Glasses may be prepared by sol-gel processing over a wide range of compositions and thick multilayer deposits may be used as waveguides for integrated optics. Doping these layers with rare-earth (RE) ions enables the fabrication of active devices for optical amplifiers; the incorporation of these ions into nanocrystallites offers possibilities for increased dopant concentration without fluorescence quenching, improved spectroscopic performance and high quantum yields. Rare-earth (RE) ions such as erbium (Er3+), ytterbium (Yb3+), neodymium (Nd3+), thulium (Tm3+), holmium (Ho3+) and praseodymium (Pr3+) have been widely used in optical applications and cover a range of wavelengths ranging from UV-visible to the near infrared. This chapter includes basic principles of fluorescence in RE doped glasses, fluorescence lifetimes, quantum yields and Judd-Ofelt analysis. A few information is given about the preparation and characterization of glasses, thin films and glass-ceramics (nanocrystallites embedded in glass matrix) prepared by sol-gel processing. The growth of nanocrystals in glassy sol-gel films through suitable heat treatments can avoid the influence of high phonon energy of silica glasses. The characterization of such materials can be evaluated by optical techniques, namely UV-Visible, FTIR, among other additional techniques that include Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM).
- rare earths
1. Light, glasses, and rare earths
The need for high-speed data associated with the advance of telecommunication systems by optical transmission and fiber-optic connections has contributed to extend the optical regime to integrated circuits ; currently, waveguides and other devices necessary for a suitable operation of integrated optical circuits in transmission systems are increasingly being investigated. In the past, coaxial (electrical) cables were used for analog and digital signal transmission over long-distance communications. But the strong attenuation of the signal (about 30 dB/km at 400-MHz cable)  imposed the use of regenerative repeaters along the entire route. Fiber optic is now an easy way to transmit information, and the next generation of long-range communications will rely on optical amplifiers to the detriment of regenerative electronic repeaters. The discovery of new glasses of exceptional optical transparency and reduced attenuation to values lower than 1 dB/km boosted the great evolution of optical communication over the last years. Inorganic glasses have been used as optical materials for a long time due mainly to its high transparency in the visible and adjacent, ultraviolet (UV) and near-infrared (NIR) ranges. However, they do not exhibit electronic transitions in this region. For these transitions to take place, controlled introduction of optically active ions is used; therefore, the optical properties of rare-earth (RE) ion-doped inorganic glasses emerged in the field of materials physics. New optical materials suitable for the development of photonic devices based on RE-doped crystal or glassy hosts have thus attracted significant scientific and technological interest. Such wide technological applications are based on the interaction of light with matter where the fluorescent behavior is essential . The most widely used RE ions in glass are erbium (Er3+) , ytterbium (Yb3+), and neodymium (Nd3+). For instance, Er3+-doped silica fiber is extensively used in optical communication; Yb3+-doped silica fiber is used in engineering materials processing, and Nd3+ doped is applied in glass lasers used for inertial confinement fusion (ICF). The invention of erbium (Er3+)-doped fiber optic amplifiers in 1987, the so-called EDFA’s, “
Among the different glass, oxide and non-oxide systems, transparent glass-ceramics (GC) offer remarkable features to the field of photonics. Glass-ceramics are a class of hybrid materials consisting of nanocrystallites embedded in a glass matrix. Transparency is a key property, in particular for dielectric optical waveguides and optical fibers, and the effect of the nanocrystals activated by RE ions on the spectroscopic properties overcomes largely those of RE ions in a glass [20, 21]. Moreover, the transparent glass-ceramics still retains the properties of a glass and can be processed and shaped by techniques used for glasses. Among the various techniques used to fabricate optical planar waveguides and photonic devices, sol-gel process with top-down and bottom-up approaches demonstrated to be a suitable route to do it [5, 10, 22]. A high doping level of RE ions can be achieved by sol-gel process. Despite the presence of hydroxyl groups (OH) of inherent character in sol-gel silicate glasses, which are extremely effective at quenching excited RE ions [3, 23], even for a few hundred ppm of OH, sol-gel is a powerful technology in the development of glasses containing RE for various types of applications.
2. Sol-gel glass: the solid with a liquid background
The rapid cooling of some molten materials (e.g., SiO2, BeF2, B2O3, As2O3, P2O5, and GeO2)  has become the traditional way to make glass, an inorganic material of fusion that has cooled to an amorphous solid without crystallization. So, unlike crystals, a glass lacks long-range order. The crystallization can be avoided when the cooling is fast, and so the viscosity of the molten liquid increases so much that the atomic
Sol-gel synthesis is nowadays used to produce glasses with a variety of compositions that are technically difficult or even impossible to produce by melting. Some binary and multicomponent oxide systems exhibit liquid-liquid immiscibility and tend to phase separate in a short range of compositions and temperatures and thus can be successfully obtained by sol-gel. This process does indeed offer great possibilities to tailor the preparation of highly homogeneous glasses, such as RE-doped glasses. When RE ions are hosted by glasses, they exchange the network cation or they act as network modifiers . It is therefore expected that the RE ions will favor non-bridging oxygen sites in the gel as the solvent is expelled. In addition, sol-gel method also offers the possibility of obtaining glassy coatings on silicon substrates to achieve planar waveguides . A method of depositing thick sol-gel coatings is illustrated in Figure 2a.
Planar waveguides have a central rectangular region of higher refractive index
The refractive index in thin films containing SiO2 (
3. Basic principles of fluorescence: specificities of sol-gel glasses
Among usual forms of light interaction with matter (absorption, reflexion, etc.), fluorescence is a very special one that can be regarded as a source of light, or, in other words, a phenomenon in which a material radiates light (emission) at longer wavelength after a brief interval (termed fluorescence lifetime), as a result of absorption of shorter wavelength. Fluorescence in optical glass is generated by the presence of RE ions. When these active ions are directly excited by incoming energy, the electron on it absorbs energy and is raised to an excited state (Figure 3a). The excited state returns to the ground state by emission of light of longer wavelength (Figure 3b). This behavior (absorption by short wave light, emission of longer wave light) is named “
The processes that occur between the absorption and fluorescence emission of light are generally illustrated by the Jablonski  diagram. Indeed, Figure 3a shows a very simplified Jablonski diagram where the transitions between states are represented as vertical lines.
Following light absorption, emission usually occurs. Prior to emission, a rapid relax to the lowest vibrational level takes place, called vibrational relaxation, which is a non-radiative transition . This process yields a relaxed excited state from which fluorescence emission originates. The fluorescence results from combined rates of radiative and non-radiative processes from a metastable excited state to the ground state (e.g., Er3+ transition 4I13/2 → 4I15/2 at 1550 nm). Radiative decay (
Glasses with the largest quantum yields, approaching unity, display emissions with major fluorescence intensity; the fluorescence lifetime (
Because the fluorescence intensity is directly proportional to the number of molecules in the excited state, lifetime measurements can be done by measuring fluorescence decay after a brief pulse of excitation . Figure 5 shows the fluorescence decay of Ho3+ (5I7) at 2000 nm, fitted with a single exponential curve, measured for Ho3+-doped sol-gel glass. However, in this case, visual inspection indicates a poor fit to the experimental data, confirmed by an
Some of the ions predominantly doping the nanocrystals and another part remain in the vitreous state, so the average distance between them increases reducing concentration quenching. This approach has been widely reported in the literature , assigning the lifetime of the remaining RE ions in the vitreous phase to the decay of the fast component, while the slow component will correspond to the ions within the crystalline environment. For the multi-exponential decay is usually defined a mean life, which is given by Eq. (10) :
OH quenching arises from the residual water, solvents, and silanol groups (Si—OH) of the early stages of sol-gel glasses. This leads to an enhancement of non-radiative decays due to the coupling between the RE states and the high vibrational energy of OH (3200 cm−1) [3, 4].
Annealing the doped gels changes the local environment of the RE ions, which results in changes in the fluorescence emission shape .
4. The 4f-4f transitions of rare earths and intensity probabilities
The REs are a group of 15 chemical elements from the Lanthanide series (atomic number, Z, ranging from 57 to 71, at the sixth row of the periodic table), starting in lanthanum (La) and ending in lutetium (Lu), which exhibit similar chemical behavior and high stability in their triply ionized form (3+). When incorporated in crystalline or amorphous hosts, the RE maintains their most stable ionized form . All REs have the general configuration [Xe]4f
5. Tailoring sol-gel-derived SiO2 glassy optical waveguides
Nowadays, great attention still is given to the development of efficient and compact planar optical waveguides, with active (doped) and/or passive (undoped) purposes, for integrated optical devices . Light propagation in passive waveguides depends strongly on the layer structure and morphology, whereas propagation losses or attenuation must be kept as low as possible. Several factors are considered to disturb the light propagation [4, 47]: (1) absorption losses (AL1) due to light absorption in the glass waveguide (due to electronic (UV-visible) absorption at the Urbach edge and vibrational (IR) absorption at the multiphonon edge); (2) scattering losses (SL1) due to refractive index fluctuations; (3) absorption losses (AL2) by impurities like OH; and (4) scattering losses (SL2) due to the imperfection of the waveguide structure such as defects or surface roughness.
Porosity, dopants, and crystalline phases within the volume of the waveguide cause volume scattering while surface scattering loss can be significant for rough surfaces. Indeed, these kinds of imperfections can appear in sol-gel glasses-derived waveguides in the form of porosity, cracks, polycrystalline phases, dust contamination, and so on, as a result of deposition operation. However, particularly for sol-gel-derived waveguides, some of the imperfections such as pores and cracks are almost inevitable, as shown in Figure 8. Moreover, the porous removal requires high annealing temperatures to densify the waveguides, and thus cracks can occur from internal stress .
The principal types of defects that may cause scattering losses in sol-gel waveguides are shown in Figure 9.
They can be classified as textural defects (cracks, pores, dust, and surface roughness), compositional defects (nanocrystallites and discontinuous refractive index), or structural defects (thickness changes, internal stress and interaction between film and substrate, etc.). The size and the optical properties of these imperfections control their degree of contribution for the scattering loss with the operational wavelength.
Furthermore, light scattering tends to increase with
Therefore, the longer wavelength range produces a weaker propagation loss. Silica glass is transparent in NIR and exhibits a non-negligible attenuation. Alternatives to this material are heavy metal fluorides (ZBLAN) glasses, transparent in the mid-infrared (MIR) wavelength range. For SiO2 glasses, here are two
Fourier transform infrared spectroscopy (FTIR) is one of the most popular analytical techniques used to study the microstructural evolution of sol-gel glasses as a function of temperature and synthesis parameters. The FTIR spectrum of 80SiO2-20TiO2 glass, as soon after deposition (wet), is shown in Figure 11a. The broad peak around 3300 cm−1 is the fundamental stretching vibration of OH group, which reveals, as expected, the presence of hydroxyl groups in the gel, the
When a non-glass-forming oxide such as Al2O3 is added to an SiO2 glass, the Si—O—Si bond is broken, which leads to the formation of two types of oxygen: the oxygen that is attached to two Si, called the bridge oxygen (BO) and the other, which is connected to one Si, called non-oxygen bridge (NBO). Each RE ion solubilized in the glass matrix needs three NBOs to compensate the 3+ charges. Since RE ions cannot induce the necessary coordination number of NBO, therefore the RE clustering becomes energetically more favorable to share the limited number of NBO. Rather, the Al3+ ions embedded in the silicate glass matrix can be incorporated in two local bonding configurations such as tetrahedrally coordinated (AlO4/2), as a network former or octahedrally coordinated (AlO6/2), as a network modifier. Therefore, Al—O—RE bonds are formed instead of RE—O—RE ones, thus increasing the RE solubility. Also, the phonon energy of the Al—O—Si bonds is smaller than that of Si—O—Si vibration. Besides, P2O5 is also used as co-doping agent to improve the fluorescent properties of RE ions in sol-gel glasses [16, 21]. In fact, the breaking up of the RE—O—RE regions and formation of RE—O—P or RE—O—Al bonds is highly likely and allows, for example, Nd3+ contents as high as 7 wt% before phase separation or clustering are observed . FTIR and X-ray photoelectron spectroscopy (XPS) can provide valuable information about bonding configuration of glasses and a quantitative estimation of the Si—O—NBO species and thus contribute to improve the design of new active waveguides. Figure 12 shows the FTIR spectra of 80SiO2-20TiO2-
The influence of the high vibrational energy of SiO2-based glasses on non-radiative relaxation can be minimized by the incorporation of RE ions into nanocrystals with low-energy phonons dispersed in the matrix. This ordered environment would avoid any RE ions clustering. The interest demonstrated in recent years by nanocomposite GC is due to the possibility of creating a low-vibrational energy neighborhood around RE ions leading to a higher luminescent efficiency. Nevertheless, a nanocrystal larger than 10–15 nm drastically increases the propagation losses. This alternative becomes especially interesting for the following reasons: (1) a host material (the nanocrystal) is used in the vicinity of the RE ions in order to optimize the spectroscopic properties; (2) the matrix is still a suitable fiber junction material, with superior processing and high stability. The optical properties of waveguides doped with nanocrystals (fluorescence spectrum, fluorescence life, and optical losses) were found to be basically dependent on the following parameters: (a) nature of the nanocrystalline phases obtained; (b) crystallite sizes; (c) volume fraction of crystalline phases dispersed in the amorphous matrix; and (d) residual concentration of RE ions retained in the amorphous matrix. The XRD patterns of 80SiO2-20TiO2-ErO1.5 sol-gel thin films in Figure 14 show different crystalline phases. At 1000°C, only anatase phase, one of the crystalline polymorphic forms of TiO2, among rutile and brookite, is evidenced by its main peak at 25.28, which corresponds to (101) plane. By increasing the annealing temperature, the decomposition of anatase phase took place and a transition from anatase to rutile crystalline phase occurs.
The main peak of the film annealed at 1050°C is at 27.44, which corresponds to rutile (110) plane. The active phase Er2Ti2O7 precipitates together with rutile at 1100°C. Both anatase and rutile are passive phases that can damage the fluorescence quantum yield by scattering losses. In fact, high treatment annealing lowers the OH content but can increase the losses scattering from non-optical crystals such as rutile.
The interest in Er2Ti2O7 is obviously because Er3+ ions are inserted in a locally well-ordered phase thus giving relatively sharp photoluminescence emissions in a wide range of spectral bands from infrared to the blue region.
Results indicated clearly that the luminescence was significantly improved while erbium was present in the form of Er2Ti2O7 crystallites dispersed in an amorphous matrix . When the content of Er2O3 (the most usual erbium precursor) greatly exceeds the solubility limit of the glass, it reacts almost entirely with P2O5 forming ErPO4 (EPO) crystallites. In the SiO2-TiO2-P2O5 system, the ErPO4 particles are crystallized during the glass-annealing process. It has been shown that ErPO4 nanocrystals resulted in an increase in the fluorescence lifetime at 1550 nm greater than 200% with a maximum value of 9 ms . Figure 15 shows the room temperature fluorescence spectra relative to the 4I13/2 → 4I15/2 obtained upon 514.5-nm excitation, for 80SiO2-20TiO2-ErO1.5 waveguides. All the spectra exhibit a main emission peak at 1530 nm with a shoulder at about 1550 nm. ErPO4, on the other hand, emits a well-detectable fluorescence band around 1535 nm, with four peaks due to the Stark splitting of the 4I13/2 level in the crystalline compound.
Glass-integrated optics is controlled by oxide glasses, in part because of advantages of production techniques. At present, the Er3+-doped optical silicate glass is a very important light-amplifying element. However, non-oxides are receiving increasing interest for optical applications. Their low phonon energies make them useful for RE doping for lasers and amplifiers, particularly for doping with Pr3+ which have rapid non-radiative relaxation rates in oxides. A suitable approach to improve fluorescence lifetime of RE ion-doped glasses is to shield ions from the glass matrix in a local environment, such as a nanocrystallite, allowing optical properties of the host glass to be maintained. For the future, sol-gel has high potential for optical devices. A deep knowledge about materials physics and quantum optics is therefore essential to achieve these challenges.
The authors gratefully acknowledge the financial support from (1) FEDER, through Programa Operacional Factores de Competitividade—COMPETE and Fundação para a Ciência e a Tecnologia—FCT, by the project UID/FIS/00068/2013 and (2) Government of the Azores (DRCT - Programme PRO-SCIENTIA, Ref. M3.3.c/Edições/009/2017). The authors also wish to thank Dr. F. Rivera-López (ULL) for providing images used in this manuscript.
Righini JC. 25 years of integrated optics: where we are and where we will go. Proc. SPIE. 2212, Linear and Nonlinear Integrated Optics, Giancarlo C. Righini, David Yevick, Editors, 2. (1994) DOI: 10.1117/12.185098
Rimoldi, B. (2016) Principles of Digital Communication: A Top-Down Approach, Published by Cambridge University Press, United Kingdom. https://doi.org/10.1017/CBO9781316337387
Digonnet MJF. Rare Earth Doped Fiber Lasers and Amplifiers. 2nd ed. New York, NY: Marcel Dekker Inc.: 1993. DOI: 10.1201/9780203904657
Righini GC. Passive and active glasses for integrated optics. In: Mazzoldi P, editor. From Galielo’s Ochialino to Optoelectronics. Singapore: World Scientific; 1993. pp. 272-294. DOI: 10.1604/9789810213329
Orignac X, Almeida RM. Silica-Based Sol-Gel Optical Waveguides on Silicon in IEE Proceedings—Optoelectronics. 1996; 143(5):287–292. DOI: 10.1049/ip-opt:19960834
Colinge J-P. Silicon-On-Insulator Technology: Materials to VLSI. Boston, MA, USA: Kluwer Academic Publishers; 2004. DOI: 10.1007/978-1-4757-2611-4
Hu H, Ricken R, Sohler W, Wehrspohn RB. Lithium niobate ridge wave-guides fabricated by wet etching. IEEE Photonics Technology Letters. 2007; 19:6. DOI: 10.1109/LPT.2007.892886
Bindra KS, Bookey HT, Kar AK, Wherrett BS. Nonlinear optical properties of chalcogenide glasses: Observation of multiphoton absorption Applied Physics Letters. 2016; 79:1939. DOI: http://dx.doi.org/10.1063/1.1402158
Kawachi M. Silica waveguides on silicon and their application to integrated-optic components. M. Optical and Quantum Electronics. 1990; 22:392. DOI: 10.1007/BF02113964
Brinker CJ, Scherer GW. Sol-Gel Science-The Physics and Chemistry of Sol-Gel Processing. San Diego; Academic Press: 1990. DOI: 10.1002/adma.19910031025
Kreibig U, Vollmer M. Optical properties of metal clusters. Spring Series in Materials Science. 1995; 25:13–201. DOI: 10.1007/978-3-662-09109-8
Diehl R. High-Power Diode Lasers: Fundamentals, Technology, Applications. Springer Berlin Heidelberg; 2000. ISBN: 978-3-540-66693-6, DOI: 10.1007/3-540-47852-3
Snitzer E. Optical maser action of Nd+3 in a barium crown glass. Physics Review Letters. 1961; 7:444. DOI: https://doi.org/10.1103/PhysRevLett.7.444
Layne CB, Lowdermilk WH, Weber MJ. Multiphonon relaxation of rare-earth ions in oxide glasses. Journal of Material Science Letters. 1994; 13:615. DOI: https://doi.org/10.1103/PhysRevB.16.10
Vasconcelos HC, Meirelles MG, Rivera-López F. Erbium photoluminescence response related to nanoscale heterogeneities in sol-gel silicates. Journal of Rare-Earths. 2013; 31:18-26. DOI: 10.1016/S1002-0721(12)60228-2
Vasconcelos HC. The effect of PO2,5 and AlO1,5 additions on structural changes and crystallization behaviour of SiO2-TiO2 sol-gel derived glasses and thin films. Journal of Sol-Gel Science and Technology. 2010; 55:126-136. DOI: 10.1007/s10971-010-2223-8
Miniscalco WJ. Erbium-doped glasses for fiber amplifiers at 1500 nm . Journal of Lightwave Technologies. 1991; 9:234-250. DOI: 10.1109/50.65882
Santa Cruz P, Morin D, Dexpert-Ghys J, Sadoc A, Glas F, Auzel F. New lanthanide-doped fluoride-based vitreous materials for laser applications. Journal of Non-Crystalline Solids. 1995; 190:238-243. DOI: 10.1016/0022-3093(95)00273-1
Kenyon AJ. Recent developments in rare-earth doped materials for optoelectronics. Progress in Quantum Electronics. 2002; 26:225-284. DOI: 10.1016/S0079-6727(02)00014-9
Santana-Alonso A, Yanes AC, Méndez-Ramos J, del-Castillo J, Rodríguez VD. Sol–gel transparent nano-glass-ceramics comprising rare-earth-doped NaYF4 nanocrystals. Physica Status Solidi (a). 2009; 206:2249-2254. DOI:10.1002/pssa.200881717
Strohhöfer C, Fick J, Vasconcelos HC, Almeida RM. Active optical properties of Er-containing crystallites in sol-gel derived glass films. Journal of Non-Crystalline Solids. 1998; 226:182. DOI: 10.1016/S0022-3093(98)00365-2
X. Orignac, D. Barbier. Potential for fabrication of sol-gel-derived integrated optical amplifiers. Proc. SPIE 2997, Integrated Optics Devices: Potential for Commercialization, Edited by S. Iraj Najafi and Mario Nicola Armenise, 271 (January 23, 1997); DOI:10.1117/12.264157
Yan Y, Faber AJ, de Waal H. Luminescence quenching by OH groups in highly Er-doped phosphate glasses. Journal of Non-Crystalline Solids. 1995; 181:283-290. DOI: 10.1016/S0022-3093(94)00528-1
Rawson H. In: Glass Chemistry. Inorganic Glass-Forming Systems. London, UK: Academic Press; 1967. pp. 123-207. DOI: 10.1007/978-3-642-78723-2_7
Weber MJ. Science and technology of laser glass. Journal of Non-Crystalline Solids. 1990; 123(1–3):208-222. DOI: 10.1016/0022-3093(90)90786-L
Scriven LE. Physics and applications of dip coating and spin coating. Materials Research Society Symposium Proceedings. 1988; 121:717. DOI: 10.1557/PROC-121-717
cbv Weisembach L, Zelinski BJ, O’Kelly J, Morreale J, Roncone RL, Burke JJ. The influence of processing variables on the optical properties of SiO2-TiO2 planar waveguides. SPIE. 1991; 1590:50. DOI: 10.1117/12.50201
McGahay V, Tomozawa M. Phase separation in rare-earth-doped SiO2 glasses. Journal of Non-Crystalline Solids. 1993; 159(3):246–252. DOI: 10.1016/0022-3093(93)90230-U
Mukher Jee SP. In: Klein LC, editor. Sol-Gel Technology for Thin Films, Fibers, Preform, Electronics and Specialty Shapes. NJ: Noyes Publications; 1988. DOI: 10.1002/adma.19890010816
García Solé J, Bausá LE, Jaque D. An introduction to the optical spectroscopy of inorganic solids. New Delhi, India: John Wiley & Sons, Ltd.; 2005. p. 283. DOI: 10.1002/0470016043.app3
Jablonski A. Über den Mechanisms des Photolumineszenz von Farbstoffphosphoren. Journal of Physics. 1935; 94:38-46. DOI: 10.1007/BF01330795
Reisfeld R. Radiative and non-radiative transitions of rare-earth ions in glasses. Structure & Bonding. 1975; 22:123-175. DOI: 10.1007/BFb0116557
Haro-González P, Lahoz F, González-Platas J, Cáceres JM, González-Pérez S, Marrero-López D, Capuj N, Martín IR. Optical properties of Er3+-doped strontium barium niobate nano-crystals obtained by thermal treatment in glass. Journal of Luminescence. 2008; 128:908-910, 2008. DOI: 10.1016/j.jlumin.2007.12.014
Lakowicz JR. Principles of Fluorescence Spectroscopy. 3rd ed. New York, NY: Springer; 2006. DOI: 10.1007/978-0-387-46312-4
Förster T. Zwischenmolekulare Energiewanderung und Fluoreszenz, Annals in Physics. 1948; 2:55. DOI: 10.1002/andp.19484370105
Dexter DL. A Theory of Sensitized Luminescence in Solids, Journal of Chemistry and Physics. 1953; 21:836. DOI: 10.1063/1.1699044
Bo F, Céline P, Jean-Luc A, Xianghua Z, Xianping F, Hongli M. Near-infrared down-conversion in rare-earth-doped chloro-sulfide glass GeS2-Ga2S3-CsCl: Er, Yb. Journal of Applied Physics. 2011; 110(11):113107. DOI: 10.1063/1.3665638
Orignac X, Barbier D, Du XM, Almeida RM, McCarthy O, Yeatman E. A Theory of Sensitized Luminescence in Solids, Optical Materials 1999; 12(1):1–18. DOI: 10.1016/S0925-3467(98)00076-7
Wybourne BG. Spectroscopic properties of rare earths. Physics Today. 1965; 18(9):70. DOI: 10.1063/1.3047727
Peacock RD, Nieboer E, Jørgensen CK, Reisfeld R. The intensity of lanthanide f↔f transitions. Structure and Bonding. 2007; 22:83–122. DOI: 10.1007/BFb0116556
Weber MJ. The Role of Lanthanides in Optical Materials. Report Number: LBL-37536. 1995. Available from: https://publications.lbl.gov/islandora/object/ir%3A102074
Dieke GH. Spectra and Energy Levels of Rare-Earth Ions in Crystals. New York, NY: Wiley; 1968. DOI: 10.1119/1.1976350
Judd BR. Optical Absorption Intensities of Rare-Earth Ions, Physics Review B. 1962; 127:750. DOI: 10.1103/PhysRev.127.750
Ofelt GS. Intensities of Crystal Spectra of Rare?Earth Ions, Journal of Chemistry and Physics. 1962; 37:511. DOI: 10.1063/1.1701366
Jorgensen CK, Reisfeld R. Judd-Ofelt parameters and chemical bonding . Journal of the Less Common Metals. 1983; 93(1):107–112. DOI: 10.1016/0022-5088(83)90454-X
Lahoz F, Capuj N, Haro-Gonzalez P, Martın IR, Perez-Rodrıguez C, Caceres JM. Stimulated emission in the red, green, and blue in a nanostructured glass ceramics, Journal of Applied Physics. 2011; 109:043102. DOI: 10.1063/1.3549157
Almeida RM, Morais PJ, Vasconcelos HC. Optical loss mechanisms in nanocomposite sol-gel planar waveguides. Proceedings of the SPIE. 1997; 3136:296–303. DOI: 10.1117/12.284127
R. M. Almeida, J. Xu, Sol?Gel Processing of Sulfide Materials, Handbook of Sol-Gel Science and Technology, Published by Springer, pp 1–26, 2016. DOI: 10.1007/978-3-319-19454-7_11-1
Davis K, Agarwal A, Tomozawa M, Hirao K. Quantitative infrared spectroscopic measurements of hydroxyl concentrations in silica glass. Journal of Non-Crystalline Solids. 1996; 203:27–36. DOI: 10.1016/0022-3093(96)00330-4
Almeida RM, Vasconcelos HC, Gonçalves MC, Santos LF. XPS and NEXAFS studies of rare-earth doped amorphous sol-gel films. Journal of Non-Crystalline Solids. 1998; 232–234:65–71. DOI: 10.1016/S0022-3093(98)00545-6
Bowron DT, Newport RJ, Rigden JS, Tarbox EJ, Oversluizen M. An X-ray absorption study of doped silicate glass, fibre optic preforms, Journal of Materials Science. 1996; 31:485. DOI:10.1007/BF01139168