Design, Preparation and Optical Properties of Bi-doped Super-Broadband Near-infrared Luminescent Materials
|Keywords||broadband near-infrared luminescence bismuth ions polycrystal glass thin-film external field upconversion luminescence|
The fast development of computer network and telecommunication technology has put forward higher demand in the speed and capacity of optical fiber transmission technology in this century. With the development of optical fiber fabrication technology, the optical fiber communication windows has covered the range from1.2to1.7μm.Whereas,due to the4f-4f forbidden transition of rare-earth ions with narrow emission band, traditional rare-earth ions-doped optical fiber amplifiers and lasers cannot satisfy the demand of modern optical fiber communication which needs optical fiber amplifiers with much higher optical gain, and much more efficienct optical fiber lasers in the optical fiber communication windows.Recently, Bismuth (Bi)-doped glasses have attracted significant attention due to their ultra-broad near-infrared (NIR) luminescence in the region from1000to1700nm with full width at half maximum (FWHM) over300nm which covers the whole windows of optical fiber communications.They have long emission lifetime of several hundred microsenconds.It is expected that Bi-doped NIR luminescent glasses can overcome the shortage of rare-earth ions-doped optical fiber amplifiers and lasers, and show promise as a new generation of ultra-broadband NIR optical fiber gain and laser media.This thesis systematically introduces the properties and mechanisms of Bi active centers,the investigation and current problems on Bi-doped NIR luminescent materials and devices, and makes a prospect on its future development. The effect of external field on the state and properties of Bi ions in Bi-doped crystals and glasses was investigated. We proposed several efficient approaches to realize tunable NIR luminescene in Bi-doped materials.The mechanisms were investigated in depth. Moreover, we developed some new methods to fabricate Bi-doped ultra-broadbad NIR luminescent thin-films and made a research on the luminescent mechanism.The sensitization functions of Bi NIR active centers to rare-earth ions were also investigated, eg. the enhanced upconversion luminescence of rare-earth ions by the sensitization of Bi NIR active centers.Thermal analysis (DTA),X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), transmission electronic microscope (TEM), Raman scattering spectroscopy, electron spin resonance (ESR), absorption/diffused reflection spectra, photoluminescence spectroscopy (PL), time resolved optical measurements were all used to study the structure and luminescence properties of the materials.We obtained a series of important conclusions and innovative results of practical significance.These results are benefit to the designs and applications of Bi-doped ultra-broadband NIR luminescent materials and devices.They will also provide a scientific reference for the research on the other luminescent p block elements.The main results and conclusions are as follows:1. The effect of external field on the state and properties of Bi ions in Bi-doped crystals and glasses were investigated:(1)γ-ray irradiation was applied to investigate the state and luminescent properties of Bi active centers in germanate polycrystals. y-ray unirradiated Y4GeO8crystal shows visible luminescence at510nm, while broadband NIR emission at1155nm with FWHM over300nm is observed in Bi-doped Y4GeO8crystal after y-ray irradiation. It is confirmed that Bi5+and Bi+are formed in the γ-ray irradiated crystals.The NIR luminescence is considered to be from3P1→3Po transition of BiT ions.NIR luminescence can be bleached with the reviving of the visible luminescence after thermal annealing at high temperature owing to the release of holes and free electrons from trapped center and electron-hole recombination.(2) The state of Bi ions and the crytallization behavior of glasses were controlled by heat-treatment process to facilitate the tuning of NIR luminescence. With the increase of the heat-treatment temperature at the first stage, Bi ions are reduced to Bi atoms and formed into Bi metallic colloids,leading to the quenching of the NIR emission. As the heat-treatment temperature continues going up,Bi metallic colloid melts and are oxidized into BiT ions and other Bi NIR active centers leading to the reviving of NIR. emission. The crytallization will also result in the increase of the relative content of Al2O3in the glassy phase, inducing the decrease of optical basicity of the glassy phase, which will be of benefit to the formation and diffusion of Bi NIR active centers.Further increasing the temperature,the precipitation of large amount of crystals will hinder the excitation of Bi NIR active centers.Moreover, the concentration quenching of Bi NIR active centers in the glassy phase will also result in the decrease of NIR emission intensity. The formation of Bi metallic colloid as nucleation agent promotes the nucleation and crystallization of MgGeO3and GeO2nanocrystals in glasses. The results help to the design of the fabrication process of Bi-doped optical fiber.2. Three approaches were proposed to realize tunable luminescence in Bi-doped glasses:(1) The partial substitution of fluoride for oxide in Bi-doped silicate glasses results in the decrease of the optical basicity and phonon energy of the glass matrix, and the reduction of the crystal field around Bi active centers simultaneously. Thus, some Bi3+and Bi2+ions in glasses are converted into low valence state Bi ions.This one-pot approach can lead to an increase of the intensity and lifetime of the near-infrared luminescence and blue-shift of the near-infrared emission peaks with the decrease of the intensity of the visible luminescence.(2) Redox reaction and sensitizor were used to modulate the luminescence.In Bi-doped fluoride silicate glasses,we used the oxidizing action of Ag+to oxide Bi metallic clusters and colloids to form Bi NIR active centers.At the same time,by controlling the formation of Ag+and molecular-like, nonplamonic Ag species (Ag2+and Ag42+),and the energy transfer from AgT and molecular-like Ag species, Bi3+and Bi2+to Bi NIR active centers,the intensity of NIR luminescence under the excitation from300to980nm are all enhanced.(3)In Bi-doped germanate glasses,by controllig Bi doping concentration, the melting temperatures and atmospheres of glasses, the concentration, energy transfer and chemical equilibrium of different Bi NIR active centers can be modulated leading to the tunable NIR luminescence in1080～1330nm. Three Bi active centers, namely Bi+T ions, Bi0atoms and Bi cluster ions,such as Bi22-dimer ions,were proposed as the probable active centers for the NIR luminescence at around1100,1260and1450nm, respectively. Compared to that of glasses,inertia or reducing atmosphere is more beneficial to the NIR emission of glasses with low Bi doping concentration.3. Ultra-broadband NIR luminescence has firstly been realized in Bi-doped germanate thin-films prepared by pulsed laser deposition for the first time:(1) The deposited germanate glass thin-films are compatible with various types of substrates, including conventional dielectrics (LaAlO3,silica) and semiconductors (Si, GaAs). The films show ultra-broadband NIR emission in the range of1000～2300nm. The emission peak position of the films can be finely tuned by changing oxygen partial pressure during the thin-film deposition, while the excitation wavelength locates from ultra-violet to near-infrared regions. The emission peak position of Bi-doped film prepared under certain oxygen partial pressure remains unchanged as the excitation wavelength changes.Most Bi NIR active centers in films may be consisted by Bi clusters.The emission at longer wavelength comes from Bi species of lower valence states compared to that at shorter wavelength.(2) Bi-doped oxyfluoride germanate thin films have emission in1000～2400nm. Systematic investigation reveals that the origin of the luminescence could be ascribed to Bi clusters.With the sensitization of Bi near-infrared active centers, enhanced broadband～2μm luminescence of Ho3+was realized in Bi/Ho co-doped film, and the energy transfer efficiency can reach up to72.6%.These results may provide promise to realize planar waveguide laser in the NIR region for the integrated optics.4. Bi-doped oxyfluoride germanate glasses show broadband NIR absorption and luminescence properties.With the sensitization by Bi NIR active centers, enhanced two-photon upconversion luminescence in green and red bands is observed in Bi/Ho co-doped glass.The upconversion excitation bands at750and970nm show a FWHM of20and45nm, respectively.Energy transfer from sensitizers, the excited-state absorption, and phonon-coupled absorption of Ho3+jointly contribute to the enhanced upconversion luminescence. The energy transfer efficiency of Bi NIR active centers is calculated to be65.3%.Our approach provides an efficient methodology to broaden the excitation bandwidth of upconversion luminescent materials, which may have the potential for promising application in solar cells.