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A monolithic Bismuth-Aluminium codoped silica glass was prepared from nano-porous silica xerogels using a conventional solution doping technique with a heterotrinuclear complex and subsequent sintering. This novel approach enabled the preparation of the silica glass which contains a single luminescent center, namely an Al-connected Bi-center.
© 2012 Optical Society of America
1. Introduction
Over the past decade there has been increasing interest in the development of Bismuth-doped fiber lasers [1–7]. Though significant progress has been achieved in this field, the origin of near-infrared (NIR) photoluminescence (PL) remains still unclear. It is worth noting, that the experimental work on bulk Bi-doped glasses was mainly performed with relatively high doping levels, typically in the range of 0.1 - 2 wt. %, while the efficient laser generation could be only achieved in fibers with low Bismuth content. Recently, it was shown [8,9] that at a large amount of Bismuth in the multicomponent glasses (few and more wt. % of Bi2O3) even nano-particles of Bismuth could be readily detected in the glassy matrix, while in a single component silica glass with a moderate content of Bismuth (∼ 150 ppm) at least two distinct types of optically active centers could be identified [10]. It is well known [11–13] that the high doping of the silica glass with rare earth elements leads to the formation of ion pairs and, as a consequence, to the quenching of photoluminescence. Obviously, the high concentration of Bismuth in silica glasses can also lead to the formation of pairs (or dimers) and aggregates of Bismuth ions creating a variety of luminescent centers. The analysis of experimental data for multicomponent Bi-doped glasses [14, 15] is even more difficult often leading to wrong conclusions. It is still unclear whether all luminescent centers in Bi-doped glasses could be identified as dimers of bismuth ions [8, 16, 17]. If so, are all these centers in the same oxidation state as recently postulated by Denker et al. [18]? In our opinion at high doping levels the luminescent centers in a multicomponent glass do not necessarily have the same nature. In particular, they can consist of the dimers of Bismuth ions, but in different oxidation states and/or in different coordination environments. Alternatively, some centers might be the dimers, while others not. To elucidate the nature of luminescent centers responsible for the lasing in Bi-doped silica fibers the development of glasses that contain only one particular species of Bismuth-connected NIR emitting centers are of primary interest. This would provide an opportunity to investigate the particular center avoiding complications because of the overlapping of absorption and PL bands and/or interaction between different luminescent species.
In this paper we report the luminescent properties of bulk dense Bi-doped silica glass containing a single luminescent center, namely Al-connected Bi-center. This Al/Bi-codoped glass was prepared by sintering nanoporous silica xerogels doped with an appropriate heterotrinuclear complex.
2. Results and discussion
The nanoporous (NP) silica xerogels were prepared using a sol-gel technique from tetraethyl orthosilicate under base-catalysis conditions [19]. The porosity of xerogels, namely, pore diameter, its specific surface area and total pore volume, can be controlled by changing the molar ratio of starting reagents and the process conditions [20]. In this study xerogels with an average pore diameter of 24 nm were used. The porosity was determined by isothermal nitrogen sorption measurements [21, 22].
As a molecular precursor we have chosen the heterotrinuclear complex [Bi2(HSal)6Al(acac)3] (hereinafter AlBi2 complex), where Hsal = O2CC6H4-2-OH and Hacac = CH3COCH2COCH3, able to accommodate in nanopores. The synthesis of this precursor was performed according to a slightly modified literature protocol [23]. The results of synthesis were verified directly by performing the single crystal X-ray diffraction analysis of the crystallized precursor. The resulting structure of the complex is shown in Fig. 1 and is very close to that reported by Thurston et al. [23].
The nanoporous silica xerogels were doped with the AlBi2 complex using a conventional solution doping technique by soaking in acetone solution of AlBi2 complex for 48 h. After the soaking the samples were dried at low temperature (< 130 °C) then heat treated in oxygen atmosphere at 700 °C for 1h. The dehydroxylation is performed in the mixture of O2/Cl2 at 900 °C for 2h. At this stage of processing due to the high volatility of Bismuth we observed the leakage of Bismuth from nanoporous glass. Such a leakage was also observed by us previously in the processing of preforms fabricated with conventional Modified Chemical Vapor Deposition (MCVD) technique and doped with a simple precursors as Bi(NO3)3. In [23] the thermal decomposition of the complex was investigated. The compound smoothly underwent decomposition on heating in air, which starts at 179 and ends up at 380 °C. Pyrolysis of the complex at 700 °C produces a mixture of the phases Bi2Al4O9 and tetragonal Bi2O3. While Bi2O3 is highly volatile, O2/Cl2 atmosphere presumably leads to the formation of new phases and to further decomposition of Bi2Al4O9. The sintering (i.e. complete shrinking of nanopores) was performed at 1300 °C in a helium atmosphere providing a transparent and colorless monolithic cylindrical preforms with a diameter of 15 mm. At the final step the preform was fused at both ends to Suprasil F300 tubes (Heraeus Tenevo LLC) and it was drawn into 2.5 mm diameter rods at about 2000 °C. The samples (NS) with dimensions of 1.6 x 1.6 x 7 mm3 for the PL experiments were cut from one of the rods and polished. Here we would like to emphasize that in contrary to [24, 25] our preforms represent the bulk dense glass and their texture properties are similar to that of the core of any sintered conventional MCVD preform. Such a preform can be easily drawn to a photonic crystal fiber (PCF) as it has been previously demonstrated for the Erbium- and Bismuth-doped glasses [19, 26].
The radial distribution of Al and Bi content (Fig. 2) was determined by Electron Probe Micro-Analysis (EPMA) in the sintered preform of 15 mm diameter before the drawing step. As the measured content of Bismuth was nearly at the detection limit of our EPMA equipment (a few ppm for Bismuth), we also performed the analysis by Two-Step Laser Mass Spectrometry (L2MS) [27] with the equipment described elsewhere [28]. The ablation of Bi/Al codoped sample and subsequent ionization of desorbed atoms were performed with two laser pulses (266 nm, 10 ns) with a delay time between pulses of 50 μs, which corresponds to the arrival time of desorbed atoms to the ionization chamber. L2MS experiment performed at several points of the sample confirmed the measurements of EPMA. The results, shown in Fig. 2, clearly indicate that the initial atomic ratio Bi:Al = 2:1 in the precursor does not hold in the sintered Al/Bi codoped silica glass. As it was mentioned above, the main reason for this is the high volatility of Bismuth, so that the processing of Al/Bi codoped silica xerogels at high temperatures, namely, dehydroxylation procedure, leads to migration of Bismuth from the pore system to the surface and to its subsequent evaporation.
Fig. 2 Radial distribution of Aluminium and Bismuth content in the sintered sample measured with EPMA and L2MS techniques.
The experiments on continuous wave (CW) PL and PL decay kinetics under pulsed excitation were performed in the photon-counting regime at room temperature with the use of a single-grating monochromator M266 (Solar LS) equipped with a nitrogen cooled photomultiplier R5509-73 (Hamamatsu Inc.). The photomultiplier was coupled to the P7887 scaler (Fast ComTec). The spectral resolution was 1 and 2.5 nm in CW and PL kinetics experiments, respectively. The overall time resolution of the system in the experiments with pulsed excitation did not exceed 2 ns, limited by the transit time spread of photo-electron pulses in the photomultiplier. All CW PL spectra were corrected for the spectral response of the system with the help of the calibrated quartz tungsten halogen lamp (Model 63976, Oriel Instruments). The experiments were performed in 90° geometry with the excitation along the short axis of the samples (1.6 mm). In all PL experiments the samples were attached to the temperature regulated copper finger with the temperature stability of 0.05 K.
In CW PL experiments the following laser sources were employed: diode-pumped solid-state (DPSS) laser (Cobolt Samba, 532 nm), tunable CW Ti:Sapphire (Model 899, Coherent Inc.) and Yb-doped fiber laser (1064 nm, Keopsys). The experiments on PL saturation were performed at 532 nm with 8 W DPSS laser (Verdi, Coherent Inc.) and variable attenuator which consisted of λ/2 wave plate mounted on a rotary stage and fixed AR coated Glan-laser prism. Such a setup allows to obtain the variable pump power in the range of 0 - 5 W with constant laser beam parameters. The beam waist radius and its location were determined with BeamScope P5 beam profiler (DataRay Inc.).
The time resolved experiments were performed with the first (8 μJ) and second (3 μJ) harmonics of the micro-chip Nd:YAG laser with a tunable repetition rate (0 - 2 kHz, < 400 ps, Teem Photonics).
In Fig. 3 we show normalized to the maximum intensity PL spectra of NS sample under laser excitation at 532, 750, 800 and 1064 nm. At 532 nm excitation wavelength (WL) the sample exhibits two PL bands: the high intensity band peaked at 1100 nm (L-band) with a full width at half maximum (FWHM) about 150 nm and the low intensity band (S-band) at 750 nm (FWHM = 76 nm) with a relative peak intensity of 2% in comparison to the principal L-band. Excitation at 750, 800 and 1064 nm leads to the shift of the L-band peak position to 1145, 1070 and 1124 nm, respectively, while its FWHM remains almost unchanged. No other NIR PL bands were observed up to 1600 nm. The intensity of the band at 1070 nm (800 nm excitation WL) is reduced by two orders of magnitude compared to that observed under excitation at 532 nm (note the high noise in the corresponding spectrum). For this reason the spectrum was recorded at the excitation power of 400 mW and without any filter before the entrance slit. This arrangement resulted in apparent increase of the signal around 1600 nm under excitation at 800 nm due to the second order diffraction of scattered pump light. It was shown previously [10, 29, 30] that the excitation of Bi-doped silica glasses and fibers at 800 nm efficiently produce the PL at 1410 -1430 nm due to the Bismuth centers in a silica subnetwork. Obviously, the Al/Bi-codoped glass developed in the present study contains predominantly Al-connected Bismuth centers.
Fig. 3 Normalized CW NIR PL spectra of the NS sample. All spectra were normalized to the maximum intensity. Excitation powers: 40 mW (532 nm); 100 mW (750 nm), 400 mW (800 nm) and 30 mW (1064 nm).
Figures 4(a) and 4(b) show the PL decay kinetics in the NS sample at various wavelengths around of 1100 and 750 nm under excitation at 532 nm (400 ps, 3 μJ, 2·105 excitation pulses for each curve). It is clearly seen that all decay kinetics in Fig. 4(a) are well fitted by single-exponential decay at any recording wavelength, but the decay time varies in the spectrum. For instance, the shortest decay time of 631 μs was measured at 1000 nm while the longest decay time of 760 μs was measured at 1200 nm. The decay kinetics curves of the low intensity band at 750 nm (Fig. 4(b)) can not be represented by single-exponential function and were fitted to the bi-exponential decay. Again we note the increase of both decay time components with the increase of recording WL.
Fig. 4 PL decay kinetics in the NS sample with an excitation wavelength of 532 nm. In color - experiment; black solid lines: a) single-exponential fit; b) bi-exponential fit. Time resolution 4 ns.
To elucidate the origin of the spectral dependence of the PL decay time we performed the experiments on PL saturation at 532 nm excitation WL. To achieve the appreciable saturation of L- and S-bands the laser beam was focused to the spot with 1/e2 waist radius of 30 μm, which was calculated on the basis of beam profile measurements in 12 points. With such a beam waist the Rayleigh length (∼ 5.3 mm) was well above the length of the NS sample (1.6 mm). In the main window of Fig. 5(a) we show normalized to the maximum intensity spectra around 1100 nm recorded at the excitation powers of 25 mW and 5 W. In the inset of Fig. 5(a) the dependence of the peak PL intensity at 1100 nm is shown. The broadening of this band by 7 nm is clearly seen in the Fig. 5(a). The shift of the peak position, if it exists, was in the limits of experimental error even though the estimated upper limit of pump intensity in our experiments (∼ 177 kW/cm2) was only 7% lower than that in the experiments performed by Bulatov et al. [14]. The saturation intensity, Isat = 3.7 ± 0.27 kW/cm2, was determined from the fit of PL intensity to the standard equation of the form IL ∼ IP/(1 + IP/Isat ), where IL and IP are the peak PL intensity and pump intensity, respectively. The obtained value of Isat is by 20% higher than the one measured in alumino-silicate fibers [14] and we consider it as overestimated. This overestimation of saturation intensity in our experiment is due to the expansion of the laser beam inside the sample and because of the uncertainty in the exact location of the beam waist. Obviously, both experimental errors are absent in the waveguide regime.
Fig. 5 Normalized CW PL spectra of the NS sample in the range of 900 −1300 nm. a) Comparison of the spectra recorded at Pp =25 mW and Pp = 5W. Inset: Power dependence of peak PL intensity. b) Comparison of the spectra recorded at 300 and 360K and at the same pump power (25 mW).
In Fig. 5(b) the normalized spectra recorded at the fixed low excitation power but at two different temperatures, 300 and 360 K, are shown. Again the broadening of the NIR PL band without apparent shift of the peak position can be readily observed, but with more pronounced relative intensity increase on the long WL side of the band.
The results of the measurements of PL saturation of the low intensity band at 750 nm are shown in Fig. 6(a). From the fit of the PL intensity dependence shown in the inset of Fig. 6(a) we obtained the value of Isat = 90 ± 5.7 kW/cm2 which we consider also as overestimated for the above reasons. In comparison to the L-band, we note the asymmetrical broadening and blue shift (∼ 5 nm) of the peak position at high excitation power. Interestingly, the long WL shoulders coincide within the experimental accuracy. The thermal broadening of S-band shown in Fig. 6(b) is significantly lower in comparison to that of L-band and does not exceed 0.5 nm at 360 K.
Fig. 6 Normalized CW PL spectra of the NS sample in the range of 610 - 900 nm. a) Comparison of the spectra recorded at Pp =25mW and Pp = 5W. Inset: Power dependence of peak PL intensity. b) Comparison of the spectra recorded at 300 and 360K and at the same pump power (25 mW).
The described experimental results can be explained in the frame of the following assumptions: (i) single Al-connected Bismuth center; (ii) doublet structure of the L-band at 1100 nm; (iii) triplet structure of the S-band at 750 nm; and, (vi) broadening of PL bands due to the thermal population of upper vibronic sublevels of the excited states. The doublet structure of the L-band was directly observed in PL experiments at high [3] and low temperatures and in the experiment on optically detected magnetic resonance [31]. The lifetimes of the individual components can be estimated as ∼630 μs (measured at 1000 nm) and ∼760 μs (measured at 1200 nm). Due to the sufficiently close values of the lifetimes, the experimental decay kinetics can be reasonably well fitted to a single exponential function at any particular WL, but, due to the WL-dependent contribution of each component, the WL-dependent lifetime should be observed. The dependence of PL peak position on the excitation WL can be explained as the interplay of the doublet structure of this band and its inhomogeneous broadening. The very close saturation parameters of two spectral components (mainly due to their close lifetimes) result only in the broadening of the L-band at high pump levels. At high pump powers the increase of the temperature in the excitation channel can not be neglected and should also contribute to the spectral broadening as it should be clear from Fig. 5(b).
The same arguments can be applied to explain the behaviour of the S-band at 750 nm. First, it was shown in the PL experiment at low temperature that this band has a triplet structure [15] (see Fig. 3(a) in the cited article). Then, from the results on the decay kinetics of present study, the shortest among three components lifetime can be estimated as ≲1.76 μs (see the data in the Fig. 4(b)) and it should be assigned to the short WL component. The lifetimes of remaining components should be in the range of 4.5 - 7.6 μs and, most probably, they are too close to be resolved in decay kinetics (at least at room temperature). Unlike the case of the L-band, there is an important difference between the first lifetime and two others. As a consequence, the saturation intensity of the first component should be much more important giving rise to the intensity of the short WL shoulder at high pump power and even to the blue shift of its peak position. Again, the increase of the temperature inside the excitation channel can contribute to the spectral broadening at high pump power.
The experimental results described above indicate that the simultaneous use of NP silica and specially designed molecular precursors containing Al and Bi ions provides a mechanism for selective formation of a single luminescent Bi-center, namely Al-connected center. The nanoporous texture of a silica xerogel due to enhanced dispersion reduces the clustering of dopants, while the heterobimetallic precursor ensures the presence of Al ions in the nearest-neighborhood of Bi ions. Though a part of Bismuth escapes from the porous system during dehydroxylation and sintering procedures, the rest of the Bi-ions remains in the coordination environment of Al ions. It is also worth making some comments on the conclusions about dimers and their oxidation state that have been reported by Denker et al. [17, 18]. Indeed, in these works Bi-doped glasses were prepared by a standard melting technique, which does not provide any selective mechanism to create predominantly one particular center and implies the formation of all types of Bismuth optical centers. Furthermore, the extinction measurements were performed at a single wavelength, namely at 500 nm, while all types of Bismuth centers exhibit absorption in the ”green” spectral region [10, 26, 29, 32]. At high doping levels dimers and aggregates of Bismuth ions can dominate in the absorption spectrum. Thus, the validity of conclusions about dimers and their oxidation state [17,18] should be confirmed by new experiments with silica glasses containing a single luminescent center. The method of the fabrication of Bi-doped glasses from nanoporous silica xerogels, presented here, not only helps to solve this task but also provides large preforms which are suitable for drawing as Bi- and rare-earth-doped fibers [19, 26]. We have successfully produced such PCF fibers with a core constituted of the described material. The results of the detailed investigation of optical properties of these fibers will be published elsewhere.
3. Conclusion
In conclusion, an Al/Bi codoped silica glass was developed with the use of nanoporous silica xerogel via conventional solution doping with a heterobimetallic complex containing Al and Bi(III) ions and subsequent sintering. The results of PL spectroscopy experiments clearly indicate that such an approach enables fabrication of a bulk glass containing a single optical center, namely, an Al-connected Bismuth center. The development of materials containing a single optical center allows us to study the origin of the NIR PL in Bi-doped silica glasses without complications related to overlapping of absorption and PL bands and/or interaction between different optical centers.
Acknowledgments
The authors are grateful to C. Focsa for measurements of Bismuth content by L2MS technique. V.B.A. is indebted to CNRS for a three months research fellowship as invited Professor at PHLAM UMR CNRS 8523. The work was supported by the “Conseil Régional du Nord/Pas de Calais” and by the “Fonds Européen de Développement Economique des Régions” (FEDER) through the “Contrat de Projets Etat Région (CPER) 2007–2013”.
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