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Ytterbium doped lithium-alumino-silicate glasses suitable for diode-pumped laser applications were investigated concerning the hydroxyl quenching of the Yb3+ fluorescence. Glasses of the nominal composition 18 mol% Li2O, 22 mol% Al2O3 and 60 mol% SiO2 with variable OH concentrations NOH (between 0.04 and 6.01 ∙ 1019 cm−3) and Yb3+ concentrations NYb (between 0.1 and 9 ∙ 1020 cm−3) were produced and a direct correlation between spontaneous emission decay rate and the product NYb ∙ NOH was observed. The radiative spontaneous emission rate in the glass host is around 1,000 s−1 (radiative lifetime 1.0 ms) and the microparameter for Yb-Yb energy migration, CYb-Yb, was found to be 1.358∙10−38 cm6 s−1. It was calculated that on average 17% of the OH groups in the glass contribute to the quenching of the Yb3+ fluorescence. By analysis of the UV edge of the glass it was concluded that melting under inert conditions leads to reduction of iron impurities to Fe2+, which can act as quenching sites for the Yb3+ ions and therefore may additionally reduce the energy storage capability of the laser material.
© 2015 Optical Society of America
Introduction
The objective of the investigation presented here was the development of amplifier materials suitable for ultra-high peak power laser applications, such as the POLARIS project [1,2]. The latter project aims at the development of a petawatt-class laser by diode pumped amplification of ultrashort pulses.
A crucial part for this goal is the choice of the amplifier material. Currently Yb3+-doped fluoride phosphate glasses (Yb:FP) and Yb3+-doped CaF2 single crystals (Yb:CaF2) are used in the POLARIS project [3–6]. In the similar project PENELOPE also Yb:CaF2 is used as ampilifier material [7]. Both, Yb:FP and Yb:CaF2, show high quantum efficiencies and long fluorescence lifetimes, even at rather high doping concentrations [8, 9]. The main flaw of these materials is their high coefficient of thermal expansion (CTE), 16∙10−6 K−1 (fluoride phosphate [10],) and 20∙10−6 K−1 (CaF2). This decreases their thermal shock resistance which is inversely proportional to the CTE [11] and hence leads to a limitation in the achievable repetition rate of the laser. Moreover, stress induced birefringence causes losses in the amplification process.
By contrast, alumino-silicate glasses which were studied recently concerning the effect of the glass structure on fluorescence lifetime and thermal expansion [12–15], show relatively small coefficients of thermal expansion. In these materials also high bending strengths are observed [16,17]. It has recently been shown that the main effect of the host glass on the fluorescence lifetime at low doping concentrations is the refractive index and the mean atomic weight of the glass composition. In this study, the behavior of lithium-alumino-silicate glasses will be presented. They possess the highest fluorescence lifetimes among all alumino-silicate glasses studied up to now [14,15].
Furthermore, the effect of increasing dopant concentrations is investigated, since high dopant concentrations are needed to overcome several problems in the amplifier setup, e.g. large B-Integral values. Earlier investigations on alumino-silicate glasses have shown that increasing Yb3+-doping concentrations result in decreasing fluorescence lifetimes [18]. This concentration quenching notably reduces the materials quantum efficiency. In Yb3+-doped materials, only a few mechanisms are suitable to effectively quench the fluorescence. The overall relaxation rate of spontaneous emission is given by:
Here k is the total relaxation rate and kr is the radiative (intrinsic) relaxation rate for spontaneous emission, kOH, kMPR, kRE, kTM are parasitic relaxation rates for OH multi-phonon relaxation, multi-phonon relaxation due to Si-O-vibrations and relaxation rates due to other rare earth ions and transition metal ions, respectively.In this paper we will mainly focus on OH quenching. Multi-phonon quenching of Yb3+ by Si-O-vibrations can be ruled out in these glasses [12,13,19]. Quenching by other rare earth or transition metal ions should be negligible because high purity raw materials have been used. Nevertheless, a hint on a comparably small influence of Fe2+ in these glasses has been found for samples with low OH concentrations. The parasitic relaxation due to OH multi-phonon relaxation can be suppressed in two ways: (I) bubbling the glass melt with dry gases and (II) introducing fluoride into the glass.
Experimental procedure
Peraluminous lithium-alumino-silicate glasses with the composition 18 mol% Li2O, 22 mol% Al2O3 and 60 mol% SiO2 were prepared with different Yb2O3 concentrations ranging from 1∙1019 to 9∙1020 Yb3+ cm−3. In order to obtain different hydroxyl concentrations in the glass (I) different raw materials, especially Al2O3 or Al(OH)3, were used and (II) the melting was conducted with additional bubbling and / or under argon atmosphere. The effect of the Yb3+ precursor was studied by using YbF3 instead of Yb2O3.
Conventional melting procedure
Glasses where melted using the common melt quenching technique from the raw materials SiO2 (Sipur A1, Schott, Germany), Al2O3 (PM-4N, Hebei Pengda New Material Tech. Co. Ltd., China) and Li2CO3 (Chemapol, Czech Republic) under ambient atmosphere. The glasses were doped by adding appropriate amounts of Yb2O3 (99.9%, Alfa Aesar GmbH & Co KG, Germany). Drying the batch before melting did not result in different OH concentrations of the final glass and was therefore not further applied. The batch was melted for 1 h at 1,650 °C and additional 2 h at 1,600 °C for homogenizing the melt under ambient atmosphere in a platinum crucible using an electric furnace. The obtained melts where cast into graphite molds, transferred to a preheated muffle furnace (730 °C) and cooled to room temperature (approx. 3 K/min). To generate high OH concentrations in the glass, another series of glasses was melted in the same way, but instead of Al2O3 as raw material, Al(OH)3 (Sumitomo Chemical, Japan) was used.
Dehydration of the glasses
The glasses were melted using the raw materials SiO2, Al2O3 and Li2CO3 and appropriate amounts of Yb2O3 or YbF3 (99.9999%) as described above. The glasses were melted in an induction furnace at temperatures of up to 1,600 °C in a dispersion hardened platinum crucible.
In a first attempt to decrease the OH concentration, nitrogen (99.999%, ≤5ppm H2O, Linde AG, Germany) was used for bubbling the melt. This was done for 3 h using a platinum tube. After that the platinum bubbling tube was removed and the melting procedure was continued for another hour in order to remove the bubbles. To minimize water uptake during this fining step, the surface of the melt was slightly purged with nitrogen through the bubbling tube.
Since this procedure led to the formation of very small bubbles (probably filled with nitrogen) and some remaining water in the glass, the bubbling gas and the atmosphere was changed.
To further decrease the OH concentration, the furnace chamber was purged with dry argon (99.998%, ≤ 5 ppm H2O, Linde AG, Germany) and the melts were bubbled using argon and/ or oxygen (99.95%, Linde AG, Germany). The bubbled melts were held one additional hour at the melting temperature under argon atmosphere without bubbling in order to remove residual bubbles (fining). Using additionally YbF3 as raw material instead of Yb2O3 further decreased the OH concentration in the glass. The melting conditions are summarized in Table 1.The respective route numbers will be used in the following for abbreviation purposes.
Table 1. Preparation of the glasses using different preparation routes; raw material for aluminum oxide (Al), doping raw material for ytterbium (Yb), applied atmosphere during melting (atmosphere), applied bubbling duration and gas (bubbling)
Afterwards the melts where poured onto preheated steel molds coated with boron nitride (HeBoCoat, Henze Boron Nitride Products AG, Germany) and transferred to a muffle furnace (730 °C). Then the furnace was switched off and the sample was allowed to cool to room temperature. The cooling rate was about 3 K/min. Bubbling the melt with light gases (N2, O2) led to residual bubbles of small diameters which were difficult to remove during the fining step. Otherwise, argon bubbling yielded larger and fewer bubbles which were more easily removable. However, a few bubbles remained nevertheless.
The obtained glasses where cut into slices with a thickness of 0.5, 4 and 10 mm and subsequently polished to optical quality. The samples were studied using UV-vis-NIR spectroscopy, FTIR transmission spectroscopy and fluorescence lifetime measurements.
UV-vis-NIR-spectra were recorded using a Shimadzu 3101 spectrometer. The FTIR transmission spectra were recorded in a SHIMADZU IRAFFINITY-1 spectrometer.
Fluorescence lifetimes, absorption and emission cross sections were determined using the setup described in [9,20] with a combination of McCumber theory [21] and Füchtbauer-Ladenburg equation [22].
Results
Determination of OH concentration
In previous investigations, it has been shown, that the water concentration of Yb3+-doped glasses plays an important role in the fluorescence behavior and notably contributes to the quenching in these glasses [23–26]. Water occurs in glasses in more or less high quantities, which depends on the used raw materials and on the applied melting procedure [27–31]. In silicate glasses water occurs as Si-OH groups, whereas different types of incorporation, e.g. as “bonded” or “free” OH groups are reported [32]. So called “free” OH groups (weakly associated) absorb infrared radiation at energies of around 3,500 cm−1. OH groups that are more strongly associated absorb at around 2,800 (“strongly associated”) and 2,350 cm−1 (“very strongly associated”). Only few studies quantify the concentration of water present in a glass. Here, the early work of H. Scholze should be mentioned, who used water partial pressure measurements and IR-spectroscopy [29]. A similar approach is used by Götz and Vosahlova [33]. Figure 1 shows the effect of the preparation route (see Table 1) on the OH band in the near infrared absorption spectra. The shown glasses are doped with 6∙1020 Yb3+ cm−3.
Fig. 1 NIR absorption spectra of the investigated lithium-alumino-silicate glass using different preparation routes (see text and Table 1). The shown glasses contain 6∙1020 Yb3+ cm−3; (inset) magnified spectra of samples with relatively low OH concentrations.
In this figure the decadic absorption coefficient α10 = log10(I0/I)/d is plotted. Here I0, I and d are the light intensity without sample, light intensity with sample and the sample thickness, respectively.
The most important feature in Fig. 1 is the absorption band at around 3,520 cm−1, since it shows the fundamental OH absorption and its vibrational (phonon) energy. The absolute height of the band and therefore the OH concentration can be varied in a wide range in this glass. A small shoulder at 2,500 cm−1 can also be seen. All glasses (except #1, black curve) exhibit the same α10 values at wave numbers below 2,750 cm−1. No reduction of this shoulder is observed upon reduction of the band maximum at 3,520 cm−1. Therefore “free” OH groups (3,520 cm−1) can be removed rather easily from the glass, whereas more strongly associated OH groups (2,800 and 2,350 cm−1) remain in the glass. At wavenumbers below 2,400 cm−1, the IR cut-off of the glasses can be found which coincides for all glasses. The determination of the OH concentration is done by evaluating the main band of the glasses at around 3,520 cm−1 and is described in the following.
Using preparation route #1 (raw material Al(OH)3), the strongest absorption at 3,520 cm−1 is observed. Using route #2 (raw material Al2O3), a strong reduction of the OH band can be seen which is even further decreased if the melt is additionally bubbled with a dry gas as done in preparation route #3 (N2-bubbling). If dry argon is used as furnace atmosphere and the melt is bubbled as in preparation route #4 (Ar atmosphere, Ar and O2 bubbling) the major part of the OH concentration can be removed. Nevertheless, a band of small intensity remains (see inset of Fig. 1). This small band, representing “free” OH groups, is removed if YbF3 is added as dopant instead of Yb2O3 (preparation route #5).
To determine the absolute OH concentration of the glasses the baseline corrected α10 values at the band maximum (around 3,520 cm−1) were divided by the so called practical molar extinction coefficient εprac published by Suzuki et al. [34] (εprac = 52 l∙mol−1∙cm−1) for a similar soda-lime-alumino-silicate glass. The obtained OH concentrations are shown in Table 2. (upper value).
Table 2. OH concentrations NOH (upper value) and measured spontaneous emission rates k (lower value; assumed error ± 5%) in dependence of Yb3+ concentration (increasing from left to right) and preparation route (see Table 1)
Fluorescence lifetimes
Figure 2 shows the fluorescence decay curves for highly doped (6∙1020 Yb3+ cm−3) glasses prepared by using different preparation routes (see Table 1) which led to different OH concentrations. In any case a mono-exponential decay was found.
Fig. 2 Fluorescence decay curves for the glasses doped with 6∙1020 Yb3+ cm−3 for different preparation routes (see Table 1).
Spontaneous emission rates (inverse fluorescence lifetimes) were obtained using a mono-exponential curve fitting procedure, via Eq. (2):
with the fluorescence intensity I and the measured spontaneous emission rate k. In Table 2, the OH concentrations and spontaneous emissions rates for all studied samples are given. The spontaneous emission rates and OH concentrations in Table 2 decrease from top to bottom (preparation route) and also increase from left to right (increasing Yb3+ concentration). The highest OH concentration was found to be 6.01∙1019 cm−3 (742 wtppm) and the lowest was 0.04∙1019 cm−3 (5 wtppm). High OH concentrations lead to high decay rates. The dependency of the spontaneous emission rate on both the OH and the Yb3+ concentration is plotted in Fig. 5.
Absorption and emission cross sections
In Fig. 3 absorption and emission cross sections of the Yb3+-doped glass samples are shown (preparation route #2, doped with 6∙1020 Yb3+ cm−3). Both were determined as described in [9,20].
Fig. 3 absorption and emission cross sections of the glass LiAS1822 (preparation route #2, doped with 6∙1020 Yb3+ cm−3).
A typical band shape for absorption and emission of Yb3+ in glasses [35], especially for alumino-silicate glasses [18], can be observed. The band shape is very similar for all analyzed glasses (route #1 - #5) and therefore only route #2 is presented. In comparison to reference materials such as CaF2 [8] or fluoride phosphate [36], the alumino-silicate glasses show different band shapes and higher absorption and emission cross sections.
The energy transfer rate PYb-Yb between Yb-ions in the case of electric dipole-dipole interaction is given by Dexter’s equation [28,37,38]:
here h is Planck’s constant, c is the speed of light, n is the refractive index of the host material (n = 1.532), R is the Yb-Yb-distance, Qa is the integrated absorption cross section, τ0 is the radiative lifetime of Yb3+ in the host material, E is the photon energy and fa(E) and fe(E) are the normalized line shape functions for absorption and emission, respectively.CYb-Yb is calculated to be 1.358 ∙ 10−38 cm6 s−1. For comparison a CYb-Yb value of 8.4∙10−39 cm6 s−1 has been calculated for fluoride glass (ZBLAN) in [39], which is somewhat lower than the value found here. Using the Wigner-Seitz-radius R = (4∙π∙NYb / 3)-1/3 as a rough estimation for the Yb-Yb distance (assuming equidistant separation), an Yb-Yb energy transfer rate of 85,779 s−1 at a doping concentration of 6∙1020 Yb3+ cm−3 is obtained. This value indicates around 86 transfers of the excitation energy between Yb3+ ions during their radiative fluorescence lifetime of 1.007 ms (993 s−1, see below) and a migration distance of 64 nm. The calculated Wigner-Seitz-radius for OH impurities lies between 1.58 nm (6.01∙1019 OH cm−3) and 8.55 nm (4∙1017 OH cm−3). Due to the large energy migration distance even a low OH concentration as 4∙1017 OH cm−3 can result in a slight OH quenching.
Effect of the preparation route on the UV-Edge
Inert melting conditions also have an effect on the spontaneous relaxation rate of the doping ions (see below) and furthermore alter the shape of the UV edge of the host material. Fig. 4 shows the UV-part of the absorption spectrum of three samples: two samples doped with 1∙1020 Yb3+ cm−3, melted under ambient atmosphere (preparation route #2) and inert atmosphere (preparation route #4) and an undoped glass (preparation route #2). In comparison to the undoped glass, an absorption band in the Yb3+-doped glasses at around 225 nm is present. Its intensity increases further if the Yb3+ doping concentration is increased (not shown). This band can be attributed to a charge transfer (CT) transition of oxygen ligands to ytterbium ions [40]. At an Yb3+ concentration of 6∙1020 cm−3, an additional band at around 330 nm appears (not shown). This may be due to the formation of Yb2+ [41,42].
Fig. 4 left: UV edge of glasses doped with 1∙1020 Yb3+ cm−3 prepared under different conditions; right: difference spectrum of glasses melted under ambient and inert atmosphere.
The comparison of samples prepared under ambient and inert atmosphere reveals small changes in the wavelength regions 225 - 300 nm and 200 - 225 nm. This can be seen as a hint to iron contaminations of the raw materials. At around 250 nm, the charge transfer transition of Fe3+-O and at around 200 nm the charge transfer transition of Fe2+-O can be observed e.g. in phosphate or fluoride phosphate glasses [43]. The intrinsic UV edge of the host glass is below 200 nm, but in this region the used spectrometer showed uncertainties. The difference spectrum (ambient minus inert condition) on the right side of Fig. 4 shows higher α10 values at around 250 nm (Fe3+) and lower values at around 200 nm (Fe2+) for preparation route #2 (black curve) compared to route #4 (red curve). This indicates a reduction of Fe3+ to Fe2+ upon inert melting conditions. CT transitions are strong transitions and even small impurity concentrations in the raw material can have a large effect in the vicinity of the UV absorption edge.
However, these statements can only be seen as a hint to an iron contamination, because other 3d-elements, other rare earth ions and even dissolved platinum also show transitions in this wavelength region, see e.g [44].
Discussion
The OH concentration can be varied in a wide range in the investigated peraluminous lithium-alumino-silicate glass. The dominant OH species is the so called “free” OH group which is coordinated with highly cross linked [SiO4] or [AlO4]- tetrahedra. After a comparably short melting time of 4 h, OH could be expelled from the melt to a great extent. The usage of fluorides additionally helped to remove OH groups, most probably by forming HF [27]. For the purpose of producing laser glasses, a bubble- and striae-free glass is required. Therefore, from a technological point of view, bubbling with easily removable gases, such as Ar, is favored over gases, that show the formation of small residual bubbles, like N2.
Fluorescence quenching
Figure 5 shows the measured spontaneous emission rates as function of the product of Ytterbium and OH concentration.
Fig. 5 Spontaneous emission rate as function of Yb3+ and OH concentrations; different preparation routes are marked; the inset shows rates at low OH concentration.
Assuming only quenching by OH groups after Yb-Yb energy transfer (electric dipole-dipole), the decay rate can be calculated by [38,45]:
Here kr is the radiative spontaneous emission rate, CYb-Yb is the microparameter for Yb-Yb energy transfer, NYb is the concentration of donor-ions (Yb3+) and Nq is the concentration of quenching sites which is proportional to the OH concentration NOH (Nq = χ∙NOH, with χ as the proportionality factor).From the linear fit in Fig. 5, a radiative spontaneous emission rate kr of 993 s−1 (radiative lifetime = 1.007 ms) and a slope of 5.83∙10−38 cm6 s−1 can be obtained. Using the calculated value of CYb-Yb from Eq. (3) a value for χ of 0.17 is calculated. Neglecting other quenching processes it can be stated, that on average 17% of the OH groups in the glass contribute to quenching of the luminescence after Yb-Yb energy migration.
The radiative fluorescence lifetime of the investigated lithium-alumino-silicate glass is around 1 ms. Other materials such as CaF2 (1.9 ms [9],) or fluoride phosphate (1.4 ms [8],) show higher fluorescence lifetime values but have smaller absorption and emission cross sections. This means (at equal doping concentration) lithium-alumino-silicate glasses potentially offer higher gain than the mentioned reference materials.
An interesting detail in Fig. 5 (inset) can be seen at very low OH concentrations (Table 2, preparation routes #4 and #5). Here somewhat higher spontaneous emission rates than predicted by the linear fit curve are observed (especially for the rightmost data point of #4 – NYb = 6∙1020 cm−3). This may be caused by the presence of iron impurities and the inert melting regime in the end of the melting procedure. As seen in Fig. 4 there may be a relatively strong reduction of iron impurities, present as Fe3+, to Fe2+. Fe2+ is very efficient in quenching the Yb3+ fluorescence, since it shows a broad d-d transition centered at around 1,000 nm [46], the wavelength region of the Yb3+ f-f-transition [39,47], but in comparison to the influence of OH in the glass this effect is small. However, up to now it is not totally clear, if this effect is due to iron impurities and the reduction to Fe2+ under inert melting conditions, but the broad absorption band of Fe2+ in the region of Yb3+ emission in combination with fast energy migration at high Yb3+ concentrations potentially lead to quenching of Yb3+ luminescence even at low impurity concentrations. It is a fine line between working under dry, inert atmospheres to avoid water contamination and reducing impurities in the material to produce unwanted parasitic quenching.
Conclusion
Peraluminous lithium-alumino-silicate glass samples of the molar composition 18% Li2O, 22% Al2O3 and 60% SiO2 with variable Yb3+ (0.1 … 9∙1020 cm−3) and OH concentrations (0.04 … 6.01∙1019 cm−3) have been prepared. We found that the spontaneous emission decay rate strongly depends on both concentrations. At relatively high Yb3+ doping concentrations even small amounts of dissolved OH can effectively quench the Yb3+ fluorescence emission. From absorption and emission spectra (cross sections), the microparameter for Yb-Yb energy migration, CYb-Yb, was calculated to be 1.358∙10−38 cm6 s−1. It was found that on average 17% of the present OH groups act as quenching centers. The radiative spontaneous emission rate (radiative lifetime), obtained from linear extrapolation, was found to be 993 s−1 (1.007 ms) for this glass composition. Furthermore it has been found that iron impurities as Fe2+ may act as quenching sites even at very low concentrations.
Acknowledgments
The research leading to these results has received funding from the European Social Fund (ESF) through the Thuringian Ministry of Economy, Employment, and Technology (project number 2011 FGR 0122) and from the Bundesministerium für Bildung und Forschung (BMBF) (03ZIK445 and 03Z1H531).
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