Papers by Giancarlo C. Righini
Applied Sciences, 2020
The optical photoluminescent (PL) emission of Yb3+ ions in the near infrared (NIR) spectral regio... more The optical photoluminescent (PL) emission of Yb3+ ions in the near infrared (NIR) spectral region at about 950–1100 nm has many potential applications, from photovoltaics to lasers and visual devices. However, due to their simple energy-level structure, Yb3+ ions cannot directly absorb UV or visible light, putting serious limits on their use as light emitters. In this paper we describe a broadband and efficient strategy for sensitizing Yb3+ ions by Ag codoping, resulting in a strong 980 nm PL emission under UV and violet-blue light excitation. Yb-doped silica–zirconia–soda glass–ceramic films were synthesized by sol-gel and dip-coating, followed by annealing at 1000 °C. Ag was then introduced by ion-exchange in a molten salt bath for 1 h at 350 °C. Different post-exchange annealing temperatures for 1 h in air at 380 °C and 430 °C were compared to investigate the possibility of migration/aggregation of the metal ions. Studies of composition showed about 1–2 wt% Ag in the exchanged s...
Micromachines, 2020
Photonic crystals (PhC) are spatially ordered structures with lattice parameters comparable to th... more Photonic crystals (PhC) are spatially ordered structures with lattice parameters comparable to the wavelength of propagating light. Their geometrical and refractive index features lead to an energy band structure for photons, which may allow or forbid the propagation of electromagnetic waves in a limited frequency range. These unique properties have attracted much attention for both theoretical and applied research. Devices such as high-reflection omnidirectional mirrors, low-loss waveguides, and high- and low-reflection coatings have been demonstrated, and several application areas have been explored, from optical communications and color displays to energy harvest and sensors. In this latter area, photonic crystal fibers (PCF) have proven to be very suitable for the development of highly performing sensors, but one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) PhCs have been successfully employed, too. The working principle of most PhC sensors is based on the f...
Micromachines, 2018
Rare earth doped materials play a very important role in the development of many photonic devices... more Rare earth doped materials play a very important role in the development of many photonic devices, such as optical amplifiers and lasers, frequency converters, solar concentrators, up to quantum information storage devices. Among the rare earth ions, ytterbium is certainly one of the most frequently investigated and employed. The absorption and emission properties of Yb3+ ions are related to transitions between the two energy levels 2F7/2 (ground state) and 2F5/2 (excited state), involving photon energies around 1.26 eV (980 nm). Therefore, Yb3+ cannot directly absorb UV or visible light, and it is often used in combination with other rare earth ions like Pr3+, Tm3+, and Tb3+, which act as energy transfer centres. Nevertheless, even in those co-doped materials, the absorption bandwidth can be limited, and the cross section is small. In this paper, we report a broadband and efficient energy transfer process between Ag dimers/multimers and Yb3+ ions, which results in a strong PL emiss...
Micromachines, 2018
Solar cells are electrical devices that can directly convert sunlight into electricity. While sol... more Solar cells are electrical devices that can directly convert sunlight into electricity. While solar cells are a mature technology, their efficiencies are still far below the theoretical limit. The major losses in a typical semiconductor solar cell are due to the thermalization of electrons in the UV and visible range of the solar spectrum, the inability of a solar cell to absorb photons with energies below the electronic band gap, and losses due to the recombination of electrons and holes, which mainly occur at the contacts. These prevent the realization of the theoretical efficiency limit of 85% for a generic photovoltaic device. A promising strategy to harness light with minimum thermal losses outside the typical frequency range of a single junction solar cell could be frequency conversion using rare earth ions, as suggested by Trupke. In this work, we discuss the modelling of generic frequency conversion processes in the context of solar cell device simulations, which can be used...
Micromachines, 2018
Microspheres made of glass, polymer, or crystal material have been largely used in many applicati... more Microspheres made of glass, polymer, or crystal material have been largely used in many application areas, extending from paints to lubricants, to cosmetics, biomedicine, optics and photonics, just to mention a few. Here the focus is on the applications of glassy microspheres in the field of energy, namely covering issues related to their use in solar cells, in hydrogen storage, in nuclear fusion, but also as high-temperature insulators or proppants for shale oil and gas recovery. An overview is provided of the fabrication techniques of bulk and hollow microspheres, as well as of the excellent results made possible by the peculiar properties of microspheres. Considerations about their commercial relevance are also added.
Journal of Physics: Conference Series, 2019
Glasses doped with rare earth elements possess unique photoluminescence properties. They find app... more Glasses doped with rare earth elements possess unique photoluminescence properties. They find application in several devices, such as lasers, optical amplifiers, and sensors. More recently, rare-earth doped glass thin films have been the subject of investigation for the development of frequency-converting layers able to increase the efficiency of silicon solar cells. Another approach to the improvement of the performance of a solar cell is based on the capture of a larger flux of light by the detector, which can be obtained by surface texture, plasmonics, or waveguide structures. Here, the recent advances in this area will be briefly reviewed.
Ceramics, 2019
Low threshold coherent emission at 1.5 µm is achieved using Er3+-doped dielectric 1D microcavitie... more Low threshold coherent emission at 1.5 µm is achieved using Er3+-doped dielectric 1D microcavities fabricated with a Radio Frequency-sputtering technique. The microcavities are composed of a half-wavelength Er3+-doped SiO2 active layer inserted between two Bragg reflectors consisting of ten, five, and seven pairs of SiO2/TiO2 layers, also doped with Er3+ ions. The morphology of the structure is inspected using scanning electron microscopy. Transmission measurements show the third and first order cavity resonance at 530 nm and 1.5 µm, respectively. The photoluminescence measurements are obtained using the optical excitation at the third order cavity resonance using a 514.5 nm Ar+ laser or Xe excitation lamp at 514.5 nm, with an excitation angle of 30°. The full width at half maximum of the emission peak at 1535 nm decreased with the pump power until the spectral resolution of the detection system was 2.7 nm. Moreover, the emission intensity presents a non-linear behavior with the pum...
The development of efficient luminescent systems, such as microcavities, solid state lasers, inte... more The development of efficient luminescent systems, such as microcavities, solid state lasers, integrated optical amplifiers, optical sensors is the main topic in glass photonics. The building blocks of these systems are glass-ceramics activated by rare earth ions because they exhibit specific morphologic, structural and spectroscopic properties. Among various materials that could be used as nanocrystals to be imbedded in silica matrix, tin dioxide presents some interesting peculiarities, e.g. the presence of tin dioxide nanocrystals allows increase in both solubility and emission of rare earth ions. Here, we focus our attention on Er3+ - doped silica – tin dioxide photonic glass-ceramics fabricated by sol-gel route. Although the SiO2-SnO2:Er3+ could be fabricated in different geometrical systems: thin films, monoliths and planar waveguides we herein limit ourselves to the monoliths. The effective role of tin dioxide as luminescence sensitizer for Er3+ ions is confirmed by spect...
Renewable and Sustainable Energy Reviews, 2017
Abstract Plasmonic structures for light manipulation at sub-wavelength scale have received great ... more Abstract Plasmonic structures for light manipulation at sub-wavelength scale have received great interest in the field of photovoltaic (PV) solar cells for their potential to significantly enhance the cell's efficiency. The performance of any solar cell is determined by the capability to absorb incoming light and produce electric charges, which, in turn, has a number of limiting factors. One is related to the ever-reducing size and acceptance angle of the active region. Another is the limited spectral sensitivity of the active material, which cannot make use of significant parts of the solar spectrum. Correspondingly, the energy harvesting may be improved in two ways, namely by adopting light trapping schemes and by exploiting spectral modification processes to shift frequencies of the solar spectrum, which are initially not absorbed, into the region of maximum absorption of the cell. Plasmonic nanoparticles (NPs) can give a significant boost to both these aspects, by scattering and concentrating the electromagnetic field into the active region of the device, and by doing that within specific spectral regions, which can be properly tuned by optimizing the size, shape, distribution of the plasmonic NPs, and by choosing the right surrounding medium. During the last ten years, many papers have been published on very specific issues, but also on general properties of plasmonics applied to solar cells, with a strong increase between 2006 and 2012, followed by a period of significant, but stable, literature productivity. Given these premises, an organized and schematic summary of the main strategies and of the recent results on the field is given in this review, where different plasmonic approaches are compared and discussed, also by recalling specific examples from the literature and providing a few key conclusions to understand the main aspects and the future perspectives of the field.
Scientific Reports, 2017
We report on nonlinear optical effects on phoxonic cavities based on hollow whispering gallery mo... more We report on nonlinear optical effects on phoxonic cavities based on hollow whispering gallery mode resonators pumped with a continuous wave laser. We observed stimulated scattering effects such as Brillouin and Raman, Kerr effects such as degenerated and non-degenerated four wave mixing, and dispersive wave generation. These effects happened concomitantly. Hollow resonators give rise to a very rich nonlinear scenario due to the coexistence of several family modes. Stimulated Brillouin Scattering (SBS) in devices that highly confine light has attracted much attention in the last years . Among these devices, whispering gallery mode resonators (WGMR) have shown to be an excellent enhancement platform to study light matter interactions such as stimulated nonlinear optical processes and frequency generation. WGMR can confine light in very small volumes and posses ultra high Q-factors . The high photon density and their long lifetime ensure an efficient nonlinear light-matter interaction and give the possibility to study, from the fundamental point of view, Kerr interactions and stimulated phenomena like Raman and Brillouin 4,7-10 . SBS is an inelastic scattering process that results from the coherent interaction of light photons and acoustic phonons. The photon-phonon interaction is enhanced due to the overlap of both waves inside the WGMR, which acts as a dual photonic-phononic or phoxonic cavity . SBS, like stimulated Raman scattering (SRS), is a pure gain process and therefore, is automatically phase-matched. SBS has one of the largest gain coefficients but a small gain bandwidth . The narrow bandwidth would require that the Brillouin frequency shift equals the free spectral range (FSR) of the WGMR, placing stringent conditions on WGMR geometries . However, because of their eccentricity, in microbubble resonators this condition can be bypassed by using high order modes with vertical FSR smaller than the fundamental FSR . WGMR have been fabricated in a large variety of geometries and materials 21 . One of the latest geometries, the microbubble 22 or microbottle 23 has attracted much attention lately. Another advantage of the hollow structure of the bubble is that it allows flowing liquids or gas inside the WGMR, giving the possibility of tailoring the dispersion 24 or resonant properties . The majority of applications of microbubble resonators (MBR) are in the sensing field: refractometers 27,28 , mass sensors 29 , temperature 30 , pressure 31 and viscosity 32 . More recently, MBR have been used for fundamental studies in nonlinear optics and lasing 34 . In refs 26 and 33, the MBR used have very thick walls (of about 10-15 μ m) compared to ours (2-4 μ m), and the SBS and FWM observed in ref. 26 are not either simultaneous or the SBS line does not act as a pump in SBS-uncoupled FWM processes. Regarding the efficiency of the SBS lines, we are presenting results that are at least more than one order of magnitude higher. Our objective here is to demonstrate that MBR with thin walls are more efficient for SBS excitation and show a richer nonlinear scenario than other on-chip WGMR. Summarizing, we experimentally show that MBR of large diameters and thin walls (2 μ m) can enhance the Brillouin lasing efficiency, cascading up to the 4 th order in both forward and backward directions. Even and odd orders are observed in both directions, but showing different lasing efficiency (even (odd) orders are more efficient in forward (backward) direction). We also report on degenerated four wave mixing (FWM) from the Brillouin laser, non-degenerated FWM, dispersive waves and SBS and SRS appearing simultaneously. Clear advantages of MBR is that their modal structure allow satisfying the different phase matching and the multi-resonant conditions required by the above mentioned NLO effects, firstly; and the hollow structure also consents to tailor dispersion or resonant conditions easily.
Crystals, 2017
Glass photonics are widespread, from everyday objects around us to high-tech specialized devices.... more Glass photonics are widespread, from everyday objects around us to high-tech specialized devices. Among different technologies, sol-gel synthesis allows for nanoscale materials engineering by exploiting its unique structures, such as transparent glass-ceramics, to tailor optical and electromagnetic properties and to boost photon-management yield. Here, we briefly discuss the state of the technology and show that the choice of the sol-gel as a synthesis method brings the advantage of process versatility regarding materials composition and ease of implementation. In this context, we present tin-dioxide-silica (SnO 2 -SiO 2 ) glass-ceramic waveguides activated by europium ions (Eu 3+ ). The focus is on the photorefractive properties of this system because its photoluminescence properties have already been discussed in the papers presented in the bibliography. The main findings include the high photosensitivity of sol-gel 25SnO 2 :75SiO 2 glass-ceramic waveguides; the ultraviolet (UV)-induced refractive index change (∆n ~-1.6 × 10 -3 ), the easy fabrication process, and the low propagation losses (0.5 ± 0.2 dB/cm), that make this glass-ceramic an interesting photonic material for smart optical applications.
Fluids, 2017
Since the pioneering work of Taylor, the analysis of flow regimes of incompressible, viscous flui... more Since the pioneering work of Taylor, the analysis of flow regimes of incompressible, viscous fluids contained in circular Couette systems with independently rotating cylinders have charmed many researchers. The characteristics of such kind of flows have been considered for some industrial applications. Recently, Taylor-Couette flows found an innovative application in the production of optical fiber nanotips, to be used in molecular biology and medical diagnostic fields. Starting from the activity of Barucci et al., the present work concerns the numerical analysis of a Taylor-Couette system composed by two coaxial counter-rotating cylinders with low aspect ratio and radius ratio, filled with three stratified fluids. An accurate analysis of the flow regimes is performed, considering both the variation of inner and outer rotational speed and the reduction of fiber radius due to etching process. The large variety of individuated flow configurations provides useful information about the possible use of the Taylor-Couette system in a wide range of engineering applications. For the present case, the final objective is to provide accurate information to manufacturers of fiber nanotips about the expected flow regimes, thus helping them in the setup of the control process that will be used to generate high-quality products.
Sensors (Basel, Switzerland), Jan 28, 2017
This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a... more This study analyzes the capabilities of a LiNbO₃ whispering gallery mode microdisc resonator as a potential bolometer detector in the THz range. The resonator is theoretically characterized in the stationary regime by its thermo-optic and thermal coefficients. Considering a Q-factor of 10⁷, a minimum detectable power of 20 μW was evaluated, three orders of magnitude above its noise equivalent power. This value opens up the feasibility of exploiting LiNbO₃ disc resonators as sensitive room-temperature detectors in the THz range.
Sensors, 2016
The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosen... more The design of Whispering Gallery Mode Resonators (WGMRs) used as an optical transducer for biosensing represents the first and crucial step towards the optimization of the final device performance in terms of sensitivity and Limit of Detection (LoD). Here, we propose an analytical method for the design of an optical microbubble resonator (OMBR)-based biosensor. In order to enhance the OMBR sensing performance, we consider a polymeric layer of high refractive index as an inner coating for the OMBR. The effect of this layer and other optical/geometrical parameters on the mode field distribution, sensitivity and LoD of the OMBR is assessed and discussed, both for transverse electric (TE) and transverse magnetic (TM) polarization. The obtained results do provide physical insights for the development of OMBR-based biosensor.
2016 IEEE 16th International Conference on Environment and Electrical Engineering (EEEIC), 2016
In this paper we present a nanosatellite for cosmic ray detection. The main payload is constitute... more In this paper we present a nanosatellite for cosmic ray detection. The main payload is constituted by a particle detector. A second payload may be constituted by the control unit of an innovative type of photovoltaic cells that will have a twofold objective: test the performances and provide additional power to the nanosatellite. It is under investigation the possibility of using a propulsion module in case, later in the phase A study, will be realized an additional advantage to transfer the orbit from circular (the most commonly used in commercial launches) to elliptical. Launch opportunities are relatively frequent since the market of nanosatellites is rapidly increasing and launch providers are including those into their business. The service module will provide power, radio communication, on board computer and attitude control. Thermal control will be achieved passively by the use of paint with proper absorptivity and emissivity and thermal blankets. The problem with this approach is that paint and solar cells have to share the external surface of the nanosatellite, unless a deployable solar array will be included. Besides being an important carrier of information from sources in our galaxy and beyond, cosmic rays may have influence in the terrestrial environment as well as in the technologically advanced human activities, such as orbiting satellites and human spaceflight. The objective of the satellite will be twofold: to train students with hands-on activities through all the phases of a space mission and the development of particle detectors as well as to study the properties of the detected cosmic rays with possible implication in fundamental physics and Earth environmental studies.
Sensors (Basel, Switzerland), Jan 31, 2016
This work shows the improvements in the sensing capabilities and precision of an Optical Microbub... more This work shows the improvements in the sensing capabilities and precision of an Optical Microbubble Resonator due to the introduction of an encaging poly(methyl methacrylate) (PMMA) box. A frequency fluctuation parameter σ was defined as a score of resonance stability and was evaluated in the presence and absence of the encaging system and in the case of air- or water-filling of the cavity. Furthermore, the noise interference introduced by the peristaltic and the syringe pumping system was studied. The measurements showed a reduction of σ in the presence of the encaging PMMA box and when the syringe pump was used as flowing system.
2016 Photonics North (PN), 2016
We report efficient generation of nonlinear phenomena related to third order optical non-linear s... more We report efficient generation of nonlinear phenomena related to third order optical non-linear susceptibility χ(3) interactions in resonant silica microspheres and microbubbles. The interactions here reported are: Third harmonic Generation (THG), Stimulated Raman Scattering (SRS), and four wave mixing processes comprising Stimulated Anti-stokes Raman Scattering (SARS) and comb generation. Unusually strong anti-Stokes components and extraordinarily symmetric spectra have been observed. Resonant SARS and SRS corresponding to different Raman bands were also observed. The lack of correlation between stimulated anti-stokes and stokes scattering spectra indicates that the signal has to be resonant with the cavity.
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Papers by Giancarlo C. Righini