In this work we present, for the first time to our knowledge, laser emission under wavelength selective laser-pulsed pumping in Nd3+-doped TeO2-ZnO-ZnF2 bulk glass for two different Nd3+ concentrations. The fluorescence properties of Nd3+ ions in this matrix which include, Judd-Ofelt calculation, stimulated emission cross-section of the laser transition and lifetimes are also presented. The site-selective emission and excitation spectra along the 4I9/2→4F3/2 absorption band show the inhomogeneous behaviour of the crystal field felt by Nd3+ ions in this fluorotellurite glass which allows for spectral tuning of the laser output pulse as a function of the pumping wavelength. The emission cross-section obtained from the Judd-Ofelt analysis and spectral data (4.9x10−20 cm2) is in fairly good agreement with the value obtained from the analysis of the laser threshold data (4x10−20 cm2).
©2013 Optical Society of America
Neodymium doped glasses have been largely investigated since neodymium has been recognized as one of the most efficient rare-earth ions for solid-state lasers due to its intense 4F3/2→4I11/2 emission at around 1.06 μm . Until now, neodymium-doped silicate and phosphate-based bulk glasses have been the most widely used laser glasses due to their good optical, thermal, and mechanical properties. Laser emission has also been obtained in other bulk glasses doped with Nd3+ such as fluorides [2–4], chalcogenides , aluminosilicates , germanates , and tellurite glasses [8–10].
The development of Nd-doped materials for laser applications requires hosts with a low phonon frequency and a low content of OH groups to minimize the multiphonon relaxation rates to enhance the quantum efficiency of emitting levels. Among oxide glasses, TeO2-based glasses combine good mechanical stability, chemical durability, high linear, and nonlinear refractive indices, together with low phonon energies (~750 cm−1) with a wide transmission window (typically 0.4-6 μm) and high rare-earth solubility [11–13]. The low phonon energy reduces the multiphonon relaxation rates and increases the quantum efficiency of the 4F3/2→4I11/2 emission of Nd3+. Furthermore, the relatively high values of the refractive index (1.8<n<2.3) [11, 12] lead to large stimulated emission cross-sections . The stimulated emission cross-sections of Nd3+ for some tellurite glasses obtained by the Judd-Ofelt (JO) approximation are even larger than for phosphate glasses due to the large refractive index . On the other hand, the presence of fluorine ions which can release hydrogen species and reduce the OH content has a great influence in the quenching processes of the radiative emission of excited levels of rare-earth ions.
Zinc tellurite binary glasses have shown interesting properties for photonic applications [15, 16]. Besides, it was found that the partial substitution of ZnO with ZnF2 significantly expands the glass forming range of the zinc tellurite system and increases the radiative lifetime of rare-earth ions [15–17].
In this work we present, for the first time to our knowledge, laser emission under wavelength selective laser-pulsed pumping in Nd3+-doped TeO2-ZnO-ZnF2 bulk glasses for two different NdF3 concentrations (1 and 2 wt%). The crystal field inhomogeneity produced by the glass matrix allows for spectral tuning of the laser output pulse. The fluorescence properties of Nd3+ ions in this matrix which include Judd-Ofelt calculation, emission peak wavelength, stimulated emission cross-section of the laser transition and lifetimes, together with the results of site-selective laser spectroscopy by exciting along the 4I9/2→4F3/2 absorption band, are also presented. The excitation spectra show the existence of a variety of crystal field site distributions for Nd3+ ions in this glass. Moreover, the spectral shift and narrowing observed in the steady-state emission spectra of the 4F3/2→4I11/2 transition under selective excitation along the 4I9/2→4F3/2 absorption band, reveal the existence of a variety of spectrally isolated distributions of Nd3+ sites in this glass which is in the origin of the spectral tuning of the laser output pulse.
2. Experimental techniques
Fluorotellurite glasses having a nominal composition of 74.6TeO2–8.8ZnO–16.6ZnF2 mol% were prepared by mixing the corresponding high purity (99.99%) oxides and fluorides (TeO2, ZnO, ZnF2 and NdF3) in an agate balls mill. The powder mixture was melted in a covered platinum crucible using an electrical vertical furnace at temperatures in the 800–850 °C range. Glass melts were homogenised by using an electrical platinum stirrer and after 20 min the glasses were poured onto a preheated brass mould. The obtained glass blocks (about 1 cm3 size) were immediately introduced into the annealing furnace, kept for 10 min at temperatures ranging from 300 to 320 °C, and then cooled down to room temperature. The glass was doped with 1 and 2 wt% (0.7 and 1.4 mol% respectively) concentrations of NdF3. The optical measurements were carried out on polished planoparallel glass slabs of about 1 mm thickness.
Conventional absorption spectra were performed with a Cary 5 spectrophotometer. The samples temperature was varied between 10 and 300 K in a continuous flow cryostat. Site-selective steady-state emission and excitation spectra were obtained by exciting the samples with a continuous wave (cw) Ti-sapphire ring laser (0.4 cm−1 linewidth) in the 780-920 nm spectral range. The fluorescence was analyzed with a 0.25 m monochromator equipped with a 1200 lines/mm diffraction grating, and the signal was detected by a Hamamatsu R5509-72 photomultiplier and finally amplified by a standard lock-in technique. The attained resolution was 0.1 nm.
Lifetime measurements were obtained by exciting the samples with a pulsed Ti-sapphire laser (10 ns pulse width) pumped by a pulsed frequency doubled Nd:YAG laser, and detecting the emission with a Hamamatsu R5509-72 photomultiplier and a Tektronix oscilloscope. Data were processed in a personal computer.
3. Results and discussion
3.1 Absorption and emission properties
The room-temperature absorption spectra were recorded in the 300-2500 nm spectral range by using a Cary 5 spectrophotometer. Figure 1 shows the room temperature absorption cross-section as a function of wavelength in the 350-950 nm spectral range. The inhomogeneously broadened bands are assigned to the transitions from the 4I9/2 ground state to the excited states of Nd3+ ions. Energy levels above 2P1/2 are not observed because of the intrinsic bandgap absorption in the host glass.
The absorption bands in Fig. 1 have been used to calculate the radiative lifetime of the 4F3/2 excited state, the branching ratios, and the radiative transition probabilities of fluorescence transitions to the lower lying 4IJ manifold, according to the Judd-Ofelt theory [18, 19]. The JO parameters obtained for this glass are Ω2 = 3.11x10−20 cm2, Ω4 = 5.00x10−20 cm2, and Ω6 = 3.87x10−20 cm2, with a root-mean-squared deviation equal to 6.03x10−7. The values of the JO parameters are in the usually observed range for tellurite glasses. However, the presence of ZnF2 in this glass, which favors the presence of fluorine ions around Nd3+, decreases the covalency degree in the rare-earth site and results in a lower value of Ω2 if compared with zinc tellurite glasses [20, 21].
The 4F3/2→4I11/2 steady-state fluorescence spectra at room temperature were measured by exciting the samples with a cw Ti-sapphire ring laser at 802 nm in resonance with the 4I9/2→4F5/2,2H9/2 absorption band. Figure 2 shows this emission for the sample doped with 1 wt% of NdF3. The emission is inhomogeneously broadened due to site-to-site variations in the local ligand field. The fluorescence band was integrated and divided by the peak intensity to yield an effective linewidth . The stimulated emission cross-section of the laser transition can be determined from spectral parameters and from the calculated radiative transition probability by making use of :Table 1 together with the effective fluorescence linewidth and peak position for the sample doped with 1 wt%. The sample doped with 2 wt% shows similar values for the peak position, linewidth, and stimulated emission cross-section. The stimulated emission cross-section in this fluorotellurite glass is in the range of the values found in tellurite glasses (3.0-5.1 pm2)  and is similar to that found in binary zinc tellurite glasses [20, 21].
The decays of the 4F3/2 state were obtained by exciting with a pulsed Ti-sapphire laser at 802 nm in the 4I9/2→4F5/2 absorption band at 295 K. Figure 3 shows the logarithmic plot of the experimental decays of the 4F3/2 level for the samples doped with 1 and 2 wt% of NdF3 at room temperature. As can be seen, the decay of the sample doped with 1 wt% is nearly single exponential whereas the decay of the sample doped with 2 wt% deviates from a single exponential at short times and the lifetime decreases from 128 to 90 μs as NdF3 concentration increases from 1 to 2 wt% respectively. The lifetime value for the sample doped with 2 wt% corresponds to the average lifetime defined by where I(t) is the intensity at time t . According to the value of the radiative lifetime the quantum efficiencies (η = τexp/τR) are around 88 and 62% for samples doped with 1 and 2 wt% respectively. This reduction of the quantum efficiency as concentration increases indicates the presence of non-radiative processes. The decay time from level 4F3/2 should be the sum of probabilities from radiative and nonradiative decays by multiphonon relaxation and by energy transfer to other Nd3+ ions. Multiphonon relaxation is expected to be small because of the energy gap between 4F3/2 and 4I15/2 levels and the values of the phonon energy involved (≅750 cm−1) . The non-exponential character of the decays and the reduction of the lifetimes as concentration increases can be due to cross-relaxation processes such as (4F3/2, 4I9/2)→ (4I15/2, 4I15/2).
The analysis of the experimental decays of the sample doped with 2 wt% of NdF3 shows that the nonradiative processes correspond to a direct transfer between Nd3+ ions. In this case, under the assumption of a dipole-dipole interaction, the donor decay curves can be accurately described by expression :Figure 4 shows a least square fit of the experimental decay for the sample doped with 2 wt% of NdF3 at room temperature. The obtained value for the interaction parameter CDA is 5.5x10−40 cm6/s, which is higher than those reported in fluoroindogallate (1.3x10−40)  and tellurite glasses (3.3x10−40)  and similar to that found in multicomponent tellurite glasses (5.5x10−40) . 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 which yield a value of 6.5 Å in this glass. The γ value for the sample doped with 1 wt% obtained from the fit to Eq. (2) is 26 s-1/2, about half the one obtained for the sample doped with 2 wt% (54 s-1/2), thus supporting the dipole-dipole transfer hypothesis.
3.2 Site-selective spectroscopy
As it is well known, in glasses, due to the site-to-site variation of the local field acting on the ions, the 4F3/2→4I11/2 transition of Nd3+ ions can show variations in peak wavelength, linewidth, and spectral profile depending on pumping wavelength. Therefore, selective excitation of Nd3+ by changing the wavelength of the pump laser provides an additional means of varying the gain profile and affecting the laser operation. To obtain information about the crystal field site inhomogeneity of Nd3+ in this fluorotellurite glass we have performed low temperature site-selective spectroscopy by using a cw Ti-sapphire ring laser with a narrow bandwidth (0.4 cm−1) as excitation source for the 4I9/2→4F3/2 transition. Figure 5 shows the low temperature (10 K) excitation spectra of the 4I9/2→4F3/2 transition obtained by collecting the luminescence at different wavelengths along the 4F3/2→4I11/2 transition for the sample doped with 1 wt%. As expected, these spectra show two main broad bands associated with the two Stark components of the 4F3/2 doublet. However, the low energy component corresponding to the 4I9/2→4F3/2 doublet, narrows and blue-shifts from around 874 nm to 869 nm as the emission wavelength decreases from 1100 to 1045 nm. In addition, at some emission wavelengths, at least two components are observed. This behavior is a consequence of contributions from Nd3+ ions in a multiplicity of environments.
The site-selective steady-state emission spectra of the laser transition were obtained at low temperature for different excitation wavelengths along the low energy component of the 4F3/2 level. As can be observed in Fig. 6 the shape, peak position, and linewidth of the emission band, change with the excitation wavelength. The spectra obtained under excitation at the low energy side of the 4I9/2→4F3/2 absorption band narrow and red shift. The wavelength of the fluorescence peak shifts from 1055 to 1064 nm by varying the excitation wavelength from 870.5 to 881 nm whereas the effective linewidth is reduced from about 24 nm to 13 nm. Similar variations in fluorescence profile, peak position, and linewidth have been observed in phosphate glasses .
These results show the inhomogeneous behavior of the crystal field felt by Nd3+ ions in this fluorotellurite glass. As can be seen in Fig. 6, only when we excite at the high energy side of the absorption band it is possible to cover the full spectral range of the Nd3+ emission probably helped by vibronic transitions. This inhomogeneous behavior is related with the influence of the pumping wavelength on the spectral behavior of the laser emission allowing some tunability of the laser emission as a function of the pumping wavelength.
3.3 Laser experiments
In order to investigate the laser emission of Nd3+ in this glass, we have pumped the 4F5/2 level, around 800 nm, with a pulsed Ti-sapphire laser of 0.1 nm spectral width and 10 ns pulse width. The samples were plates, with two polished faces and thicknesses of 2 and 4 mm, and NdF3 concentrations of 1 wt% and 2 wt%. To avoid thermal damage, the sample was put out of the pump focus, being the pumping beam diameter on the sample approximately 1.2 mm. A quasi confocal symmetrical resonator was built using two concave and HR mirrors of 50 cm of curvature radius at a distance of 40 cm each other, in a longitudinal pumping scheme. In order to minimize the intra-cavity losses, the sample was placed at the center of the resonator and oriented at Brewster angle with respect to the resonator axis. The output laser pulses spectra were recorded with a diode-array Hamamatsu-Triax 190 spectrum analyzer using a 1200 lines/mm diffraction grating, whereas the temporal evolution of both the pumping and laser output was recorded by detecting the radiation diffused by the sample with a fast photodiode (rise and fall times less than 300 ps) connected to a digital oscilloscope.
Laser emission was obtained when the pump energy was above the threshold value of 27 mJ for the sample of 1% and 4 mm thickness, and slightly higher for the other thickness and concentration. This fact can be related to the relatively high absorption and possible defects concerning the optical quality of the samples. The observed delay time between pump and output laser was in the 1 μs range for the different samples and pumping levels, and the temporal width of the obtained laser pulses was about 250 ns, as can be seen in Fig. 7.
We can estimate the stimulated emission cross-section from threshold measurements and optical density measurements using the expression 9] and in fairly good agreement with the value obtained from the absorption and emission data (Table 1)
The laser output spectra were recorded as a function of pumping wavelength along the 4I9/2→4F5/2 absorption band. As can be seen in Fig. 8 stimulated emission occurs around 1059 nm and the maximal output intensity is achieved when pumped at 802 nm.
As an example Fig. 9 shows three series of laser emission spectra obtained at different excitation wavelengths for the sample doped with 1 wt% of NdF3. The lowest emission peak wavelength, situated around 1058.5 nm, was obtained by exciting around 800 nm. The maximal output energy is obtained by pumping at 802 nm, which corresponds to the maximum of the absorption band. For longer pumping wavelengths, up to 805 nm, the wavelength of the emission peak shifts to higher values ranging from 1058.5 to 1061 nm. However, for excitation wavelengths longer than 806 nm, the emission peak slightly shifts again to shorter wavelengths. This behavior has also been observed in phosphate glasses  and attributed to the existence of well defined inhomogeneous Nd3+ site distributions. On the other hand, the spectral effective linewidth of the laser emission varies from 1.5 to 3.1 nm in the 1 wt% NdF3-doped sample, showing the high crystal field inhomogeneity of the glass matrix.
The spectral behavior of the stimulated emission under wavelength selective pumping follows similar trends to those of the spontaneous emission under wavelength selective pumping. However, the tunability of the emission peak for the stimulated emission occurs in a shorter range due to gain limitations.
Absorption and luminescence measurements have been performed in TeO2-ZnO-ZnF2 fluorotellurite glass doped with 1 and 2 wt% NdF3 by using steady-state and time-resolved laser spectroscopy. The JO intensity parameters and radiative transition probabilities have been calculated. The stimulated emission cross-section for the laser transition calculated from spectral data gives a value of 4.9x10−20 cm2 in reasonably good agreement with the value estimated from laser threshold data (4x10−20 cm2). The time evolution of the decays from state 4F3/2 is consistent with a dipole-dipole direct energy transfer mechanism.
The site-selective emission and excitation spectra show the inhomogeneous behavior of the crystal field felt by Nd3+ ions in this fluorotellurite glass. This is related with the influence of the pumping wavelength on the spectral behavior of the laser emission which allows for spectral tuning of the laser output as a function of the pumping wavelength.
The relatively high value of the stimulated emission cross-section of the laser transition suggests that these glasses are promising laser materials. Though the high value of both linear and nonlinear refractive indices can produce self focusing, and becomes a handicap for laser operation, this drawback may be overcome by using these glasses in fiber and waveguide devices.
This work was supported by the Spanish Government MEC under Projects No. MAT2009-14282-C02-02, MAT2009-14282-C02-01, FIS2011-27968, Consolider SAUUL CSD2007-00013, and Basque Country Government (IT-659-13 and SAIOTEK S-PE12UN016).
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