Open Access
Add to BibSonomy
Abstract
The IR absorption spectrum of high purity photo-thermo-refractive (PTR) glasses melted under atmosphere containing different concentrations of water is studied. The absorption spectra of the water and glass matrix are separated and deconvoluted to elementary Gaussian bands. The water absorption spectrum includes 11 bands with a maxima at 4320, 3560, 3053, 2897, 2815, 2765, 2525, 2315, 2219, 2150, and 1908 nm. The current technology provides a decrease of the decimal absorption coefficient at 2800 nm from 0.4 to 0.04 cm-1. The PTR glass matrix absorption spectrum includes three bands at 4855, 3950, and 3250 nm (2060, 2532, and 3077 cm-1). The long wavelength absorption edge for the PTR glass matrix is 2900 nm for the decimal absorption coefficient 0.01 cm-1, 3600 nm for 0.1 cm-1, and 4200 nm for 1 cm-1.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Water content in glass is an important problem for various optical applications since the optical absorption of OH groups and water molecules has high intensity bands in near and middle infrared range. Photo-thermo-refractive (PTR) glass is one of the volume holographic materials, where phase holograms are recorded by refractive index change after exposure to optical radiation and thermal treatment. The induced refractive index changes in PTR glass can reach 1000 ppm, e.g. surveys [1,2]. One of the main advantages of PTR glass is low absorption in a region from 360 to 2000 nm which gives an opportunity to use PTR glass in high-power laser systems. One of the challenges is an extension of an operational window to long wavelength region for applications in 2000-4000 nm range.
PTR glass is a Na2O–ZnO–Al2O3–SiO2 glass doped with anions F- and Br- along with cations Ag+ and Ce3+. A long wavelength absorption edge in PTR glass starts at 2700 nm [1]. Decimal absorption coefficient (internal optical density/thickness) gradually increases from 0.2 to 1 cm-1 in the range of 2800-4000 nm. This absorption was ascribed to admixture of hydroxyl groups. However, no detailed study of long wavelength absorption spectra in this glass was undertaken.
Water absorption in silicate glass has been a wide-studied topic at least for the last 50 years. It is well-known that admixture of water in silicate glasses results in three broad overlapped absorption bands with maxima in vicinity of 2800, 3600, and 4300 nm, and a number of low intensity bands in shorter wavelength range, e.g. [3–5]. Analysis of the aforementioned absorption spectra was studied in Na2O-CaO-MgO-SiO2 glasses [6] in the range from 1000 to 3000 nm. It was shown that the absorption band with maximum at 2800 nm has a complex shape. Therefore, four Gaussian bands were required to model it. Four more low intensity absorption bands assigned to water were found in a short wavelengths range at 2570, 2240, 2160, and 1430 nm. Absorption spectra in this glass doped with different concentrations of iron and water were studied at longer wavelengths [7]. A method of extraction of glass matrix, iron and water absorption spectra from experimental ones was developed and spectra of all those components were separated. Deconvolution of experimental spectra of each component to Gaussian functions was processed. A number of overlapped vibrational bands of glass matrix were identified from 4000 to 7000 nm. They were assigned to combinations of the main vibrational states of silicate matrix. The IR absorption spectrum of Fe2+ has a broad band with the main maximum at 1200 nm and two shoulders in vicinity of 2200 and 3200 nm. Five Gaussian components were used to model this spectrum. The broad band at 1200 nm is attributed to the absorption of Fe2+ ions in octahedral site symmetry. The band with the maximum 2200 nm and a shoulder at about 3200 nm is attributed to an absorption of Fe2+ ions in octahedral site symmetry [8]. The spectrum of water species has three main bands with maxima at 2800, 3450, and 4250 nm. Two long wavelength bands are well modelled by single Gaussian functions. Absorption of water at longer wavelengths could not be extracted because of masking by intensive vibrational bands of silicate glass matrix. The short wavelength band in vicinity of 2800 nm required four Gaussian functions for modelling. Small absorption bands similar to those described in Ref. [6] were found in the short wavelength region.
Nature of water related absorption bands were discussed in multiple publications. The bands at 2800, 3600, and 4300 nm were initially attributed to stretching vibrations of OH-groups [4]. Later, two types of water species in glass are considered, e.g. Ref. [9]. They are hydroxyl (O-H) groups responsible for long wavelength absorption bands and molecular water [H2Omol] responsible for a complex band with experimental maximum at 2800 nm. The small bands at shorter wavelengths are usually considered as combinations of basic vibrations of hydroxyl groups and molecular water. According to Ref. [4], there is no absorption bands of O-H groups at wavelengths longer than 4300 nm. However, the later work [10] shows, that silicate glass has absorption bands of (O-H) groups beyond 4300 nm.
The goal of the current work is a detailed analysis of the long-wavelength absorption spectrum of PTR glass. No efforts to ascribe absorption bands to specific vibrations of hydroxyl groups and water molecules are expected and all absorption bands related to these species are described in this work as water bands.
2. Experimental
PTR glass with composition 15Na2O–5ZnO–4Al2O3–70SiO2–5NaF–1KBr–0.01Ag2O–0.01CeO2 (mol %) was melted in an electric furnace in a platinum crucible with stirring by a platinum stirrer. Regular PTR glass (R) was melted in conventional atmosphere. Dehydrated (D) glass was melted under a fused silica dome with dry air purging. For optical absorption measurements, two types of polished samples with thickness of 8 and 1 mm were produced. Absorption spectra in 8 mm thick samples were measured from 250 to 2700 nm, in 1 mm thick samples - from 2700 to 4700 nm. The spectra from 250 to 3200 nm were measured at Perkin Elmer Lambda 950 spectrophotometer. The spectra at the range from 3200 to 4700 nm were measured at Fourier Transform IR spectrophotometer (FTIR) Perkin Elmer Spectrum 100 Optica. Decimal absorption coefficient (internal optical density divided by thickness in centimeters) was calculated.
3. Experimental results and modeling
The absorption spectra of R and D glasses from 250 to 3200 nm are demonstrated in Fig. 1. Both glasses have very low absorption in visible and near IR spectral regions. UV absorption spectra are the same in both glasses. An absorption band with maximum at 305 nm is assigned to a photosensitizer Ce3+, short wavelength absorption edge is assigned to Ce4+ [11]. Absorption edge in IR region is usually assigned to water admixture in glass [6,7]. It can be seen that the intensity of bands at the range from 1500-3200 nm for regular glass is higher than those for dehydrated one.
The main approach for detailed analysis is the same as in Ref. [6]: comparison of absorption spectra for glasses melted in different conditions (regular and dry atmosphere) where water concentration should be different. In order to separate the absorption spectra of glass matrix from the inherent absorption of water, it is necessary to ensure that the properties of glass matrix do not change if the melting atmosphere is varied. Water concentration in silicate glasses usually does not exceed a few ppm, e.g. [12]. We can suppose that such concentration of water should not change the shape of intrinsic vibrational spectra of glass matrix. However, one could suppose that the interaction of water in furnace atmosphere with fluorinated PTR glass melt could produce very volatile hydrofluoric acid (HF). As a result, concentration of fluorine in glass and absorption spectrum of glass matrix could be changed. To check stability of fluorine concentration in glass, the following result observed in Ref. [11] was used: position of Ce3+ absorption band is shifted to a short wavelength region with the rate of 1.5 nm/mol.% NaF when fluorine concentration increased. The UV absorption spectra of dehydrated and regular PTR glass in vicinity of Ce3+ band are demonstrated in Fig. 2(a).
Fig. 2. UV absorption spectra of dehydrated (D) and regular (R) PTR glass (a) and their derivatives over wavelength (b).
These absorption spectra derivatives over wavelength are shown in Fig. 2(b). One can see that maxima of Ce3+ absorption band ($\frac{{dA}}{{d\lambda }} = 0$) have the same positions for glasses with different water concentration. This fact is the evidence that fluorine concentration in glasses was not changed by melting in atmosphere with different water concentration. Thus, spectral shapes of glass matrix and water could be assumed identical in all glasses.
The IR absorption spectra of dehydrated and regular PTR glasses are shown in Fig. 3. As it can be seen from Fig. 3(a), there is no detectable absorption in both glasses below 1500 nm. The absorption above 1500 nm gradually grows to longer wavelengths showing several shoulders. There is a sharp step in the absorption spectra at 2700 nm. Absorption of regular glass is significantly higher than that of the dehydrated one. The spectra become close to each other at wavelengths beyond 4200 nm.
On the basis of the results observed in Ref. [6,8], one can suppose that absorption in IR region is a combination of vibrational bands of glass matrix, and absorption of admixtures of water and iron. Current technology of PTR glass provides concentration of iron about 1 ppm [1,2] and its absorption could not be detected with conventional spectrophotometry. Therefore, absorption spectra of two glasses with different water concentration Cw1 and Cw2 could be modelled as a sum of water and matrix:
Here, A1 and A2 are absorption spectra of glasses. The supposition is made on the basis of our previous consideration that absorption spectra of glass matrix (Am) and specific absorption spectra of water (Asw) are the same in both glasses. Thus, subtraction Eq.(1) from Eq.(2) provides a function that does not include absorption of glass matrix. This means that it gives spectral shape of water absorption. However, both concentrations of water in the glasses and specific absorption of water are unknown. Therefore, absolute values of absorption correspond to an unknown water concentration.
Such a differential spectrum for regular (A2) and dehydrated (A1) glasses is shown in Fig. 4. It could be assigned to the water absorption (Eq. (3)). It shows well-known bands of water with maxima at 4300 and 3550 nm and a sharp shoulder in vicinity of 2800 nm. Detectable absorption of rather complex shape could be seen from 2700 to 1500 nm. Deconvolution of this spectrum to Gaussian components is done in accordance with formula:
Fig. 4. Absorption spectrum of water species produced as difference between experimental spectra of regular and dehydrated glasses: a - within the range 1500-2800 nm, b – within the range 2500-4500 nm. The red curve (1) is an experimental spectrum, the black curve (2) is the sum of Gaussian components according to Table 1.
Table 1. Parameters of Gaussian functions used for deconvolution of water absorption spectrum in PTR glass.
Parameters of Gaussian functions are shown in Table 1. They are depicted in Fig. 4. The sum of eleven Gaussian bands describes the differential experimental absorption spectrum with accuracy about 3%.
This means that while the experimental spectra show only three maxima, real number of transitions in different structural units containing water and hydroxyl is larger. No water absorption bands at the wavelengths above 4500 nm were extracted because glass matrix absorption in this spectral area is too high and differential spectrum becomes too noisy.
It was supposed above that only glass matrix and water contribute to IR absorption spectra, while absorption of glass matrix and spectral shape of water are the same in both studied glasses. A linear combination of a glass absorption spectrum A2 and a spectrum of water with unknown coefficient k is:
According to Eq. (3), (A2-A1) is a non-normalized water absorption. To eliminate a water component in this linear combination of spectra (Eq. (5)), the expression in square brackets should be equal to zero and the following condition should be satisfied as:
The ratio of water concentrations in the studied glasses is unknown. However, the problem could be solved by enumeration of k values. To determine k, a number of linear combinations of spectra according to the left side of Eq. (5) are calculated and depicted in Fig. 5.
Fig. 5. Linear combinations of absorption spectra of R glass (A2) and water (A2-A1) in accordance with Eq. (5).
The goal is to find a value of k, when the features of water spectrum would disappear from this linear combination. The most distinguished feature of water absorption is a sharp shoulder in vicinity of 2800 nm (Fig. 1 and 3). This shoulder is distinctly seen for k=0 that is just the absorption spectrum of R glass. When k increased, this shoulder decreases. For k=1.185, this shoulder completely disappears; for k=1.6, this shoulder is negative. This means that for k=1.185, the water absorption is eliminated from the spectrum. According to Eq. (5), the residual absorption should be considered as the spectrum of glass matrix. This spectrum is depicted in Fig. 6. Parameters of deconvolution of this spectrum with Gaussian functions (Eq. (4)) are shown in Table 2 and Gaussian curves are depicted in Fig. 6. PTR glass matrix absorption at wavelengths shorter than 2600 nm is too small to analyze with the used measurement technique. While coefficient k is known, it becomes possible to calculate water absorption spectra in glasses. A combination of Eq. (4) and Eq. (5) provides water absorption spectra in regular (AswCw2) and dehydrated glass (AswCw1).
Fig. 6. Absorption spectrum of PTR glass matrix and its deconvolution to Gaussian bands in standard (a) and logarithmic (b) scale. 1 – experimental spectrum, 2 – sum of Gaussian bands, 3,4,5 – Gaussian bands according to Table 2.
Table 2. Parameters of Gaussian bands used for deconvolution of PTR glass matrix spectrum
Figure 7 demonstrates the absorption spectra of PTR glasses and contributions of water and glass matrix absorption. As it can be seen, water absorption provides major contribution to absorption for the wavelengths shorter 3700 nm for regular glass while for dehydrated glass – for λ<3000 nm only. It is important that PTR glass matrix starts detectable absorption at λ>3000 nm with the first maximum at 3250 nm. At wavelengths beyond 4500 nm, absorption of water is too small comparing to that of PTR glass matrix and could not be extracted from experimental spectra.
Fig. 7. Absorption spectra of regular (a) and dehydrated (b) glasses along with contributions of glass matrix and water.
4. Discussion
The similar to Ref. [6,12] approach of comparison spectra of PTR glasses melted in atmosphere with different concentration of water enabled separation of absorption spectra of glass matrix and water admixtures. The experimental water spectrum (Fig. 4) demonstrates three wide intensive overlapped absorption bands with maxima in vicinity of 4320, 3560, and 2815 nm (2315, 2809, and 3552 cm-1). Two long wavelength bands are well-modelled by Gaussian functions with the same maxima (Table 1). Description of the short wavelength band that looks as a sharp shoulder requires at least four Gaussian functions with maxima at 3053, 2897, 2815, and 2765 nm (3275, 3452, 3552, and 3617 cm-1). A short wavelength absorption tail of water could be described by Gaussian functions with maxima at 2525, 2315, 2219, 2150, and 1908 nm (3960, 4319, 4506, 4651, 5241 cm-1).
Water absorption bands near 4300 and 3600 nm are well known [3,7,9] and usually are ascribed to stretching modes of Si-O-H groups [9]. Maxima of these bands in PTR glass differ from similar bands in Na2O-CaO-MgO-SiO2 glass for 70-80 nm [7]. Description of water complex spectral shape in 3000-2700 nm region in different silicate glasses [6,9] and PTR glass requires four Gaussian functions. Their positions differ for 1-15 nm in different glasses.
Three bands with maxima from 2200 to 2600 nm observed in this work are similar those in Ref. [6 and 9]. However, absorption from 2300 to 1500 nm is described by a wide band with maximum at 2160 nm in Na2O-CaO-MgO-SiO2 glass [7] while it required two bands with maxima at 2150 and 1908 nm for PTR glass. However, accuracy of deconvolution in the range of absorption below 0.01 cm-1 does not allow to discriminate between these results. Well-known water absorption band at 1420 nm [6] was not detected in PTR glass. This could be a result of smaller concentration of water and thinner samples used for measurements.
Absorption spectrum of PTR glass matrix in range from 2500 to 5000 nm is described with high precision by three Gaussian bands (Fig. 6 and Table 2). The long wavelength band with maximum at 4855 nm (2060 cm-1) is close to similar bands in silica [9,13, and 5] and multicomponent silicate glass [7]. It is usually assigned to the second overtone of fundamental vibrations of SiO4 tetrahedra. The shorter wavelength band in Na2O-CaO-MgO-SiO2 glass has maximum at 4166 nm (2400 cm-1) [7]. In PTR glass, the similar absorption band has maximum at 3950 nm (2532 cm-1) that is significantly shifted to short wavelengths. The shortest revealed absorption band in PTR glass matrix at 3250 nm (3077 cm-1) is similar to that in Na2O-CaO-MgO-SiO2 glass [14]. Such a band was not detected in Ref. [7] because it was masked by high absorption of water. It is important to note that while long wavelength absorption edge of conventional PTR glass is about 2700 nm, utmost values of long wavelength edge are 2900 nm for absorption coefficient 0.01 cm-1 and 3600 nm for 0.1 cm-1.
5. Conclusion
IR absorption spectrum of high purity photo-thermo-refractive (PTR) glass is a combination of absorption of uncontrolled admixture of different water species and glass matrix. A comparative study of glasses containing different concentration of water allowed separating the absorption spectra of water and glass matrix. The spectra are deconvoluted to elementary Gaussian bands. Water absorption spectrum includes two absorption bands with maxima at 4320 and 3560 nm, a sharp shoulder near 2800 nm that is described by four Gaussian bands at 3053, 2897, 2815, and 2765 nm, and a nonmonotonic decay curve that is described by Gaussian functions with maxima at 2525, 2315, 2219, 2150, and 1908 nm (3960, 4319, 4506, 4651, 5241 cm-1). The current technology provides decrease of decimal absorption coefficient at 2800 nm down to 0.04 cm-1. PTR glass matrix absorption spectrum includes three bands at 4855, 3950, and 3250 nm. Long wavelength absorption edge for PTR glass matrix is 2900 nm for decimal absorption coefficient 0.01 cm-1 and 3600 nm for 0.1 cm-1.
Funding
IPG Photonics.
Acknowledgements
Authors thank Larissa Glebova for help in dehydrated glass technology development and Artem Mandryk for help in glass fabrication.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
References
1. L. B. Glebov, “Photosensitive glass for phase hologram recording,” Glass Sci. Technol. 71C, 85–90 (1998).
2. L. B. Glebov, “Volume holographic elements in a photo-thermo-refractive glass,” Journal of Holography and Speckle 5, 1–8 (2008). [CrossRef]
3. C. R. Bamford, Colour Generation and Control in Glass (Elsevier Scientific Publishing Co., 1977)
4. H. Scholze, Glass: Nature, Structure and Properties (Springer-Verlag, 1991)
5. A. M. Efimov, Optical Constants of Inorganic Glasses (CRC Press, 1995)
6. E. N. Boulos, L. B. Glebov, and T. V. Smirnova, “Absorption of iron and water in the Na2O-CaO-MgO-SiO2 glasses. part 1. separation of ferrous and hydroxyl spectra in the near IR region,” J. Non-Cryst. Solids 221(2-3), 213–221 (1997). [CrossRef]
7. L. B. Glebov and E. Boulos, “Absorption spectra of iron and water in silicate glasses,” in Proc. of International Commission on Glass Conference Glass in the New Millenium (2001), S4–2.
8. W. E. Jackson, F. Farges, M. Yeager, P. A. Mabrouk, S. Rossano, G. A. Waychunas, E. I. Solomon, and G. E. Brown, “Multi-spectroscopic study of Fe(II) in silicate glasses: Implications for the coordination environment of Fe(II) in silicate melts,” Geochim. Cosmochim. Acta 69(17), 4315–4332 (2005). [CrossRef]
9. A. M. Efimov, V. G. Pogareva, and A. V. Shashkin, “Water-related bands in the IR absorption spectra of silicate glasses,” J. Non-Cryst. Solids 332(1-3), 93–114 (2003). [CrossRef]
10. A. M. Efimov and V. G. Pogareva, “Water related IR absorption spectra for some phosphate and silicate glasses,” J. Non-Cryst. Solids 275(3), 189 (2000). [CrossRef]
11. M. L. Brandily-Anne, J. Lumeau, L. N. Glebova, and L. B. Glebov, “Specific absorption spectra of cerium in multicomponent silicate glasses,” Journal of Non-Crystalline Solids 356(44-49), 2337–2343 (2010). [CrossRef]
12. J. M. Jewell, M. S. Spess, and J. E. Shelby, “Effect of water concentration on the properties of commercial soda-lime-silica glasses,” J. Am. Ceram. Soc. 73(1), 132–135 (1990). [CrossRef]
13. T. Izawa, N. Shibata, and A. Takeda, “Optical attenuation in pure and doped fused silica in the IR wavelength region,” Appl. Phys. Lett. 31(1), 33–35 (1977). [CrossRef]
14. M. Brandily-Anne, L. N. Glebova, J. Lumeau, and L. B. Glebov, “IR absorption edge of multicomponent silicate glasses,” in 8th Pacific RIM Conference (2009), paper PACRIM8-S23-004-2009.
