- Email: [email protected]
Site-effects on the laser emission of Nd3+ ions in a new fluoride glass
Journal of Non-Crystalline Solids 213 & 214 Ž1997. 271–275
Site-effects on the laser emission of Nd 3q ions in a new fluoride glass J. Azkargorta a , I. Iparraguirre a , R. Balda a , J. Fernandez ´ b b E. Denoue , J. Lucas ´ a
a,)
, J.L. Adam b,
Departamento de Fısica Aplicada I, Escuela T.S. de Ingenieros Industriales y de Telecomunicacion, ´ ´ UniÕersidad del Paıs ´ Vasco, Alameda de Urquijo s r n, 48013 Bilbao, Spain b Laboratoire des Verres et Ceramiques, Campus de Beaulieu, 35042 Rennes cedex, France ´
Abstract Site-selective spectroscopy and stimulated emission experiments were performed in the 4 F3r2 ™4 I 11r2 laser transition of Nd in a new fluoride glass with composition Žin mol%. 30BaF2 –18InF3 –12GaF3 –20ZnF2 –9LuF3 –6ThF4 –4MnF2 –1NdF3 ŽBIGaZLuTMn.. The experiments show the existence of simultaneous laser emission from two distinguishable subsets of Nd 3q sites. The stimulated emission outputs are at 1050.2 and 1054.9 nm. The temporal behavior of the laser emission spectra shows wavelength dependence in accordance with the existence of broad Nd 3q site distributions. 3q
PACS: 42.55.Nw; 42.65.Ft; 78.47.p; 78.90.q t
1. Introduction A simple approach to the study of glass structure has conventionally considered the existence of some short-range ordering around the so-called networkforming ŽNWF. elements. In most cases, the network is built up by an arrangement of first neighbor coordination polyhedra, and their interconnection and, therefore, network-dimensionality, is adjusted by the network-modifying ŽNWM. elements such as alkali or alkaline-earth ions. In this picture, known as the ‘continuous random network’ ŽCRN., the NWM ions are supposed to be in a random spatial distribution. However, recent results w1x about different kinds
)
Corresponding author. Fax: q34-4 441 4041; e-mail: [email protected].
of glasses make clear that local ordering around NWM ions is greater than predicted by the CRN model because they do not spread uniformly throughout the glass. The optical properties of rare-earth doped glasses are closely related to the local structure and bonding at the ion site w2x and for this reason have been commonly used as probes for local ordering. Although rare earths are not randomly distributed throughout the glass they may enter as former ions. Their optical spectra show, even at low concentrations, an inhomogeneous broadening which is the evidence of large site-to-site crystal field variations. Several laser spectroscopic techniques, such as fluorescence line narrowing ŽFLN., spectral hole burning, etc., w3,4x have been developed to isolate individual subsets of ions within the inhomogeneously broadened distribution. In recent FLN investigations,
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 6 7 0 - 9
272
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
Durville et al. w5x found anomalies in the homogeneous linewidth of the 5 D 0 ™7 F0 transition of Eu3q at positions of spectral structure within the inhomogeneous band profile for both lithium silicate oxide glass and zirconium–barium–lanthanum–aluminium ŽZBLA. fluoride glass. The authors interpret these anomalies as possible abrupt changes in local site symmetry which might occur when active ions change from network-former to modifiers. Actually, rare earth ions have been identified in both positions in zirconium based fluoride glasses w6x. In recent works w7–12x, some of the present authors have studied the optical properties of Cr 3q, Nd 3q and Eu3q ions in two families of heavy-metal and transition-metal fluoride glasses by using time resolved laser spectroscopy. The wavelength dependence of 4 T2 ™4A 2 Cr 3q emission and the site-to-site variations in the local environment of the Eu3q ions in heavy-metal fluoride glasses Žby using time-resolved laser-excited fluorescence line narrowing techniques. supplied evidence about the existence of two main statistical site distributions together with a high local ordering for the studied ions in these fluoride glasses. In a more recent investigation w13x laser action was obtained for the first time in Nd 3q doped BIGaZLuTMn by flashlamp pumping, and site-dependent effects on time-resolved stimulated emission were pointed out. In this work, we present new results on luminescence and stimulated emission experiments performed in the 4 F3r2 ™4 I 11r2 laser transition of Nd 3q in this new fluoride sample, whose composition is Žin mol% . 30BaF2 – 18InF 3 – 12GaF3 – 20ZnF2 – 9LuF3 –6ThF4 –4MnF2 –1NdF3 ŽBIGaZLuTMn.. For the first time in this sample, the experiments show simultaneous laser emission around two different wavelengths Ž1050.2 and 1054.9 nm.. These emissions could be associated with different subsets of Nd 3q sites in the glass, showing that ions in different regions of the inhomogeneous distribution act independently and are able to burn two holes in the saturated inversion at frequencies which correspond to the center of the two main atomic transitions associated with the two different Nd 3q subsets. This interpretation also considers the accidental coincidence of stimulated emission from Nd 3q in different sites and involving transitions between different pairs of energy levels.
2. Experimental techniques Laser emission was obtained in a one meter confocal resonator with mirrors having 99% reflectivities at 1.06 mm by pumping an uncoated glass rod 6.35 mm in diameter and 42 mm long with flashlamps. The laser emission threshold was achieved with a lamp discharge energy of 34 J. The maximum output energy, about 2 mJ, was obtained with the greatest available discharge energy Ž60 J.. The laser spectrum was recorded by using a one meter spectrometer ŽSpex 1702r04. with variable slits and a cooled extended infrared photomultiplier connected to a fast digital oscilloscope. The attained resolution was 0.02 nm. The steady-state emission measurements were made using the intermediate tuning range Ž800–920 nm. of a Ti-sapphire ring laser Ž12 GHz bandwidth. as exciting light. Fluorescence was analyzed with a 0.22 meter monochromator ŽSpex.. The signal was detected by a extended IR photomultiplier ŽHamamatsu R7102. and finally amplified by a standard lock-in technique.
3. Results 3.1. Laser spectra Two laser emission lines were observed at 1050.2 and 1054.9 nm with similar threshold input energies. An average of 24 pulses at each selected wavelength was made to display the spectral shape of the laser output. The measured single pulse energy dispersion at each selected wavelength was about 5%. Fig. 1 shows the resulting spectrum for two different input energies. Some interesting features are noticeable: The line at the longer wavelength is wider than the other one. The ratio between the two intensity peaks does not change within errors of measurement, when increasing pump power. Both lines broaden as pump power increases, but this effect is greater in the low energy laser line. A slight shift in the peak positions Žabout 0.1 nm. is also observed when increasing pump power. The analysis of the time resolved Nd:BIGaZLuTMn lasing pulse averaged over 24 pump pulses shows the existence of wavelength depen-
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
273
high energy wing last more than those at the low energy side of the stimulated emission band. Fig. 2 also displays the normalized delivered output energy at the two wavelengths. As can be seen, at the shorter wavelength the energy delivering process occurs for a longer time, and, as a consequence, some blue shift is present as a function of time for this emission peak. 3.2. Spectroscopic measurements The low temperature excitation spectra of the F3r2 level shown in Fig. 3, were obtained with a tunable Ti-sapphire ring laser. These spectra, which correspond to different emission wavelengths, show, as expected, two main broad bands associated with the two Stark components of the 4 F3r2 doublet. The low energy one clearly shows the existence of at least two components. This behavior is a consequence of contributions from Nd 3q ions residing in a multiplicity of environments. The monochromatic 4
Fig. 1. Laser output spectra at two different input energies: 45 J Ž=. and 60 J Ž(. of flashlamp discharge.
dence. Fig. 2 shows, as an example, the relaxation oscillations at two different wavelengths at the high and low energy wing of the 1050.2 emission peak. As can be observed, the oscillations recorded at the
Fig. 2. Laser output intensities Žtop. and normalized delivered output energies Žbottom. averaged for 24 pulses as a function of time for two wavelengths at both sides of the emission centered at 1050.2 nm.
Fig. 3. Low temperature steady-state Ž4.2 K. excitation spectra of 4 I 9r 2 ™4 F3r2 transition for luminescence monitored at different wavelengths within the 4 F3r 2 ™4 I 11r2 emission band.
274
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
radiation excites an isochromat corresponding to a subset of sites, which may not be physically identical. Therefore, the emission line profile is a composite of emissions from two or more statistical site distributions which may have different natural homogeneous linewidths. The inhomogeneous character of the Nd 3q luminescence in the glass matrix, can be also analyzed by investigating the decay rate of the metastable state, by using non-resonant site-selective excitation in the 4 I 9r2 ™ Ž4G5r2 , 4 G 7r2 . hypersensitive transition at low temperature. The lifetime Žwhich remains single exponential. does not exhibit a monotonic variation with wavelength as expected for only one kind of statistical site distribution for Nd 3q ions Žsee Fig. 5 of reference w13x.. The change of the fluorescence decay times amounts to 15% in the excitation wavelength interval studied. The steady-state emission measurements were obtained from excitation in the 4 I 9r2 ™4 F3r2 transi-
Fig. 4. Low temperature steady-state Ž4.2 K. emission spectra of the 4 F3r 2 ™4 I 11r2 transition for different excitation wavelengths along the low energy Stark component of the 4 F3r 2 level.
tion. Fig. 4 shows the steady-state emission spectra at different excitation wavelengths measured at 4.2 K. As can be observed the shape of the emission band changes and develops a second peak as the excitation goes to low energy through the low energy Stark component of the 4 F3r2 doublet. The wavelength positions of these two peaks roughly agree with those found for the two laser lines.
4. Discussion The problem of energy extraction from inhomogeneously broadened lasing media is of importance, not only from a fundamental point of view to understand the complexities of a glass, but also for achieving practical applications. If the inhomogeneous medium is under large-signal gain operation, monochromatic extraction selectively de-excites those ions which are resonant with the beam, causing hole burning and reduced energy extraction compared to a homogeneous system of the same initial small-signal gain w4x. Nevertheless, when the ratio between homogeneous and inhomogeneous linewidths is appropriate there may be a few resonant sites with cross-sections large enough to lase. This may be the case in our system which has two distinguishable features in the emission spectra near the lasing wavelengths. In this energy region, as shown in Fig. 3, the low temperature 4 I 9r2 ™4 F3r2 excitation spectra also develop two peaks in the low Stark component of the 4 F3r2 state. Moreover, as we mentioned before, the nonmonotonic step-like variation of the lifetimes with wavelength across the 4 I 9r2 ™ Ž4 G5r2 , 4 G 7r2 . hypersensitive transition cannot be explained if just one kind of site distribution were present. In addition, comparison of the time resolved lasing pulse at the high-energy side Ž1049.6 nm. and at the low-energy side Ž1050.6. of the high-energy laser peak Ž1050.2 nm. Žsee Fig. 2., clearly shows that at the shorter wavelength the energy delivering process occurs for a longer time, in agreement with the different lifetimes of field sites. We emphasize that the interpretation of the experimental results given above does not exclude the possibility of an accidental coincidence of stimulated emissions from Nd 3q ions in different sites and involving identical or different pairs of Stark levels.
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
275
5. Conclusions
References
In summary, we believe that our experiments give further insight into the inhomogeneity problem in glasses and have proved the existence of two distinguishable site distributions for Nd 3q in a fluoride glass. Although the precise nature of the atomic ordering in these glasses is not yet well known, recent 19 F NMR studies in zirconium–barium– lanthanum fluoride glass w14x have revealed the presence of three discrete fluoride-ion sites in the glass structure which could give account of the differences in bonding coordination for Nd 3q ions in these glasses.
w1x J. Wang et al., J. Non-Cryst. Solids 163 Ž1993. 261, and references therein. w2x M.J. Weber, J. Non-Cryst. Solids 123 Ž1990. 208. w3x M.J. Weber, in: Laser Spectroscopy of Solids 49, ed. W.M. Yen and P.M. Selzer ŽSpringer, Berlin, 1981. p. 189, and references therein. w4x M.J. Weber, J. Non-Cryst. Solids 47 Ž1982. 117. w5x F. Durville, U.S. Dixon and R.C. Powell, J. Lumin. 36 Ž1987. 221. w6x J. Lucas et al., J. Non-Cryst. Solids 27 Ž1978. 273. w7x R. Balda et al., Phys. Rev. B44 Ž1991. 4759. w8x J. Fernandez et al., J. Non-Cryst. Solids 131–133 Ž1991. ´ 1230. w9x M.J. Elejalde et al., Phys. Rev. B46 Ž1992. 5169. w10x M.A. lllarramendi, R. Balda and J. Fernandez, Phys. Rev. ´ B47 Ž1993. 8411. w11x R. Balda et al., J. Phys.: Condens. Matter 6 Ž1994. 913. w12x R. Balda, J. Fernandez, H. Eilers and W.M. Yen, J. Lumin. ´ 59 Ž1994. 81. w13x J. Azkargorta, I. Iparraguirre, R. Balda, J. Fernandez, E. ´ Denoue and J.L Adam, IEEE J. Quant. Electron. 30 Ž1994. ´ 1862. w14x D.R. MacFarlane, J.O. Browne, T.J. Bastow and F.W. West, J. Non-Cryst. Solids 108 Ž1989. 289.
Acknowledgements The investigations were supported by the Comision Interministerial de Ciencia y Tecnologıa ´ of the Spanish Government ŽMAT93-0434. and by Accion ´ Integrada Hispano-Francesa ŽHF-82B..
Site-effects on the laser emission of Nd 3q ions in a new fluoride glass J. Azkargorta a , I. Iparraguirre a , R. Balda a , J. Fernandez ´ b b E. Denoue , J. Lucas ´ a
a,)
, J.L. Adam b,
Departamento de Fısica Aplicada I, Escuela T.S. de Ingenieros Industriales y de Telecomunicacion, ´ ´ UniÕersidad del Paıs ´ Vasco, Alameda de Urquijo s r n, 48013 Bilbao, Spain b Laboratoire des Verres et Ceramiques, Campus de Beaulieu, 35042 Rennes cedex, France ´
Abstract Site-selective spectroscopy and stimulated emission experiments were performed in the 4 F3r2 ™4 I 11r2 laser transition of Nd in a new fluoride glass with composition Žin mol%. 30BaF2 –18InF3 –12GaF3 –20ZnF2 –9LuF3 –6ThF4 –4MnF2 –1NdF3 ŽBIGaZLuTMn.. The experiments show the existence of simultaneous laser emission from two distinguishable subsets of Nd 3q sites. The stimulated emission outputs are at 1050.2 and 1054.9 nm. The temporal behavior of the laser emission spectra shows wavelength dependence in accordance with the existence of broad Nd 3q site distributions. 3q
PACS: 42.55.Nw; 42.65.Ft; 78.47.p; 78.90.q t
1. Introduction A simple approach to the study of glass structure has conventionally considered the existence of some short-range ordering around the so-called networkforming ŽNWF. elements. In most cases, the network is built up by an arrangement of first neighbor coordination polyhedra, and their interconnection and, therefore, network-dimensionality, is adjusted by the network-modifying ŽNWM. elements such as alkali or alkaline-earth ions. In this picture, known as the ‘continuous random network’ ŽCRN., the NWM ions are supposed to be in a random spatial distribution. However, recent results w1x about different kinds
)
Corresponding author. Fax: q34-4 441 4041; e-mail: [email protected].
of glasses make clear that local ordering around NWM ions is greater than predicted by the CRN model because they do not spread uniformly throughout the glass. The optical properties of rare-earth doped glasses are closely related to the local structure and bonding at the ion site w2x and for this reason have been commonly used as probes for local ordering. Although rare earths are not randomly distributed throughout the glass they may enter as former ions. Their optical spectra show, even at low concentrations, an inhomogeneous broadening which is the evidence of large site-to-site crystal field variations. Several laser spectroscopic techniques, such as fluorescence line narrowing ŽFLN., spectral hole burning, etc., w3,4x have been developed to isolate individual subsets of ions within the inhomogeneously broadened distribution. In recent FLN investigations,
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 6 7 0 - 9
272
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
Durville et al. w5x found anomalies in the homogeneous linewidth of the 5 D 0 ™7 F0 transition of Eu3q at positions of spectral structure within the inhomogeneous band profile for both lithium silicate oxide glass and zirconium–barium–lanthanum–aluminium ŽZBLA. fluoride glass. The authors interpret these anomalies as possible abrupt changes in local site symmetry which might occur when active ions change from network-former to modifiers. Actually, rare earth ions have been identified in both positions in zirconium based fluoride glasses w6x. In recent works w7–12x, some of the present authors have studied the optical properties of Cr 3q, Nd 3q and Eu3q ions in two families of heavy-metal and transition-metal fluoride glasses by using time resolved laser spectroscopy. The wavelength dependence of 4 T2 ™4A 2 Cr 3q emission and the site-to-site variations in the local environment of the Eu3q ions in heavy-metal fluoride glasses Žby using time-resolved laser-excited fluorescence line narrowing techniques. supplied evidence about the existence of two main statistical site distributions together with a high local ordering for the studied ions in these fluoride glasses. In a more recent investigation w13x laser action was obtained for the first time in Nd 3q doped BIGaZLuTMn by flashlamp pumping, and site-dependent effects on time-resolved stimulated emission were pointed out. In this work, we present new results on luminescence and stimulated emission experiments performed in the 4 F3r2 ™4 I 11r2 laser transition of Nd 3q in this new fluoride sample, whose composition is Žin mol% . 30BaF2 – 18InF 3 – 12GaF3 – 20ZnF2 – 9LuF3 –6ThF4 –4MnF2 –1NdF3 ŽBIGaZLuTMn.. For the first time in this sample, the experiments show simultaneous laser emission around two different wavelengths Ž1050.2 and 1054.9 nm.. These emissions could be associated with different subsets of Nd 3q sites in the glass, showing that ions in different regions of the inhomogeneous distribution act independently and are able to burn two holes in the saturated inversion at frequencies which correspond to the center of the two main atomic transitions associated with the two different Nd 3q subsets. This interpretation also considers the accidental coincidence of stimulated emission from Nd 3q in different sites and involving transitions between different pairs of energy levels.
2. Experimental techniques Laser emission was obtained in a one meter confocal resonator with mirrors having 99% reflectivities at 1.06 mm by pumping an uncoated glass rod 6.35 mm in diameter and 42 mm long with flashlamps. The laser emission threshold was achieved with a lamp discharge energy of 34 J. The maximum output energy, about 2 mJ, was obtained with the greatest available discharge energy Ž60 J.. The laser spectrum was recorded by using a one meter spectrometer ŽSpex 1702r04. with variable slits and a cooled extended infrared photomultiplier connected to a fast digital oscilloscope. The attained resolution was 0.02 nm. The steady-state emission measurements were made using the intermediate tuning range Ž800–920 nm. of a Ti-sapphire ring laser Ž12 GHz bandwidth. as exciting light. Fluorescence was analyzed with a 0.22 meter monochromator ŽSpex.. The signal was detected by a extended IR photomultiplier ŽHamamatsu R7102. and finally amplified by a standard lock-in technique.
3. Results 3.1. Laser spectra Two laser emission lines were observed at 1050.2 and 1054.9 nm with similar threshold input energies. An average of 24 pulses at each selected wavelength was made to display the spectral shape of the laser output. The measured single pulse energy dispersion at each selected wavelength was about 5%. Fig. 1 shows the resulting spectrum for two different input energies. Some interesting features are noticeable: The line at the longer wavelength is wider than the other one. The ratio between the two intensity peaks does not change within errors of measurement, when increasing pump power. Both lines broaden as pump power increases, but this effect is greater in the low energy laser line. A slight shift in the peak positions Žabout 0.1 nm. is also observed when increasing pump power. The analysis of the time resolved Nd:BIGaZLuTMn lasing pulse averaged over 24 pump pulses shows the existence of wavelength depen-
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
273
high energy wing last more than those at the low energy side of the stimulated emission band. Fig. 2 also displays the normalized delivered output energy at the two wavelengths. As can be seen, at the shorter wavelength the energy delivering process occurs for a longer time, and, as a consequence, some blue shift is present as a function of time for this emission peak. 3.2. Spectroscopic measurements The low temperature excitation spectra of the F3r2 level shown in Fig. 3, were obtained with a tunable Ti-sapphire ring laser. These spectra, which correspond to different emission wavelengths, show, as expected, two main broad bands associated with the two Stark components of the 4 F3r2 doublet. The low energy one clearly shows the existence of at least two components. This behavior is a consequence of contributions from Nd 3q ions residing in a multiplicity of environments. The monochromatic 4
Fig. 1. Laser output spectra at two different input energies: 45 J Ž=. and 60 J Ž(. of flashlamp discharge.
dence. Fig. 2 shows, as an example, the relaxation oscillations at two different wavelengths at the high and low energy wing of the 1050.2 emission peak. As can be observed, the oscillations recorded at the
Fig. 2. Laser output intensities Žtop. and normalized delivered output energies Žbottom. averaged for 24 pulses as a function of time for two wavelengths at both sides of the emission centered at 1050.2 nm.
Fig. 3. Low temperature steady-state Ž4.2 K. excitation spectra of 4 I 9r 2 ™4 F3r2 transition for luminescence monitored at different wavelengths within the 4 F3r 2 ™4 I 11r2 emission band.
274
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
radiation excites an isochromat corresponding to a subset of sites, which may not be physically identical. Therefore, the emission line profile is a composite of emissions from two or more statistical site distributions which may have different natural homogeneous linewidths. The inhomogeneous character of the Nd 3q luminescence in the glass matrix, can be also analyzed by investigating the decay rate of the metastable state, by using non-resonant site-selective excitation in the 4 I 9r2 ™ Ž4G5r2 , 4 G 7r2 . hypersensitive transition at low temperature. The lifetime Žwhich remains single exponential. does not exhibit a monotonic variation with wavelength as expected for only one kind of statistical site distribution for Nd 3q ions Žsee Fig. 5 of reference w13x.. The change of the fluorescence decay times amounts to 15% in the excitation wavelength interval studied. The steady-state emission measurements were obtained from excitation in the 4 I 9r2 ™4 F3r2 transi-
Fig. 4. Low temperature steady-state Ž4.2 K. emission spectra of the 4 F3r 2 ™4 I 11r2 transition for different excitation wavelengths along the low energy Stark component of the 4 F3r 2 level.
tion. Fig. 4 shows the steady-state emission spectra at different excitation wavelengths measured at 4.2 K. As can be observed the shape of the emission band changes and develops a second peak as the excitation goes to low energy through the low energy Stark component of the 4 F3r2 doublet. The wavelength positions of these two peaks roughly agree with those found for the two laser lines.
4. Discussion The problem of energy extraction from inhomogeneously broadened lasing media is of importance, not only from a fundamental point of view to understand the complexities of a glass, but also for achieving practical applications. If the inhomogeneous medium is under large-signal gain operation, monochromatic extraction selectively de-excites those ions which are resonant with the beam, causing hole burning and reduced energy extraction compared to a homogeneous system of the same initial small-signal gain w4x. Nevertheless, when the ratio between homogeneous and inhomogeneous linewidths is appropriate there may be a few resonant sites with cross-sections large enough to lase. This may be the case in our system which has two distinguishable features in the emission spectra near the lasing wavelengths. In this energy region, as shown in Fig. 3, the low temperature 4 I 9r2 ™4 F3r2 excitation spectra also develop two peaks in the low Stark component of the 4 F3r2 state. Moreover, as we mentioned before, the nonmonotonic step-like variation of the lifetimes with wavelength across the 4 I 9r2 ™ Ž4 G5r2 , 4 G 7r2 . hypersensitive transition cannot be explained if just one kind of site distribution were present. In addition, comparison of the time resolved lasing pulse at the high-energy side Ž1049.6 nm. and at the low-energy side Ž1050.6. of the high-energy laser peak Ž1050.2 nm. Žsee Fig. 2., clearly shows that at the shorter wavelength the energy delivering process occurs for a longer time, in agreement with the different lifetimes of field sites. We emphasize that the interpretation of the experimental results given above does not exclude the possibility of an accidental coincidence of stimulated emissions from Nd 3q ions in different sites and involving identical or different pairs of Stark levels.
J. Azkargorta et al.r Journal of Non-Crystalline Solids 213 & 214 (1997) 271–275
275
5. Conclusions
References
In summary, we believe that our experiments give further insight into the inhomogeneity problem in glasses and have proved the existence of two distinguishable site distributions for Nd 3q in a fluoride glass. Although the precise nature of the atomic ordering in these glasses is not yet well known, recent 19 F NMR studies in zirconium–barium– lanthanum fluoride glass w14x have revealed the presence of three discrete fluoride-ion sites in the glass structure which could give account of the differences in bonding coordination for Nd 3q ions in these glasses.
w1x J. Wang et al., J. Non-Cryst. Solids 163 Ž1993. 261, and references therein. w2x M.J. Weber, J. Non-Cryst. Solids 123 Ž1990. 208. w3x M.J. Weber, in: Laser Spectroscopy of Solids 49, ed. W.M. Yen and P.M. Selzer ŽSpringer, Berlin, 1981. p. 189, and references therein. w4x M.J. Weber, J. Non-Cryst. Solids 47 Ž1982. 117. w5x F. Durville, U.S. Dixon and R.C. Powell, J. Lumin. 36 Ž1987. 221. w6x J. Lucas et al., J. Non-Cryst. Solids 27 Ž1978. 273. w7x R. Balda et al., Phys. Rev. B44 Ž1991. 4759. w8x J. Fernandez et al., J. Non-Cryst. Solids 131–133 Ž1991. ´ 1230. w9x M.J. Elejalde et al., Phys. Rev. B46 Ž1992. 5169. w10x M.A. lllarramendi, R. Balda and J. Fernandez, Phys. Rev. ´ B47 Ž1993. 8411. w11x R. Balda et al., J. Phys.: Condens. Matter 6 Ž1994. 913. w12x R. Balda, J. Fernandez, H. Eilers and W.M. Yen, J. Lumin. ´ 59 Ž1994. 81. w13x J. Azkargorta, I. Iparraguirre, R. Balda, J. Fernandez, E. ´ Denoue and J.L Adam, IEEE J. Quant. Electron. 30 Ž1994. ´ 1862. w14x D.R. MacFarlane, J.O. Browne, T.J. Bastow and F.W. West, J. Non-Cryst. Solids 108 Ž1989. 289.
Acknowledgements The investigations were supported by the Comision Interministerial de Ciencia y Tecnologıa ´ of the Spanish Government ŽMAT93-0434. and by Accion ´ Integrada Hispano-Francesa ŽHF-82B..