Energy Test of an Efficient Random Laser Emission Collecting System

The problem of light collection in random lasers (RLs) is addressed. As the radiation emitted by this system is Lambertian due to its spatial incoherence, a device based on an ellipsoidal revolution mirror is designed, developed, and tested in order to optimize the harvesting of the radiation emitted by the RL. The system provides a simple injection procedure of the emitted energy at the entrance of a multimode optical fiber. The results obtained show that the device has a net energy efficiency of 35%, close to the theoretically expected one. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported LicenseThis work was supported by the Basque Government PIBA2018-24, Spanish Government MINECO under Project No. MAT2017-87035-C2-2-P (AEI/FEDER, UE), and Basque Country University (UPV/EHU) PPG17/07 and GIU17/01

Role of Eu2+ and Dy3+ Concentration in the Persistent Luminescence of Sr2MgSi2O7 Glass-Ceramics

In this study, glass-ceramics based on Sr2MgSi2O7 phosphor co-doped with Eu/Dy were obtained from the sintering and crystallisation of glass powders. The glasses were melted in a gas furnace to simulate an industrial process, and the dopant concentration was varied to optimise the luminescence persistence times. The doped parent glasses showed red emission under UV light excitation due to the doping of Eu3+ ions, while the corresponding glass-ceramics showed persistent blue emission corresponding to the presence of Eu2+ in the crystalline environment. The dopant concentration had a strong impact on the sintering/crystallisation kinetics affecting the final glass-ceramic microstructure. The microstructures and morphology of the crystals responsible for the blue emission were observed by scanning electron microscopy–cathodoluminescence. The composition of the crystallised phases and the distribution of rare-earth (RE) ions in the crystals and in the residual glassy phase were determined by X-ray diffraction and energy dispersive X-ray analysis. The emission and persistence of phosphorescence were studied by photoluminescence.This research was funded by MICINN under projects PID2020-115419GB-C-21/C-22/AEI/ 10.13039/501100011033 and PID2019-107439GB-I00 and by the project PIE-CSIC 201960E016. And The APC was funded by PID2020-115419GB-C-21/C-22/AEI/ 10.13039/501100011033

Structure and luminescent properties of Sm/Dy-doped Sr2MgSi2O7 glass-ceramics

Sm3+-doped and Sm3+/Dy3+ codoped SiO2–SrO–MgO glasses were prepared by conventional melt quenching and Sr2MgSi2O7 based glass–ceramics from sintering and crystallization of the glass powders. The thermal, structural, and optical properties of the glasses and glass–ceramics were investigated as a function of the dopant concentration. The optical characterization includes the photoluminescence spectra and the lifetimes of the 4G5/2 (Sm3+) and 4F9/2 (Dy3+) excited states. In Sm3+ single-doped samples, the emission intensity increases up to a concentration of 0.3 mol% Sm3+ ions and then decreases due to nonradiative energy transfer processes. The emission spectra in the glass–ceramics show a more resolved structure and higher intensity compared to the glass samples, suggesting a different and crystalline environment for the Sm3+ ions. The non-radiative processes also influence the experimental decays of the glass samples which deviate from a single exponential with lifetimes decreasing as Sm3+ concentration increases. The emission and excitation spectra of the codoped samples do not show significant energy transfer between Sm3+ and Dy3+ ions. Different emitting colors can be obtained in the codoped glasses by changing the excitation wavelength. The studied glass–ceramics could be applied as enamels on ceramic or metallic substrates.Funding from MICINN under projects PID2020-115419GB-C-21/C-22/AEI/ 10.13039/501100011033, PID2019-107439GB-I00 and PIE-CSIC 201960E016 is acknowledged

Novel Sol-Gel Route to Prepare Eu3+-Doped 80SiO2-20NaGdF4 Oxyfluoride Glass-Ceramic for Photonic Device Applications

Oxyfluoride glass-ceramics (OxGCs) with the molar composition 80SiO2-20(1.5Eu3+: NaGdF4) were prepared with sol-gel following the “pre-crystallised nanoparticles route” with promising optical results. The preparation of 1.5 mol % Eu3+-doped NaGdF4 nanoparticles, named 1.5Eu3+: NaGdF4, was optimised and characterised using XRD, FTIR and HRTEM. The structural characterisation of 80SiO2-20(1.5Eu3+: NaGdF4) OxGCs prepared from these nanoparticles’ suspension was performed by XRD and FTIR revealing the presence of hexagonal and orthorhombic NaGdF4 crystalline phases. The optical properties of both nanoparticles’ phases and the related OxGCs were studied by measuring the emission and excitation spectra together with the lifetimes of the 5D0 state. The emission spectra obtained by exciting the Eu3+-O2− charge transfer band showed similar features in both cases corresponding the higher emission intensity to the 5D0→7F2 transition that indicates a non-centrosymmetric site for Eu3+ ions. Moreover, time-resolved fluorescence line-narrowed emission spectra were performed at a low temperature in OxGCs to obtain information about the site symmetry of Eu3+ in this matrix. The results show that this processing method is promising for preparing transparent OxGCs coatings for photonic applications.This project received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement no. 739566. The authors acknowledge financial support from MICINN under projects PID2020-115419GB-C21-C22/AEI/10.13039/501100011033 and University of the Basque Country (Project GIU21/006)

Er/Yb co-doped LiYF4 transparent oxyfluoride glass-ceramics with up-conversion optical properties

Transparent oxyfluoride glass-ceramics doped with rare earth ions (RE3+) are promising materials for photoluminescence up- and down-conversion. In this study, glass compositions within the system 40SiO2–25Al2O3–18Li2O–7LiF–10YF3 (mol.%) doped with ErF3 and codoped with ErF3/YbF3 were prepared by melt-quenching method and subjected to thermal treatment at temperatures above glass transition (Tg + 35 °C) for long dwell times to obtain the corresponding glass-ceramics. The formation of LiYF4 and LiAlSiO4 nanocrystals was confirmed by XRD analysis after thermal treatment at 540 °C for 20 h. The increase in the treatment time up to 80 h resulted in the enhancement of the UC luminescence yield and the decrease of the Red to Green ratio (R/G) emission intensity. Due to the nano-sized crystals, the glass-ceramic products were transparent (%T) in near-infrared (NIR) and visible spectral region with %T remaining approximately 85% and 75%, respectively, after 20 h treatment. However, the visible window transparency reduced, with %T dropping to around 50% after 80 h due to the increase in crystal size and crystalline fraction. The influence of Yb3+ co-doping on the up-conversion (UC) luminescence has been investigated in the glass-ceramics and compared to the parent glasses, confirming that Yb3+ ions were also a key factor for facilitating up-conversion via energy transfer (ET), leading to a greater luminescence yield than for Er3+ single doped glass-ceramics and tuning of R/G intensities.This work was supported by a part of the European Union’s Horizon 2020 research and innovation program [grant number 739566]; the MICINN [grant number PID2020-115419GBC-21/C-22/AEI/10.13039/501100011033]; and the project VEGA 1/0476/22

Non-Linear Optical Properties of Er3+–Yb3+-Doped NaGdF4 Nanostructured Glass–Ceramics

Transparent oxyfluoride glass–ceramics containing NaGdF4 nanocrystals were prepared by melt-quenching and doped with Er3+ (0.5 mol%) and different amounts of Yb3+ (0–2 mol%). The selected dopant concentration the crystallization thermal treatments were chosen to obtain the most efficient visible up-conversion emissions, together with near infrared emissions. The crystal size increased with dopant content and treatment time. NaGdF4 NCs with a size ranging 9–30 nm were obtained after heat treatments at Tg + 20–80 °C as confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Energy dispersive X-ray analysis shows the incorporation of rare earth ions into the NaGdF4 nanocrystals. Near-infrared emission spectra, together with the up-conversion emissions were measured. The optical characterization of the glass–ceramics clearly shows that Er3+ and Yb3+ ions are incorporated in the crystalline phase. Moreover, visible up-conversion emissions could be tuned by controlling the nanocrystals size through appropriated heat treatment, making possible a correlation between structural and optical properties.This research was funded by Spanish National projects MAT2017-87035-C2-1-P/2-P (AEI/FEDER, UE), Basque Country University PPG17/07 and GIU17/014 and Basque Government PIBA2018-24. This study is part of the dissemination activities of project FunGlass. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 739566. This study was also created in the frame of the project Centre for Functional and Surface Functionalized Glass (CEGLASS), ITMS code is 313011R453, operational program Research and innovation, co-funded from European Regional Development Fund

Crystallization Process and Site-Selective Excitation of Nd3+ in LaF3/NaLaF4 Sol–Gel-Synthesized Transparent Glass-Ceramics

In this study, transparent oxyfluoride glass-ceramics (GCs) with NaLaF4 nanocrystals (NCs) were prepared by the sol–gel method for the first time. Three different molar ratios of La(CH3COO)3/Na(CH3COO) were used to obtain the GCs, which were sintered at 450, 550 and 650 °C for 1 min. X-ray diffraction (XRD) was employed to follow the evolution of the xerogel during the heat treatments and to study crystal growth for the three temperatures. In all cases, the LaF3 crystalline phase was present, but crystallization of NaLaF4 was only promoted at 650 °C. Thermogravimetric and thermodifferential analysis (TGA-DTA) and Fourier transform infrared spectroscopy (FTIR) were used to analyze the crystallization process. High-resolution transmission electron microscopy (HRTEM) was employed to confirm NaLaF4 crystallization and determine the size distribution. The incorporation of Nd3+ ion into NaLaF4 and LaF3 nanocrystals was confirmed by site-selective emission and excitation spectra. The Nd3+ emission intensities in both phases depend not only on the NaLaF4/LaF3 ratio but also on their emission efficiencies.The authors acknowledge financial support from MINECO under projects MAT2017-87035-C2-1-P/-2-P (AEI/FEDER, UE), and Basque Government PIBA2018-24. This article is a part of the dissemination activities of the project FunGlass, which has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 739566

Spark plasma sintering and optical properties of Tm3+ and Tm3+ /Yb3+ doped NaLaF4 transparent glass-ceramics

Tm3+ doped oxyfluoride glass-ceramics (GCs) containing NaLaF4 nanocrystals (NCs) have been obtained by spark plasma sintering (SPS). First, the precursor glasses were melted and then milled and sieved to a suitable particle size. Glass powder pellets were sintered by spark plasma sintering under vacuum conditions. The SPS processing parameters (temperature, pressure, and holding time) were optimized to obtain transparent glass-ceramics. The times of SPS processing are considerably shorter compared with those for the preparation of these GCs by conventional thermal treatment. All glass-ceramics contain nanocrystals of the β- NaLaF4 phase with an average crystal size of 20 nm, but the more highly doped samples (2Tm3+ and 0.5Tm3+/2Yb3+) show evidence of the presence of another phase corresponding with α-NaLaF4. The luminescence properties of the near infrared (NIR) emissions of Tm3+ for different concentrations reveal the presence of concentration quenching of the 3H4 and 3F4 levels. The analysis of the decay from the 3H4 level with increasing concentration is consistent with a dipole-dipole quenching process assisted by energy migration, whereas the self-quenching of the 3F4→3H6 emission can be attributed to fast diffusion. Energy transfer between Yb3+ and Tm3+ ions is confirmed by the NIR and upconverted (UC) emissions after Yb3+ excitation at 975 nm. No UC emission is observed under 791 nm excitation of Tm3+ ions.Funding from MICINN under project PID2020–115419 GB-C-21/C-22/AEI/10.13039/501100011033 is acknowledged. This paper is part of the dissemination activities of project FunGlass. This project has received funding from the European Union´s Horizon 2020 research and innovation program under grant agreement No 739566

Transparent Glass-Ceramics Produced by Sol-Gel: A Suitable Alternative for Photonic Materials

Transparent glass-ceramics have shown interesting optical properties for several photonic applications. In particular, compositions based on oxide glass matrices with fluoride crystals embedded inside, known as oxyfluoride glass-ceramics, have gained increasing interest in the last few decades. Melt-quenching is still the most used method to prepare these materials but sol-gel has been indicated as a suitable alternative. Many papers have been published since the end of the 1990s, when these materials were prepared by sol-gel for the first time, thus a review of the achievements obtained so far is necessary. In the first part of this paper, a review of transparent sol-gel glass-ceramics is made focusing mainly on oxyfluoride compositions. Many interesting optical results have been obtained but very little innovation of synthesis and processing is found with respect to pioneering papers published 20 years ago. In the second part we describe the improvements in synthesis and processing obtained by the authors during the last five years. The main achievements are the preparation of oxyfluoride glass-ceramics with a much higher fluoride crystal fraction, at least double that reported up to now, and the first synthesis of NaGdF4 glass-ceramics. Moreover, a new SiO2 precursor was introduced in the synthesis, allowing for a reduction in the treatment temperature and favoring hydroxyl group removal. Interesting optical properties demonstrated the incorporation of dopant ions in the fluoride crystals, thus obtaining crystal-like spectra along with higher efficiencies with respect to xerogels, and hence demonstrating that these materials are a suitable alternative for photonic applications.This work was supported by MINECO under projects MAT2013-48246-C2-1-P, MAT2013-48246-C2-2-P, and MAT2017-87035-C2-1-P/-2-P (AEI/FEDER, UE) and Basque Country Government IT-943-16 and PPG17/07. The authors are grateful for access to the Spanish Beamline (SpLine) at the ESRF facilities in Grenoble to perform experiments MA-3350 and 25-01-1014. Jose Joaquin Velazquez also acknowledges MINECO for Grant FPDI-2013-16895

Random Laser Action in Nd:YAG Crystal Powder Jon

This work explores the room temperature random stimulated emission at 1.064 mu m of a Nd:YAG crystal powder (Neodymium-doped yttrium aluminum garnet) in a very simple pump configuration with no assistance from an internal mirror. The laser threshold energy as a function of pump beam area and pump wavelength has been measured, as well as the temporal dynamics of emission pulses. The absolute energy of stimulated emission and the absolute laser slope efficiency have been measured by using a method proposed by the authors. The results show a surprising high efficiency that takes the low Nd3+ ion concentration of the crystal powder into account.This work has been partially supported by the Spanish Ministerio de Economia y Competitividad, MINECO project MAT2013-48246-C2-2-P, the Basque Country Government, project IT-943-16, and Saiotek S-PE11UN072 and S-PC13UN017