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The effects of single femtosecond laser pulse irradiation (130 fs pulse duration, 800 nm center wavelength) on the structure of binary lithium silicate glasses of varying chemical compositions were investigated by micro-Raman spectroscopy. Permanent modifications were generated at the surface of the glass samples with varying laser fluences in the ablative regime and evaluated for changes in the corresponding Raman band positions and bandwidths. For increasing laser fluences, the position of certain Raman bands changed, indicating an increase in the mass density of the glass inside the irradiated area. Simultaneously, the widths of all investigated bands increased, indicating a higher degree of disorder in the glass structure with respect to bond-angle and bond-length variations.
©2013 Optical Society of America
1. Introduction
The nonlinear absorption of femtosecond (fs) laser pulses and their subsequent relaxation processes allow the modification of optical transparent materials like inorganic glasses in very small volumes either as ablation on glass surfaces [1,2] or as modification inside the glass volume [3–6]. Laser ablation can be used to fabricate microchannels on or inside glass for microfluidic analytical devices [6]. Non-ablative modifications inside the bulk of glasses or dielectrics on the other hand are used to produce photonic devices such as optical waveguides [6,7], optical couplers [6–8] or holograms for microholographic data storage [9].
However, a lack in the understanding of the laser-glass interaction still limits the accuracy and the efficiency of photonic devices. Recent studies of the effects of femtosecond laser pulse irradiation on the glass structure revealed a large variety of transient and permanent modification of the remaining glass material [10], such as shock wave generation [11], void creation [11] or permanent changes in the electronic structure of the residual surface material [12 and this work]. However, only little work was devoted to the role of the chemical glass composition in terms of energy absorption [13], and permanent volume modification [14].
Raman spectroscopy is known as a powerful tool to analyze structural changes of amorphous solids such as glasses with respect to their composition [15–17]. It was successfully used to characterize structural changes caused during the glass manufacturing process by variation of the temperature or cooling rate [18,19] or changes in the chemical stability [20]. Moreover, micro-Raman spectroscopy was successfully employed to investigate the fs-laser-induced material modification in glasses. As an example, Chan et al. [21] showed that mass densification occurred in amorphous fused silica upon fs-laser pulse irradiation. Little et al. [22] studied waveguides written by a train of fs-laser pulses in BK7 glass using kHz and MHz pulse repetition rates. Ehrt et al. [23] provided evidence for refractive index changes upon fs-laser pulse irradiation in several technical glasses of discrete chemical compositions.
In this work, ablation spots have been processed on the surface of binary silicate glasses with systematically varying lithium dioxide content. Those samples have a well investigated glass structure [24]. Micro-Raman spectroscopic characterizations were performed and indicate a change in the material structure of the residual glass left at the surface after the irradiation with fs-laser pulses.
2. Experimental
Lithium silicate glasses with the composition xLi2O·(100-x)SiO2 (with x = 25, 34 and 40 mol%) - named LiSi(100-x) in this work - were prepared by mixing reagent grade raw materials of Li2CO3 and SiO2 and by melting in a platinum crucible for 1 hour. The glass melt was quenched between two steel plates and subsequently annealed at temperatures approx. 10°C above the glass transformation temperature (Tg). Then, the manufactured glass plates were cut into pieces of 20x20x2 mm3 and polished with water free lubricant to avoid a possible modification of the glass surfaces. The polishing process assured an optical quality grade of the glass surfaces. The intended glass composition was confirmed by chemical analyses based on X-ray fluorescence and inductively coupled plasma optical emission spectrometry (Table 1). Characterization of the thermophysical properties, i.e., the linear expansion coefficient (α) and Tg was done by dilatometric measurements. The results are compiled in Table 1 together with the measured mass density (ρ), determined experimentally by the Archimedes principle. All values fit well with data available in the literature [24]. High-purity fused silica samples (Suprasil, Heraeus GmbH, Germany) were selected as reference material.
Table 1. Composition of the investigated glasses as determined by chemical analysis by X-ray fluorescence and inductively coupled plasma analysis and properties (glass transformation temperature (Tg), thermal expansion coefficient (α) and mass density (ρ)) of the investigated glasses determined by dilatometric measurement and Archimedes’ principle
The polished glass samples were irradiated by single fs-laser pulses from a Ti:sapphire regenerative laser amplifier system (Spectra Physics GmbH, Spitfire) emitting pulses at 800 nm center wavelength and with ~130 fs pulse duration. The laser pulse energies were attenuated by a combination of a half-wave plate and a linear polarizer. The fs-laser pulses were then focused by a spherical lens (f = 80 mm) yielding a Gaussian-like beam profile with a 1/e2-beam radius w0~16.5 µm at the sample surface. Laser peak fluences were determined from the laser pulse energy measurements according to a method proposed by Liu (“D2-method”) [25]. The irradiation peak fluences of the laser pulse started at about 22.5 J/cm2 and were reduced stepwise until no surface modification was detectable by optical microscopy. At the given experimental conditions smooth ablation craters with diameters between 4 µm and 28 µm and depths up to 350 nm were generated at the glass surfaces. Typical depths of the residual fs-laser modified near surface layer in dielectrics are expected to lie between some hundreds of nanometers up to a few micrometers [12,26]. All irradiations were performed in air and the spots were separated by >90 µm to avoid any spatial spot overlap.
Micro-Raman spectra were acquired in the wavenumber range between 200 cm−1 and 1400 cm−1 using a Horiba LabRam HR800 vis Spectrometer (Jobin-Yvon-Horiba, Longjumeau, France) in the center of the ablation spots and at the non-irradiated surface as references. For that, the 473 nm line of a continuous wave diode laser was used as an excitation source. This radiation was focused on the sample surface by a long working distance microscope objective (Olympus, 100x LWD, numerical aperture NA = 0.8) illuminating a circular area of nominally ~0.7 µm in diameter. The positioning accuracy of the sample surface in the focal plane of the objective was better than 1 µm. The backscattered radiation was collected with the same microscope objective along with a confocal aperture of 100 µm diameter. An optical grating of 1800 grooves/mm was selected in the spectrometer which was calibrated with the pronounced Raman peak of single-crystalline silicon positioned at 520.7 cm−1. For calibration, a separate single-crystalline silicon wafer was used. The Raman signal scattered from the glass samples was recorded by a CCD camera for 50 seconds. The wavenumber resolution of the Raman device was better than 1 cm−1. Given the transparency of the investigated glass samples in the visible spectral range, the Raman information depth was mainly limited by the Rayleigh length of the microscope objective (<3.4 µm).
3. Results and discussion
Figure 1 shows micro-Raman spectra of the three non-irradiated lithium silicate glasses of systematically varying lithium dioxide content along with that of fused silica as a reference.
Fig. 1 Normalized and baseline-corrected Raman spectra of three silicate glasses (non-irradiated) containing various amounts of lithium in the wavenumber range between 200 cm−1 and 1400 cm−1. For comparison, the reference spectrum of Fused Silica is shown. For improved visibility the individual curves are vertically shifted by 0.3.
Except for fused silica, the raw data were normalized to the maximum Raman signal at ~1100 cm−1. A constant offset value was subtracted to our measurements to obtain the background free micro-Raman spectra presented in Fig. 1. The correction offset takes into account the instrumental background as well as the effect of fluorescence. The corrected Raman spectra were deconvoluted using several Gaussian-type band elements in a non-linear least squares calculation by the Levenberg-Marquardt fitting method [29]. Table 2 compiles all Raman bands used here for the deconvolution procedure along with their physical origin.
Table 2. Assignment and origin of the peaks used for deconvolution of the Raman spectra
Fused silica forms a three-dimensional network of silicon surrounded by four oxygen ions which bind covalently in a tetrahedral shape. Those oxygen ions are called “bridging oxygens” (BOs). As seen in the reference spectrum of Fig. 1, the Raman bands of fused silica can be found mainly in the low frequency region between 200 cm−1 and 600 cm−1 and at about 800 cm−1. With the exception of three (labeled A, C, E in Fig. 2), all bands in the fused silica spectrum are assigned to Si-O-Si bending or stretching vibrations (labeled B, H/H’, L, and N in Fig. 2). Band C and band E are assigned to either 4- or 3-membered ring structures of silicon tetrahedra.
Fig. 2 Deconvoluted Raman spectra of the non-irradiated LiSi60 [(a) - green], LiSi66 [(b) - blue], LiSi75 [(c) - red], and fused silica [(d) - grey] glasses. In all graphs the black lines represent the normalized and baseline-corrected raw data. The investigated bands referred in this paper are band B (solid orange line) and band M (solid cyan line).
An additional band (A) appears at about 380 cm−1 in fused silica samples and the LiSi75 glass with 25 mol% lithium dioxide which is caused by Si-O-H wagging [20,30]. Adding gradually more lithium dioxide weakens the glass network. Indeed, the ionic nature of the Li-O bond implies that the oxygen ions involved are connected covalently to the silicon ions at one end only. Such oxygen ions are called “non bridging oxygens” (NBOs) and convert the three-dimensional silicon network to a more chainlike silicon structure. The decrease of connectivity in the glass network associated to the presence of NBOs causes an alteration of the glass properties. Namely, Tg increases with the concentration of NBOs while α and ρ decrease (see Table 1).
In the common nomenclature the structure of silicon bond arrangements of tetrahedral shape to the neighboring oxygen are labeled Qn-structure, with n = 0 - 4 according to the number of BOs. Adding network modifiers, such as lithium, substantially changes the Qn-structure and consequently creates additional bands in the Raman spectrum as it can be seen in the detailed deconvolution of the Raman spectra presented in Fig. 2. In the low wavenumber region between 400 cm−1 and 700 cm−1 two new bands appear at about 580 cm−1 (band D) and 650 cm−1 (band F) that are indicative of vibrational modes of Q3 (band D) and Q2 (band F) structures. In the high wavenumber region the vibrational stretching modes of the Q-structures result in additional bands in the Raman spectrum. Those new structural elements, caused by adding Li2O to pure silica, can also be found for Q2 stretching modes at 950 cm−1 (band K) and for Q3 stretching modes at 1070 cm−1 (band M). To match the measured Raman data, an additional band had to be fitted at about 890 cm−1 that can be assigned to either Q0 (band I) or Q1 (band J) structural elements [27,30]. A more accurate assignment was not possible here. One can underline that with increasing lithium content in the binary Li2O-SiO2 glasses, the bands assigned to the Qn (n = 0 - 3) structures rise in intensity while several pure silica bands are not observed anymore (like bands A, C, E, and H’).
The high frequency regime of stretching vibrations is well suited to observe changes in the Qn-structure [20,27,30]. Accordingly, this region was analyzed in more detail for the investigated glasses before and after irradiation by single fs-laser pulses with laser fluences between 6 J/cm2 and 22.5 J/cm2. In silicate melts the following equilibrium has been proposed [33,34]:
It was shown that with increasing temperature of the melts the equilibrium shifts to the right hand side of Eq. (1). The high wavenumber range in the Raman spectra was deconvoluted using the bands (I/J, K, L, M, N) as listed in Table 2.The fractions FQn of the Qn-units in the glass were determined from the measured Raman band areas S(Qn) of the single modes and were calculated for each n as
Figure 3 shows the FQn values in the glasses LiSi60 (Fig. 3(a)), LiSi66 (Fig. 3(b)), and LiSi75 (Fig. 3(c)) as a function of the irradiation fs-laser fluence ϕ0. The reference value for the non-irradiated glass is indicated at ϕ0 = 0 J/cm2. As a first result it can be stated, that no significant changes in the Qn-structure of the individual glasses can be detected after the irradiation, indicating that these structural blocks remain intact upon fs-laser exposure. A comparison of the FQn for the three different glasses is presented in Fig. 3(d). While the LiSi60 and LiSi66 glasses behave generally very similar, the high silica-content glass LiSi75 exhibits somewhat increased fractions of Q3 and Q4-structures and a reduced Q2 fraction.Fig. 3 Fraction F of Qn units after fs-laser pulse irradiation of the glasses LiSi60 (a), LiSi66 (b), and LiSi75 (c). A comparison of the different glass compositions is given in (d).
In order to address more subtle structural changes upon fs-laser pulse irradiation, we closely studied Raman bands B and M (see Table 2). More specifically, we looked into the center position and the half width at half maximum (HWHM) of Raman bands B and M when varying the incident laser fluence. The center position of band B and M for different fluences is reported in Figs. 4(a)-4(d) and Figs. 4(e)-4(g), respectively. The increase of their HWHM is shown in Figs. 5(a)-5(d) and Figs. 5(e)-5(g).
Fig. 4 Changes of peak center positions of Raman band B (Q4 bending) of fs-laser irradiated LiSi60 [(a) - green], LiSi66 [(b) - blue], LiSi75 [(c) - red] glasses and fused silica [(d) - black] and Raman band M (Q3 stretching) of fs-laser irradiated LiSi60 [(e) - green], LiSi66 [(f) - blue], and LiSi75 [(g) - red] glasses as a function of the laser fluence. A reference value for the non-irradiated material and its standard deviation is given at 0 J/cm2. The grey vertical lines indicate the ablation threshold fluence according to [13]. The dashed black lines guide the eye.
Fig. 5 Changes of half width at half maximum (HWHM) of Raman band B (Q4 bending) of fs-laser irradiated LiSi60 [(a) - green], LiSi66 [(b) - blue], LiSi75 [(c) - red] glasses and fused silica [(d) - black] and Raman band M (Q3 stretching) of fs-laser irradiated LiSi60 [(e) - green], LiSi66 [(f) - blue], and LiSi75 [(g) - red] glasses as a function of the laser fluence. A reference value for the non-irradiated material and its standard deviation is given at 0 J/cm2. The grey vertical lines indicate the ablation threshold fluence according to [13]. The dashed black lines guide the eye.
The deconvoluted Raman spectrum of fused silica is shown in Fig. 2(d). The B band of the unmodified glass is located at 453 cm−1 with a HWHM of 42 cm−1. It represents mainly oxygen displacement and is generally assigned to oxygen motion in the plane bisecting the Si-O-Si linkage [37]. Upon fs-laser modification this band is shifted to 457 cm−1 and shows no change in the HWHM. According to Hemley et al. [35], this can be related to a small decrease in bond-angle but no significant change in the bond-angle distribution of Si-O-Si bending vibration upon fs-laser modification of fused silica, indicating a deformation of the SiO2 network. Hehlen et al. [39] and Deschamps et al. [40] have also investigated changes of Raman spectra of densified silica showing a shift of the center of band B to higher wavenumbers and, therefore, a decrease of the Si-O-Si bond-angle with increasing density.
By adding lithium dioxide to fused silica, the silicon network evolves to a more chainlike structure. Band B of the LiSi75 composition (Fig. 2(c)), containing 75 mol% SiO2, is located at 454 cm−1 (similar to fused silica) with a much larger HWHM of about 66 cm−1 indicating a much larger bond-angle distribution of the Si-O-Si linkage. Upon fs-laser modification the B band shows the same change of its center position as in fused silica. Moreover, the HWHM increases to 72 cm−1. Therefore, we deduce that the silicon network in LiSi75 exhibits a change in angle and angle distribution of the Si-O-Si linkage upon fs-laser modification. Additionally, when adding lithium dioxide, a new Raman peak (band M) appears at 1082 cm−1 with a HWHM of 40 cm−1. This band is assigned to Si-O-Li stretching vibrations. Upon fs-laser irradiation, the center position of the M band in LiSi75 decreases slightly while the HWHM increases. So additionally to the effects shown in fused silica, LiSi75 exhibits a change in angle distribution of the Si-O-Si bending mode and a change in the bond-length and bond-angle distribution of the Si-O-Li stretching mode.
The LiSi66 and LiSi60 glasses show a further peak center increase of the B band due to the chemical changes. The center wavenumber is now located at 496 cm−1 for LiSi66 and 501 cm−1 for LiSi60 with a HWHM of about 42 cm−1 for both compositions. However, in comparison to the high silicate glasses, band B shows now change in its center position (Figs. 4(a), 4(b)), but an increase of its HWHM (Figs. 5(a), 5(b)). So the silicon network exhibits no change of its average angle, though an increase of the angle distribution is observed.
Similar results can be found in the literature for the conventional (non-laser based) treatment of lithium silicate glasses. Kitamura et al. [31] have investigated structural changes of lithium disilicate glasses that were mechanically densified with pressures up to 6 GPa in a multi-anvil high-pressure apparatus. After densification they observed that with increasing density, the B band shifts towards larger wavenumbers while the M band shifts towards smaller wavenumbers. Similarly, McMillian et al. [36] have established a correspondence between a decrease of the Si-O-Si angle and a positive shift of the B band in fused silica. In conclusion, our studies show that a positive shift of the B band could be a signature of a densified glass material. Another possible explanation could be the influence of the thermal history of the laser-modified glasses. Bressel et al. [41] have investigated the fictive temperature effect. The fictive temperature is the last temperature at which glasses reached equilibrium state before a rapid quench freezes its structure. Those authors showed, that the shift of band B and the Si-O-Si bond angle decrease can be explained with an increase of the fictive temperature of fs-laser modified glasses. Therefore, based on Figs. 4(c)-4(g), we deduce that densification of the silicon network occurs in glasses with a high silicon content, in good agreement with previous works from Chan et al. [21]. This densification is seen as a compensation of transient stress applied to the material by the subsequent relaxation processes after the absorption of the fs-laser pulse. Additionally thermal effects can change the Raman spectra of fs modified glasses and have to be taken into account.
In the low silica compositions (LiSi66 and LiSi60), no such change in position of the Raman band B can be detected (Figs. 4(a) and 4(b)) indicating a different compensation of the laser-induced stress due to its high content of network modifying oxides and, therefore, more ionic bonds. Given the similarity of the observations reported in [31] and [37] to our Raman spectroscopic results, a glass densification supposedly occurs in the center of the fs-laser irradiated spots at the lithium silicate surfaces. Apparently, the Si-NBO bonds respond first to the fs-laser-induced effects before the Si-O-Si bonds (band B) tend to reduce internal stresses. The evaluation of the bandwidth of the Si-O-Si bending peak (band B) shows a broadening with increasing laser fluence. This is in contrast to McMillan et al. [37] who have shown that with higher mechanical pressure and consequently increasing density, the Raman signal is shifted towards higher wavenumbers while the bandwidth becomes smaller. In contrary to McMillian et al., Okuno et al. [38] have shown in their work related to shockwave densified fused silica that at shockwave pressures below about 18 GPa the Raman band position and the bandwidth of the Si-O-Si bending mode (band B) both show a moderate increase as it also takes place in this work. However, as the conditions under which a shockwave is formed may include pressure and also thermal effects, local changes in the fictive temperature of the glass cannot be excluded here.
In summary, we explain the shifts of band B and M by a pressure-induced deformation of the local bonds of a structurally modified lithium silicate glass. The broadening of the Raman bandwidth is a clear indicator for an increasing bond-angle distribution of Si-O-Si angles and consequently for a higher degree of disorder in the glass network. The structural reaction to the fs-laser-induced stress is therefore depending on the glass composition. In glasses with high NBO concentrations, the ionic bonds react in the first place and are able to compensate in large part the induced pressure. The processes of energy deposition and relaxation after ultrashort laser pulse irradiation are very fast and involve many physical and chemical processes. However we speculate that the shockwave emerging from the laser-heated and ablating volume is modifying the surrounding residual surface material. Additionally, a different material solidification or stress distribution after excitation compared to the cooling during the manufacturing process might also be considered.
4. Conclusion
The impact of single femtosecond laser pulse irradiation on lithium silicate glasses of varying chemical composition was studied by means of micro-Raman spectroscopy. For increasing laser fluences the Raman analysis revealed a characteristic shift of specific Raman bands along with their broadening. Although upon the laser-irradiation no variation occurred in the distribution of the Qn-units of the individual glasses, changes in the Si-O-Si bending frequencies and in the Si-NBO stretching frequencies were detected. The broadening of the Raman bands revealed an increased bond-angle variation as indication for a larger degree of disorder in the laser-modified region. All those effects are indicative of a material densification in fused as it was also found in the single component SiO2 glass by Chan et al. [21]. In binary silicate glasses the changes in the Raman spectra are a mixture of densification and fictive temperature effects. Comparing our findings to the results acquired on glasses densified by mechanical pressure [31,37] implies that this densification after fs-laser pulse irradiation is induced by a pressure wave or by thermal excitation along with subsequent material relaxation processes.. More specifically, binary Li2O-SiO2 glass compositions with a high Li2O content of more than 34 mol% did not exhibit alterations of the Si-O-Si bending mode while the Si-NBO (Q3) stretching mode was shifted after laser-irradiation. Apparently, if the content of NBOs inside the glass network is large enough, the pressure induced by densification can relax only via changes of the NBO structural elements. At a lower Li2O content of 25 mol%, the NBO bonds cannot relax the laser-induced densification entirely and consequently, the SiO2 network has to be modified.
Acknowledgments
The authors would like to thank R. Schadrack (BAM) for the dilatometric measurements and S. Richter (IKTS) for the Raman spectroscopic measurements. This work was supported by the German Science Foundation (DFG) under grants no. EB 248/4-2 and EI 110/30-2.
References and links
1. A. Ben-Yakar, R. L. Byer, A. Harkin, J. Ashmore, H. A. Stone, M. Shen, and E. Mazur, “Morphology of femtosecond-laser-ablated borosilicate glass surfaces,” Appl. Phys. Lett. 83(15), 3030–3032 (2003). [CrossRef]
2. P. Rudolph, J. Bonse, J. Krüger, and W. Kautek, “Femtosecond- and nanosecond-pulse laser ablation of bariumalumoborosilicate glass,” Appl. Phys., A Mater. Sci. Process. 69(7), S763–S766 (1999). [CrossRef]
3. A. M. Streltsov and N. F. Borrelli, “Study of femtosecond-laser-written waveguides in glasses,” J. Opt. Soc. Am. B 19(10), 2496–2504 (2002). [CrossRef]
4. O. M. Efimov, L. B. Glebov, K. A. Richardson, E. Van Stryland, T. Cardinal, S. H. Park, M. Couzi, and J. L. Brunéel, “Waveguide writing in chalcogenide glasses by a train of femtosecond laser pulses,” Opt. Mater. 17(3), 379–386 (2001). [CrossRef]
5. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics 2(4), 219–225 (2008). [CrossRef]
6. M. Masuda, K. Sugioka, Y. Cheng, N. Aoki, M. Kawachi, K. Shihoyama, K. Toyoda, H. Helvajian, and K. Midorikawa, “3-D microstructuring inside photosensitive glass by femtosecond laser excitation,” Appl. Phys., A Mater. Sci. Process. 76(5), 857–860 (2003). [CrossRef]
7. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef] [PubMed]
8. S. Nolte, M. Will, J. Burghoff, and A. Tünnermann, “Femtosecond waveguide writing: a new avenue to three-dimensional integrated optics,” Appl. Phys., A Mater. Sci. Process. 77(1), 109–111 (2003). [CrossRef]
9. H. J. Eichler, P. Kuemmel, S. Orlic, and A. Wappelt, “High-density disk storage by multiplexed microholograms,” IEEE J. Sel. Top. Quantum Electron. 4(5), 840–848 (1998). [CrossRef]
10. D. Puerto, J. Siegel, W. Gawelda, M. Galvan-Sosa, L. Ehrentraut, J. Bonse, and J. Solis, “Dynamics of plasma formation, relaxation and topography modification induced by femtosecond laser pulses in crystalline and amorphous dielectrics,” J. Opt. Soc. Am. B 27(5), 1065–1076 (2010). [CrossRef]
11. A. Mermillod-Blondin, J. Bonse, A. Rosenfeld, I. V. Hertel, Yu. P. Meshcheryakov, N. M. Bulgakova, E. Audouard, and R. Stoian, “Dynamics of femtosecond laser induced voidlike structures in fused silica,” Appl. Phys. Lett. 94(4), 041911 (2009). [CrossRef]
12. T. Seuthe, M. Höfner, F. Reinhardt, W. J. Tsai, J. Bonse, M. Eberstein, H. J. Eichler, and M. Grehn, “Femtosecond laser-induced modification of potassium-magnesium silicate glasses: an analysis of structural changes by near edge x-ray absorption spectroscopy,” Appl. Phys. Lett. 100(22), 224101 (2012). [CrossRef]
13. M. Grehn, W. J. Tsai, M. Höfner, T. Seuthe, J. Bonse, A. Mermillod-Blondin, A. Rosenfeld, J. Hennig, A. W. Achtstein, C. Theiss, U. Woggon, M. Eberstein, and H. J. Eichler, “Nonlinear optical properties of binary and ternary silicate glasses upon near-infrared femtosecond pulse laser irradiation,” AIP Conf. Proc. 1464, 660–670 (2012). [CrossRef]
14. Y. Dai, B. Zhu, J. Qiu, H. Ma, B. Lu, S. Cao, and B. Yu, “Direct writing three-dimensional Ba2TiSi2O8 crystalline pattern in glass with ultrashort pulse laser,” Appl. Phys. Lett. 90(18), 181109 (2007). [CrossRef]
15. S. A. Brawer and W. B. White, “Raman spectroscopic investigation of the structure of silicate glasses. I. The binary alkali silicates,” J. Chem. Phys. 63(6), 2421–2432 (1975). [CrossRef]
16. B. O. Mysen, D. Virgo, and C. M. Scarfe, “Relations between the anionic structure and viscosity of silicate melts—a Raman spectroscopic study,” Am. Mineral. 65, 690–710 (1980).
17. J. C. Phillips, “Structure and selectively enhanced Raman spectra of high-silica alkali ailicate glasses,” Phys. Rev. B 32(8), 5350–5355 (1985). [CrossRef]
18. T. Maehara, T. Yano, and S. Shibata, “Structural rules of phase separation in alkali silicate melts analyzed by high temperature Raman spectroscopy,” J. Non-Cryst. Solids 351(49-51), 3685–3692 (2005). [CrossRef]
19. J. Tan, S. Zhao, W. Wang, G. Davies, and X. Mo, “The effect of cooling rate on the structure of sodium silicate glass,” Mater. Sci. Eng. B 106(3), 295–299 (2004). [CrossRef]
20. L. Robinet, C. Coupry, K. Eremin, and C. Hall, “The use of Raman spectrometry to predict the stability of historic glasses,” J. Raman Spectrosc. 37(7), 789–797 (2006). [CrossRef]
21. J. W. Chan, T. R. Huser, S. H. Risbud, and D. M. Krol, “Modification of the fused silica glass network associated with waveguide fabrication using femtosecond laser pulses,” Appl. Phys., A Mater. Sci. Process. 76(3), 367–372 (2003). [CrossRef]
22. D. J. Little, M. Ams, S. Gross, P. Dekker, C. T. Miese, A. Fuerbach, and M. J. Withford, “Structural changes in BK7 glass upon exposure to femtosecond laser pulses,” J. Raman Spectrosc. 42(4), 715–718 (2011). [CrossRef]
23. D. Ehrt, T. Kittel, M. Will, S. Nolte, and A. Tünnermann, “Femtosecond-laser-writing in various glasses,” J. Non-Cryst. Solids 345-346, 332–337 (2004). [CrossRef]
24. H. Scholze, Glass—Nature, Structure and Properties (Springer-Verlag, 1988).
25. J. M. Liu, “Simple technique for measurements of pulsed Gaussian-beam spot sizes,” Opt. Lett. 7(5), 196–198 (1982). [CrossRef] [PubMed]
26. J. Siegel, D. Puerto, W. Gawelda, G. Bachelier, J. Solis, L. Ehrentraut, and J. Bonse, “Plasma formation and structural modification below the visible ablation threshold in fused silica upon femtosecond laser irradiation,” Appl. Phys. Lett. 91(8), 082902 (2007). [CrossRef]
27. D. W. Matson, S. K. Sharma, and J. A. Philpotts, “The structure of high silica alkali silicate glasses—a Raman spectroscopic investigation,” J. Non-Cryst. Solids 58(2-3), 323–352 (1983). [CrossRef]
28. S. K. Sharma, J. F. Mammone, and M. F. Nicol, “Raman investigation of ring configurations in vitreous silica,” Nature 292(5819), 140–141 (1981). [CrossRef]
29. D. W. Marquardt, “An algorithm for least-squares estimation of nonlinear parameters,” J. Soc. Ind. Appl. Math. 11(2), 431–441 (1963). [CrossRef]
30. P. McMillan, “Structural studies of silicate glasses and melts-applications and limitations of Raman spectroscopy,” Am. Mineral. 69, 622–644 (1984).
31. N. Kitamura, K. Fukumi, H. Mizoguchi, M. Makihara, A. Higuchi, N. Ohno, and T. Fukunaga, “High pressure densification of lithium silicate glasses,” J. Non-Cryst. Solids 274(1-3), 244–248 (2000). [CrossRef]
32. G. S. Henderson, G. M. Bancroft, and M. E. Fleet, “Raman spectra of gallium and germanium substituted silicate glasses: variations in intermediate range order,” Am. Mineral. 70, 946–960 (1985).
33. B. O. Mysen and J. D. Frantz, “Raman spectroscopy of silicate melts at magmatic temperatures: Na2O-SiO2, K2O-SiO2 and Li2O-SiO2 binary compositions in the temperature range 25–1475°C,” Chem. Geol. 96(3-4), 321–332 (1992). [CrossRef]
34. B. O. Mysen and J. D. Frantz, “Silicate melts at magmatic temperatures: in-situ structure determination to 1651°C and effect of temperature and bulk composition on the mixing behavior of structural units,” Contrib. Mineral. Petrol. 117(1), 1–14 (1994). [CrossRef]
35. R. J. Hemley, H. K. Mao, P. M. Bell, and B. O. Mysen, “Raman spectroscopy of SiO2 glass at high pressure,” Phys. Rev. Lett. 57(6), 747–750 (1986). [CrossRef] [PubMed]
36. P. McMillan, B. T. Poe, P. Gillet, and B. Reynard, “A study of SiO2 glass and supercooled liquid to 1950 K via high temperature Raman spectroscopy,” Geochim. Cosmochim. Acta 58(17), 3653–3664 (1994). [CrossRef]
37. P. McMillan, B. Piriou, and R. Couty, “A Raman study of pressure-densified vitreous silica,” J. Chem. Phys. 81(10), 4234–4236 (1984). [CrossRef]
38. M. Okuno, B. Reynard, Y. Shimada, Y. Syono, and C. Willaime, “A Raman spectroscopic study of shock-wave densification of vitreous silica,” Phys. Chem. Miner. 26(4), 304–311 (1999). [CrossRef]
39. B. Hehlen, “Inter-tetrahedra bond angle of permanently densified silicas extracted from their Raman spectra,” J. Phys. Condens. Matter 22(2), 025401 (2010). [CrossRef] [PubMed]
40. T. Deschamps, C. Martinet, D. R. Neuville, D. de Ligny, C. Coussa-Simon, and B. Champagnon, “Silica under hydrostatic pressure: a non continous medium behavior,” J. Non-Cryst. Solids 355(48-49), 2422–2424 (2009). [CrossRef]
41. L. Bressel, D. de Ligny, C. Sonneville, V. Martinez, V. Mizeikis, R. Buividas, and S. Juodkazis, “Femtosecond laser induced density changes in GeO2 and SiO2 glasses: fictive temperature effect,” Opt. Mater. Express 1(4), 605–613 (2011). [CrossRef]
