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 non-exponential 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
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 1600–1900 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 non-linearities [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) . 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 3F4→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 780° 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 410° C, and further cooled at a rate of 3° 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 0.8×1019, 1.6×1019, 3.2×1019, 0.8×1020, and 1.6×1020 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 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.
3.1 Absorption and emission properties
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, 3H4, 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.
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´ , 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=4.09×10-20 cm2, Ω4=1.36×10-20 cm2, and Ω6=1.19×10-20 cm2, with a root-mean-squared deviation equal to 3.88×10-7. These values are in agreement with those previously reported in tellurite glasses . 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.
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 3F4→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 ,
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 ∆λeff is the effective linewidth. The effective linewidth (105 nm) of the transition has been calculated by using the relation and it is broader by nearly 30 nm than the one in fluoride glasses. This makes this niobic tellurite glass attractive for broadband amplifiers specially in the wavelength range that overlaps the conventional band of erbium doped fiber amplifiers. The maximum emission cross-section is 0.4×10-20 cm2 which is sligthly higher than the one found in other tellurite glasses and twice the one of ZBLAN glass . 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 1530–1570 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.
We have also obtained the room temperature stimulated emission cross section of the 3F4→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 emission cross section is 0.92×10-20 cm2. As for the 3H4→3F4 transition, this value is more than twice the one of ZBLAN glass .
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%.
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% correspond to the average lifetime defined by
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.
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 . 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 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 . In the diffusion limited relaxation model, in the case of the dipole-dipole interaction, the quenching rate is given by,
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.
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,
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 the expression , where N is the concentration and CDA is the energy transfer 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)×10-39 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 C DA and τR from R 6 0=τR CDA. The obtained 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)  and much shorter than the 17.9 Å value recently reported for thulium doped TeO2-CdCl2 glass .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 .
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) . 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 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 ,
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)×1020 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  and Er-doped phosphate glasses . As can be seen from Fig. 10, in these glasses the diffusion limited hypothesis gives a rather good description of the experimental results.
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 3F4→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.
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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