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Density changes of GeO2 and SiO2 glasses subjected to irradiation by tightly focused femtosecond pulses are observed by Raman scattering. It is shown that densification caused by the void formation in GeO2 glass is very similar to the changes under hydrostatic pressure. In contrast, the experimental observations in SiO2 glass could be explained by pressure effect or by the fictive temperature anomaly, i. e., a resultant smaller specific volume of the glass (a denser phase) at a high thermal quenching rate. Density changes of GeO2 and SiO2 glasses are opposite upon close-to-equilibrium heating; this gives new insights into the mechanisms of densification under highly non-equilibrium conditions: fs-laser induced micro-explosions, heating and void formation. The pressure and temperature effects of glass modification by ultra-short laser pulses are discussed considering applications in optical memory, waveguiding, and formation of micro-optical elements.
©2011 Optical Society of America
1. Introduction
Structural modifications of glasses and crystals by direct laser writing with ultra-short laser pulses has a wide range of applications in waveguiding, lasing, and micro-/opto-fluidics [1–10]. Densification of glasses and, consequently, an increase of the refractive index is useful for waveguiding and fabrication of micro-optical elements. However, high intensity laser pulses are causing similar structural damage of dielectric matrices as γ-rays or particle beams (electrons and neutrons) [11] since light-field driven electrons can reach high energies of up to ∼ 0.5 eV at breakdown conditions. Structural point defects like atomic voids, interstitials and the density redistribution of constituent atoms/ions due to pressure and temperature effects need a better understanding for micro-optical applications and fabrication of photonic devices. This research could also provide answers to unknown mechanisms of the formation of natural glasses, called tectites, which are believed to be created by meteorite impacts [12, 13].
A pure silica glass has an anomalous fictive temperature, T f, behavior [14], since the largest mass density is observed at elevated formation temperatures (high cooling rates of glass formation). Hence, ultra-fast thermal quenching can help to create densified shells around the voids formed by in-bulk micro-explosions in pure silica glasses and in quartz. The 3D localized thermal annealing realized via a multi-photon seeding of an avalanche ionization, as observed in photo-polymers [15,16], creates an effective 3D localized hot-spot which can be used to directly heat silica glass and cause an increase of its density after fast quenching. In view of recently observed densification of silica below the threshold of shock wave generation at tight focusing and the simultaneous heating [17], formation of a dense phase due to the anomalous fictive temperature effect is highly probable. How an ultra-fast rate of thermal quenching after the micro-explosion affects the glass structure and at which fictive temperature, Tf, [14] the glass is retrieved has to be better understood [18,19]. The Tf of shock amorphised crystals around the void-structures and their density could provide a method to estimate the pressure-temperature (p,T)-conditions of glass formation.
SiO2 and GeO2 are two technologically important materials for the direct laser writing of waveguides using fs-laser pulses. Structural analysis and understanding of the fundamental densification mechanisms of glass [18] in fs-laser structured regions will provide insights for the best compositional formulation of the glass and exposure conditions for the waveguide recording. Pressure induced birefringence around fs-laser structured volumes [20] or the densification of glass can be used to create a set of micro-optical elements and tools for microfluidics.
Here, we demonstrate that the compression of GeO2 glass network around the micro-explosion sites is induced by fs-pulses. This is the first direct observation of densification in GeO2 glass confirmed by corresponding changes of several characteristic Raman lines. In particular, the D2-band of three-ring tetrahedral-(GeO)3 increases in intensity and its position moves to the larger Raman shift wavenumbers. Comparison with hydrostatic pressure induced changes of Raman bands corroborates the presence of augmented pressure around fs-laser irradiated sites. The present analysis explains the densification of silica reported recently [17]. We discuss how formation of waveguides by laser writing could be controlled via a glass composition, which defines the fictive temperature behavior of the material, and/or by thermal conditions: heating and quenching rates. This can open new ways of material processing.
2. Methods
2.1. Sample Preparation and Characterization
Femtosecond laser pulses, 800 nm/150 fs (Hurricane, Spectra Physics), were tightly focused [21, 22] with an objective lens of numerical aperture NA = 1.4 at 5–10 μm depth inside glass samples in order to minimize spherical aberration and axial elongation of the focal region [23]. Only the regions without strong crack formation were investigated. The regions were laser modified at pulse energies larger than those required for the void formation [24, 25]; a single-pulse-per-void irradiation mode was used if not indicated otherwise. Separation of ∼ 5 μm between adjacent irradiation spots was chosen in order to avoid thermal annealing of previously exposed locations (Fig. 1(a)). The effects of close proximity of irradiation spots, thermal annealing, and crystallization we have reported elsewhere [26].
Fig. 1 (a) An optical image of a GeO2 region modified by single pulses of 300 nJ/pulse energy (at the entrance of microscope), 800 nm wavelength and 150 fs pulse duration focused at 10 μm depth. (b) Map of the region boxed in (a) at the 520 cm−1 D2-band. (c) Raman spectra of laser irradiated regions at different pulse energies 200, 300, and 400 nJ and at different hydrostatic pressures; measured using 532 and 633 nm wavelength illumination. Arrows in (c) shows the observed tendencies with increasing pulse energy and/or pressure. Wavelengths of laser irradiation for Raman measurements are denoted.
Raman scattering was measured using Aramis (Nano-Optic Devices) and Renishaw dedicated microscope-based setups at a NA = 0.5 – 0.9 focusing. The wavelength of excitation for Raman scattering measurements was 633 nm or 532 nm whichever caused less of a background luminescence.
Germanium dioxide from Alfa Aesar labeled 99.98% was used as a starting product. The glass sample was prepared by melting the starting powder at 1400°C in a Pt crucible during 36 h and quenched by dripping the bottom of the crucible in water. A perfectly transparent bubble free glass was obtained. Permanently densified samples were obtained at 9.6 GPa and 15.4 GPa in a diamond anvil cell as described in ref. [26]. The laser-structured vitreous silica samples were compared to a permanently densified and quenched samples. The silica glass of ρ = 2.25 g/cm3 density was obtained in a belt press at 3 GPa and 400°C as described in ref. [27]. The quenched sample was heat treated at 1500°C for ∼ 1 h and then quenched in water [28].
2.2. Theory: Fictive Temperature
The fictive temperature, Tf, is an imprint of the thermal history of glass preparation. It is the last temperature at which glass reached equilibrium state with the supercooled liquid before a rapid quench to room conditions. Usually, at slow thermal cooling rate a more dense glass can be obtained, i. e., the specific volume is smaller in Volume ∝ Temperature dependence upon cooling from liquid phase as observed in most of silicate glasses [29].
There is a known exception from this behavior in the case of pure SiO2 glass [30–32] with the highest density observed at Tf = 1500°C [31]. The following dependence
has been confirmed for the Tf range between 1000 and 1500° [14] (in the normal behavior Eq. (1) has a “-” sign.). This anomalous behavior is reversed in silica with 3 mol% of F or Cl [33]. It has been established that GeO2 glass [29] as well as silicates show larger density at smaller Tf, the most common behavior.In silica, as the Tf increases the following changes in Raman scattering can be recognized: (i) the shift of broad band at 440 cm−1, the (Si-O-Si) stretching mode, to a higher Raman shifts, (ii) the area under the specific D1 and D2 bands increases (the D1 and D2 corresponds to the 4 and 3 tetrahedral ring structures, respectively), (iii) the TO component of the fundamental stretching of the Si-O-Si bridges shifts to lower Raman shifts, (iv) the Si-O-Si bond angle decreases, (v) the Rayleigh scattering is increasing [28].
The mass density, ρ, is related to the refraction index, n, via Lorentz-Lorenz equation, which for the molar refractivity, A, reads [34]: , where N is the molecular number density, α is the polarizability, W is the molecular weight. In terms of a refractive index change, Δn, induced by the density change, Δρ, it reads [17]: , where 0-index indicates the initial properties of material. As mass density increases, the refractive index becomes larger. Hence, by controlling Tf it is possible to tune the refractive index. In the case of fs-laser writing, high flexibility in choice of writing geometry or energy deposition helps to form waveguides in very different materials: crystals and glasses. The unique feature of fs-laser structuring is that thermal quenching is very fast and a non-equilibrium state of matter can be quenched and retrieved to room conditions. Materials can be shock-liquidized by fs-pulses and afterwards thermally quenched at a record high rate [35]. We put forward a conjecture that some of the amorphous phases in the fs-laser structured regions could behave as high-Tf glasses. In silica this can explain formation of waveguiding regions at the center of inscribed line (at the center of the focus) and is consistent with recent results [17].
3. Results and Discussion
3.1. Femtosecond Laser Structured GeO2
Figure 1 shows observed changes in Raman spectrum caused by fs-laser pulses and permanent densification retrieved upon compression up to 15 GPa in GeO2 glass. The pressure induced changes [36, 37] have the same trend as those observed from irradiated regions. Namely, the tetrahedral distortion, a decrease of symmetry is signified by a broad angle distribution decrease of the 420 cm−1 band (symmetric stretching), an increase of the 860/970 cm−1 bands (asymmetric stretching), broadening of the 860 cm−1 band, a Raman shift increase of 420 cm−1 band, and a Raman shift decrease of 860/970 cm−1 bands. In addition, D2 band at 520 cm−1, assigned to 3-membered GeO3-rings, increases in intensity and Raman shift proportionally to the stress applied (c). Mapping of the laser-structured location (see (b)) at this D2 band qualitatively confirms presence of the denser regions at the irradiation sites.
The laser power dependence on densification is depicted in Fig. 2. The intensity of D2-band become stronger for the highest pulse energies above 600 nJ. At the highest pulse energy of 700 nJ/pulse (before an onset of strong crack formation) crystallization of GeO2 is observed judging from an increase of the D2 band in between of the two void structures [37]. This might be caused by a heat effect due to comparatively close proximity of irradiation spots. High laser energies or small spot separations can induce crystallization, quartz-like GeO2, and lattice rupture as we reported elsewhere [26]. This modification between the void-structures is of thermal nature and also might be due to the fictive temperature effect. Apart of this long-range phenomenon at the energies higher than 600 nJ, a recognizable increase of the D2 band intensity is observed as well as a shift of the 860 cm−1 band towards the lower wavenumbers for the energies higher than 500 nJ (not shown here). This implies that the structural changes of glass can be controlled by changing the pulse energy. Interestingly, there were no measurable difference of D2 band intensity for the single and double pulse per void-structures at the smallest pulse energy (Fig. 2).
Fig. 2 (a) An optical image of a GeO2 region modified by single pulses of 300 nJ/pulse energy (at the entrance of microscope), 800 nm wavelength and 150 fs pulse duration focused at 10 μm depth. (b) Cross section of the two void structures (marked in (a)) at the 520 cm−1 D2-band vs pulse energy. Arrows in (b) denote main tendency of Raman intensity vs pulse energy.
3.2. Femtosecond laser structured SiO2
In silica modified by tightly focused fs-laser pulses, densification can be caused by micro-explosion triggered shock-compaction or due to ultra-fast thermal quenching and resultant high Tf. Figure 3 shows changes of Raman spectra from fs-laser structured regions. The used pulse energy was larger than that required for the void formation. The observed modifications, the Raman shift increase of the band at 440 cm−1 and the increase of D2 band intensity, are consistent with both the high temperature [38] and the high pressure effects [39]. No changes in the D1 band seems to be more consistent with a moderate densification of the sample, ρ = 2.25 g/cm3, however, it is difficult to discriminate densification from a pure thermal quenching effect. Evolution of volume with thermal quenching rate is schematically represented in Fig. 4 for the normal (a) and anomalous (b) glass transitions. In agreement with the anomalous behavior of silica, the denser silica glass ρ 2 > ρ 1 is formed at fast quenching rate and larger fictive temperature Tf 2 > Tf 1 (see, the ABC path in Fig. 4). The formation of more dense silica regions around voids will therefore benefit from both effects: high Tf as well as high pressure induced densification.
Fig. 3 Raman spectra from different SiO2 glasses (Eqs. (1,2)): fs-laser treated (density ρ = 2.201 g/cm3, fictive temperature Tf = 1175°C), hydrostatic pressure-densified ρ = 2.25 g/cm3, thermally quenched (ρ = 2.202 g/cm3, Tf = 1350°C) and initial silica (ρ = 2.198 g/cm3, Tf = 925°C). Pulse energy was 200 nJ, wavelength 800 nm, pulse duration 150 fs, lateral separation between irradiation sites was 2 μm and intra layer separation 2.5 μm (ten layer structure). The inset shows an optical image of a 80 × 80 μm2 area packed with void-structures.
Fig. 4 Typical normal (a) and anomalous (b) glass transition Volume ∝ Temperature dependence observed in most of glasses (a) and silica (b), respectively (adopted from ref. [13, 31]). For silica: higher Tf corresponds to the larger density glass (anomalous behavior); The crystal melting is a first order phase transition and marked by the dotted-line; Tm is the melting temperature. Note, volume is increasing upwards (a) and downwards (b); a darker shade background corresponds to the higher density, ρ. The cycle ABC schematically shows how laser-induced melting and fast quenching results in waveguiding structure (higher mass density glass) in material with anomalous fictive temperature behavior.
A direct correlation between the maximum of the band at 440 cm−1 assigned to the (Si-O-Si) stretching mode, and the Tf was proposed [38]:
By assuming in first approximation that thermal effect is predominant in the laser irradiated region, it is therefore possible to estimate the Tf of the sample (Fig. 3) as follow: the Tf of the initial silica was 925 ± 50°C, the laser treated regions 1175 ± 50°C, and thermally quenched silica 1350 ± 50°C using Eq. (2) and relate it to the density changes according to Eq. (1).The change of fictive temperature can also be related to the induced change of refractive index since dn/dTf was estimated to be 1.6 × 10−6K−1 [31]. Applied here, the detected index variation associated with the laser treatment is 4 × 10−4 (Δn ≃ +0.03 %) in silica glass for single-pulse irradiation. It is important to recall that the volume occupied by void-structures is smaller than that measured by Raman scattering and that all the obtained results are averaged between the initial glass and the void-structure. Therefore the laser treatment has increased the refractive index in silica glass by at least 0.03 %. Locally some regions with higher refractive index and the mass density are present. The density change can explain compaction of the shell material around the void. Possibility to create glasses at high thermal quenching rate is here demonstrated.
It would be informative to measure Raman scattering from the inner part of the void-structures using a higher spatial resolution and sensitivity of detection. New insights into the mechanisms of glass transition could be obtained. A possibility to tune the refractive index change should exist using Ge-doped SiO2 glass. Indeed Ge addition, an heavier atom, significantly increases the refractive index. As demonstrated earlier [40], the fictive temperature has the same effect in these glasses as in silica. This can be utilized to create waveguides in Ge-doped SiO2 glasses via the fictive temperature anomaly when the Ge-to-Si ratio is suitable. By coupling the pressure and fictive temperature effects induced by laser treatment new methods to control the final density and refractive index can be obtained. Presented here analysis of Raman scattering and determination of fictive temperature, Tf, in pure silica and germania glasses is consistent with the first observation of refractive index increase in fs-laser inscribed waveguides [41] and more recent thermal effects of fs- and ps-laser writing in glasses [17, 42–44].
The two effects useful for waveguide formation: the anomalous fictive temperature and compression are very difficult to distinguish and can probably coexist on a scale of ∼ 100 nm below the used Raman spatial resolution. The main difference between the two process is a change of the width of the main-band which decreases significantly under densification and do not change with the fictive temperature (see, Fig. 3). As the pressure caused change is a normal behavior of glass, the anomalous behavior could be related to the anomalous fictive temperature effect. Further studies are required to advance an understanding of this potentially useful behavior in glasses.
4. Conclusions
Observation of GeO2 compression by fs-laser triggered micro-explosions is confirmed when separation between laser irradiated regions is large (> 5 μm). Structural changes are proportional to the pulse energy above 500 nJ as observed by intensity increase of the D2 and a shift of the 860 cm−1 band towards lower wavenumbers. Modifications of SiO2 glass induced by fs-laser irradiation at tight focusing are consistent with formation of the high Tf-regions. Those regions are prospective for waveguiding since silica is an anomalous-Tf medium and a higher temperature causes formation of the higher density glass.
Further studies of Ge-doped silica are required to quantify the contribution of pressure densification and transition from anomalous to normal Tf behavior. This would allow to find an optimal Ge-doping concentration for fs-recording of waveguides. The outlined mechanism of creating required density of a glass structure (a waveguide or an optical element) via controlled thermal treatment using direct laser writing could open new avenues in material engineering for waveguides, sensors, and formation of 3D optical elements. The proposed mechanism is consistent with already reported results [17, 41]. The very same principles of 3D direct writing by hot-spot polymerization [16, 45, 46] can be extended to structuring of glasses, glass-ceramics, and crystals; e.g., a sol-gel transition leading to densification and glass formation can be laser controlled by a laser hot-spot scanning. The controlled fast thermal quenching can be used to recover functional optical structures and devices from glasses whose composition and fictive temperature behavior are pre-engineered and optimized for fs-laser structuring - a novel approach in engineering of optical materials.
Acknowledgments
This work was supported by CNRS, CECOMO and LLP - Leonardo da Vinci. SJ is thankful for the visiting professorship at Lyon-I University and for a laser access at Photon Process lab of Hokkaido University for preparation of some of the samples. Authors are thankful to Professors Micheline Boudeulle for discussions of structural properties of silicas, to Bernard Champagnon and Sylvie Le Floch for the densified silica glass sample.
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