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Homogeneous line width in a zinc borate glass activated by Eu3+
J O U R N A L OF
ELSEVIER
Journal of Non-Crystalline Solids 220 (1997) 217-221
Homogeneous line width in a zinc borate glass activated by Eu 3+ M. Sbetti a, E. Moser
a, M. Montagna a, M. Ferrari b,* S. Chaussedent c,,, M. Bettinelli d
a Dipartimento di Fisica, lstituto Nazionale per la Fisica della Materia, Uniuersith di Trento, l~ia Sommarit,e 14, 1-38050 Pot'o, Trento, ltaly b Consiglio Nazionale delle Ricerche CeFSA, Centro Fisica Stati Aggregati, via Sommarive 14, 1-38050 Porto, Trento, Italy c Dipartimento di Fisica, Istituto Nazionale per la Fisica della Materia, Universith di Padova, l:ia Marzolo 8, 1-35131 Padova, Italy a Facoltgt di Scienze MM.FF.NN., lstituto Policattedra, Unit,ersith di Verona, 1-37134 Verona, Italy
Received 10 March 1997
Abstract In this work we report on the temperature dependence of the homogeneous line width ( F h) in 4 Z n O . 3B203:Eu 3+ glass in the region between 30 K and room temperature. The time resolved resonant fluorescence line narrowing technique was applied to the 7F0 ~ 5 D 0 transition of Eu 3+. We obtain that F h follows the commonly found behaviour T 2 in the whole range of temperature (30-300 K). The F h is comparable to that measured in borate glasses with a small amount of network modifier. This result, together with a linear increase of F h with the excitation energy within the inhomogeneous profile, is indicative of a specific local environment around the Eu 3+ ion in the zinc borate glass. © 1997 Elsevier Science B.V.
1. Introduction
Glasses have a structural disorder which gives rise to inhomogeneous line broadening. The inhomogeneous line width ( F i) is due to the site dependence of the static crystal field and does not depend strongly on the temperature. On the contrary, the homogeneous line width ( F h) shows a power law temperature dependence F h = A T " with a = 1 in the very low temperature range and with ce-~ 2 at higher
* Corresponding author. Tel.: +39-461 881 684; fax: +39-461 881 680; e-mail: [email protected]. 1 Permanent address: P.O.M.A. Facult~ des Sciences, Universit~ d' Angers, 2 Bd. Lavoisier, F-49045 Angers cedex, France.
temperatures ( T > 20 K) (for a review see Refs. [1-3]). In developing glasses activated by trivalent lanthanide ions for applications such as optical materials, it is important to have information about the interaction occurring between the active ion and the host [4,5]. The dynamical interactions affect F h of the optical transitions and the line-narrowing methods provide information [6-8]. Moreover, the knowledge of the ratio of homogeneous-to-inhomogeneous line widths is of interest for glass lasers [9,10] and optical data storage [11] where a large F i to F h ratio is required. For temperatures higher than 20 K the time resolved resonant fluorescence line narrowing (TRRFLN) technique is the most suitable to obtain directly the temperature dependence of the homogeneous line width.
0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 2 8 4 - 6
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M. Sbetti et al. / Journal of Non-C~stalline Solids 220 (1997) 217-221
The spectroscopic properties of the glass used in this work have been investigated in detail in a previous work [12]. The zinc borate sample doped with 0.5% Eu 3+ had a relatively strong 5D 0 -->TF 0 e m i s s i o n transition and no spectral energy transfer was observed. The decay curves of the luminescence from the 5D 0 state can be described by a single exponential and the lifetimes are within errors of measurements, the same at room temperature (1.62 ms) and at 12 K (1.65 ms) [12]. These properties make the sample a good candidate for TRRFLN measurements.
2. Experimental The 4ZnO3B203 glass doped with 0.5% of Eu 3+ was the same used in a previous work [12]. The luminescence spectra in the temperature range between 90 K and room temperature were obtained using a dye laser (FL2001), operating with Rhodamine 6G, pumped by the 308 nm line of an excimer laser (EMG50), giving 12 ns pulses and a line width of 0.04 cm -l. To eliminate Rayleigh scattering, the luminescence from the sample was focused on a chopper blade which was synchronised in antiphase with the laser pulse. The collected luminescence was passed through a double monochromator (Spex 1402) with a dispersion of 15 c m - 1 / m m in the yellow region of the spectrum around 17300 cm ~. For this setup the maximum resolution was 0.4 c m - ~. The spectra were collected with a delay of 0.3 ms and a gate of 5 ms. In the experiment performed at low temperature, the exciting source was a single mode ring dye laser (CR699-21), operating with Rhodamine 6G, pumped by the 514.5 nm line of a Ar + laser (Innova 20). The actual line width of the laser was 3 MHz. A chopper was used both to gate the exciting light and to block Rayleigh scattering from the sample. The exciting pulse width was 2.1 ms with a period of 14.2 ms. Detection occurred with a delay of 0.9 ms from the cut-off of the pulse and a gate of 10 ms. The resonant emission was measured by a plane Fabry-Perot (Burleigh RC150) interferometer and dispersed by a monochromator (JY HRS4). The actual finesse of the interferometer was 33. The free spectral range was adjusted between 1.25 and 0.25
cm -] at each temperature, according to the line width to be measured. For both the experimental configurations the signal from the photomultiplier (RCA 31034A02) was analysed by a photon counting system (home made time resolved) and the acquisition process was controlled by a personal computer. The measured spectra were fitted by the convolution of a Lorentzian with the instrumental response profile. The homogeneous line width was determined from the half width at half maximum of the Lorentzian shape [ 13].
3. Results Fig. l shows the dependence of the room temperature homogeneous line width of Eu 3+, on the laser excitation energy across the inhomogeneous profile of the 7FU~ S D 0 transition. F h changes, at room temperature, by a factor larger than two within the inhomogeneous profile, linearly increasing with the transition frequency. In the inset the emission spec-
4.0
'E o
3.5
¢-
•
¢-
3.0
17200
17400
17600
._1 -I
o
2.5
t"
o'} o
E 2.o
o "1-
1.5
li~
17280
,
r
17320
,
i
17360
,
i
17400
Excitation Energy [cm-~] Fig. 1. Dependence of the homogeneous line width at room temperature, in the 0.5% 4ZnO. 3BzO3:Eu 3+ glass, on the excitation energy within the 5D 0 ~--7F0 inhomogeneous profile. The inset shows the emission spectrum of the 5D 0 -->7F0 transition recorded at 12 K after excitation at 308 nm.
M. Sbetti et al. / Journal of Non-CD,stalline Solids 220 (1997) 217-221
219
4. Discussion
J
"13 1 ....1 0 CD c~D 0
0.1
E 0
"1-
0"0120
. . . . . . . . .100 T [K]
3 ;o ....
i; O0
Fig. 2. Temperature dependence of the SD0 ----)7F0 homogeneous line width at 17320 cm ~, in the 0.5% 4ZnO. 3B203:Eu 3+ glass. Triangles and open circles denote measurements performed by the two different experimental configurations that use pulsed and CW dye laser, respectively. Solid circles and solid squares reproduce the data obtained by Morgan and El-Sayed [15] for borate glasses of molar composition 90B20~.10Na20 and 70B20 a.30Na20, respectively. Our data are fitted to a function, F h = AT 2, shown by the curve.
trum recorded at 12 K after excitation at 308 nm by the excimer laser is reported. The 5D 0 ---->7Fo inhomogeneous band peaks at 17320 c m - i and has a F i of about 130 cm I In the low energy region of the spectrum, the emission from the 5D o level towards the lowest Stark component of the 7F L state overlaps the emission due to the 5D 0 ----~7F 0 transition. Because of this the T R R F L N can be frustrated by accidental coincidence due to thermal population of the low lying 7F l Stark component [14]. For this reason we have measured F h values for excitation energies greater than 17280 cm 1. Fig. 2 shows the temperature dependence of the homogeneous line width at 17320 c m - ~. The straight line is the data fit obtained by the power law function F h = A T % T h e fit gives A = (2.0 _+ 0.3) × 10 -5 cm l / K ~ and o~= 2 . 0 + 0 . 1 . This value corresponds to A = ( 0 . 6 _ + 0 . 1 ) M H z / K 2. Fig. 2 also shows the data obtained by Morgan and E1-Sayed [15] for two borate glasses with a different amount of Na20.
The practically linear increase of the homogeneous line width when the transition frequency increases within the inhomogeneous profile, as shown in Fig. 1, has been already observed by Avouris et al. [16] and is often a property of the VF0 ~ S D 0 transition of Eu 3+ in glasses. In particular, our result is close to the one obtained in a sodium borate glass with a small amount of Na~O [17]. A more or less similar variation has been observed in silicate and borate glasses [16,17]. Brecher and Riseberg have observed a similar behaviour for the transition probability by life time measurements [18]. Furthermore, by site selective spectroscopy, taking the 5D o -'~7Fj luminescence at different excitation frequencies and by a crystal field analysis in C2,, site symmetry, they found a correlation between the local distortion around the Eu 3+ ion and the transition frequency. We have performed similar spectroscopic measurements coupled with a crystal field analysis and we have shown that the local structure of the Eu 3+ ions in the zinc borate glass agrees with the Brecher and Riseberg model [12]. We observed that the Eu 3+ sites, whose 7F o ~---~5I)o transition lies at larger energy, correspond to more distorted local environments. The results shown in Fig. 1 confirm the observation of Avouris et al. [16] that a correlation exists between the crystal field and the value of F h. Fig. 2 shows a T 2 dependence of the homogeneous line width in the range 3 0 - 3 0 0 K. Actually, we observe that the two sets of data (circles and triangles) are not exactly connected. In the region around 100 K the data taken with the pulsed laser are systematically greater than those taken with the CW laser. This effect can be easily explained. The linear behaviour of the transition probability and of the local distortion as a function of the excitation frequency is the result of averaging over a large number of ions [19]. The actual distribution of the excited and observed sites depends on the times of the measurement determined by the width of the pulse and by the delay time. Shorter pulses and smaller delays will favour detection of ions which have larger oscillator strengths and, as a consequence, larger homogeneous line widths. The effect is well shown by comparing spectra taken at different de-
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M. Sbetti et a l . / Journal of Non-Co'stalline Solids 220 (1997) 217-221
lays: the line width decreases as the delay increases. It should be noted that in the presence of spectral energy transfer the opposite effect is observed. The nearly T 2 dependence of the homogeneous line width over the considered temperature range is not surprising and is reported and discussed in many articles [3]. However, a complete description of the optical dephasing mechanisms is still lacking. At low temperature (T < 5 K), where F hot T ~ with 1 < o~ < 1.3, dephasing is due to the dynamics of local defects in the glass structure, which are typical of the topological disorder. The dynamics of the glass among nearly equivalent configurations are simply described by the two level systems (TLS) model, which was introduced to account for low temperature thermal properties [20,21]. The low temperature behaviour of F h is accounted for by models based on the dipole-dipole interaction between the dipole associated with the optical transition of the rare-earth ion and the dipole associated with the oscillations of the TLS [22]. At higher temperature phonon Raman scattering should give dephasing in glasses as in crystals producing the observed F h oc T 2 behaviour. Nevertheless, a model based on TLS active in the whole temperature range has been recently proposed [23]. As already observed by Morgan et al. [17], a description based on a dipole-dipole coupling between the EU 3+ ion and TLS can explain the linear frequency dependence of the F h within the inhomogeneous profile. Ions in more distorted sites, having a larger dipole strength, will have a larger homogeneous line width. In the zinc borate glass the F n of the 7F0 ~ 5 D 0 transition of Eu 3+ appears to be smaller than that measured in other inorganic glasses [1]. A smaller F h, measured at 300 K, in comparison to the one measured in silicate glasses has been previously reported for sodium borate glasses [15,17]. A full comparison with other borate glasses is impossible because, at least to our knowledge, no information is available about F h of the 7F0 ~ S D 0 transition of Eu 3+ in the temperature range below 300 K. Morgan and EI-Sayed [15] have measured the temperature dependence of F h in the region 300-700 K for two borate glasses. The values, taken from Ref. [15], are plotted in Fig. 2. F h is larger in the borate glass with higher Na20 content. A linear increase of F h with the Na20 amount has been reported by Morgan
et al. [17]. The inhomogeneous linewidth also increases with NazO concentration, for small content but decreases at larger content [25]. This decrease of /'i with the content of the network modifier, which is similar to the one observed in silicate and germanate glasses, has been assigned to an increase of the number of non-bridging oxygens produced by the network modifier. This increase causes a decrease of the linkage among tetrahedra and an increase of network flexibility around the rare earth ion. The increase of F~ in borate glasses at small Na20 content, as it increases, has a different origin. In fact, as the Na-- content increases, the population of BO4 tetrahedra increases. This increase reduces the flexibility of the network causing a larger distribution in the crystal field strength and larger F i. Furthermore, an increase of the B O a units bonded to the E u 3+ ion gives rise to a larger F h [17]. Our results for F n in the zinc borate glass (Fig. 2) agree with those for the sodium borate glasses at small content of network modifier. This effect suggests that in the 4ZnO. 3B203 glass the Eu 3+ is mainly bonded to oxygens belonging to threefold coordinated boron atoms and coordination by B O 4 units is not important. This suggestion is in agreement with previous observations. Harris et al. have remarked that the 4ZnO. 3B203 undoped glass has a NMR response due to boron atoms in threefold coordination and a small fraction of borons in fourfold coordination [24]. We have observed a phonon sideband coupled to the 5Do~--7F0 transition and assigned it to an E u - O stretching vibration where the oxygen belongs to BO~- units [12]. Although F h is small, F i is quite large (130 cm 1), larger than the largest one measured in borate glasses [25]. This magnitude gives a very important ratio, F . , / F h, of about 60 at room temperature, taking for F h the value on the maximum of the 5D 0 ~ T F 0 band. The origin of the site dependence of F h is not explained. First the relative importance, as a function of the temperature, of the two dephasing mechanisms, involving two phonon scattering and TLS modes respectively, is not known. For both mechanisms F h should increase as the local distortion around the chromophore increases since the coupling strength increases. However one should also consider the local dynamics which are correlated to the structure. In particular, the microscopic nature of the
M. Sbetti et al. / Journal of Non-Co'stalline Solids 220 (1997) 217-221
TLS, their density and their location near a chromophore should be investigated.
5. Conclusions At room temperature, a linear increase of F h with the frequency across the inhomogeneous profile of t h e 7F 0 ~--~5D0 transition indicates a site dependence of the dephasing processes. In the whole range of temperatures F h follows the power law temperature dependence F h = (0.6 + 0 . 1 ) T (2"°+ O.l) MHz. A F i to F h ratio of about 60 is measured at room temperature. The small F h width is due to particular local structures around the Eu 3+ ion which is surrounded by a network where the number of nonbridging oxygens is small. The Eu 3+ ion appears to be mainly coordinated by units containing boron atoms in threefold coordination.
Acknowledgements S.C. acknowledges support by the I N F M research contract 'Preparation and characterization of rareearths-doped amourphous systems for applications in the field of active optical devices'.
References [1] R.M. Macfarlane, R.M. Shelby, J. Lumin. 36 (1987) 179. [2] W.M. Yen, R.T. Brundage, J. Lumin. 36 (1987) 209.
221
[3] M.J. Weber, M.D. Sturge, eds., Optical Linewidths in Glasses, J. Lumin. 36 (1987) and references therein. [4] M.J. Weber, J. Non-Cryst. Solids 47 (1982) 117. [5] M.J. Weber, J. Non-Cryst. Solids 123 (1990) 208. [6] W.M. Yen, P.M. Seizer, eds., Laser Spectroscopy of Solids (Springer, Berlin, 1981). [7] J.L. Skinner, W.E. Moerner, J. Phys. Chem. 100 (1996) 13251. [8] T. Okuno, K. Tanaka, K. Koyama, M. Namiki, T. Suemoto, J. Lumin. 58 (1994) 184. [9] J.M. Pellegrino, W.M. Yen, MJ. Weber, J. Appl. Phys. 51 (1980) 6332. [10] S.A. Brawer, M.J. Weber, Appl. Phys. Lett. 35 (1979) 31. [11] W.E. Moerner, ed., Persistent Spectral Hole-Burning: Science and Applications (Springer, Berlin, 1988). [12] M. Bettinelli, A. Speghini, M. Ferrari, M. Montagna, J. Non-Cryst. Solids 201 (1996) 211. [13] T. Kushida, E. Takushi, Phys. Rev. BI2 (1975) 824. [14] MJ. Weber, J.A. Paisner, S.S. Sussman, W.M. Yen, L.A. Riseberg, C. Brecher, J. Lumin. 12&13 (1976) 729. [15] J.R. Morgan, M.A. E1-Sayed, Chem. Phys. Lett. 84 (1981) 213. [16] P. Avouris, A. Campion, M.A. El-Sayed, J. Chem. Phys. 67 (1977) 3397. [17] J.R. Morgan, E.P. Chock, W.D. Hopewell, M.A. El-Sayed, R. Orbach, J. Phys. Chem. 85 (1981) 747. [18] C. Brecher, L.A. Riseberg, Phys. Rev. BI3 (1976) 81. [19] G. Cormier, J.A. Capobianco, C.A. Morrison, A. Monteil, Phys. Rev. B48 (1993) 16290. [20] P.W. Anderson, B.I. Halperin, C.M. Varma, Philos. Mag. 25 (1972) 1. [21] W.A. Phillips, J. Low Temp. Phys. 7 (1972) 351. [22] P. Reineker, K. Kassner, in: Optical Spectroscopy of Glasses, ed. I. Zschokke (Reidel, Dordrecht, 1986) p. 65. [23] P. Neu, A. Wiirger, Europhys. Lett. 29 (1995) 561. [24] I.A. Harris Jr., P.J. Bray, Phys. Chem. Glasses 25 (1984) 69. [25] S. Todoroki, K. Hirao, N. Soga, J. Appl. Phys. 72 (1992) 5853.
ELSEVIER
Journal of Non-Crystalline Solids 220 (1997) 217-221
Homogeneous line width in a zinc borate glass activated by Eu 3+ M. Sbetti a, E. Moser
a, M. Montagna a, M. Ferrari b,* S. Chaussedent c,,, M. Bettinelli d
a Dipartimento di Fisica, lstituto Nazionale per la Fisica della Materia, Uniuersith di Trento, l~ia Sommarit,e 14, 1-38050 Pot'o, Trento, ltaly b Consiglio Nazionale delle Ricerche CeFSA, Centro Fisica Stati Aggregati, via Sommarive 14, 1-38050 Porto, Trento, Italy c Dipartimento di Fisica, Istituto Nazionale per la Fisica della Materia, Universith di Padova, l:ia Marzolo 8, 1-35131 Padova, Italy a Facoltgt di Scienze MM.FF.NN., lstituto Policattedra, Unit,ersith di Verona, 1-37134 Verona, Italy
Received 10 March 1997
Abstract In this work we report on the temperature dependence of the homogeneous line width ( F h) in 4 Z n O . 3B203:Eu 3+ glass in the region between 30 K and room temperature. The time resolved resonant fluorescence line narrowing technique was applied to the 7F0 ~ 5 D 0 transition of Eu 3+. We obtain that F h follows the commonly found behaviour T 2 in the whole range of temperature (30-300 K). The F h is comparable to that measured in borate glasses with a small amount of network modifier. This result, together with a linear increase of F h with the excitation energy within the inhomogeneous profile, is indicative of a specific local environment around the Eu 3+ ion in the zinc borate glass. © 1997 Elsevier Science B.V.
1. Introduction
Glasses have a structural disorder which gives rise to inhomogeneous line broadening. The inhomogeneous line width ( F i) is due to the site dependence of the static crystal field and does not depend strongly on the temperature. On the contrary, the homogeneous line width ( F h) shows a power law temperature dependence F h = A T " with a = 1 in the very low temperature range and with ce-~ 2 at higher
* Corresponding author. Tel.: +39-461 881 684; fax: +39-461 881 680; e-mail: [email protected]. 1 Permanent address: P.O.M.A. Facult~ des Sciences, Universit~ d' Angers, 2 Bd. Lavoisier, F-49045 Angers cedex, France.
temperatures ( T > 20 K) (for a review see Refs. [1-3]). In developing glasses activated by trivalent lanthanide ions for applications such as optical materials, it is important to have information about the interaction occurring between the active ion and the host [4,5]. The dynamical interactions affect F h of the optical transitions and the line-narrowing methods provide information [6-8]. Moreover, the knowledge of the ratio of homogeneous-to-inhomogeneous line widths is of interest for glass lasers [9,10] and optical data storage [11] where a large F i to F h ratio is required. For temperatures higher than 20 K the time resolved resonant fluorescence line narrowing (TRRFLN) technique is the most suitable to obtain directly the temperature dependence of the homogeneous line width.
0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 ( 9 7 ) 0 0 2 8 4 - 6
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M. Sbetti et al. / Journal of Non-C~stalline Solids 220 (1997) 217-221
The spectroscopic properties of the glass used in this work have been investigated in detail in a previous work [12]. The zinc borate sample doped with 0.5% Eu 3+ had a relatively strong 5D 0 -->TF 0 e m i s s i o n transition and no spectral energy transfer was observed. The decay curves of the luminescence from the 5D 0 state can be described by a single exponential and the lifetimes are within errors of measurements, the same at room temperature (1.62 ms) and at 12 K (1.65 ms) [12]. These properties make the sample a good candidate for TRRFLN measurements.
2. Experimental The 4ZnO3B203 glass doped with 0.5% of Eu 3+ was the same used in a previous work [12]. The luminescence spectra in the temperature range between 90 K and room temperature were obtained using a dye laser (FL2001), operating with Rhodamine 6G, pumped by the 308 nm line of an excimer laser (EMG50), giving 12 ns pulses and a line width of 0.04 cm -l. To eliminate Rayleigh scattering, the luminescence from the sample was focused on a chopper blade which was synchronised in antiphase with the laser pulse. The collected luminescence was passed through a double monochromator (Spex 1402) with a dispersion of 15 c m - 1 / m m in the yellow region of the spectrum around 17300 cm ~. For this setup the maximum resolution was 0.4 c m - ~. The spectra were collected with a delay of 0.3 ms and a gate of 5 ms. In the experiment performed at low temperature, the exciting source was a single mode ring dye laser (CR699-21), operating with Rhodamine 6G, pumped by the 514.5 nm line of a Ar + laser (Innova 20). The actual line width of the laser was 3 MHz. A chopper was used both to gate the exciting light and to block Rayleigh scattering from the sample. The exciting pulse width was 2.1 ms with a period of 14.2 ms. Detection occurred with a delay of 0.9 ms from the cut-off of the pulse and a gate of 10 ms. The resonant emission was measured by a plane Fabry-Perot (Burleigh RC150) interferometer and dispersed by a monochromator (JY HRS4). The actual finesse of the interferometer was 33. The free spectral range was adjusted between 1.25 and 0.25
cm -] at each temperature, according to the line width to be measured. For both the experimental configurations the signal from the photomultiplier (RCA 31034A02) was analysed by a photon counting system (home made time resolved) and the acquisition process was controlled by a personal computer. The measured spectra were fitted by the convolution of a Lorentzian with the instrumental response profile. The homogeneous line width was determined from the half width at half maximum of the Lorentzian shape [ 13].
3. Results Fig. l shows the dependence of the room temperature homogeneous line width of Eu 3+, on the laser excitation energy across the inhomogeneous profile of the 7FU~ S D 0 transition. F h changes, at room temperature, by a factor larger than two within the inhomogeneous profile, linearly increasing with the transition frequency. In the inset the emission spec-
4.0
'E o
3.5
¢-
•
¢-
3.0
17200
17400
17600
._1 -I
o
2.5
t"
o'} o
E 2.o
o "1-
1.5
li~
17280
,
r
17320
,
i
17360
,
i
17400
Excitation Energy [cm-~] Fig. 1. Dependence of the homogeneous line width at room temperature, in the 0.5% 4ZnO. 3BzO3:Eu 3+ glass, on the excitation energy within the 5D 0 ~--7F0 inhomogeneous profile. The inset shows the emission spectrum of the 5D 0 -->7F0 transition recorded at 12 K after excitation at 308 nm.
M. Sbetti et al. / Journal of Non-CD,stalline Solids 220 (1997) 217-221
219
4. Discussion
J
"13 1 ....1 0 CD c~D 0
0.1
E 0
"1-
0"0120
. . . . . . . . .100 T [K]
3 ;o ....
i; O0
Fig. 2. Temperature dependence of the SD0 ----)7F0 homogeneous line width at 17320 cm ~, in the 0.5% 4ZnO. 3B203:Eu 3+ glass. Triangles and open circles denote measurements performed by the two different experimental configurations that use pulsed and CW dye laser, respectively. Solid circles and solid squares reproduce the data obtained by Morgan and El-Sayed [15] for borate glasses of molar composition 90B20~.10Na20 and 70B20 a.30Na20, respectively. Our data are fitted to a function, F h = AT 2, shown by the curve.
trum recorded at 12 K after excitation at 308 nm by the excimer laser is reported. The 5D 0 ---->7Fo inhomogeneous band peaks at 17320 c m - i and has a F i of about 130 cm I In the low energy region of the spectrum, the emission from the 5D o level towards the lowest Stark component of the 7F L state overlaps the emission due to the 5D 0 ----~7F 0 transition. Because of this the T R R F L N can be frustrated by accidental coincidence due to thermal population of the low lying 7F l Stark component [14]. For this reason we have measured F h values for excitation energies greater than 17280 cm 1. Fig. 2 shows the temperature dependence of the homogeneous line width at 17320 c m - ~. The straight line is the data fit obtained by the power law function F h = A T % T h e fit gives A = (2.0 _+ 0.3) × 10 -5 cm l / K ~ and o~= 2 . 0 + 0 . 1 . This value corresponds to A = ( 0 . 6 _ + 0 . 1 ) M H z / K 2. Fig. 2 also shows the data obtained by Morgan and E1-Sayed [15] for two borate glasses with a different amount of Na20.
The practically linear increase of the homogeneous line width when the transition frequency increases within the inhomogeneous profile, as shown in Fig. 1, has been already observed by Avouris et al. [16] and is often a property of the VF0 ~ S D 0 transition of Eu 3+ in glasses. In particular, our result is close to the one obtained in a sodium borate glass with a small amount of Na~O [17]. A more or less similar variation has been observed in silicate and borate glasses [16,17]. Brecher and Riseberg have observed a similar behaviour for the transition probability by life time measurements [18]. Furthermore, by site selective spectroscopy, taking the 5D o -'~7Fj luminescence at different excitation frequencies and by a crystal field analysis in C2,, site symmetry, they found a correlation between the local distortion around the Eu 3+ ion and the transition frequency. We have performed similar spectroscopic measurements coupled with a crystal field analysis and we have shown that the local structure of the Eu 3+ ions in the zinc borate glass agrees with the Brecher and Riseberg model [12]. We observed that the Eu 3+ sites, whose 7F o ~---~5I)o transition lies at larger energy, correspond to more distorted local environments. The results shown in Fig. 1 confirm the observation of Avouris et al. [16] that a correlation exists between the crystal field and the value of F h. Fig. 2 shows a T 2 dependence of the homogeneous line width in the range 3 0 - 3 0 0 K. Actually, we observe that the two sets of data (circles and triangles) are not exactly connected. In the region around 100 K the data taken with the pulsed laser are systematically greater than those taken with the CW laser. This effect can be easily explained. The linear behaviour of the transition probability and of the local distortion as a function of the excitation frequency is the result of averaging over a large number of ions [19]. The actual distribution of the excited and observed sites depends on the times of the measurement determined by the width of the pulse and by the delay time. Shorter pulses and smaller delays will favour detection of ions which have larger oscillator strengths and, as a consequence, larger homogeneous line widths. The effect is well shown by comparing spectra taken at different de-
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M. Sbetti et a l . / Journal of Non-Co'stalline Solids 220 (1997) 217-221
lays: the line width decreases as the delay increases. It should be noted that in the presence of spectral energy transfer the opposite effect is observed. The nearly T 2 dependence of the homogeneous line width over the considered temperature range is not surprising and is reported and discussed in many articles [3]. However, a complete description of the optical dephasing mechanisms is still lacking. At low temperature (T < 5 K), where F hot T ~ with 1 < o~ < 1.3, dephasing is due to the dynamics of local defects in the glass structure, which are typical of the topological disorder. The dynamics of the glass among nearly equivalent configurations are simply described by the two level systems (TLS) model, which was introduced to account for low temperature thermal properties [20,21]. The low temperature behaviour of F h is accounted for by models based on the dipole-dipole interaction between the dipole associated with the optical transition of the rare-earth ion and the dipole associated with the oscillations of the TLS [22]. At higher temperature phonon Raman scattering should give dephasing in glasses as in crystals producing the observed F h oc T 2 behaviour. Nevertheless, a model based on TLS active in the whole temperature range has been recently proposed [23]. As already observed by Morgan et al. [17], a description based on a dipole-dipole coupling between the EU 3+ ion and TLS can explain the linear frequency dependence of the F h within the inhomogeneous profile. Ions in more distorted sites, having a larger dipole strength, will have a larger homogeneous line width. In the zinc borate glass the F n of the 7F0 ~ 5 D 0 transition of Eu 3+ appears to be smaller than that measured in other inorganic glasses [1]. A smaller F h, measured at 300 K, in comparison to the one measured in silicate glasses has been previously reported for sodium borate glasses [15,17]. A full comparison with other borate glasses is impossible because, at least to our knowledge, no information is available about F h of the 7F0 ~ S D 0 transition of Eu 3+ in the temperature range below 300 K. Morgan and EI-Sayed [15] have measured the temperature dependence of F h in the region 300-700 K for two borate glasses. The values, taken from Ref. [15], are plotted in Fig. 2. F h is larger in the borate glass with higher Na20 content. A linear increase of F h with the Na20 amount has been reported by Morgan
et al. [17]. The inhomogeneous linewidth also increases with NazO concentration, for small content but decreases at larger content [25]. This decrease of /'i with the content of the network modifier, which is similar to the one observed in silicate and germanate glasses, has been assigned to an increase of the number of non-bridging oxygens produced by the network modifier. This increase causes a decrease of the linkage among tetrahedra and an increase of network flexibility around the rare earth ion. The increase of F~ in borate glasses at small Na20 content, as it increases, has a different origin. In fact, as the Na-- content increases, the population of BO4 tetrahedra increases. This increase reduces the flexibility of the network causing a larger distribution in the crystal field strength and larger F i. Furthermore, an increase of the B O a units bonded to the E u 3+ ion gives rise to a larger F h [17]. Our results for F n in the zinc borate glass (Fig. 2) agree with those for the sodium borate glasses at small content of network modifier. This effect suggests that in the 4ZnO. 3B203 glass the Eu 3+ is mainly bonded to oxygens belonging to threefold coordinated boron atoms and coordination by B O 4 units is not important. This suggestion is in agreement with previous observations. Harris et al. have remarked that the 4ZnO. 3B203 undoped glass has a NMR response due to boron atoms in threefold coordination and a small fraction of borons in fourfold coordination [24]. We have observed a phonon sideband coupled to the 5Do~--7F0 transition and assigned it to an E u - O stretching vibration where the oxygen belongs to BO~- units [12]. Although F h is small, F i is quite large (130 cm 1), larger than the largest one measured in borate glasses [25]. This magnitude gives a very important ratio, F . , / F h, of about 60 at room temperature, taking for F h the value on the maximum of the 5D 0 ~ T F 0 band. The origin of the site dependence of F h is not explained. First the relative importance, as a function of the temperature, of the two dephasing mechanisms, involving two phonon scattering and TLS modes respectively, is not known. For both mechanisms F h should increase as the local distortion around the chromophore increases since the coupling strength increases. However one should also consider the local dynamics which are correlated to the structure. In particular, the microscopic nature of the
M. Sbetti et al. / Journal of Non-Co'stalline Solids 220 (1997) 217-221
TLS, their density and their location near a chromophore should be investigated.
5. Conclusions At room temperature, a linear increase of F h with the frequency across the inhomogeneous profile of t h e 7F 0 ~--~5D0 transition indicates a site dependence of the dephasing processes. In the whole range of temperatures F h follows the power law temperature dependence F h = (0.6 + 0 . 1 ) T (2"°+ O.l) MHz. A F i to F h ratio of about 60 is measured at room temperature. The small F h width is due to particular local structures around the Eu 3+ ion which is surrounded by a network where the number of nonbridging oxygens is small. The Eu 3+ ion appears to be mainly coordinated by units containing boron atoms in threefold coordination.
Acknowledgements S.C. acknowledges support by the I N F M research contract 'Preparation and characterization of rareearths-doped amourphous systems for applications in the field of active optical devices'.
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