Spectroscopy and concentration quenching of the
infrared emissions in Tm3+-doped TeO2-TiO2Nb2O5 glass
Rolindes Balda1,2*, Joaquín Fernández1,2, Sara García-Revilla1, and Jose M. FernándezNavarro3
1
Departamento Fisica Aplicada I, Escuela Superior de Ingenieros, Alda. Urquijo s/n 48013 Bilbao, Spain
Unidad Física de Materiales CSIC-UPV/EHU and Donostia International Physics Center, Apartado 1072, 20080
San Sebastian, Spain
3
Instituto de Optica, Consejo Superior de Investigaciones Científicas, Serrano 121, 28006 Madrid, Spain
*Corresponding author: [email protected]
2
Abstract: In this work, we report the optical properties of Tm3+ ions in
tellurite glasses (TeO2-TiO2-Nb2O5) for different Tm3+ concentrations
ranging between 0.05 and 1 wt%. Judd-Ofelt intensity parameters have been
determined to calculate the radiative transition probabilities and radiative
lifetimes of excited states. The stimulated emission cross-sections of the
infrared emissions at 1487 nm and 1800 nm have been determined from the
line shape of the emission spectra and the lifetimes of levels 3H4 and 3F4
respectively. The emission spectra obtained under 793 nm excitation reveal
the existence of energy transfer via cross-relaxation among Tm3+ ions. As a
result, the intensity of the infrared 3H4→3F4 emission at 1487 nm decreases
in relation to the one at 1800 nm, as concentration increases. The nonexponential character of the decays from the 3H4 level with increasing
concentration indicates the presence of a dipole-dipole quenching process
assisted by energy migration. The self-quenching of the 3F4→3H6 emission
at 1800 nm can be attributed to limited diffusion within the active centers.
©2007 Optical Society of America
OCIS codes: (140.3380) Laser Materials; (160.5690) Rare earth doped materials; (300.6280)
Spectroscopy, fluorescence and luminescence
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28 May 2007 / Vol. 15, No. 11 / OPTICS EXPRESS 6750
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1. Introduction
In the last years the rapid expansion of bandwidth requirements for telecommunications in the
1400-1600 nm low-loss optical fiber transmission window has generated considerable interest
in Tm3+ ions. The 3H4→3F4 transition at around 1470 nm allows a band extension in the
spectral range corresponding to the S-band amplifier region, on the short wavelength side of
the conventional erbium-doped fiber amplifier C band at 1530-1570 nm. On the other hand,
the thulium emission around 1800 nm is of interest to extend lasing capability into the 1600#81812 - $15.00 USD
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1900 nm atmospheric window [1-3]. However, there are some problems to develop efficient
amplifiers using Tm3+-doped glasses. Two important factors to be considered in developing
more efficient optical devices based on rare-earth ions are the glass host and the active ions
concentration. The host matrix should have a low phonon energy to minimize multiphonon
relaxation rates because the energy difference between the 3H4 and the next lower-lying 3H5 is
not large (≈4300 cm-1). To avoid this problem, glasses with low phonon energies such as
fluorides, tellurites, chalcogenides, and heavy metal oxide glasses are required.
Tellurite glasses have been a subject of increasing interest for optoelectronics applications,
especially because of their high refractive index and low phonon energies [4,5]. Moreover,
these glasses combine good mechanical stability, chemical durability, and high linear and
nonlinear refractive indices, with a wide transmission window (typically 0.4-6 μm), which
make them promising materials for photonic applications such as upconversion lasers, optical
fibre amplifiers, non linear optical devices, and so on [6-14]. Broadband Er-doped fiber
amplifiers have been achieved by using tellurite-based fibers as the erbium host [11,12].
Among the different compositions studied, niobic tellurite glasses have proven to possess a
large transparency window as well as a high refractive index and high stability [15-18].
Besides, the addition of TiO2 produces a further increase of the linear and nonlinear refractive
indices. The high linear index increases the local field correction at the rare earth site leading
to large radiative transition probabilities, whereas the non-linear one enhances the optical nonlinearities [15,17,19].
The second factor to be considered is the ion concentration. When the activator ion
concentration in glass becomes high enough, ions interact and ion-ion energy transfer occurs.
The energy transfer processes reduce the lifetime and consequently the efficiency of the 3H4
level due to the well-known cross relaxation process (3H4, 3H6→3F4, 3F4) [4]. In this process
part of the energy of an ion in the 3H4 level is transferred to another ion in the ground state
with both ions ending up in the 3F4 level.
In this work, we present together with the spectroscopic properties of Tm3+ ions in niobic
tellurite glass (80TeO2-5TiO2-15Nb2O5), the effect of concentration on the 3H4→3F4 and
3
F4→3H6 emissions for different Tm3+ concentrations (0.05, 0.1, 0.2, 0.5, and 1 wt%) and at
different temperatures between 10 K and 295 K. As the Tm3+ ions concentration is increased
both infrared emissions show concentration quenching. In the case of the 1487 nm emission
the non-exponential character of the decays from the 3H4 level with increasing concentration,
together with the dependence of the quenching rates on Tm3+ concentration, indicate the
presence of a dipole-dipole quenching process in the framework of a diffusion-limited regime.
The average critical distance, which indicates the extent to which the energy transfer can
occur, has been obtained at different temperatures and compared with other glasses.
Concerning the 1800 nm emission, the analysis of the experimental decays as a function of
concentration indicates that the self-quenching can be attributed to limited diffusion within the
active centers.
2. Experimental techniques
The glass was prepared by melting a 10 g batch of high purity TeO2 (Alfa 99.99), Nb2O5 (Alfa
99.995), and TiO2 (Sigma Aldrich 99.99) reagents and heating them in a platinum crucible
placed in a Thermostar vertical furnace, at 780º C during 30 min in air atmosphere. The melt
was stirred with a platinum rod and then poured onto a preheated brass plate, annealed 15 min
at 410º C, and further cooled at a rate of 3º C/min down to room temperature. The glass was
doped with 0.05, 0.1, 0.2, 0.5, and 1 wt% of Tm2O3 (Alfa 99.999) with an accuracy of 1% for
the less concentrated sample. The Tm3+ concentrations are based on the dopant added. No
post-melting analysis was made. These concentrations correspond to 0.8x1019, 1.6x1019,
3.2x1019, 0.8x1020, and 1.6x1020 ions/cm3. Finally the samples were cut and polished for
optical measurements.
Conventional absorption spectra were performed with a Cary 5 spectrophotometer. The
steady-state emission measurements were made with a Ti-sapphire ring laser (0.4 cm-1
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linewidth) in the 760-940 nm spectral range as exciting light. The fluorescence was analyzed
with a 0.25 monochromator, and the signal was detected by a PbS detector. Lifetime
measurements were obtained by exciting the samples with a Ti-sapphire laser pumped by a
pulsed frequency doubled Nd:YAG laser (9 ns pulse width), and detecting the emission with
an extended IR Hamamatsu R5509-72 photomultiplier. Data were processed by a Tektronix
oscilloscope.
3.
Results
3.1 Absorption and emission properties
2.5
1G
4
T=295 K
20
2.0
1G 3F
3
4
2,3 H 4
1.5
16
3H
5
1.0
3F
4
E (103 cm-1)
Absorption cross section (10-20cm2)
The room temperature absorption spectra were obtained for all samples in the 400-2000 nm
range with a Cary 5 spectrophotometer. Its spectral resolution was 0.5 nm at wavelengths
below 1100, and 2 nm, above. Figure 1(a) shows the room temperature absorption cross
section as a function of wavelength. The spectrum is characterized by six bands corresponding
to the transitions starting from the 3H6 ground state to the different higher levels 1G4, 3F2, 3F3,
3
H4, 3H5, and 3F4. Energy levels higher than 1G4 are not observed because of the intrinsic
bandgap absorption in the host glass. The integrated absorption coefficient for different
absorption bands shows a linear dependence on concentration, which indicates that the
relative concentrations of Tm3+ ions are in agreement with the nominal values. Figure 1(b)
shows the energy level diagram showing the positions of the J states of the Tm3+ ions in this
glass derived from the absorption spectrum.
3F
2,3
3H
12
793 nm
4
1487 nm
3H
5
8
0.5
3F
4
4
1800 nm
0.0
400
600
800 1000 1200 1400 1600 1800 2000
3H
0
6
Wavelength (nm)
(a)
(b)
3+
Fig. 1. (a) Room temperature absorption cross-section of Tm in TeO2-TiO2-Nb2O5 glass. (b)
Energy level diagram obtained from the absorption spectrum.
Data from the spectrum in Fig. 1(a), together with the value of the refractive index
(n=2.191) have been used to calculate the radiative transition rates by using the Judd-Ofelt
(JO) theory [20,21]. To obtain the contribution to the integrated absorption coefficient
corresponding to levels 3F2 and 3F3 a Gaussian fit method has been used to separate the
overlapping peaks into two independent ones. The absorption bands measured are all
dominated by electric dipole transitions except the transition 3H6→3H5, which contains
electric-dipole and magnetic-dipole contributions. The magnetic-dipole contribution, fmd, can
be obtained from the equation fmd=nf´ [22], where n is the refractive index of the studied glass
and f´ is a quantity calculated on the basis of energy-level parameters for lanthanide aquo
ions. The electric dipole oscillator strength for this transition is then obtained by subtracting
the calculated magnetic-dipole contribution from the experimental oscillator strength. By
using a least squares fitting of calculated and experimental oscillator strengths, the JO
parameters obtained for this glass are Ω2=4.09x10-20 cm2, Ω4=1.36x10-20 cm2, and
Ω6=1.19x10-20 cm2, with a root-mean-squared deviation equal to 3.88x10-7. These values are
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in agreement with those previously reported in tellurite glasses [23]. The error analysis of the
measured quantities used in the JO calculation gives an accuracy of 5%.
The radiative transition probabilities for all excited levels of Tm3+ can be calculated by
using the JO parameters. The radiative transition probabilities, the branching ratios, and the
radiative lifetimes of some selected levels of Tm3+ in TeO2-TiO2-Nb2O5 (TTN) glass are
shown in Table 1.
Table 1. Predicted radiative transition rates, lifetimes, and branching ratios of Tm3+ in TTN glass.
Energies (cm-1)
Arad (s-1)
G4 → H6
3
F4
3
H5
3
H4
3
F3
3
F2
21236
15461
12978
8707
6799
6245
2588.7
397.7
1804.3
639.3
137.4
36.1
H4 →3H6
3
F4
3
H5
12610
6725
4242
H5 →3H6
3
F4
F4 →3H6
Transitions
1
3
3
3
3
τrad (ms)
0.178
β (%)
46
7
32
11
2.4
0.6
3098.7
259.7
49.3
0.293
91
7.6
1.4
2484
8368
656.5
6.9
1.49
99
1
5555
558.7
1.78
100
The infrared emissions in the 1300-2200 nm spectral range were obtained for all samples
at room temperature by exciting at 793 nm. Figure 2 shows the fluorescence spectra
corresponding to the 3H4→3F4 and 3F4→3H6 transitions normalized to the 3H4→3F4 transition
for the samples doped with 0.1, 0.2, 0.5, and 1 wt% of Tm2O3. The spectra show a strong
emission band centered around 1487 nm which corresponds to the 3H4→3F4 transition
together with a less intense emission band centered around 1800 nm and corresponding to the
3
F4→3H6 transition. The peak position and the bandwidth do not change with Tm3+
concentration. However, the ratio of the integrated emission intensity of transition 3H4→3F4 to
that of transition 3F4→3H6 decreases with increasing Tm3+ concentration. This reduction of the
intensity of the 1487 emission with concentration has been attributed to cross relaxation
between 3H4→3F4 and 3F4→3H6 transitions [4,25]
The room temperature stimulated emission cross section of the 3H4→3F4 laser transition
has been obtained by using the following expression [26],
σ se =
λ p4
β
8πn 2 c τ R Δλeff
(1)
where λp is the peak fluorescence wavelength, β is the branching ratio for the transition, n is
the index of refraction of the host matrix, c the velocity of light, τR the radiative lifetime of
the emitting level, and Δλeff is the effective linewidth. The effective linewidth (105 nm) of the
transition has been calculated by using the relation Δλ = I (λ )dλ and it is broader by nearly
eff
∫
I max
30 nm than the one in fluoride glasses. This makes this niobic tellurite glass attractive for
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broadband amplifiers specially in the wavelength range that overlaps the conventional band of
erbium doped fiber amplifiers. The maximum emission cross-section is 0.4x10-20 cm2 which is
sligthly higher than the one found in other tellurite glasses and twice the one of ZBLAN glass
[27]. The gain bandwidth of an amplifier is determined by the width of the emission spectrum
and the emission cross-section. Using the figure of merit (FOM) for bandwidth as the product
of the stimulated emission cross-section and FWHM, this value is nearly three times larger
than in fluoride glass. Assuming that the FOM for bandwidth is an indication of the
achievable gain band, the obtained value for these niobic-tellurite glasses suggests that these
glasses may provide extended short wavelength gain of the erbium-doped C band at 15301570 nm. Figure 3 shows the spectral overlap between the 3H4→3F4 and 4I13/2→4I15/2
transitions of Tm3+ and Er3+ ions respectively in TTN glasses.
1.2
Intensity (arb. units)
3H →3F
4
4
T= 295 K
λ =793 nm
exc
1.0
0.8
3F →3H
6
4
1 wt%
0.5 wt%
0.2 wt%
0.1 wt%
0.6
0.4
0.2
0.0
1300 1400 1500 1600 1700 1800 1900 2000 2100 2200
Wavelength (nm)
Normalized Intensity (arb. units)
Fig. 2. Room temperature emission spectra of Tm3+ ion in TTN glass for the samples doped
with 0.1, 0.2, 0.5 and 1 wt% of Tm2O3.
T = 295 K
3H →3F
4
4
4
4I
13/2→ I15/2
0.8
0.4
0.0
1300 1350 1400 1450 1500 1550 1600 1650 1700
Wavelength (nm)
Fig. 3. Spectral overlap between the H4→3F4 and 4I13/2→4I15/2 normalized emissions of Tm3+
and Er3+ ions respectively.
3
We have also obtained the room temperature stimulated emission cross section of the
F4→3H6 laser transition by using expression (1). In this case, the branching ratio for an
emission between the first excited level and the ground state is equal to unity. The maximum
3
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emission cross section is 0.92x10-20 cm2. As for the 3H4→3F4 transition, this value is more
than twice the one of ZBLAN glass [28].
3.2 Lifetimes
The lifetime values of the 3H4 level were obtained for different Tm3+ concentrations as a
function of temperature by exciting at 793 nm. The fluorescence lifetime at low concentration
and temperature is equal to the calculated radiative lifetime (293.5 μs); however, as
concentration increases, the lifetime decreases even at low temperature, which indicates the
presence of nonradiative energy transfer processes. The fluorescence decays for the samples
doped with 0.05, 0.1, 0.2, and 0.5% of Tm2O3 can be described at all temperatures by an
exponential function to a good approximation. For the sample doped with 1 wt% the decays
become non exponential. As an example, Fig. 4 shows the logarithmic plot of the
experimental decays of the 3H4 level at 295 K for the samples doped with 0.1, 0.5, and 1 wt%.
T= 295 K
Ln I(t) (arb. units)
0.0
3H →3F
4
4
-4.0
0.1%
-8.0
1%
0.5%
-12.0
0.0
0.5
1.0
1.5
Time (ms)
2.0
Fig. 4. Logarithmic plot of the fluorescence decay of the 3H4 level obtained under excitation at
793 nm at room temperature for the samples doped with 0.1, 0.5, and 1 wt%.
Figure 5 shows the lifetime values of the 3H4 level between 10 K and 295 K for the
samples doped with 0.1, 0.2, 0.5, and 1 wt%. The lifetime of the sample doped with 0.05%
(not shown in the figure) is equal to the radiative lifetime at low temperature. However it
shows a small decrease at room temperature probably related to the presence of some impurity
ions. In fact, infrared transmission measurements reveal the presence of a band around 3 μm
corresponding to OH- impurities. The lifetime values for the sample doped with 1 wt%
I (t )dt
correspond to the average lifetime defined by τ = ∫
I0
The decays of the 3F4 level were obtained for all samples at 295 K by exciting at 793 nm.
The decays show an initial rise, due to the lifetime of the 3H4 level, followed by the decay. The
decays of the samples doped with 0.05, 0.1, 0.2, and 0.5 wt% can be described by an
exponential function to a good approximation. However, the decay of the sample doped with
1 wt% deviates from a simple exponential. As an example, Fig. 6 shows the logarithmic plot
of the experimental decays of the 3F4 level at 295 K for the samples doped with 0.1 and 1
wt%. The lifetimes decrease from 2.16 ms to 1.56 ms as concentration increases from 0.05 to
1 wt% of Tm2O3.
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350
3H
Lifetime (μs)
300
4
0.1%
0.2%
250
0.5%
200
1%
150
100
0
50
100
150
200
Temperature (K)
250
300
Fig. 5. Temperature dependence of the 3H4 level lifetime for the samples doped with 0.1, 0.2,
0.5, and 1 wt% of Tm2O3.
-2
T= 295 K
Ln [I (t) (arb. units)]
-4
3F →3H
4
6
-6
-8
0.0
0.4
0.8
1.2
-10
-12
λexc= 793 nm
-14
λem= 1660 nm
0.1%
1%
-16
0
2
4
6
8
Time (ms)
10
12
14
Fig. 6. Logarithmic plot of the fluorescence decay of the 3F4 level obtained under excitation at
793 nm at room temperature for the samples doped with 0.1 and 1 wt%. The inset shows the
rise times.
4. Discussion
4.1 Concentration quenching of the 3H4 emission
As we mentioned in Section 3, the emission from the 3H4 level shows concentration
quenching, and as concentration rises the 1487 nm emission becomes less intense whereas the
relative intensity of the 1800 nm emission increases. This concentration quenching is also
reflected by the decrease of the experimental lifetimes. This behavior has been previously
observed in Tm3+-doped systems and attributed to cross-relaxation between Tm3+ ions [4]. In
this process part of the energy of an ion in the 3H4 level is transferred to another ion in the
ground state with both ions ending up in the 3F4 level (3H4, 3H6→3F4, 3F4). This process
reduces the lifetime of the 3H4 level and consequently the efficiency of the 1487 nm emission.
The characteristic decay time of the 3H4 level should be governed by a sum of probabilities
for several competing processes: radiative decay, nonradiative decay by multiphonon
relaxation, and by energy transfer to other Tm3+ ions. In these tellurite glasses nonradiative
decay by multiphonon relaxation is expected to be small because the energy difference
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between 3H4 and 3H5 levels is 4240 cm-1 and the energy of the highest phonons is about 780
cm-1. This corresponds to 5.4 phonons, which indicates that multiphonon relaxation process is
weak and can be neglected in this case. Energy transfer processes such as cross-relaxation are
generally described in terms of three limiting cases: (i) direct relaxation, (ii) fast diffusion,
and (iii) diffusion limited relaxation [29]. In the diffusion limited relaxation model, in the
case of the dipole-dipole interaction, the quenching rate is given by,
1 1
(2)
−
= KN A N D
τ τR
where τR is the intrinsic decay time, K is a constant involving donor-donor and donor-acceptor
transfer constants, and NA and ND are the acceptor and donor concentrations respectively. In
our case the donor and acceptors are the Tm3+ ions and the equation gives the quenching rate
as a function of the square of concentration.
In the case of very fast diffusion, the decay of the donor fluorescence is purely exponential
and the quenching rate shows a linear dependence on concentration. Figure 7 shows the
quenching rate of the 3H4 level as a function of the square of Tm3+ concentration at room
temperature. The intrinsic decay time τR corresponds to the lifetime of the lowest concentrated
sample which is equal to the radiative lifetime (293.5 μs). As can be observed, in this
concentration range the quenching rate shows a linear dependence on the square of
concentration which indicates that the behavior is close to a dipole-dipole quenching
mechanism in the framework of a limited-diffusion regime. The same behavior is observed at
low temperature.
3000
[1/τ-1/τR] x103s-1
3H →3F
4
4
295 K
2000
1000
0
0
1
2
3
[Tm3+concentration (1020cm-3)]2
Fig. 7. Quenching rates of the 3H4 emission as a function of the square of Tm3+ concentration at
room temperature.
Taking into account the existence of energy migration among donors, we have used the
Yokota-Tanimoto and Burshtein expressions to fit the donor fluorescence decay for an energy
transfer assisted by donor migration [30,31]. The best agreement between experimental data
and theoretical fit is obtained with the expression corresponding to the Burshtein model,
⎛
⎞
t
(3)
I (t ) = I 0 exp⎜⎜ − − γ t − Wt ⎟⎟
τ
⎝
⎠
R
where τR is the intrinsic lifetime of donor ions, γ characterizes the direct energy transfer, and
W represents the migration parameter. In the case of dipole-dipole interaction, γ is given by
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Received 3 Apr 2007; revised 11 May 2007; accepted 13 May 2007; published 17 May 2007
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the expression γ = 4 π 3 / 2 N C 1DA/ 2 , where N is the concentration and CDA is the energy transfer
3
microparameter. Figure 8 shows the fit for the sample doped with 1% wt. These results
indicate that the electronic mechanism of energy transfer is a dipole-dipole interaction in the
framework of a diffusion-limited regime. From the fitting in Fig. 8, the value obtained for the
energy transfer microparameter is (1.14±0.07)x10-39 cm6/s and the migration transfer rate was
found to be (579±10) s-1.
The value for the critical radius R0, which is defined as the distance at which the
probability of the cross-relaxation process becomes equal to the intrinsic decay rate of the
metastable level, can be calculated in terms of CDA and τR from R 6 = τ C . The obtained
0
R
DA
value for the critical transfer radius in this glass is 8.3±0.02 Å which means that energy
transfer can occur among ions located within this distance. This value is larger than the one
reported in the case of thulium chalcogenide glass (7.3 Å) [32] and much shorter than the 17.9
Å value recently reported for thulium doped TeO2-CdCl2 glass [33]. The critical distance
decreases from 8.3 Å at 295 K to 7.0 Å at 10 K indicating that the cross-relaxation process is
less efficient at low temperature. On the other hand the migration transfer rate increases from
116 to 579 s-1 as temperature increases from 10 K to 295 K. The increase of the migration
transfer rate with temperature has been examined by Weber in Ref. 29. This rate depends on
the frequencies, linewidths, and probabilities of the transitions involved in the process. At low
temperature only the lowest Stark levels are populated and the number of resonant transitions
giving rise to energy migration are reduced [29].
1
Ln [I (t) (arb. units)]
T=295 K
-1
λ
-3
λ
exc=
em=
793 nm
1487 nm
a
-5
-7
-9
-11
0.0
0.4
0.8
1.2
1.6
2.0
Time (ms)
Fig. 8. Experimental emission decay curve of level 3H4 for the sample doped with 1 wt% of
Tm2O3 at room temperature and the calculated fit with equation (3) (solid line).
4.2 Concentration quenching of the 3F4 emission
Concerning the 1800 nm emission, the relative luminescence intensity increases whereas its
lifetime decreases with concentration. In this case, in which level 3F4 is the first excited state,
the quenching of luminescence when active ion concentration increases can not be due to
cross-relaxation between various excited states, and it has been mainly considered as due to
diffusion towards unidentified impurities (such as OH or other impurities present in the
starting materials) [34]. The problem can be separated into two cases: diffusion limited regime
and fast diffusion. The first case is considered to occur when the order of magnitude for
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Received 3 Apr 2007; revised 11 May 2007; accepted 13 May 2007; published 17 May 2007
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transfer probability between sensitizers and sensitizer to activator are the same. As we have
seen, in the case of the diffusion limited situation the quenching rate is proportional to the
square of concentration. As it is shown in Fig. 9 the quenching rate of level 3F4 shows a linear
behavior as a function of the square of Tm3+ concentration, which indicates that we are
dealing with a diffusion limited regime. In such a case, and assuming a dipole-dipole
interaction, the self quenching behavior can be described by [34],
(4)
τw
τ (N ) =
2
⎡
9 ⎛ N ⎞ ⎤
⎜
⎟ ⎥
⎢1 +
2π ⎜⎝ N 0 ⎟⎠ ⎥
⎢
⎣
⎦
where τw is the measured lifetime at low concentration (2.16 ms) and N0 is a critical sensitizer
concentration for self-quenching. The longer measured lifetime compared with the radiative
one could be due to radiative trapping. Figure 10 shows the experimental values together with
the fit to expression (4). The critical concentration is (3.08±0.17)x1020 ions/cm3. This value is
an indication of the self-quenching. The obtained value for the critical concentration is higher
than the values obtained in Tm-doped lead-niobium-germanate glass [35] and Er-doped
phosphate glasses [36]. As can be seen from Fig. 10, in these glasses the diffusion limited
hypothesis gives a rather good description of the experimental results.
200
[1/τ-1/τR] x103s-1
3F →3H
4
6
295 K
100
0
0
1
2
3
[Tm3+concentration (1020cm-3)]2
Fig. 9. Quenching rates of the 3H4 emission as a function of the square of Tm3+ concentration at
room temperature.
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2.4
295 K
Lifetime (ms)
3F →3H
4
6
2.0
1.6
1.2
0.0
0.5
1.0
Tm3+concentration
1.5
2.0
(1020cm-3)
Fig. 10. Experimental lifetimes of level 3F4 at room temperature as a function of Tm2O3
concentration and the calculated fit with Eq. (4).
5. Conclusions
Absorption and luminescence measurements have been performed in Tm3+ doped niobic
tellurite glasses. The Judd-Ofelt intensity parameters and radiative transition rates have been
calculated. The infrared emissions at 1487 and 1800 nm have been characterized for
concentrations ranging from 0.05 to 1 wt% of Tm2O3. Fluorescence measurements show that
the 1487 nm emission is broader by nearly 30 nm in this glass if compared to fluoride glass
and the stimulated emission cross section is twice which makes these glasses attractive for
broadband amplifiers specially in the wavelength range that overlaps the conventional band of
the erbium doped fiber amplifier.
The 3H4→3F4 emission intensity at 1487 nm was found to decrease in relation to the
3
F4→3H6 emission at 1800 nm as thulium concentration increases, due to the presence of
cross-relaxation processes. An analysis of the fluorescence decays of the 3H4→3F4 emission as
a function of concentration reveals that the electronic mechanism responsible for the ion-ion
interaction is a dipole-dipole quenching process in the framework of a diffusion-limited
regime.
The self-quenching of the 1800 nm emission can be attributed to limited diffusion within
the active centers. This means that the probability for the diffusive steps between active
centers is of the same order of magnitude than the one for quenching between impurities and
centers.
Acknowledgments
This work has been supported by the Spanish Government (Ref.: MAT2005-06508-C02-02
and MAT2004-03780) and the Basque Country University (Ref.: UPV13525/2001).
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(C) 2007 OSA
Received 3 Apr 2007; revised 11 May 2007; accepted 13 May 2007; published 17 May 2007
28 May 2007 / Vol. 15, No. 11 / OPTICS EXPRESS 6761