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Comparison of laser properties of rare earths in oxide and fluoride glasses
Journal of the Less-Common Metals, 126 (1986) 187-194
COMPARISON OF LASER PROPERTIES AND FLUORIDE GLASSES*
R. REISFELD”,
187
OF RARE EARTHS IN OXIDE
M. EYAL” and C. K. JQRGENSEN”
a Department o~i~or~a~~c and Analy~~cul Chemistry, The Hebrew University of Jerusalem, JerusaEem
91904 (Israel) ’ Dkpartement de Chimie M&ale, (Switzerland)
Analytique et Appliqute,
tiniversite* de GenPve, CHl2II
Geneva 4
(Received March 3,1986)
Summary Laser properties of fluoride and oxide glasses containing neodymium(III), and holmium(II1) depend on the pumping efhciency, lifetimes of the excited and terminal levels and stimulated peak cross-sections. We have determined these properties in oxide glass (tellurite) having a composition of 35Zn0:65TeO, and fluoride glass (ZBLA) having composition 57ZrF, : 34BaF, : 5LaF, : 4AlF,, or (PBLA) having a composition 36PbF, : 24ZnF, : 35GaF, : 3YF, : 2AlF, for neodymium(II1) and holmium(II1). It was shown that multiphonon relaxation (in the case of energy difference less than 3000 cm- ‘) is always orders of magnitude lower in the fluoride glass than in the tellurite glass, which was chosen as a model for oxide glass. No significant difference was found for concentration quenching between the two kinds of glasses.
1. Introduction Fluoride glasses containing about 50 mol.% ZrF,, which can be replaced by HfF, or ThF, [l-3] (ZBLA glass), have been considered as materials for fibre optics in the range 0.335 urn [4]. Another important category of fluoride glasses containing zinc(I1) (or manganese(II)), gaIlium(II1) and lead(I1) fluorides (PBLA glass) was invented [5, S] at the University of Maine, Le Mans. The absorption spectra and luminescence of 4f” erbium(II1) and 3ds manganese(II), and the mutual energy transfer between excited states of these two species were studied in such a glass [7]_ In ZBLA glass, the luminescence of 4f2 praseodymium(III)
*Paper presented at the 17th Rare. Earth Hamilton, Ontario, Canada, June 9 12.1986. 0022-5088/86/%3.50
Research
Conference,
McMaster
?I Elsevier Sequoia/Printed
University,
in The Netherlands
188
[S, 91, 4P europium(II1) [lo, 111, 4f” holmium(II1) [12,13] and erbium(II1) [1416] occurs from more highly excited J levels than usual, the lower limit still allowing perceptible luminescence for the energy gap between the emitting J level and the closest lower-lying J level being 2000 cm- ’ (0.25 eV), some 2 to 4 times smaller than in nearly all other glasses and crystals. It may be noted that this rich emission spectrum from several J levels besides the ground state is observed also at room temperature. The superior optical characteristics of fluoride glasses for IR fibre optic applications also provide an ideal medium or host enabling the glass to be integrated into a system acting as a laser light source as well as the actual waveguide material. Laser properties of neodymium(II1) in YLiF, crystals were investigated by Rines et al. [17] and of holmium(II1) in YLiF, crystals sensitized with erbium(II1) and thulium(II1) by Knights et al. [18]. We feel that fluoride glass can serve as a laser host in a similar way to crystals. The spectroscopic and fluorescent properties of neodymium(II1) containing fluorozirconate glasses have been reported by Weber in work with the Lucas-Poulain group at Rennes [19] and optical absorption of 3d transition metal ions such as iron, cobalt, nickel and copper by Ohishi et al. [20]. Here we calculated laser properties of neodymium(III)- and holmium(III)- doped fluoride glasses and compared these properties with tellurite and ED-2 laser glass. The procedure of such calculation has been described elsewhere [al-241. 2. Intensity parameters and radiative transitions of neodymium(II1) holmium(II1)
and
The absorption spectra of neodymium(II1) and holmium(II1) serve as a basis for a complete set of predictions of transition rates within the 4f3 configuration of neodymium(II1) and the 4f” configuration of holmium(II1). The procedure is based on the Judd-Ofelt theory and is described in detail elsewhere [15,21]. According to theory, the otherwise forbidden transitions within the f-f configuration of rare earths become slightly allowed by admixing of wavefunctions of the f-f configuration with odd components of the crystal field potential. The intraconfigurational transitions then become subject to a new set of selection rules and oscillator strengths of the transitions depend parametrically on the three phenomenological parameters R,, 52,, C&. Reduced matrix elements for the transitions are almost invariant in respect to the crystal field strength [al] and have been tabulated for all rare earth ions [25]. The optical transitions of rare earths in solids are predominantly of electric dipole character and their spectral intensities can be described using the treatment of Judd and Ofelt. In this approach the line strength S of a transition between two J states is given by the sum of products of empirical intensity parameters R, and matrix elements of tensor operators UC’)of the form S,,(aJ:bJ’)
= e’~R,l(aJI
where t = 2,4,6
IO’)1 IbJ’)lz
(1)
The values of Q, are obtained from a least-squares fit of measured and calculated absorption line strengths and typically have an experimental uncertainty of about 10%. The integrated intensities of the absorption bands yield 0 which are an effective average over the different rare earth environments in the glass. The most significant factor determining the values is the strengths of the odd-order terms in the expansion of the local field at the rare earth sites. These in turn are affected by the nearest-neighbour anion(s) and cations. For a given glass former systematic changes of R, have been observed with the changes in the size and charge of network modifier ions [22]. The three R are determined by solving an overdetermined set of linear equations built by equating the measured oscillator strengths with the sum of products of the unknown R with the appropriate reduced matrix elements. The three R found from the solution are put into a computer program that calculates all the radiative transition rates possible in the system analyzed. The intensity parameters for neodymium(II1) and holmium(II1) in fluoride glasses are given in Table 1. These were used for calculation of transition rates between relevant levels using [22] A(aJ:
bS)
=
64n4v3
n(n2 + 2)2
3hc3(2J + 1)
9
S,, + n3Smd
C-9
where S,, is the magnetic dipole contribution TABLE 1 Intensity parameters for neodymium(II1) and holmium(II1) in fluoride and tellurite glasses Glass *Ho* ZBLA ZBLA ZnTe *Nd* PBLA ZBLA ZBLA ZnTe
Reference
a2 (pm*)
% (pm’)
Q, (pm*)
12 22 22
2.28 2.43 f 0.03 6.92 f 0.22
2.08 1.67+0.09 2.81+ 0.33
1.73 1.84f0.03 1.42+0.20
26 26 19 26
1.01 kO.28 l.lOkO.25 1.95*0.2ci 4.66kO.57
3.73 * 0.35 3.80+0.30 3.6510.38 5.08kO.83
6.19kO.43 5.53kO.20 4.17+0.17 6.08+0.51
ZnTe, 35Zn0:65TeO,. ZBLA, 57.OZrF,:34.OBaF,:3.OLaF,:4.OAlF,:2.0(NdF,, PBLA, 36PbF,:24ZnFZ:35GaF,:2AlFx:3YF,:2LaF,:2NdFj
3. Non-radiative
HoF,) mol.“,,. mol.“,,.
transitions
The calculation
of non-radiative transfer rates due to multiphonon decay, by subtracting the calculated radiative rates from the reciprocals of the integrated lifetimes. The two parameters of the multiphonon decay were calculated from the plot of W,,, us. energy gap AE from the emitting W,, was accomplished
190
level to the next lower level: the parameter a from the slope of the plot and the electronic factor B from its intercept. The parameters obtained for fluoride B = 1.63 f 0.1 x lOi’, are: a = 0.0053 * 0.0005, (PBLA) and glasses a = 0.0052 f 0.0005, B = 1.59 + 0.1 x lOi (ZBLA), and for tellurite glass: CI= 0.0047 and B = 6.3 x 10”. The parameters are substituted into W,,, = Bexp( - aAE)
(3)
where AE is the energy gap from the electronic level to its next lower neighbour. The entire set of transition rates is calculated, now with the non-radiative transition rates included [24]. The result of such a procedure is shown in ref. 26, Table 4, for neodymium(II1) in PBLA glass, where in addition to 4F,,, emission also the emission from 4D3,2 and 2P3,2 is shown. The outstanding property of fluoride glasses is the relatively long-lived luminescence from levels which are separated only by a small energy gap to the next lower level [13]. In PBLA and ZBLA glass we were able to measure the lifetimes of emissions from two levels; the thermalized 4D,,2 (361 nm) and 2P,,2 (387 nm). The first three emission lines are identified as belonging to the transitions: 4D,,2 + 41,,2 (361 nm), 4Dj,2 ---*41, ii2 (381 nm) and 4D,,2 -+ 41,,,2 (412nm). The second group of lines belongs to the transitions from 2P3,2. In addition to multiphonon decay, cross-relaxation between two ions of the same nature at high concentration lowers the lifetime of emitting level. The cross-relaxation between a pair of rare earth ions is graphically presented in ref. 24, Fig. 3. The measured luminescence lifetime is related to the total relaxation rate by l/r = C W,, + CA + Per = l/z, + Per
(4)
where CA is the total radiative rate, C W,,, is the non-radiative rate and PC, is the rate of cross-relaxation between adjacent ions, and 7. is the intrinsic lifetime. The critical radius R, for cross-relaxation is defined by P,,(RoW70)
=
(5)
1
R. being the critical distance at which the probability
for cross-relaxation PC, equals the sum of radiative and multiphonon relaxations. The cross-relaxation channel for neodymium(II1) is 4F,,, + 411si2 = 4I912 -+ 411 512 and for holmium(II1) (5S2,5F4) + 51, = 51, + 514. The critical radii in various glasses are presented in Table 2. 4. Laser action The formula for laser peak cross-section
is [27]
where 3,(cm) is the emission wavelength; Ai (cm) is the full width at half height of the emission band; n is the refractive index; A (s- ‘) is the radiative transfer probability.
191
Threshold P
= W%l th
power for transverse pumping is
+ L,,,) x 10- ’ 2&,5,1Focc,
(7)
where L,,, is the resonant power wavelength which is defined as
loss due to self-absorption
at the laser
L,,, = 21aN&/Z
(3)
where N is the number density of lasing lines, & the Boltzmann factor for the terminal laser level, Z the partition function, 1 the length of laser resonator cavity, L, the non-resonant loss which is mainly due to the absorption of the medium and loss at mirrors, i,, the pumping wavelength, zy the lifetime of the lasing level, F the Boltzmann population fraction of the lasing level and CQ, the absorption coefficient of the pumped level which is obtained by dividing the optical density of the sample by its thickness. TABLI? 2 Critical glasses
radii for cross-relaxation
Compound
*Ho (III)* 0.23;ZnTe O.S/ZnTe 2.7;ZnTe 2.0iZBLA *Nd (III)* 0.5t ‘ZnTe 1.6;ZnTe 2.7,‘ZnTe 0.5,‘ZBLA 2.OIPBLA
Concentration (102” cm-“)
of neodymium(II1)
and holmium(II1)
Critical radius [A]
~intr,nric (w)
0.48 1.90 5.80 3.20
3.32 f 1.32 4.2lkO.44 3.44 +0.56 5.54 f 0.28
1.10 3.50 5.80 0.85 4.02
4.74kO.11 5.07kO.36 5.81 kO.25 3.72 f 0.80 5.07 k 0.61
ZnTe, 35Zn0:65Te02. ZBLA, 57.OZrF,:34.OBaF,:3.OLaF,:4.OAlF, mol.“,,. PBLA. 36PbF,:24ZnFz:35GaF,:2A1F,:3YF,:2LaF,:2NdF,
in fluoride and tellurite
Excitation
TlllM‘“W3 b)
A.(nm)
13.5 13.5 13.5 332
12.3 10.2 9.2 105
445 445 445 440
187 187 187 455 345
178 130 102 443 264
579 579 579 576 576
mol.“,,.
Tables 3 and 4 present the comparison of peak cross-section and threshold power for laser action of neodymium(II1) and holmium(II1) in fluoride, oxide and chalcogenide glasses for transverse pumping. From the tables it can be seen that the laser characteristics for neodymium(II1) in fluoride glasses are quite similar and even better than in ED-2 glass. The laser cross-section obtained in this work for neodymium(II1) in tellurite glass is similar to that obtained by Michel et al. [30] in lithium tellurite, where laser action was observed with a very low threshold. The threshold for laser emission due to %, + 51, emission at 745 nm is extremely high in singly doped glass, however, it can be significantly lowered by co-doping the glass with erbium(II1) as found with YLiF, [31].
4F3,2-+411iiz 4F,,2-+41, ,,2 4F3,Z-+411i,r 4F3,2-+41, ,,z 4F,,,--+41, i/z
ZBLA PBLA ED-2 ZnTe GLS
1049 1039 1060 1660 1077
(nm)
Peak emission
2.72 4.02 1.83 3.46 2.63
Concentration (cm - s) x 102*
of neodymium(II1)
glasses
3.14 3.57 1.27 4.73 14.50
wt.%.
Absorption coefficient at 806 nm (cm-l)
in fluoride
GLS, 3Ga,S,:0.85LaS,:O.l5Nd,S,. ZBLA, 57.OZrF,:34.OBaF,:3.OLaF,:4.OAlF~:B.ONdF, mol.%. PBLA, 36PbF,: 24ZnF~:35GaF~:2AlF~:3YF~:2LaF~:2NdF~ mol.%. ED-2,60Si0,:27.5Li,0:10Ca0:2.5A1,0,:0.16Ce0, mol.%, 2.012Nd,O, ZnTe, 35Zn0:65TeO, mol.%, ZNd,O, wt.%. aThis work.
Assignment
Host
Spectroscopic and laser properties chalcogenide and oxide glasses
TABLE 3
26.7 33.0 27.8 29.0
(nm)
Al
2.9 2.75 2.9 3.6 7.95
a(cm’) x 10-20
as compared
57 112 173 93 11.3
Pth (W cm- 2, L,=l% L, = 0.2%
with
400 190 300 130 100
19 a 27 a 28
Reference
TABLE 4
1990
51,p51, *
ZnTe
3.49
3.20 3.20 3.49
Concentration (cm-j) x 1020
ZBLA, 57.0ZrF,:34.0BaFZ:3.0LaF,:4.0A1F,:2.0HoF, *This work. *Calculated for 77 K.
ZnTe, 35Zn0, 65TeOZ.
745 1990 745
5s,m“I, 51,~51,* 5S,m51,
ZBLA ZBLA ZnTe
(mn)
Peak emission
Assignment
0.17
165
2.57 mol.“,.
1.93 0.26 1.90
0 (cm’) x 10~20
10.0 155 11.0
(nm)
Ah
1.43 1.43 2.57
Absorption coefficient at 540 nm (cm-‘)
4297 _
1123
300
354 _
P,, (W cm-*) L, = 1% L, = 0% L, = 07” L, = 37;
and laser properties of holmium (III) in fluoride glass as compared with tellurite glass
Host
Spectroscopic
110 4200 13.5 2900
51 (ns)
a a 29 a
Reference
194
References
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
M. Poulain, M. Poulain and J. Lucas, Mater. Res. Bull., lO(1975) 243. M. Poulain, M. Chanthanasinh and J. Lucas, Mater. Res. Bull., 12(1977) 151. M. Poulain, and J. Lucas, Verres Refract., 32 (1978) 505. D. C. Tran, G. H. Sigel, Jr., and B. Bendow, J. Lightwaue Technology, LT-2 (1984) 566. J. P. Miranday, C. Jacoboni and R. De Pape, J. Non-tryst. Solids, 43 (1981) 393. J. P. Miranday, C. Jacoboni and R. De Pape, Rev. Chim. Miner., 16(1979) 277. R. Reisfeld, E. Greenberg, C. Jacoboni, R. De Pape and C. K. JCrgensen, J. Solid State Chem., 53 (1984) 236. M. Eyal, E. Greenberg, R. Reisfeld, and N. Spector, Chem. Phys. Lett., 117(1980) 108. J. L. Adam, and W. A. Sibley, J. Non-tryst. Solids, 76(1985) 267. R. Reisfeld, E. Greenberg, R. N. Brown, M. G. Drexhage and C. K. J@gensen, Chem. Phys. Lett., 95(1983) 91. B. Blanzat, L. Boehm, C. K. J#rgensen, R: Reisfeld and N. Spector, J. Solid State Chem., 32 (1980) 185. K. Tanimura, M. D. Shinn, W. A. Sibley, M. G. Drexhage and R. N. Brown, Phys. Reu. B, 30 (1984) 2429. R. Reisfeld, M. Eyal, E. Greenberg and C. K. Jdrgensen, Chem. Phys. Lett., 118(1985) 25. R. Reisfeld, G. Katz, C. Jacoboni, R. De Pape, M. G. Drexhage, R. N. Brown and C. K. Jorgensen, J. Solid State Chem., 48 (1983) 323. R. Reisfeld, G. Katz, N. Spector, C. K. Jorgensen, C. Jacoboni and R. De Pape, J. Solid State Chem., 41(1982) 253. M. D. Shinn, W. A. Sibley, M. G. Drexhage and R. N. Brown, Phys. Reu. B., 27(1983) 6635. G. A. Rines, M. D. Thomas, M. G. Knights and E. P. Chicklis, Digest of Technical Papers of CLEO, Baltimore, MD, WJ2 (1985) 94. M. G. Knights, J. Mosto and E. P. Chicklis, Digest of Technical Papers of CLEO, Baltimore, MD, WJl(l985) 94. J. Lucas, M. Chanthanasinh, M. Poulain, M. Poulain, P. Brun and M. J. Weber, J. Non-tryst. Solids, 27(1978) 273. Y. Ohishi, S. Mitachi, T. Kanamori and T. Manabe, Phys. Chem. Glasses, 24 (1983) 135. R. Reisfeld and C. K. Jorgensen, Lasers and Excited States of Rare Earths, Springer, Berlin, 1977. R. Reisfeld, J. Less-Common Met., 112(1985) 9. R. Reisfeld, Proc. Int. Symp. on Rare Earth Spectroscopy, Wroclaw, 1984, World Scientific, Singapore, 1985 p. 587. R. Reisfeld and M. Eyal, J. Phys. (Paris), Colloq., 7(1985) 349. C. W. Nielson and G. F. Koster, Spectroscopic coefficients for the p”, d” and f” Configurations, Massachusetts Institute of Technology Press, Cambridge, MA, 1964. R. Reisfeld, NATO AR W, Algarve, Portugal, 1986. S. E. Stokowski and M. J. Weber, Laser Glass Handbook M-95, Lawrence Livermore National Laboratory, CA, 1979. A. Bornstein and R. Reisfeld, J. Non-tryst. Solids, 50 (1982) 23. R. Reisfeld and Y. Kalisky, Chem. Phys. Lett., 75(1980) 443. J. C. Michel, D. Morin and F. Auzel, Rev. Phys. Appl., 13 (1978) 859. R. C. Eckhardt, L. Esterowitz and R. E. Allen, in Rare Earths in Modern Science and Technology, Plenum, New York, 1980, p. 631.
COMPARISON OF LASER PROPERTIES AND FLUORIDE GLASSES*
R. REISFELD”,
187
OF RARE EARTHS IN OXIDE
M. EYAL” and C. K. JQRGENSEN”
a Department o~i~or~a~~c and Analy~~cul Chemistry, The Hebrew University of Jerusalem, JerusaEem
91904 (Israel) ’ Dkpartement de Chimie M&ale, (Switzerland)
Analytique et Appliqute,
tiniversite* de GenPve, CHl2II
Geneva 4
(Received March 3,1986)
Summary Laser properties of fluoride and oxide glasses containing neodymium(III), and holmium(II1) depend on the pumping efhciency, lifetimes of the excited and terminal levels and stimulated peak cross-sections. We have determined these properties in oxide glass (tellurite) having a composition of 35Zn0:65TeO, and fluoride glass (ZBLA) having composition 57ZrF, : 34BaF, : 5LaF, : 4AlF,, or (PBLA) having a composition 36PbF, : 24ZnF, : 35GaF, : 3YF, : 2AlF, for neodymium(II1) and holmium(II1). It was shown that multiphonon relaxation (in the case of energy difference less than 3000 cm- ‘) is always orders of magnitude lower in the fluoride glass than in the tellurite glass, which was chosen as a model for oxide glass. No significant difference was found for concentration quenching between the two kinds of glasses.
1. Introduction Fluoride glasses containing about 50 mol.% ZrF,, which can be replaced by HfF, or ThF, [l-3] (ZBLA glass), have been considered as materials for fibre optics in the range 0.335 urn [4]. Another important category of fluoride glasses containing zinc(I1) (or manganese(II)), gaIlium(II1) and lead(I1) fluorides (PBLA glass) was invented [5, S] at the University of Maine, Le Mans. The absorption spectra and luminescence of 4f” erbium(II1) and 3ds manganese(II), and the mutual energy transfer between excited states of these two species were studied in such a glass [7]_ In ZBLA glass, the luminescence of 4f2 praseodymium(III)
*Paper presented at the 17th Rare. Earth Hamilton, Ontario, Canada, June 9 12.1986. 0022-5088/86/%3.50
Research
Conference,
McMaster
?I Elsevier Sequoia/Printed
University,
in The Netherlands
188
[S, 91, 4P europium(II1) [lo, 111, 4f” holmium(II1) [12,13] and erbium(II1) [1416] occurs from more highly excited J levels than usual, the lower limit still allowing perceptible luminescence for the energy gap between the emitting J level and the closest lower-lying J level being 2000 cm- ’ (0.25 eV), some 2 to 4 times smaller than in nearly all other glasses and crystals. It may be noted that this rich emission spectrum from several J levels besides the ground state is observed also at room temperature. The superior optical characteristics of fluoride glasses for IR fibre optic applications also provide an ideal medium or host enabling the glass to be integrated into a system acting as a laser light source as well as the actual waveguide material. Laser properties of neodymium(II1) in YLiF, crystals were investigated by Rines et al. [17] and of holmium(II1) in YLiF, crystals sensitized with erbium(II1) and thulium(II1) by Knights et al. [18]. We feel that fluoride glass can serve as a laser host in a similar way to crystals. The spectroscopic and fluorescent properties of neodymium(II1) containing fluorozirconate glasses have been reported by Weber in work with the Lucas-Poulain group at Rennes [19] and optical absorption of 3d transition metal ions such as iron, cobalt, nickel and copper by Ohishi et al. [20]. Here we calculated laser properties of neodymium(III)- and holmium(III)- doped fluoride glasses and compared these properties with tellurite and ED-2 laser glass. The procedure of such calculation has been described elsewhere [al-241. 2. Intensity parameters and radiative transitions of neodymium(II1) holmium(II1)
and
The absorption spectra of neodymium(II1) and holmium(II1) serve as a basis for a complete set of predictions of transition rates within the 4f3 configuration of neodymium(II1) and the 4f” configuration of holmium(II1). The procedure is based on the Judd-Ofelt theory and is described in detail elsewhere [15,21]. According to theory, the otherwise forbidden transitions within the f-f configuration of rare earths become slightly allowed by admixing of wavefunctions of the f-f configuration with odd components of the crystal field potential. The intraconfigurational transitions then become subject to a new set of selection rules and oscillator strengths of the transitions depend parametrically on the three phenomenological parameters R,, 52,, C&. Reduced matrix elements for the transitions are almost invariant in respect to the crystal field strength [al] and have been tabulated for all rare earth ions [25]. The optical transitions of rare earths in solids are predominantly of electric dipole character and their spectral intensities can be described using the treatment of Judd and Ofelt. In this approach the line strength S of a transition between two J states is given by the sum of products of empirical intensity parameters R, and matrix elements of tensor operators UC’)of the form S,,(aJ:bJ’)
= e’~R,l(aJI
where t = 2,4,6
IO’)1 IbJ’)lz
(1)
The values of Q, are obtained from a least-squares fit of measured and calculated absorption line strengths and typically have an experimental uncertainty of about 10%. The integrated intensities of the absorption bands yield 0 which are an effective average over the different rare earth environments in the glass. The most significant factor determining the values is the strengths of the odd-order terms in the expansion of the local field at the rare earth sites. These in turn are affected by the nearest-neighbour anion(s) and cations. For a given glass former systematic changes of R, have been observed with the changes in the size and charge of network modifier ions [22]. The three R are determined by solving an overdetermined set of linear equations built by equating the measured oscillator strengths with the sum of products of the unknown R with the appropriate reduced matrix elements. The three R found from the solution are put into a computer program that calculates all the radiative transition rates possible in the system analyzed. The intensity parameters for neodymium(II1) and holmium(II1) in fluoride glasses are given in Table 1. These were used for calculation of transition rates between relevant levels using [22] A(aJ:
bS)
=
64n4v3
n(n2 + 2)2
3hc3(2J + 1)
9
S,, + n3Smd
C-9
where S,, is the magnetic dipole contribution TABLE 1 Intensity parameters for neodymium(II1) and holmium(II1) in fluoride and tellurite glasses Glass *Ho* ZBLA ZBLA ZnTe *Nd* PBLA ZBLA ZBLA ZnTe
Reference
a2 (pm*)
% (pm’)
Q, (pm*)
12 22 22
2.28 2.43 f 0.03 6.92 f 0.22
2.08 1.67+0.09 2.81+ 0.33
1.73 1.84f0.03 1.42+0.20
26 26 19 26
1.01 kO.28 l.lOkO.25 1.95*0.2ci 4.66kO.57
3.73 * 0.35 3.80+0.30 3.6510.38 5.08kO.83
6.19kO.43 5.53kO.20 4.17+0.17 6.08+0.51
ZnTe, 35Zn0:65TeO,. ZBLA, 57.OZrF,:34.OBaF,:3.OLaF,:4.OAlF,:2.0(NdF,, PBLA, 36PbF,:24ZnFZ:35GaF,:2AlFx:3YF,:2LaF,:2NdFj
3. Non-radiative
HoF,) mol.“,,. mol.“,,.
transitions
The calculation
of non-radiative transfer rates due to multiphonon decay, by subtracting the calculated radiative rates from the reciprocals of the integrated lifetimes. The two parameters of the multiphonon decay were calculated from the plot of W,,, us. energy gap AE from the emitting W,, was accomplished
190
level to the next lower level: the parameter a from the slope of the plot and the electronic factor B from its intercept. The parameters obtained for fluoride B = 1.63 f 0.1 x lOi’, are: a = 0.0053 * 0.0005, (PBLA) and glasses a = 0.0052 f 0.0005, B = 1.59 + 0.1 x lOi (ZBLA), and for tellurite glass: CI= 0.0047 and B = 6.3 x 10”. The parameters are substituted into W,,, = Bexp( - aAE)
(3)
where AE is the energy gap from the electronic level to its next lower neighbour. The entire set of transition rates is calculated, now with the non-radiative transition rates included [24]. The result of such a procedure is shown in ref. 26, Table 4, for neodymium(II1) in PBLA glass, where in addition to 4F,,, emission also the emission from 4D3,2 and 2P3,2 is shown. The outstanding property of fluoride glasses is the relatively long-lived luminescence from levels which are separated only by a small energy gap to the next lower level [13]. In PBLA and ZBLA glass we were able to measure the lifetimes of emissions from two levels; the thermalized 4D,,2 (361 nm) and 2P,,2 (387 nm). The first three emission lines are identified as belonging to the transitions: 4D,,2 + 41,,2 (361 nm), 4Dj,2 ---*41, ii2 (381 nm) and 4D,,2 -+ 41,,,2 (412nm). The second group of lines belongs to the transitions from 2P3,2. In addition to multiphonon decay, cross-relaxation between two ions of the same nature at high concentration lowers the lifetime of emitting level. The cross-relaxation between a pair of rare earth ions is graphically presented in ref. 24, Fig. 3. The measured luminescence lifetime is related to the total relaxation rate by l/r = C W,, + CA + Per = l/z, + Per
(4)
where CA is the total radiative rate, C W,,, is the non-radiative rate and PC, is the rate of cross-relaxation between adjacent ions, and 7. is the intrinsic lifetime. The critical radius R, for cross-relaxation is defined by P,,(RoW70)
=
(5)
1
R. being the critical distance at which the probability
for cross-relaxation PC, equals the sum of radiative and multiphonon relaxations. The cross-relaxation channel for neodymium(II1) is 4F,,, + 411si2 = 4I912 -+ 411 512 and for holmium(II1) (5S2,5F4) + 51, = 51, + 514. The critical radii in various glasses are presented in Table 2. 4. Laser action The formula for laser peak cross-section
is [27]
where 3,(cm) is the emission wavelength; Ai (cm) is the full width at half height of the emission band; n is the refractive index; A (s- ‘) is the radiative transfer probability.
191
Threshold P
= W%l th
power for transverse pumping is
+ L,,,) x 10- ’ 2&,5,1Focc,
(7)
where L,,, is the resonant power wavelength which is defined as
loss due to self-absorption
at the laser
L,,, = 21aN&/Z
(3)
where N is the number density of lasing lines, & the Boltzmann factor for the terminal laser level, Z the partition function, 1 the length of laser resonator cavity, L, the non-resonant loss which is mainly due to the absorption of the medium and loss at mirrors, i,, the pumping wavelength, zy the lifetime of the lasing level, F the Boltzmann population fraction of the lasing level and CQ, the absorption coefficient of the pumped level which is obtained by dividing the optical density of the sample by its thickness. TABLI? 2 Critical glasses
radii for cross-relaxation
Compound
*Ho (III)* 0.23;ZnTe O.S/ZnTe 2.7;ZnTe 2.0iZBLA *Nd (III)* 0.5t ‘ZnTe 1.6;ZnTe 2.7,‘ZnTe 0.5,‘ZBLA 2.OIPBLA
Concentration (102” cm-“)
of neodymium(II1)
and holmium(II1)
Critical radius [A]
~intr,nric (w)
0.48 1.90 5.80 3.20
3.32 f 1.32 4.2lkO.44 3.44 +0.56 5.54 f 0.28
1.10 3.50 5.80 0.85 4.02
4.74kO.11 5.07kO.36 5.81 kO.25 3.72 f 0.80 5.07 k 0.61
ZnTe, 35Zn0:65Te02. ZBLA, 57.OZrF,:34.OBaF,:3.OLaF,:4.OAlF, mol.“,,. PBLA. 36PbF,:24ZnFz:35GaF,:2A1F,:3YF,:2LaF,:2NdF,
in fluoride and tellurite
Excitation
TlllM‘“W3 b)
A.(nm)
13.5 13.5 13.5 332
12.3 10.2 9.2 105
445 445 445 440
187 187 187 455 345
178 130 102 443 264
579 579 579 576 576
mol.“,,.
Tables 3 and 4 present the comparison of peak cross-section and threshold power for laser action of neodymium(II1) and holmium(II1) in fluoride, oxide and chalcogenide glasses for transverse pumping. From the tables it can be seen that the laser characteristics for neodymium(II1) in fluoride glasses are quite similar and even better than in ED-2 glass. The laser cross-section obtained in this work for neodymium(II1) in tellurite glass is similar to that obtained by Michel et al. [30] in lithium tellurite, where laser action was observed with a very low threshold. The threshold for laser emission due to %, + 51, emission at 745 nm is extremely high in singly doped glass, however, it can be significantly lowered by co-doping the glass with erbium(II1) as found with YLiF, [31].
4F3,2-+411iiz 4F,,2-+41, ,,2 4F3,Z-+411i,r 4F3,2-+41, ,,z 4F,,,--+41, i/z
ZBLA PBLA ED-2 ZnTe GLS
1049 1039 1060 1660 1077
(nm)
Peak emission
2.72 4.02 1.83 3.46 2.63
Concentration (cm - s) x 102*
of neodymium(II1)
glasses
3.14 3.57 1.27 4.73 14.50
wt.%.
Absorption coefficient at 806 nm (cm-l)
in fluoride
GLS, 3Ga,S,:0.85LaS,:O.l5Nd,S,. ZBLA, 57.OZrF,:34.OBaF,:3.OLaF,:4.OAlF~:B.ONdF, mol.%. PBLA, 36PbF,: 24ZnF~:35GaF~:2AlF~:3YF~:2LaF~:2NdF~ mol.%. ED-2,60Si0,:27.5Li,0:10Ca0:2.5A1,0,:0.16Ce0, mol.%, 2.012Nd,O, ZnTe, 35Zn0:65TeO, mol.%, ZNd,O, wt.%. aThis work.
Assignment
Host
Spectroscopic and laser properties chalcogenide and oxide glasses
TABLE 3
26.7 33.0 27.8 29.0
(nm)
Al
2.9 2.75 2.9 3.6 7.95
a(cm’) x 10-20
as compared
57 112 173 93 11.3
Pth (W cm- 2, L,=l% L, = 0.2%
with
400 190 300 130 100
19 a 27 a 28
Reference
TABLE 4
1990
51,p51, *
ZnTe
3.49
3.20 3.20 3.49
Concentration (cm-j) x 1020
ZBLA, 57.0ZrF,:34.0BaFZ:3.0LaF,:4.0A1F,:2.0HoF, *This work. *Calculated for 77 K.
ZnTe, 35Zn0, 65TeOZ.
745 1990 745
5s,m“I, 51,~51,* 5S,m51,
ZBLA ZBLA ZnTe
(mn)
Peak emission
Assignment
0.17
165
2.57 mol.“,.
1.93 0.26 1.90
0 (cm’) x 10~20
10.0 155 11.0
(nm)
Ah
1.43 1.43 2.57
Absorption coefficient at 540 nm (cm-‘)
4297 _
1123
300
354 _
P,, (W cm-*) L, = 1% L, = 0% L, = 07” L, = 37;
and laser properties of holmium (III) in fluoride glass as compared with tellurite glass
Host
Spectroscopic
110 4200 13.5 2900
51 (ns)
a a 29 a
Reference
194
References
8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
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