Cooling rate calibration and mapping of ultra-short pulsed laser modifications in fused silica by Raman and Brillouin spectroscopy

Abstract

This paper focuses on the preparation of a new extended set of calibrations of cooling rate (fictive temperature) in fused silica determined by inelastic light scattering and its subsequent use to characterize the local cooling rate distribution in ultra-short pulsed (USP) laser modification. In order to determine the thermal history (e.g. cooling rate and fictive temperature) of fused silica, high-resolution inelastic light-scattering experiments (Raman and Brillouin spectroscopy) were investigated. Calibrations were performed and compared to the existing literature to quantify structural changes due to a change of fictive temperature. Compared to existing calibrations, this paper provides an extension to lower and higher cooling rates. Using this new set of calibrations, we characterized a USP laser modification in fused silica and calculated the local fictive temperature distribution. An equation relating the fictive temperature (Tf ) to cooling rates is given. A maximum cooling rate of 3000 K min−1 in the glass transition region around 1200 °C was deduced from the Raman analysis. The Brillouin observations are sensitive to both the thermal history and the residual stress. By comparing the Raman and Brillouin observations, we extracted the local residual stress distribution with high spatial resolution. For the first time, combined Raman and Brillouin inelastic light scattering experiments show the local distribution of cooling rates and residual stresses (detailed behavior of the glass structure) in the interior and the surrounding of an USP laser modified zone.

High‐sensitivity Raman imaging of the surface of casted glass plates

“…This result is in agreement with 29 Si NMR and Raman investigations of the influence of fictive temperature in different silicate systems [21,31‐33] . Additionally, σ values were also reported to increase with fictive temperature [28] . From these observations, we infer that our thermal treatment properly relieved thermal stresses but was not sufficiently long to achieve full reorganization of the local structure.…”

“…We did not attempt to correlate our Raman parameters with precise values of fictive temperature as this procedure would require a calibration curve and was thus beyond the scope of this work. It should be noted however that the range of σ values (∼10 cm −1 ) obtained from our analysis is quite large compared to what was obtained in T f mapping across ultra‐short pulse (USP) laser modified samples [28] . Moreover, it is unlikely that our process achieved comparable cooling rates and thus large T f fluctuations.…”

“…[21,[31][32][33] Additionally, values were also reported to increase with fictive temperature. [28] From these observations, we infer that our thermal treatment properly relieved thermal stresses but was not sufficiently long to achieve full reorganization of the local structure. These local fluctuations are reflected in the Raman spectra, and Raman images thus reveal the fingerprint of T f variations at the surface of the glass piece.…”

“…It was used as an indicator of the densification ratio of vitreous silica at room temperature, [27] and more recently for local mapping of fictive temperature variations in fused silica. [28] In Figure 7, the representation of this parameter indicates that increases by ∼10 cm −1 across the surface of the sample. The map exhibits the same well-resolved pattern as the intensity at 950 cm −1 , which emphasizes the strong correlation between the increase of both parameters and the casting process.…”

“…We probed the position of the low‐wavenumber band by following the center of mass of the 200–730 cm −1 envelope, introducing Raman parameter σ described by Deschamps et al [27] This parameter accounts for variations of network connectivity occurring upon changes in the medium‐range order. It was used as an indicator of the densification ratio of vitreous silica at room temperature, [27] and more recently for local mapping of fictive temperature variations in fused silica [28] . In Figure 7, the representation of this parameter indicates that σ increases by ∼10 cm −1 across the surface of the sample.…”