Structural relaxation phenomena in silicate glasses modified by irradiation with femtosecond laser pulses

www.nature.com/scientificreports OPEN received: 03 May 2016 accepted: 31 January 2017 Published: 07 March 2017 Structural relaxation phenomena in silicate glasses modified by irradiation with femtosecond laser pulses Thomas Seuthe1, Alexandre Mermillod-Blondin2, Moritz Grehn3, Jörn Bonse4, Lothar Wondraczek5 & Markus Eberstein1 Structural relaxation phenomena in binary and multicomponent lithium silicate glasses were studied upon irradiation with femtosecond (fs) laser pulses (800 nm central wavelength, 130 fs pulse duration) and subsequent thermal annealing experiments. Depending on the annealing temperature, microRaman spectroscopy analyses evidenced different relaxation behaviours, associated to bridging and non-bridging oxygen structures present in the glass network. The results indicate that the mobility of lithium ions is an important factor during the glass modification with fs-laser pulses. Quantitative phase contrast imaging (spatial light interference microscopy) revealed that these fs-laser induced structural modifications are closely related to local changes in the refractive index of the material. The results establish a promising strategy for tailoring fs-laser sensitivity of glasses through structural mobility. The physical mechanisms governing the laser matter interaction with dielectrics are manifold1. Their sequence begins with nonlinear absorption of the laser radiation, resulting in the ultrafast formation of an electron-hole plasma confined in the solid2,3. Electron-phonon coupling transfers the electronic energy to the lattice, provoking a temperature increase and local melting of the glass4. In the melt, structural disorder appears as the ions gain mobility. The presence of a hot volume reaching temperatures up to several thousands of Kelvin5 surrounded by a cold environment results in huge spatial temperature gradients. As a consequence, cooling at rates as high as 108 Ks−1, the glass constituents ‘freeze’ in their actual configuration, determining the structural state of the fs-laser modified material. Note that these cooling rates exceed by several orders of magnitude what is usually obtained by conventional manufacturing techniques5. The fs-laser induced structural modification translates into a refractive index change which is the cornerstone of a multitude of applications such as optical waveguides6,7, optical couplers8,9, or multiplexers for micro-holographic data storage10. Several approaches have been employed to study fs-laser induced structural and/or chemical modifications. X-ray absorption near edge structure (XANES) spectroscopy has proven to be successful in estimating specific bond length variations in laser-irradiated potassium-magnesium silicate glasses11. Elemental mapping using energy dispersive X-ray analysis (EDX or EPMA) enabled the visualization of spatial ion migration in several glasses12,13. In the optical domain, Raman spectroscopy is known as a powerful tool to analyse structural changes in amorphous solids (such as glasses) with respect to their composition14–19. Moreover, micro-Raman spectroscopy was successfully employed to investigate fs-laser-induced material modifications in different glasses20,21. In this work, we take benefit of the structural sensitivity of micro-Raman spectroscopy along with thermal annealing at various temperatures above (see the Supplementary Information) and below the glass transformation temperature (Tg) to probe the structural relaxation phenomena in two distinct fs-laser irradiated lithium silicate glasses, namely LS (24 mol% Li2O and 76 mol% SiO2) and LNMS (13 mol% Li2O, 15 mol% Na2O, 9 mol% MgO and 63 mol% SiO2). The binary lithium silicate glass (LS) and the multicomponent glass (LNMS) were 1 Fraunhofer IKTS, Fraunhofer Institute for Ceramic Technologies and Systems, Winterbergstraße 28, 01277 Dresden, Germany. 2Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Max-Born-Straße 2a, 12489 Berlin, Germany. 3Technische Universität Berlin, Department of Optics and Atomic Physics, Straße des 17. Juni 135, 10623 Berlin, Germany. 4Bundesanstalt für Materialforschung und –prüfung (BAM), Unter den Eichen 87, 12205 Berlin, Germany. 5Otto-Schott-Institute of Materials Research, Fraunhoferstraße 6, 07743 Jena, Germany. Correspondence and requests for materials should be addressed to M.E. (email: ) Scientific Reports | 7:43815 | DOI: 10.1038/srep43815 1 www.nature.com/scientificreports/ Figure 1. Baseline-corrected and normalized Raman spectra of the glasses (blue solid lines) LS (a) and LNMS (b) before modification with a fs-laser pulse. These reference spectra are deconvoluted into several individual bands with Gaussian shape (black solid lines) for evaluation of changes and the resultant fit (red dashed line). Middle: Spectral changes after irradiation obtained by subtracting the spectra measured before and after irradiation. Bottom: Spectral changes after irradiation and annealing obtained by subtracting the spectra measured before and after irradiation and thermal treatment. The annealing times and temperatures were 240 min at 0.81 ×  Tg (LS) and 60 min at 0.94 ×  Tg (LNMS). selected for this study as they both exhibit characteristic changes in the local Si-O-Si bond angles and in the Si-O* bond strengths upon fs-laser induced material modification22. Those modifications can be directly probed via micro Raman spectroscopy and allow a qualification of the structural state of the material. Different annealing behaviours (associated with α- and β-relaxations processes) were identified above and below Tg. Complementary quantitative phase contrast imaging measurements reveal the correlation between structural arrangement and refractive index. Results Raman spectra of the pristine glasses. Figure 1 (top) shows the baseline-corrected and normalized Raman spectra (blue solid lines) of the non-irradiated glasses LS (a) and LNMS (b) determined experimentally. The black solid lines represent specific Raman bands associated to individual stretching and bending motions of specific sub-structures in silicate glasses. With the exception of two defect types (D1, D2, associated with bands C and E), those sub-structures are called Qn-structural elements, where 0 ≤  n ≤ 4 is the number of bridging oxygen ions per tetrahedral cell23. The red dashed lines are the least-squares-fitted linear combination of all bands used for deconvolution (see Table 1). These reference spectra can be split into two sections. The low-frequency section from 200 cm−1 to about 850 cm−1 is attributed mostly to bending vibrations of various glass sub-structures (bands A to H). The most interesting band within this section is band B, centred at about 450 cm−1, which is associated to Si-O-Si bending vibrations. Specifically, changes of the centre position of band B reflects changes in the Si-O-Si angle14,24. Additionally it has to be mentioned, that band C, associated with 4-membered rings of SiO4 tetrahe (...truncated)