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Physics » "Luminescence - An Outlook on the Phenomena and their Applications", book edited by Jagannathan Thirumalai, ISBN 978-953-51-2763-5, Print ISBN 978-953-51-2762-8, Published: November 10, 2016 under CC BY 3.0 license. © The Author(s).

Chapter 7

Luminescent Glass for Lasers and Solar Concentrators

By Meruva Seshadri, Virgilio de Carvalho dos Anjos and Maria Jose Valenzuela Bell
DOI: 10.5772/65057

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Schematic energy levels of rare earth ions.
Figure 1. Schematic energy levels of rare earth ions.
Schematic representation of (a) random network of alkali silicate and (b) glass structure of different atomic neighborhoods (3D).
Figure 2. Schematic representation of (a) random network of alkali silicate and (b) glass structure of different atomic neighborhoods (3D).
Schematic diagram of ion‐ion interaction in the case of Nd3+ and Er3+ ions.
Figure 3. Schematic diagram of ion‐ion interaction in the case of Nd3+ and Er3+ ions.
Schematic representation of upconversion mechanisms. (a) GSA/ESA process; (b) ETU process; (c) cooperative process.
Figure 4. Schematic representation of upconversion mechanisms. (a) GSA/ESA process; (b) ETU process; (c) cooperative process.
(a) Absorption and (b) emission spectra of Nd3+‐doped phosphate glass [33].
Figure 5. (a) Absorption and (b) emission spectra of Nd3+‐doped phosphate glass [33].
Variation of (a) Ω2 with oscillator strength (fexp) of HST and (b) Ω2 with band position (Eexp) of HST in alkali and mixed alkali phosphate glasses [45].
Figure 6. Variation of (a) Ω2 with oscillator strength (fexp) of HST and (b) Ω2 with band position (Eexp) of HST in alkali and mixed alkali phosphate glasses [45].
Rare earth‐doped fibers working wavelength ranges.
Figure 7. Rare earth‐doped fibers working wavelength ranges.
Erbium‐doped fiber amplifier (EDFA).
Figure 8. Erbium‐doped fiber amplifier (EDFA).
Simplified energy band diagram of Er3+‐doped silica fiber. (a) Three‐level process. (b) Two‐level process.
Figure 9. Simplified energy band diagram of Er3+‐doped silica fiber. (a) Three‐level process. (b) Two‐level process.
Absorption and gain spectra of EDFA.
Figure 10. Absorption and gain spectra of EDFA.
(a) Schematic diagram of luminescent solar concentrator. (b) Emission of solar radiation.
Figure 11. (a) Schematic diagram of luminescent solar concentrator. (b) Emission of solar radiation.
Absorption and emission spectra of Nd3+ and Yb3+ ions.
Figure 12. Absorption and emission spectra of Nd3+ and Yb3+ ions.

Luminescent Glass for Lasers and Solar Concentrators

Meruva Seshadri, Virgilio de Carvalho dos Anjos and Maria Jose Valenzuela Bell
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Rare earth‐doped glasses find applications in numerous photonic devices including color displays, infrared solid‐state lasers, and indicators, among many. In this chapter, we will present and discuss several luminescent glasses doped with rare earth ions in their trivalent form (RE3+) with general background and technological perspectives. Initially, we begin with a short introduction of RE3+ electronic energy‐level structure in solids followed by the discussion of structural feature of glass lasers. While the lasing properties are mainly governed by the solubility of the ions and phonon interactions, the issue of ion interactions in solid hosts will be addressed since they hardly depend on the type of materials. Spectroscopic properties of Nd3+‐doped phosphate glasses are discussed in the framework of Judd‐Ofelt theory. Rare earth‐doped optical amplifiers are tackled from a technological point of view, as well as luminescent solar concentrators for enhancement of solar efficiency.

Keywords: glasses, rare earth ions, luminescence, optical amplifiers, solar concentrators

1. Introduction

A large number of ions from the lanthanide (rare earth) and actinide groups of periodic table exhibit laser action when doped into a large number of host crystals or glasses. The construction of high‐power crystalline lasers requires substantial crystal sizes. In practice, crystals are grown by the small seed crystals while pulling the larger crystal from the melt. These methods are very difficult if larger crystals are needed. When glasses are used as host media, the environments of the ions vary much more than in a crystalline material because of the random structural character of the glass matrix. All glass lasers until present date have used trivalent lanthanides as the active ions. Glass plays many varied roles in rare earth laser systems, as it can be made with uniformly distributed rare earth concentrations and has great potential as a laser host medium. In addition, rare earth‐doped fibers have received growing attention recently. They can be used as amplifiers in optical communication systems and as optical sources. Glass lasers are substantially inhomogeneously broadened, and usually present more broader line width than Nd:YAG ones. The laser transition line width and shape typically vary from one glass to another glass matrix. In this way, innovation of glass lasers remains a vibrant area in the development of science and technology. The luminescent glass materials are used to develop solid‐state lasers operating in visible and NIR regions due to their potential applications in the fields of medical, eye safe lasers, atmosphere pollution monitoring, energy converters and telecommunications. Sharp fluorescent lines, strong absorption bands, and reasonable high quantum efficiency will determine the adequacy of laser material.

In this contribution, the introduction consists of brief review of electronic energy‐level structure of RE3+ in solids, structural features, and general considerations in laser glasses. Laser glasses and amplifiers are studied to know the spectroscopic parameters using spectroscopic techniques. Especially, the current achievements on Nd3+‐doped phosphate glass lasers will be discussed and numerous data will be presented. Er3+‐doped glass fiber amplifiers subject will also be addressed. Finally, we will discuss rare earth‐based luminescence concentrators for photovoltaic applications.

1.1. Rare earth ions as luminescent centers

Trivalent rare earth ions are well known for their special optical properties, which result from the fact that the electronic transitions within the 4f shell occur at optical frequencies. The 4f shell is shielded by the completely filled 5s and 5p shells, so the frequencies of the transitions are almost independent of the host. Generally, the electric dipole transitions are forbidden due to equal parity of the electronic levels within the 4f shell. Those transitions are possible in solids that have slightly mixed odd‐parity wave functions. As a result, the absorption and emission cross sections are small (~10-20 or 10-21 cm2), and the lifetime of the luminescent level is relatively long ranging from microseconds to several milliseconds. The influence of the electric field around the ions removes the degeneracy of the 4f levels, resulting in a Stark splitting of the levels. Figure 1 shows the energy levels of the trivalent lanthanide ions (4f configurations). This figure provides useful information to predict and/or to make a proper assignment of the emission spectra corresponding to trivalent rare earth ions in crystals or glasses.

The energy levels of the 4f shell arise from spin‐spin and spin‐orbit interactions and are often denoted using Russell‐Saunders notation 2S+1LJ [1, 2], in which S is the total spin angular momentum, L is the total orbital angular momentum, and J is the total angular momentum. There exist 14 rare earth elements all having a different number of electrons in the incompletely filled 4f shell. However, due to the shielding by the outer lying shells, the magnitude of the splitting is small, resulting in relatively narrow lines.

Table 1 displays some important emission transitions of rare earth ions and their technological interest. More general views about the spectra and energy levels of rare earth ions can be found in literature [912]. The large number of excited states that are suitable for optical pumping and subsequent decay to metastable states having high quantum efficiencies and narrow emission lines is favorable for achieving laser action.


Figure 1.

Schematic energy levels of rare earth ions.

RE3+ ionsTransitionsWavelength (nm)Application
Pr3+1G43H51300Optical amplifier [3]
Nd3+4F3/24I11/21064Solid‐state lasers [4]
Eu3+5D07F2615Displays, lighting [5]
Tb3+5D47F5545Lighting [6]
Dy3+6F11/2+6H9/26H15/21300Optical amplifier [7]
Er3+4I13/24I15/21530Optical amplifier [8]
Tm3+3H43F41480Optical amplifier [7]
Yb3+2F5/22F7/2980Sensitizer [6]

Table 1.

Important emission lines of some lanthanide ions.

1.2. Glasses for lasers and amplifiers: structural features

Glasses are widely used nowadays and present several applications in different fields of life. There are several definitions for glassy materials. Grouping them one can define glass as an inorganic product of melting which has been cooled to a solid without crystallization [13]. It means that glass looks like an undercooled liquid. An undercooled liquid can crystallize at any moment but at room temperature it is not possible. Therefore, glass is an amorphous material as it does not exhibit long range order of atoms in a lattice. The transition of a glass melt to a crystallized state at the crystallization temperature does not take place if the cooling of the glass is fast. Below the crystallization temperature, glass behaves like a fluid down to the transformation temperature. Below this temperature, glass has the properties of a solid. Glass can also be defined as an amorphous solid lacking completely, in long range, periodic atomic structure and exhibiting a region of glass transformation behavior.

In general, glasses can be prepared either from high quality, chemically pure components or from a mixture of far less pure minerals. The batch materials can be divided in to five categories depending on the property of components used in the glass preparation: (1) glass formers, (2) flux, (3) property modifier, (4) colorant, and (5) fining agent. Every glass contains one or more components, which serve as the primary source of the structure. The same component may be classified into different categories when used for different purposes. For example, alumina serves as a glass former in aluminate glasses but in most silicate glasses, it works as property modifier [14]. Zachariasen [15] noted that those crystalline oxides that form open, continuous networks tended to form glasses and those glass‐forming networks were associated with ions with particular coordination numbers (CN). The primary glass formers in commercial oxide glasses are silica (SiO2), boric oxide (B2O3), and phosphoric oxide (P2O5), in addition to other compounds GeO2, Bi2O3, As2O3, Sb2O3, TeO2, Al2O3, Ga2O3, and V2O5 which act as glass formers under certain circumstances.


Figure 2.

Schematic representation of (a) random network of alkali silicate and (b) glass structure of different atomic neighborhoods (3D).

These oxide glass formers play an important structural role in glasses when used as network or intermediate oxide modifiers. Thus, the oxide network modifiers will create strong bridging oxygen (BO) bonds in between the glass‐forming polyhedra and weak nonbridging oxygen (NBO) bonds (see in Figure 2a). Moreover, the oxide modifiers will control many useful properties, lowering the melting temperature that is highly useful for developing technologically used glasses. The intermediate oxides also modify glass properties due to their coordination numbers and bond strengths in between the network formers and network modifiers. Table 2 reports some of the oxide‐based modifier glasses for realization of structure influence on various properties.

Glass compositionD (g/cm3)Tg (°C)ndτ (ms)
60SiO2‐20Al2O3‐20Li2O [16]2.406931.5312.65
20Na2O [16]2.458111.5072.45
20MgO [16]2.558271.5482.43
20CaO [16]2.618681.5572.37
20ZnO [16]2.847421.5732.51
20La2O3 [16]4.088631.7092.01
71SiO2‐14Al2O3‐15MgO [17]2.448311.5202.48
10CaO [17]2.618291.549
10BaO [17]2.868411.5492.39
15SrO [17]2.738341.549
TBZN—0.05Ho2O3 [18]4.24828617.4

Table 2.

Density (D), transition temperature (Tg), refractive index (nd), and lifetime of respective ions (τ599 nm(Sm3+) [16, 17]; τ660 nm(Ho3+) [18]) in different oxide modifier‐based glass compositions.

Owing to superior physical properties such as high thermal expansion coefficients, low melting and softening temperatures, and high ultraviolet transmission, phosphate glasses have several important attributes over conventional silicate and borate glasses. However, the poor chemical durability of these early optical glasses has temporarily discouraged for their further development. Interest in the amorphous alkali phosphates was stimulated in the 1950s by their use in a variety of industrial applications, including sequestering agents for hard water treatments and dispersants for clay processing and pigment manufacturing [19]. Studying such materials, Van Wazer [20] established the foundations for much of our current understanding of the nature of phosphate glasses. Kordes and co‐workers [21, 22] re‐examined the alkaline earth phosphate glasses, including UV‐transmitting compositions and observed some “anomalous” trends in properties.

Phosphate glasses have unique characteristics that include high transparency, high thermal stability, low refractive index and dispersion, and high gain density due to high solubility for lanthanide ions and hence find growing field of applications [2326]. An important step in laser glasses occurred in 1967 when phosphate‐based compositions were first explored [27]. Several phosphate glasses find considerable applications in optical data transmission, detection, and sensing [28]. The phosphate glasses also find applications in fast ion conductors, optical filters, reference electrodes, stable storage medium for immobilizing high‐level nuclear waste [29] and as additives in the manufacturing of insulating polymeric cables. Certain phosphate vitreous electrolytes are being used increasingly in electrochemical sensors, in prototype batteries, and in electrochemical devices. There have been many excellent reviews of structural studies on phosphate glasses including that of Van Wazer [20, 30, 31]. A schematic structure of different atomic neighborhoods (covalent and ionocovalent) in rare earth (Er3+)‐doped phosphate glass is shown Figure 2b.

The large concentration of lasing atoms can be doped easily into different glasses, which can be made in different shapes and sizes depending on technological needs. The high‐power laser threshold is possible from the ions‐doped glasses compared to the same ions‐doped crystal due to their large absorbing capability of incident energy and energy level broadening. The electronic transitions of rare earth ions have 4f‐4f and 4f‐5d configurations, which are weakly affected by the host material, since the 4f electrons are effectively screened by outer, filled electron shells. Thus, these transitions cause absorption and fluorescence patterns from the ultraviolet (UV) to infrared regions and they are narrow. Two important aspects of the optical behavior of rare earth ions are determined by the host material. One is the electric dipole transitions which occur between 4f states are strictly forbidden for an isolated ion, since the parity of the electronic configuration must change with an electric dipole transition. However, the strengths of these electric dipole transitions remains relatively weak due the perturbative nature of the admixing states, and as consequence, the radiative lifetimes of excited rare earth ions can be relatively long (∼10-3 ms). Another aspect is related to the nonradiative rates originated from relaxation of the excited states of the rare earth ions, which is determined by the host material. Recently, Babu et al. [32] had studied different fluorophosphate glasses doped with 0.5 mol% of Er3+ ions through the Judd‐Ofelt and McCumbers theories for potential broadband optical fiber lasers and amplifiers. Praseodymium (Pr3+)‐doped high‐aluminum phosphate (HAP) glasses with excellent chemical durability for thermal ion‐exchanged optical waveguide have been investigated by Tian et al. [33]. In order to improve the solar cell efficiency, potential downconversion was studied in GeS2‐Ga2S3‐CsCl glass for modifying solar spectrum and are found quantum yield to be below 1200 and 1650 nm are 51 and 76%, respectively [34].

2. Limiting factors in rare earth‐doped glasses

2.1. RE3+ ions solubility in glasses

Higher concentration of RE3+ ions in glasses lead to clusters. Thus, the clusters of rare earth ion serve as luminescent quenchers, either by increasing ion‐ion interactions between rare earth ions or by forming rare earth compounds that are not optically active. Therefore, much effort has been done into developing suitable host glass compositions for various rare earth doping levels. Some of the technological limitations for the development of new glasses are due to the rare earth ion density required for laser action. Currently, high‐power lasers used Nd3+doped phosphate glasses and their nominal Nd doping levels in the range of about 3–4.2 × 1020 cm-3.

2.2. Phonon interaction

Since the multiphonon decay process is essentially a competing process to the fluorescence, a low multiphonon decay rate can lead to an increased fluorescence efficiency for many important rare earth transitions. The efficiency of nonradiative transition rate depends on the energy gap between the ground and excited states, as well as the vibrational energy of the oscillators. The nonradiative transition decay rate can be expressed in terms of multiphonon relaxation, concentration quenching, energy transfer to another doping impurity such as transition metal ions or other rare earth ions, and energy transfer to hydroxyl groups OH- [35]. The quenching efficiency is strongly dependent on the number of vibrational quanta that are needed to bridge the gap between the lowest emitting level and the highest nonemitting level of the lanthanide ion. Generally, hosts with low phonon energy tend to have a low multiphonon decay rate; thus, selection of lower phonon hosts such as fluoride or tellurite glasses can reduce the contribution of multiphonon relaxation and allow important radiative transitions.

2.3. Ion‐ion interaction

Ion‐ion interaction is due to multipolar interactions between neighbor rare earth ions. The analysis of ion‐ion interaction and energy transfer provides essential information to the applications of laser glasses and display devices. In such process, the excitation energy transfers from an excited donor to a nearby unexcited activator (acceptor). Many theories have been put forward to give formulas for the rate of energy transfer by electric dipole‐dipole interaction (n = 6), electric dipole‐quadrupole interaction (n = 8), and quadrupole‐quadrupole interactions (n = 10). These transfer mechanisms differ from one another in the dependence of the transfer rate on donor‐acceptor distance, but common to all is the condition that an overlap between the donor emission spectrum and the acceptor absorption spectrum is essential for the transfer to occur. Such resonant transfer is analyzed most frequently through luminescence measurements: donor molecules are excited in the presence of acceptor ions, and the luminescence yield of donor and/or acceptor and the decay time of donor luminescence are measured as functions of the acceptor concentration. A more detailed description of ion‐ion interactions is given by Inokuti‐Hirayama theory [36], which explicitly deals with the dynamics of energy migration.

Among many energy transfer theories, Förster [37] and Dexter [38] theory on energy transfer is one of the most widely employed and the probability rate of energy transfer can be determined as [39] follows:


where R is the distance of separation between donor and acceptor, CDA is the energy transfer constant (cm6/s), and n¯[= 1/e(ω0/KT)1] is the average occupation of phonon mode at temperature T. Then, the energy transfer constant is expressed as follows:


The critical radius of the interaction can be obtained by the product of energy transfer constant and intracenter lifetime of the donor excited level (i.e., RC6=CDA × τD).

In addition to the aforementioned mechanisms, in glasses containing only one RE3+ species (for example Nd3+, Er3+), a number of different ion‐ion interactions occur such as energy migration, cross‐relaxation among others. The most important are outlined below.

2.4. Energy migration

Energy migration is strongly dependent on rare earth ion concentration and due to the dipole‐dipole Förster and Dexter interactions. An excited ion in the metastable state can interact with the nearby ground state ion then promoting it to the excited level (see Figure 3). The successive energy transfers between the ions increase the probability of nonradiative decay that lead to decrease the fluorescence efficiency [40, 41].


Figure 3.

Schematic diagram of ion‐ion interaction in the case of Nd3+ and Er3+ ions.

2.5. Cross relaxation

Cross relaxation involves the same RE ions and their schematic representation, which is shown in Figure 3. An excited state of ion A that gives half its energy to ion B is in its ground state. So that both ions end up in the intermediate energy level, from which they relax rapidly to the ground state through nonradiative relaxation. This relaxation is usually observed for Nd3+‐doped glasses that cause concentration quenching.

2.6. Upconversion mechanisms

By Excited State Absorption (ESA): The simplest representation of upconversion mechanism in trivalent Er3+ ion through the ground‐state absorption (GSA)/excited state absorption (ESA) is shown in Figure 4a. The first excitation photon is absorbed by the ground‐state N0 and populates the intermediate‐state N1. Provided the lifetime of N1 is long enough, a second incident photon can be absorbed, exciting further the ion from its intermediate‐state N1 to a higher‐lying excited state N2, from which upconversion luminescence arises.


Figure 4.

Schematic representation of upconversion mechanisms. (a) GSA/ESA process; (b) ETU process; (c) cooperative process.

By sensitized energy transfer upconversion (ETU): Sensitized energy transfer upconversion was first introduced by Auzel who called ETU from Addition of Photon for Energy Transfer (APTE) from (the French ‘Addition de Photons par Transferts d ‘Energie'). Upconversion involves nonradiative energy transfers between a sensitizer (e.g., Yb3+) and an activator (e.g., Er3+, Ho3+, Tm3+). Usually, sensitizer has a strong absorption cross section at the excitation wavelength. Once the sensitizer is excited (after absorption of an incident photon), it relaxes to a lower‐energy state (the ground state in the case of Yb3+) by transferring its energy to a neighboring activator, raising the latter to a higher‐energy state (see Figure 4b). These energy transfer processes are generally based on electric dipole‐dipole interactions.

By cooperative luminescence: Figure 4c shows the schematic representation upconversion by cooperative luminescence. Two excitation photons sequentially absorbed by two different ions (e.g., two Yb3+ ions, but the two ions do not need to be the same species), moving both of them into their excited state. Then, both ions decay simultaneously to their ground state, with the emission of one single photon that contains the combined energies of both ions. The cooperative emission occurs from a virtual level, and it explains why the emission probability is rather low.

3. Spectroscopic properties of Nd3+‐doped phosphate glasses

3.1. Judd‐Ofelt theory and intensity parameters

Optical spectroscopy often used to measure optical absorption of trivalent rare earth ions in UV‐vis‐NIR regions, from which the effect of a host matrix on the local environment of a given rare earth cation with its first neighbor anions such as oxygen can be elucidated using the theory proposed by Judd [42] and Ofelt [43]. Each observed transition corresponds to a transition between two spin‐orbit coupling levels. The Judd‐Ofelt theory has been applied for the interpretation of these transitions by the three mechanisms: (1) magnetic dipole transitions, (2) induced electric dipole transitions, and (3) electric quadrupole transitions. The quantitative analysis of the intensities of these f‐f transitions in the rare earth ions has been provided independently by Judd [42] and Ofelt [43]. The basic idea of Judd and Ofelt is that the intensity of f‐f electric dipole transitions can arise from the admixture into the 4fN configuration of opposite parity (e.g., 4fN-1n1d1 and 4fN-1n1g1). According to Judd‐Ofelt theory, the intensity of magnetic and electric dipole transition is represented as follows:

This means that experimentally measured oscillator strengths could be expressed to a good approximation in terms of absorption of light by electric dipole (fed) and magnetic dipole mechanisms. But, the order of magnitude of magnetic dipole oscillator strengths (fmd) is found to be very low for the observed intensities of rare earth ions and thus will not be considered further. The calculated oscillator strengths are therefore approximately equal to electric dipole oscillator strengths, that is


where υ is the mean energy of the transition ψJψJ, and Uλ2 is the squared reduced matrix element of the unit tensor operator of the rank, λ (=2, 4 and 6). T2, T4, and T6 are related to the radial part of 4fN wave functions, the refractive index of the medium, and the ligand‐field parameters that characterize the environment of the ion. These quantities are treated as parameters and are determined from the experimental oscillator strengths.

The intensity of an absorption band is expressed in terms of a quantity called the “oscillator strength.” Experimentally, the oscillator strength (f) is a measure of the intensity of an absorption band and is proportional to the area under the absorption peak. The area under an absorption peak is a better measure of the intensity than the molar absorptivity at the peak maximum, because the area is the same for both the resolved and unresolved band. The oscillator strength (f) of each absorption band is expressed in terms of absorption coefficient α(λ) at a particular wavelength λ and is given by [44]


where m and e are the mass and charge of the electron, c is the velocity of light, and N is the density of the absorbing ions obtained from the rare earth ion concentration and glass density values, respectively. ∫ε(ν)dν represents the area under the absorption curve. The molar absorptivity ε(ν) of an absorption band at energy ν (cm-1) is given by


where C concentration of rare earth ions per unit volume, l optical path length, and log(I0I) is the optical density.

As an example, optical absorption measurements were made at room temperature in the wavelength region 300–900 nm for an Nd3+‐doped phosphate glass using JASCO V‐630, UV‐vis spectrophotometer. Figure 5 shows (a) absorption and (b) emission spectra of Nd3+‐doped phosphate glass. The validity of Judd‐Ofelt theory is determined by the root‐mean‐square (δrms) deviation between the measured and calculated oscillator strengths by the relation [45].


Using the experimental oscillator strengths (fexp), a best set of Judd‐Ofelt intensity parameters Ωλ (λ = 2, 4, and 6) for the Nd3+ ion‐doped glass were determined using the procedure followed in Ref. [46]. The Judd‐Ofelt intensity parameters represent the square of the charge displacement due to the induced electric dipole transition. The advantage of Ωλ parameters is that a set of parameters is needed for describing both the absorption and emission processes. The Ωλ parameters are important for the investigation of the local structure and bonding in the vicinity of rare earth ions. Reisfeld [47] indicated that Ω2 parameter is sensitive to both asymmetry and covalency at rare earth sites. Oomen and van Dongen [48] pointed out that the rigidity or long range effects of glass hosts were responsible for changes in Ω6. The Ω4 parameters are affected by factors causing changes in both Ω2 and Ω6. Table 3 shows Judd‐Ofelt intensity parameters Ωλ (λ = 2, 4, and 6) for the Nd3+‐ion doped various phosphate glasses.


Figure 5.

(a) Absorption and (b) emission spectra of Nd3+‐doped phosphate glass [33].

Glass matrixΩ2Ω4Ω6Ω46
40P2O5‐20Al2O3‐40Na2O [49]4.706.05.401.11
41P2O5‐17K2O‐9.5CaO‐8Al2O3‐24aF2 [50]5.407.036.511.07
58.5P2O5‐17K2O‐14.5SrO2‐9Al2O3 [51]6.743.866.350.60
75NaPO3‐24LiF3 [52]3.444.146.280.65
(65P2O5‐15Na2O)‐15Li2O [45]4.323.666.000.61
(65P2O5‐15Na2O)‐15Na2O [45]5.424.938.060.61
(65P2O5‐15Na2O)‐15K2O [45]7.688.9611.710.76
(65P2O5‐15Na2O)‐7.5Li2O‐7.5Na2O [45]4.013.695.920.62
(65P2O5‐15Na2O)‐7.5Li2O‐7.5K2O [45]6.426.158.960.69
(65P2O5‐15Na2O)‐7.5Na2O‐7.5K2O [45]4.903.886.180.63

Table 3.

Judd‐Ofelt intensity parameters (Ωλ, λ = 2, 4, and 6 × 10-20 cm2) in 1 mol% Nd2O3‐doped phosphate glasses.

The covalency between the rare earth ion and the surrounding oxygen in the glass modifies the intensity of hypersensitive transitions (HST), as suggested by Reisfeld and co‐worker [47, 48]. This can be observed more clearly in RE3+‐doped alkali‐ and mixed alkali‐based glasses. For example, in Nd3+ ion, the transition 4I9/24G5/2 + 2G7/2 is found to be hypersensitive by selection rule, ΔJ ≤ 2, ΔL ≤ 2, and ΔS = 0. The observed oscillator strengths of the hypersensitive transition are higher when compared to other bands (see inset Figure 5a). Figure 6 shows variation of J‐O parameter, Ω2 with (a) oscillator strength (b) energy (cm-1) of HST transition of Nd3+ ion for alkali, mixed alkali phosphate glasses. From Figure 6a, among the alkali glass matrices, the oscillator strength of HST is found to be lower in lithium phosphate glass matrix and higher in potassium glass matrix. But the oscillator strength of HST is found to be higher in lithium‐potassium phosphate glass matrix among the mixed alkali glass matrices. Thus, the results of oscillator strength show that potassium is playing active role in enhancing the oscillator strengths of Nd3+ ion in potassium phosphate glass matrices (K, Li‐K, and Na‐K) compared to other glass matrices. In lithium phosphate glass matrix, oscillator strength of HST indicates the lower crystal field symmetry at Nd3+ ion site.


Figure 6.

Variation of (a) Ω2 with oscillator strength (fexp) of HST and (b) Ω2 with band position (Eexp) of HST in alkali and mixed alkali phosphate glasses [45].

The hypersensitive transition can also give the information regarding the covalency nature of rare earth‐ligand interaction, which can be determined by the shift of hypersensitive band position to lower or higher wavelength due to nephelauxetic effect. From Figure 6b, it is observed that for lithium to sodium glass matrices, the peak wavelength of HST shifts toward higher wavelength side and the Ω2 parameter also increases indicating that the structural changes are not influencing the covalence bond. For lithium to potassium and lithium‐potassium glass matrices, the peak wavelength of HST does not change but the Ω 2 parameter increased indicating that the some structural changes are influencing the Nd‐O bond. For sodium to potassium and sodium‐potassium glass matrices, the peak wavelength of HST shifts toward lower wavelength side but Ω2 parameter increased (for Na to K) and decreased (for Na to Na‐K) indicating that the structural changes are influencing the Nd‐O bond.

Due to the zero values of certain reduced matrix elements ‖Uλ‖of Nd3+ ion, certain lasing transitions can be uniquely characterized by the ratio of intensity parameters Ω4 and Ω6, which is known as spectroscopic quality factor (χ) and are shown in Table 2. If χ < 1, the 4F3/24I11/2 transition shows stronger intensity than that of 4F3/24I9/2 transition. From the Table 2 data, the obtained χ value is <1 for Nd3+‐doped phosphate glasses, indicating that the intensity of 4F3/24I11/2 transition at 1064 nm will be stronger for various phosphate glasses [53].

3.2. Fluorescence analysis and radiative properties

The emission spectra of Nd3+‐doped phosphate glass recorded at room temperature in the wavelength region 800–1500 nm under excitation wavelength, 514.5 nm of Ar3+ laser are shown in Figure 5b. Emission spectra show three emission peaks due to the transitions, 4F3/24I9/2, 4F3/24I11/2, and 4F3/24I13/2 nearly centered at 902, 1069, and 1340 nm, respectively. The stimulated emission cross section is an important parameter and its value is related to the rate of energy extraction from the optical material. The Judd‐Ofelt theory can be applied to laser glasses and can successfully account for the induced emission cross sections that are observed.

The efficiency of a laser transition is evaluated by considering the stimulated emission cross section, and it is related to the radiative transition probability. It can be obtained from the emission spectra using Fuchtbauer‐Ladenburg method [54]


where λp is peak wavelength, and Δλeff is the effective line width. The effective line width Δλeff is obtained from

ΔλeffI(λ)Imax dλ

where I(λ) is the integrated fluorescence intensity and Imax is the peak fluorescence intensity. The radiative transition probability Arad(ψJ, ψ'J') for emission from an initial excited state ψJ to a final ground state ψ'J' is


Where the factor n(n2+2)29 represents the local field correction term for the ion in a medium, υ is the energy of transition, and n is the refractive index of the glass.

The radiative lifetime (τR) of an excited state ψ'J' is calculated from


The fluorescence branching ratio, βR, predicts the relative intensity of lines from a given excited states and characterizes the lasing potency of that particular transition. In order to choose suitable lasing transition, one has to select the transition having branching ratio >0.5 and the energy difference of about 3000 cm-1 between the emitting level and the next lower level. The fluorescence branching ratio (βR) is given by


Table 4 shows critical parameters to the laser designer such as branching ratios and radiative life times, peak stimulated emission cross sections of 1062 nm laser line in various phosphate glasses.

Glassesτexp (μs)τcal (μs)η (%)βrad (%)Δλ (nm)σp (cm2)
55P2O5‐17K2O‐11.5Bao‐6BaF2‐9Al2O3 [55]210189905925.16.23
58.5P2O5‐17K2O‐14.5MgO‐9Al2O3 [56]262249746828.84.41
57P2O5‐14.5K2O‐28.5BaO [57]178430415329.32.78
0.4MgO‐0.4BaF2‐0.1Al(PO3)3‐0.1Ba(PO3)3 (wt.%) [58]1853086049322.97
APG1 [59]38536110627.83.4
APG2 [59]46445610131.52.4
HAP4 [60]35027.03.6
HAP3 [60]38037210227.93.2
Q89 [61]35021.23.8

Table 4.

Laser emission properties at 1060 nm of Nd3+‐doped phosphate glasses

τexp: radiative life time; τrad: radiative lifetime from J‐O theory; η: quantum efficiency; β: branching ratio; σP: peak emission cross section (×1022 cm2). (APG1, APG2, HAP4, HAP3, and Q89 are commercial phosphate glasses).

4. Rare earth‐doped optical amplifiers

Rare earth‐doped fibers have received growing attention recently. They can be used as amplifiers in optical communication systems and as optical sources. Optical amplifiers amplify an incident weak light signal through the process of stimulated emission. The main ingredient of any optical amplifier is the optical gain realized when the amplifier is optically pumped to achieve population inversion. The optical gain, in general, depends mainly on the doping material, on the frequency (or wavelength) of the incident signal, and also on the local beam intensity at any point inside the amplifier. By a proper choice of the doping materials, the amplifier characteristics such as the operating wavelength and the gain bandwidth can be modified. Many different dopants such as erbium (Er3+), holmium (Ho3+), neodymium (Nd3+), samarium (Sm3+), thulium (Tm3+), and ytterbium (Yb3+) can be used to realize fiber amplifiers operating at different wavelengths covering a wide region extending over 0.5–3.5 μm and the most commonly used hosts are silicate, phosphate, fluoride, and tellurite glasses. Figure 7 shows working of active fibers within a specific wavelength range determined by various rare earth ions. Specially, optical telecommunication transmission [wavelength division multiplexing (WDA)] systems work in the conventional C‐band (1530–1565 nm) telecommunication window. This band can easily be observed in erbium (Er3+) ions among the rare earths and is the most useful dopant for commercial optical amplifiers and erbium‐doped fiber amplifiers (EDFAs) that are made from silicate and phosphate glass matrices. The schematic representation of Er3+‐doped fiber amplifiers (EDFA) is shown in Figure 8. An erbium‐doped fiber is pumped optically by an infrared laser sources at 980 or 1480 nm and are compatible with InGaAs and InGaAsP laser diodes. The three‐level pumping process in EDF is illustrated in Figure 9.


Figure 7.

Rare earth‐doped fibers working wavelength ranges.


Figure 8.

Erbium‐doped fiber amplifier (EDFA).


Figure 9.

Simplified energy band diagram of Er3+‐doped silica fiber. (a) Three‐level process. (b) Two‐level process.

When a laser source tuned at 980 nm is used to pump the EDF, the Er3+ ions move from ground level (4I15/2) to excited level (4I15/2). The ions stay excited level only about 10-6 s and after that they decay into a metastable level through multirelaxation process. In this process, the energy loss is turned into mechanical vibrations in the fiber. Finally, ions stay in the order of 10-3 s in metastable level, which is longer than the ion lifetime in the excited level and decay to the ground level with emission of photons in the 1530 nm wavelength region. Therefore, under 980 nm pumping, almost all the ions will be accumulated in the metastable level, and the three‐level system can be simplified into two levels for most of the practical applications, as shown in Figure 9b. Whereas in the case of 1480 nm wavelength pumping, ions excited from ground level to the metastable level directly. Therefore, 1480 nm pumping is more efficient than 980 nm pumping because it does not involve the nonradiative transition and is often used for high‐power optical amplifiers. However, amplifiers with 1480 nm pump usually have higher noise than the ones pumped at 980 nm.

Optical pumping provides the necessary population inversion between the energy levels E1 and E2, which in turn provides the optical gain defined as g = σ(N2 - N1), where σ is the transition cross section. Normally, in laser and amplifiers, the gain coefficient decreases as signal power increases and is called as gain saturation. The gain coefficient can be defined as [62] follows:


where G0 is the small‐signal gain coefficient at a given wavelength, P is the signal power, and Ps is the saturated signal power. The absorption and broadened gain spectra of EDFA (see Figure 10) are the advantages for Wavelength Division Multiplexing (WDM) applications.


Figure 10.

Absorption and gain spectra of EDFA.

Wave division multiplexing optical transmission system requires a flat gain spectrum of EDFA across the whole usable bandwidth. It is difficult to achieve the gain flatness in WDM system because EDFA has the narrow high gain in the C‐hand wavelength region (1530–1570 nm) centered at 1550 nm. In recent decades, many glass hosts for Er3+ ions have been investigated to realize the optical amplifier. For the flattened gain performance of optical amplifier, Er3+doped fluoride [63], tellurite [64], and bismuth [65] based glasses are capable of realizing a flat gain over a broadband width of 1530–1560 nm.

5. Luminescent solar concentrators (LSC)

A schematic of luminescent solar concentrator (LSC) is shown in Figure 11a. LSC is working based on the absorption of sun light by a transparent material that has been doped with high quantum efficiency luminescent ions and subsequently re‐emit by transparent material. The resulting luminescence is propagate by total internal reflection in transparent material and concentrated onto the solar cell, which attached to the edge of the transparent material to convert trapped emission light into electricity. The LSC was first proposed in late 1970s [66]. A typical design consists of a polymer plate doped with a luminescent material, such as a fluorescent organic dye, with solar cells optically matched to the plate edges. In recent decades, luminescent transparent material extends to availability of inorganic luminescence materials such as semiconductor quantum dots (QDs) [67], rare earth‐doped materials [68] and semiconductor polymers [69]. Particularly, rare earth‐doped glasses are still great interest due to their potential use as solid‐state lasers and luminescence solar concentrators.


Figure 11.

(a) Schematic diagram of luminescent solar concentrator. (b) Emission of solar radiation.

Figure 11b shows emission spectra of (AM1) solar radiation. It is known that the efficiency of solar cell is mainly limited by the loss of photons with much higher energy than the band gap of the photovoltaic solar cell. Normally, photovoltaic cells are made with silicon, which absorb photons only with energy greater than 1.1 eV and it is improving the performance of LSC. The luminescent material usually absorbs all wavelengths below 950 nm. Above 950 nm, the luminescence consists of a strong emission band in the range from 950 to 1000 nm [70]. This region is more reliable to increase the spectral response of the Si Photo‐voltaic cell in LSC (see shaded region in figure). In addition the number of photons in the LSC is double while extending the absorption from visible (300–600 nm) out to the NIR (300–900 nm) range. The extending wavelength increased nonradiative recombination due to increasing molecular dimensions and decreasing probability of radiative transitions [71]. This leads to decrease in fluorescence quantum yield (FQY). In rare earth‐doped materials, the FQY vary greatly depending on host materials and concentration, but values >90% have been reported in glass substrates [71]. The FQY is defined as follows:

ηFQY= NoofemittedphotonsNo.ofabsorbedphotons×100%


Figure 12.

Absorption and emission spectra of Nd3+ and Yb3+ ions.

A high quantum yield (~95–100%) is essential for good LSC performance. Rare earth ions such as neodymium (Nd3+) and ytterbium (Yb3+) in glassy hosts exhibit high FQY (~90%) values due to range at 800–1300 nm (Nd3+) and 1000 nm (Yb3+) emission peaks. When Nd3+/Yb3+ co‐doped glasses are used as LCS, Nd3+ acts as sensitizer due to its numerous absorption bands in visible region, and Yb3+ acts as activator due to its single emission peak at ~1000 nm. Moreover, this material absorbs 20% of the solar spectrum in the range 440–980 nm range [72]. Figure 12 shows absorption (for Nd3+) and emission (for Nd3+, Yb3+) spectra for realizing the LCS. It is observed that the emission peak at ~1000 nm is matched perfectly to the maximum spectral response of a Si‐Solar cell.

6. Conclusions

In summary, this chapter consists of a brief discussion on electronic energy‐level structure of RE3+ in solids, structural features, and general considerations in lasers glasses such concentration quenching, phonon interaction, ion‐ion interaction, and upconversion mechanisms. Judd‐Ofelt intensity parameters (Ωλ, λ = 2, 4, and 6) on Nd3+‐doped phosphate glasses and relevant theory were considered. We also included a study on the covalency between rare earth ion and the surrounding oxygen in the glass that increases the intensity of hypersensitive transitions for the case of alkali‐ and mixed alkali phosphate‐based glasses. Laser parameters such as emission cross sections (σp), radiative lifetime (τ), branching ratios (β), peak bandwidth (Δλ), and quantum efficiency on Nd3+‐doped phosphate glasses are reported. Laser emission process in three‐level and two‐level‐based silica‐EDFA was discussed as well as significant features of rare earth‐doped luminescent solar concentrators.


The authors would like to thank to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), PNPD/CAPES, and FAPEMIG.


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