Transparent glass-ceramics combine interesting properties of both amorphous and crystalline phases and offer specific characteristics of capital importance in photonics. More important, photonic glass-ceramics can tailor and enhance the spectroscopic properties of the rare-earth ions depending on their compositions and nature. In this work, we studied SnO-nanocrystal-based transparent glass-ceramic planar waveguides activated by rare-earths to give solutions for the problems mentioned above and enhance the rare-earth luminescence efficiency for integrated photonics. SiO-SnO:Er planar waveguides containing 30 mol% SnO nanocrystals were fabricated by sol-gel method and dip-coating technique. The planar waveguides were assessed by various characterization techniques to ensure the applicability of such glass-ceramics for integrated photonics. The experimental assessment of the SiO-SnO:Er planar waveguides focused on the key considered photonic characteristics including the structural, morphological, spectroscopic, and especially optical waveguiding properties. The photoluminescence measurements put in evidence the role of SnO nanocrystals as efficient Er luminescence sensitizers. Moreover, the incorporation of Er ions in SnO nanocrystals was demonstrated to reduce the effect of non-radiative relaxation processes on the luminescence of the Er ions and thus led to higher luminescence efficiency. Majority of the Er ions (97%) was confirmed to be imbedded in the SnO nanocrystals. The SiO-SnO:Er glass-ceramic planar waveguides have confined propagation modes, step-index profile with high confinement of 82% at 1542 nm and especially, low losses of 0.6 ± 0.2 dB/cm at 1542 nm.

The development of the information technology and, very recently, of new application scenarios like Internet of Things (IoT) and Industry 4.0 has pushed the research towards new technological platforms. In this frame, the spatial freedom permitted by flexible short-range connections and flexible devices has become as important as "classical" parameters such as low weight, low power consumption, and electromagnetic immunity. Accordingly, first the flexible electronics and later the flexible photonics have produced innovative components and devices. The present paper aims at presenting a brief overview of this broad area, underlining the achievements and the remaining challenges in the different routes to the manufacturing of flexible photonic devices. Material platforms remain at the core of such developments, and it is interesting to note that glassy materials constitute a fundamental piece in the present and future scenario.