In this work we present a detailed study about the influence of the host matrix in the spectroscopic and laser properties of Nd3+ in three different fluoride glasses. Site-selective time-resolved techniques have been used to investigate the crystal field changes felt by the Nd3+ ion as a consequence of glass inhomogeneity. Stimulated emission experiments performed under selective wavelength laser pumping show the existence of bichromatic emission from two distinguishable site distributions for Nd3+ in fluoride glasses. This result can be explained by the moderate inter-site energy transfer among Nd3+ ions found in these systems.
©2008 Optical Society of America
From the first report on laser action in glass 47 years ago , the search for new laser glass matrices, new laser ions, and new laser transitions still remains of great interest for a wide field of applications, such as high power lasers for industrial applications, inertial confinement fusion research, glass fiber lasers and amplifiers emitting at different wavelengths for telecommunications, biomedical and environment applications, laser cooling, optical sensors, and more recently femtosecond fiber lasers.
Referring to the laser devices, most of the glass lasers have used trivalent lanthanides as active ions, due to low coupling of the rare earth (RE) with the host vibrations, which moderates the nonradiative emission from the excited electronic level and therefore increases the quantum efficiency of the emitting level. On the other hand, the optical properties of REdoped glasses are closely related to local structure and bonding at the ion site and for this reason have been commonly used as probes for local ordering. Although rare earth in glasses may or may not enter as former ions, their optical properties show, even at low concentrations, an inhomogeneous broadening which is the evidence of large site-to-site crystal field variations. As a consequence, rare earth ions may offer some laser tunability as a result of inhomogeneous broadening of the optical transitions and, moreover, the laser parameters can be somewhat controlled by varying the chemical composition of the host glass. Among lanthanides, Neodymium is one of the most investigated active ions not only by its research and industrial applications, but also because the trends in properties varying with composition observed for Nd3+ are also applicable to other rare earths .
The inhomogeneous nature of the spectroscopic properties of rare-earth ions in glasses due to site effects, which is good in optical pumping with broad band sources for lasers operating under small-signal gain conditions, may seriously affect the optimum energy extraction at large-signal gain operation because the gain is no longer simply proportional to the stored energy . In this case, the distribution of the spectroscopic parameters from site to site must be considered. These effects can be observed and quantified by using fluorescence linenarrowing (FLN) techniques [4,5] which allow to obtain a detailed information about the local field, ion-ion and ion-host interaction processes.
Ion-ion interactions in highly concentrated neodymium materials is a matter of practical as well as theoretical importance. Due to the inherent disorder of glass, ions in nearby sites may be in physically different environments with greatly varying spectroscopic properties. Therefore, in addition to causing a spatial migration of energy, the transfer may also produce spectral diffusion within the inhomogeneously broadened spectral profile . The migration of the electron excitation over the inhomogeneous profile (spectral migration) determines the effectiveness of the stimulated emission generation (amplification) .
On the other hand, much work has been done over the last years in order to search for new compositions leading to stable and robust low phonon solids for rare earth doping. Among all, fluoride glasses have been a subject of interest in the investigation of passive and active optical applications including optical waveguides, optical amplifiers, and laser hosts materials . Their ability to incorporate a substantial amount of rare earth ions which become part of the glass-forming framework, their wide infrared transparency in the fiber configuration (0.3-5 μm) which allows pumping and lasing over a wide spectral range, and their high emission efficiency due to multiphonon emission rates which are lower than in other glasses, make them attractive candidates for laser applications . Laser action has been previously reported in RE-doped heavy-metal fluoride glasses, both in bulk and fiber form [9-16]. The lasing properties of Nd3+ have been reported in fluorozirconate glasses in a fiber configuration [14-15], in bulk form , and in glass microspheres . In a previous work , the authors have demonstrated laser action in a Nd3+-doped BIGaZLuTMn fluoride glass in bulk, pumped by flashlamps.
In the first part of this work we present a review of the optical properties of Nd3+ in some emblematic heavy metal and transition metal fluoride glasses. The study includes, together with general spectroscopic properties, such as emission peak wavelengths, stimulated emission cross-sections of the laser transition, and site-selective spectroscopy of Nd3+ in these matrices, a detailed analysis of the site-dependent energy diffusion among Nd3+ ions by using time resolved fluorescence line narrowing (TRFLN) spectroscopy. The results of this study reveal the influence of crystal field inhomogeneity at the Nd3+ sites on the laser performance of these materials.
The final part of the work is devoted to the generation of lasing action under wavelength selective laser-pulsed excitation in the three different fluoride glass samples studied. The results are discussed in order to analyze differences and similarities among them and with respect to other glass hosts. The main issue of this analysis is the experimental demonstration of the effect of the inhomogeneous site properties on the laser emission in these materials, in particular, the effect of the pumping wavelength on the spectroscopic behavior of the laser output. As we shall see, these results allow us to understand why and how the laser emission in these systems can be tuned and how this tunability is associated with the presence of two main groups of centers which correspond to Nd3+ ions located in different low-symmetry crystal field environments and which are responsible for the bichromatic laser emission observed in these systems.
The discussion includes a comparison between the results obtained in these fluoride glasses and those found by the authors in Nd3+-doped disordered yttrofluorite crystals which shows the great role played by crystal field inhomogeneities on the laser properties of glasses. It is worthy to mention that the experiments performed under laser-pulsed excitation closely resemble the system response for a Q-switch operation.
2. Materials and experimental spectroscopic techniques
Heavy-metal fluoride glass samples doped with different neodymium concentrations (0.1, 1, 2, 3 and 5 mol%) were prepared at the University of Rennes (France), whereas transition metal fluoride glass doped with 0.1, 1, 2, and 3 mol% was prepared at the University of Maine (France). The molar composition of the samples studied is given in Table 1.
The samples temperature was varied between 4.2 and 300 K in a continuous flow cryostat. Site-selective steady-state emission and excitation spectra were obtained by exciting the samples with a Ti-sapphire ring laser (0.4 cm-1 linewidth) in the 780-920 nm spectral range. The fluorescence was analyzed with a 0.25 m monochromator, and the signal was detected by a Hamamatsu R5509-72 photomultiplier and finally amplified by a standard lock-in technique.
Time resolved resonant fluorescence line narrowed emission measurements were obtained by exciting the samples with a Ti-sapphire laser, pumped by a pulsed frequency doubled Nd:YAG laser (9 ns pulse width), and detecting the emission with a Hamamatsu R5108 photomultiplier provided with a gating circuit designed to enable gate control from an external applied TTL level control signal. Data were processed by an EGG-PAR boxcar integrator.
3. Spectroscopic characterization of Nd3+-doped fluoride glasses
3.1. General spectroscopic properties
In this sub-section we present a summary of the general spectroscopic properties of the Nd3+-doped fluoride glasses which have been already worked out by some of the authors in a previous publication .
Data from the absorption spectra measurements together with the values of the refractive indices and Nd3+ ion concentrations were used to calculate the radiative transition rates and branching ratios for the fluorescence from the 4F3/2 to the 4IJ states by using the Judd-Ofelt theory . The calculated radiative lifetimes and the stimulated cross section for the laser transition are presented in Table 2 for the three glasses, together with the effective fluorescence linewidth. Since the emission bands are slightly asymmetric, an effective linewidth was determined by integrating the fluorescence line shape and dividing by the intensity at the peak wavelength of the fluorescence emission.
The decays from level 4F3/2 as a function of temperature were performed with a narrow-band tunable dye laser, by exciting the samples at the 4I9/2→4G5/2 absorption band in the 4.2-300 K range, and were found to be single exponential at all temperatures and concentrations . The experimental lifetimes of the samples doped with 1 mol% of NdF3 are included in Table 2.
3.2. Site-selective spectroscopy
The inhomogeneous character of the Nd3+ luminescence in the glass matrix was analyzed by taking advantage of the tunability and narrow bandwidth of the Ti:sapphire laser. Figure 1 shows, as an example, the low temperature (4.2 K) excitation spectra of the 4I9/2→4F3/2 transition obtained at different emission wavelengths along the 4F3/2→4I11/2 transition for BIG glass doped with 1 mol% of NdF3. These spectra show, as expected, two main broad bands associated with the two Stark components of the 4F3/2 doublet; however, the low energy one clearly shows the existence of at least two components. This behavior is a consequence of contributions from Nd3+ ions in a multiplicity of environments. The monochromatic radiation excites an isochromat corresponding to a subset of sites, which may not be physically identical. Therefore, the emission line is a composite of emissions from two or more statistical site distributions which may have different natural homogeneous linewidths.
In the same way, the steady-state emission spectra of the 4F3/2→4I11/2 laser transition were obtained at low temperature for different excitation wavelengths along the low energy component of 4F3/2. As can be observed in Fig. 2, the shape of the emission band in BIG glass changes and develops a second peak as excitation goes to low energy. A similar behavior is repeated in all fluoride glasses measured.
3.3. Time-resolved fluorescence line narrowing (TRFLN) spectroscopy
It is worth noticing that the above mentioned inhomogeneities observed in the optical properties of Nd3+-doped fluoride glasses are closely related to local structure and bonding at the ion site. As we mentioned above, the existence of different crystal field sites may produce spectral broadening and/or multiple emission lines with different spectral features which can influence the energy extraction from the material as well as the laser emission wavelength when it is used as a lasing medium. As a consequence, the knowledge of the precise crystal field structure of the rare-earth in a given material is of paramount importance to understand its potentialities for lasing applications.
To better understand the spectral features of our Nd3+-doped fluoride glasses we have performed time-resolved-fluorescence-line-narrowing (TRFLN) spectroscopy at 4.2 K by exciting with a monochromatic laser pulse (0.1 nm spectral width) inside the inhomogeneous broadened 4I9/2→4F3/2 absorption band of Nd3+ ions and resonantly collecting the emitted luminescence at different times. As an example, Fig. 3 shows, together with the non narrowed 4F3/2→4I9/2 emission, the TRFLN emission spectra of BIG glass obtained 5 μs after the laser pulse by exciting at different wavelengths in the low energy component of the 4F3/2 doublet. As can be seen, the TRFLN spectra change with excitation wavelength and show two contributing components: the one on the high energy side of the spectra is the FLN line with a width around 8 cm-1 corresponding to the resonant emission to the lowest Stark component of the ground state. The line position is determined by the wavelength of the pumping radiation. In addition to this line we observe a broad non selected emission which corresponds to background fluorescence due to ions excited by energy transfer and/or to non resonant emissions. It is noticeable that the features of this broad emission change as a function of the excitation wavelength showing the crystal field glass inhomogeneity.
As time delay increases the relative intensity of the narrow line and the broad component changes and the later becomes stronger, indicating the existence of energy transfer between discrete regions of the inhomogeneous broadened profile. This effect increases with concentration and produces a relative increase of the broad emission with respect to the narrow band; moreover the transfer process appears at shorter time delays. Figure 4(a) shows typical results for BIG glass with three different NdF3 concentrations obtained at two different time delays (5 μs and 500 μs) after the laser pulse by exciting at 870 nm.
The energy transfer process is also dependent on the excitation wavelength. The 4F3/2→4I9/2 spectra performed by exciting at different wavelengths along the low energy Stark component of the 4I9/2→4F3/2 absorption band show that the broad emission decreases as the wavelength of the excitation radiation increases, because the excitation energy can migrate mainly in one direction. Figure 4(b) shows this excitation energy dependence for the BIG sample doped with 3 mol%. The spectra were obtained at low temperature and at two different time delays (5μs and 500 μs) after the laser pulse.
The time evolution of the 4F3/2→4I9/2 TRFLN spectra can be used to analyze the energy transfer electronic mechanism, because it is produced by a combination of radiative decay and nonradiative transfer to other nearby ions. Subsequent fluorescence from the acceptor ions replicates the inhomogeneously broadened emission profile, and shows that transfer is not only to resonant sites but to the full range of sites within the inhomogeneous profile (see Fig. 4). In this case a quantitative measure of the transfer is provided by the ratio of the narrow line intensity to the total fluorescence intensity in the inhomogeneous band.
If the dispersion in the radiative decay rate is neglected, the Föster formula  for dipoledipole transfer allows to write the following relationship between the integrated background and narrow line intensities:
where γ represents the average transfer rate parameter at a given laser energy EL.
We have analyzed the TRFLN spectra obtained at different time delays between 5 and 600 μs according to this equation. As an example, Fig. 5(a) shows the results for BIG samples doped with 1, 3, and 5 mol% at low temperature under excitation at 872 nm. As can be observed the linear fit to t1/2 indicates that a dipole-dipole interaction mechanism among the Nd3+ ions dominates in this time regime. The γ values for this excitation wavelength were found to be 9.2, 22, and 52 s-1/2 for the samples doped with 1, 3, and 5 mol% respectively. These values indicate that energy transfer among Nd3+ ions is weak at concentrations up to 3 mol% and increases with concentration. Moreover, the analysis of the TRFLN spectra obtained at different excitation wavelengths shows that the transfer rate depends on excitation energy. As an example, and according to equation (1), the analysis of the TRFLN spectra for a BIG sample doped with 3 mol% at 4.2 K, shows that the energy migration rate increases with increasing excitation energy (see Fig. 5(b)). The value of the average transfer rate increases from 22 s-1/2 to 88 s-1/2 when the excitation wavelength decreases from 872 to 868 nm. This rise in the energy migration rate is due to the increasing number of possible acceptors. The average energy transfer rates found in these glasses are lower than those found in fluoroarsenate  and germanate glasses  for a similar concentration of Nd3+ ions.
Finally, and referring to the site dependence of the spectroscopic properties of Nd3+ in fluoride glasses, we can conclude that in spite of the strong inhomogeneous broadening, the inter-site energy transfer is moderate even at high rare-earth concentrations. As will be seen in the next section, this behavior explains why it is possible to tune the laser emission in these fluoride glasses by varying the excitation wavelength.
4. Stimulated emission experiments under wavelength selective pumping
In order to investigate the laser spectral dynamics of these materials under selective pumping conditions, we have used a 9 ns pulse-width Ti-sapphire laser (spectral halfwidth < 0.1 nm and about 30 mJ pulse energy) to pump the 4F3/2 or 4F5/2 levels of Nd3+ ions, around 860 and 790 nm respectively (depending on the absorption properties of the specific sample). The glass samples were polished slabs doped with different Nd3+ concentrations (1-5 mol %). A 15 cm long longitudinal symmetric confocal resonator with two high reflectance mirrors was implemented. The samples were placed at Brewster angle to minimize the resonator losses and situated slightly out of the pump focus to avoid thermal damage. The output laser pulses spectra were recorded with a diode-array Hamamatsu-Triax 190 spectrum analyzer by using a 1200 lines/mm diffraction grating, whereas the temporal evolution of the pumping and laser output was recorded with a fast fotodiode connected to a digital oscilloscope. The observed time delay between pump and output laser pulses was in the 150-400 ns range for the different samples and pumping levels. The pump energies used in our experiments are below the ones needed for saturation conditions. The typical absorbed pump pulse energy is about 50% of the incident one, which could give an excited state population of the order of 15% of the ground state population.
The results of these laser experiments show the critical influence of the pumping wavelength on the spectral behavior of the laser emission as well as the effect of Nd3+ concentration. As an example, Fig. 6 proves, in a Nd3+-doped BIG glass with 5 mol%, that it is possible to obtain laser action independently in two different spectral domains, separated about 8 nm, or to lase both lines simultaneously, the so called bichromatic laser emission, by using selective wavelength pumping. It is worthy to mention that there is not a significant dependence of the lasing spectral position on the pump energy (the maximum spectral shift is ~1 nm).
Figure 7 displays a 3D picture of the excitation wavelength dependence of the laser spectra obtained by plotting the laser emission intensity as a function of both excitation and emission wavelengths for the glass samples (a) BIG (1 mol%), (b) PZG (2 mol%), and (c) ZBLAN (2 mol%) when pumping into the 4F5/2 level. The slight differences among them are due to the different optical qualities of the materials and sample thickness but, essentially, all these fluoride glass samples show a similar behavior as far as laser emission is concerned. On the other hand, when pumping the 4F3/2 level in more concentrated Nd3+-doped samples, the resultant spectra show small differences with respect to those obtained by pumping the 4F5/2 state. Figure 8(a) shows, as an example, the behavior of the 5 mol% NdF3-doped BIG sample where a pronounced excitation valley in the long wavelength peak, corresponding to the absorption fall between the two 4F3/2 Stark components, is clearly observed. It is also worth mentioning the abrupt extinction of the laser emission at the long pumping wavelengths.
These results obtained in Nd3+-doped fluoride glasses point out to a direct relation between the site effects and the spectral behavior of the laser output. Moreover, they confirm the high degree of local order present in these systems in spite of their amorphous nature. It is worthy mentioning that in previous works [23,24] the authors have investigated the critical influence of the pumping wavelength on the spectral behavior of the laser emission in disordered Nd3+-doped yttrofluorite crystals. As in the present case, two main site distributions were found for the rare earth which can be lased, either independently or simultaneously, by exciting in a narrow wavelength range around 867 nm. Moreover, as we found for heavy Nd-doped glass samples, an enhancement of the longer laser line with respect to the shorter one was found as concentration increases. Figure 8 shows the laser emission of the 8% NdF3-Yttrofluorite sample and the 5% NdF3-BIG glass for comparison.
It is important to remark that the same kind of laser experiments performed by our group in other Nd3+-doped glass hosts such as oxyfluorides , fluoroarsenates, and tellurites  showed a different behavior with no significant changes in the laser emission spectra when the pumping wavelength was varied whatever the pumping level used. As an example Fig. 9, shows the 3D laser emissions of an oxyfluoride and a fluoroarsenate glasses obtained with the same experimental setup as the one used for fluoride glasses. Only slight line shifts, or little changes in line-width are observed due to the spectral inhomogeneity of the glass and to the variations of absorption cross section with the pumping wavelength.
We have presented in this work a spectral and dynamic study of the spontaneous and stimulated emissions of Nd3+-doped fluoride glasses which confirms the existence of two main broad distributions of crystal field sites for the rare earth.
The effect of the Nd3+ concentration in the glasses has been investigated by using site-selective time-resolved spectroscopy. In spite of the strong inhomogeneous broadening associated to the existence of two main broad site distributions of crystal field sites, the intersite energy transfer is moderate even at rare-earth concentrations as high as 5 mol%. This is the main reason why it is possible to obtain laser emission in these fluoride glasses in different spectral ranges, depending on the excitation energy.
Short laser pulse pumped experiments have shown the presence of two broad distinguishable laser lines corresponding to the above mentioned broad site distributions found in these glasses. Wavelength resolved pump excitation in these glasses allows for selecting the laser emission wavelength.
The behavior of the laser emission in Nd3+-doped fluoride glasses is similar to the one found in Nd3+-doped mixed fluoride crystals, which points out to similarities between the crystal field felt by the rare earth in disordered crystals and glasses, and clarifies the bichromatic emission observed in both systems.
This work was supported by the Basque Country Government (IT-331-07). The authors want to thank Prof. Jean-Luc Adam and Prof. Brigitte Boulard for supplying the glass samples.
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