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The structural and mechanical characteristics of ultrafast lasers inscribed Er2O3-doped GeGaS chalcogenide glass waveguides have been investigated with the aid of micro-Raman spectroscopy and nanoindentation. The mechanical property data indicates two distinct regions within the waveguides, with the top of the waveguide having considerably lower elastic modulus and hardness. Raman spectroscopy also confirms the existence of such distinct regions. These results are interpreted in terms of structural modifications made by the laser inscriptions to the glassy network.
© 2016 Optical Society of America
1. Introduction
Chalcogenide glasses are potential candidates for all-optical devices and mid infrared fiber lasers, because of their high third order optical nonlinearity χ(3) [1,2], good transmittance in the infrared region [3], and low phonon energy [4]. Glassy sulphides, selenides and tellurides have good transmittance up to the wavelength ~11, ~15, and ~20 µm respectively in the IR region. Their low phonon energy makes them good host material for rare earth ions such as Er and Yb, and rare earth doped crystalline and glassy chalcogenides have wide application as amplifiers [5,6] and lasers [7, 8]. GeGaS glasses are of particular research interest due to their enhanced rare-earth ions solubility [9] and their ability for supercontinuum generation [10].
Direct laser writing using ultrafast lasers is a proven technique for making optical waveguides and lasers in transparent materials like glass and ceramics. This reduces heat diffusion from the processed area to its surrounding regions, thereby enabling high quality micro/nano fabrication [11–13]. Another important aspect of direct laser writing is that it can induce a strong absorption even in transparent materials due to nonlinear multiphoton absorption [14]. Upon focusing the ultrafast lasers inside transparent materials, the nonlinear absorption can modify the material within the focal volume of the laser. This allows fabricating integrated optical elements such as waveguides on the surface or deep inside transparent materials, without the need for complex lithography and clean room facilities, making this technique cost effective.
In the present work, the spatial variation in the mechanical behaviour of the optical straight waveguides inscribed using femto-second laser pulses in Er2O3 doped GeGaS glass, has been investigated by using nanoindentation technique. Results show that the waveguides have two distinct regions, which are due to photo induced modifications. Such modifications are generally associated with changes in density and refractive index [15], mass transfer [16], the fluidity of the glasses [17], among other things. Micro-Raman studies have been carried out in the present samples to understand the structural modifications due to laser irradiation, which results in the mechanical property variations.
2. Experiments
2.1 Glass synthesis
Bulk glasses were prepared by vacuum-sealed melt quenching method. Appropriate quantities of high purity (99.999%) constituent elements were sealed in an evacuated quartz ampoule at 10−5 Torr and slowly heated at a rate 100 °C.h−1 in a rocking furnace. The ampoules were maintained above the melting temperature of the constituents for 12 h at 10 rpm to ensure the homogeneity of the melt. They were subsequently quenched in air to get bulk glassy samples. The amorphous nature of the as-quenched samples was confirmed by X-ray diffraction technique. Bulk Glass samples were subsequently cut and polished to optical quality.
2.2 Laser inscription of the waveguides
The optical straight waveguides were written in a 6 mm long Er2O3 doped GeGaS glass sample using a master oscillator power amplified Yb-doped fiber laser (IMRA µJewel D400). A central wavelength of 1047 nm was used for waveguide inscription at a repetition rate of 100 kHz. The pulse duration was 350 fs at the full-width half maximum and the polarization was set to circular. Using a 0.67 NA aspheric lens the laser pulse was focused to a spot of 1 µm and 100 µm below the sample surface. Computer controlled Aerotech x-y-z air bearing stages were used to translate the sample about the laser spot. Transverse geometry was used to write the waveguides where the sample was translated perpendicular to the laser propagation direction. Each waveguide was separated by 50 µm to avoid the overlapping and to reduce the stress influence on the subsequent waveguide. Waveguides were written in batches with pulse energies varying from 1.81 µJ down to 100 nJ, decreasing by 15% for each subsequent batch. Four translations speeds 4, 6, 10, and 18 mm·s−1 were used to inscribe each batch of waveguides. After the inscription, the waveguide facets were ground and polished to remove any tapering close to the facet. The photo induced structure was imaged with an optical microscope and the mode field image of the waveguide was recorded using a 1550 nm light source butt-coupled to the waveguide from one end and an Electrophysics-7290A IR Vidicon camera, connected to a frame grabber card, at the other end. The images were gamma corrected and normalized for the nonlinear response of the Vidicon camera. The corresponding images are shown in Fig. 1(a) and Fig. 1(b) respectively.
Fig. 1 (a) An optical micrograph of the single mode waveguide structure inscribed at 1 µJ and translation speed of 4 mm/s under white light illumination and (b) its near field image at 1550 nm wavelength.
2.3 Nanoindentation studies
A Triboindenter (Hysitron, Minneapolis, USA) with in situ imaging capability was used to record the load, P, and the corresponding depth of penetration, h, with resolutions of ~1 nN and ~0.2 nm respectively, was utilized for conducting the nanoindentation experiments. A Berkovich diamond tip indenter with a tip radius of 100 nm was used. The loading and the unloading rates were equal at 0.8 mN/s with a peak load of 8 mN, and the hold time at the peak load was set to 10 s. The indentations were performed on different parts of the optically modified, inverted tear drop shaped waveguides as well as on the bulk of the sample for reference values. From the measured P-h responses, elastic modulus, E, and hardness, H, were extracted by employing the Oliver-Pharr method [18].
2.4 Raman spectroscopy
Micro-Raman spectroscopic measurements were made on the waveguides using a Horiba JobinYvon (LabRAM HR) Raman Spectrometer in the back scattering mode. The scattered light was detected with the aid of a triple monochromator and a CCD cooled to −70°C. The sample was illuminated by the 532 nm line of an argon ion laser focused using 50x objective. The spectral resolution for the recorded Stokes-side Raman was ~0.6 cm−1. All the spectra were recorded using 2 mW of laser power (by using neutral density filter D1) in order to avoid undesired irradiation-induced heating. Spot size of the laser used in the Raman setup was ~2 µm. Data acquisition time of 5 s was employed to record the spectrum.
3. Results and discussion
The inverted tear dropped structure observed in Fig. 1 is due to the transverse geometry used for inscribing the waveguides. Though the longitudinal geometry has the advantage of better cross-sectional symmetry, the waveguide length is limited by the working distance of the objective used for focusing the writing laser. This can be overcome by using transverse geometry in which the waveguide symmetry is compromised due to the fact that the focus diameter of the laser beam (2ω0, where ω0 is the radius of the focus diameter) is much smaller than the confocal parameter (b = 2πω02/λ) [19]. It is observed that the diameter of the waveguide structure obtained is directly related to the net fluence used for inscribing the waveguides.
In nanoindentation studies, two distinct regions, region A and region O, are observed in the waveguide structure with different values of E and H. Region A corresponds to the whole waveguide structure and region O corresponds to the top of the waveguide. The load, P, vs. displacement, h, obtained for region A, region O and bulk glass are shown in Fig. 2(a). A smooth P-h behaviour is observed in all these regions with significant levels of residual depths upon complete unloading. The images of the indentation impressions show neither corner cracking nor pile-up of the material against the indenter. The measured values of E and H at different regions in the waveguide (region A and region O) are shown in Fig. 2(b). It is observed that both E and H are similar at different places of the waveguide and the bulk glass, except at region O where the values of E and H are found to be lesser than that of the bulk values. The average values of E and H in region A are 21.84 ± 0.13 and 2.28 ± 0.05 GPa respectively, which are similar to those of the bulk of the sample. In contrast, E and H of region O are 19.3 and 1.87 GPa respectively. These are 12% and 18% lower than those measured in region A or in the bulk, and are significantly larger than the scatter associated with the nanoindentation results.
Fig. 2 (a) Representative load, P, vs. displacement, h, curves for region A, region O and bulk glass (b) Value of E and H at different points inside the waveguide (region O and region A) chosen randomly.
Figure 3 shows the micro-Raman spectra taken at region O, region A and bulk glass. While the spectra obtained in the region A and bulk are identical, the spectrum obtained from the region O is significantly different, indicating to photo induced local structural changes during the waveguide writing. These differences in the spectra are mainly due to the difference in the thermal process happening at these regions during the laser inscription on the glass material [20]. At the focus, the concentrated laser energy impinges on a small area for a very short time, and hence the substrate melts locally and re-solidifies in a short time as the laser focus is shifted to the subsequent point for creating the waveguide. This resembles melt quenching which is used in the manufacture of the bulk glass itself, and hence the bulk glass and the region A tend to have similar Raman spectra. At region O, probability of multi photon absorption decreases as the beam size increases in comparison with the focal zone. The larger beam size and the overlapping of the beam cause thermal energy stay for much longer duration at this region in contrast to the laser focus. Hence the thermal process happening at region O resembles annealing more than quenching. Thus, the structural re-organisation associated with annealing results in an area with different Raman spectra.
To gain further insights, the Raman spectra obtained at region O and region A are deconvoluted and are shown in Fig. 4(a) and Fig. 4(b) respectively. The spectrum in the region A shows peaks at 341.3, 456.6, 590.3 and 743.1 cm−1. The peak at ~341.3 cm−1 can be assigned to symmetrical stretching vibration of GeS4 and GaS4; since the atomic masses of Ge and Ga and the vibrational frequencies of GeS4 and GaS4 tetrahedra are nearly same, their spectra are indistinguishable. The peak at ~455 cm−1 is due to the vibrations of GeS4 and GaS4 tetrahedra interconnected by sulphur bridging atoms [21–23]. The bands at 590 and 743 cm−1 are due to the stretching vibration involving oxygen and dopant atom, similar to that seen in the Raman spectra of Er2O3 doped Te glasses [24]. In contrast, the spectrum at region O yields peaks at 340.3, 479.1, 612.2 and 710 cm−1. The shift in the Raman mode to 479.1 cm−1 (from 455 cm−1) is possibly due to vibrations in S8 rings and sulphur chains, as there are more sulphur units due to breaking of bridging sulphur between the GeS4/GaS4 tetrahedra [21]. The basic structural units of GeGaS glasses are GeS4 and GaS4 tetrahedra, which are connected through bridging sulphur to form a three dimensional structure [21]. In case of rare-earth doped GeGaS glasses, the doped ions cause the formation of non-bridging sulphurs and play a charge compensating role. When GeGaS is doped with Er2O3, SO3 trigonal pyramids, associated with non-bridging oxygen atoms, are formed [24]. These, in turn, cause a large shift to the Raman modes towards higher frequencies. The intensity of band at 340 cm−1 decrease drastically compared to GeS and GeGaS samples, due to the breaking of GeS/GaS bonds, releasing sulphur to form SO3 triagonal pyramids.
Fig. 4 (a) Deconvoluted Raman spectra at the region A of the glass (b) at the region O (c) DSC thermogram of as-quenched Er2O3 doped GeGaS glass and (d) deconvoluted Raman spectra of the bulk glass annealed at 500 °C.
Differential Scanning Calorimetric (DSC) studies have been performed on the sample using Mettler Toledo DSC1. The crystallization temperature of the sample is found to be well beyond 500 °C as depicted in Fig. 4(c), the DSC thermogram of as-quenched Er2O3 doped GeGaS glass. To understand the structural changes during annealing, Raman spectra have been taken on the sample annealed at 500 °C. It is observed that when sample is annealed at 500 °C, the Raman peak at 590 cm−1 is red-shifted to 602 cm−1 as observed in Fig. 4(d). For the spectra taken at region O of the waveguide, the 590 cm−1 peak is further red-shifted to 612 cm−1 (Fig. 4(b)). It is evident that during the inscription, the laser induced heat at region O is much more than 500 °C, closer to its crystallization temperature and the Fig. 4(d) is intermediate between Fig. 4(a) and Fig. 4(b). Intensity as well as the area of the peak at ~590 cm−1 decreases as the annealing temperature increases.
The Raman spectra obtained at the region O for waveguides inscribed at different translational speeds and different pulse energies are shown in Fig. 5(a) and Fig. 5(b) respectively. As the translational speed increases, an increase in intensity is observed at 340 cm−1 band. This is due to the breaking of GeS/GaS bonding in GeS4/GaS4 tetrahedra due to annealing at the region O. With an increase in the translational speed, the photo induced structural changes due to thermal annealing decrease, as the time of annealing would be less, leading to an increase in intensity of 340 cm−1 band as observed in Fig. 5(a). Simiarly, with increase in pulse energy more GeS/GaS bonds are broken, leading to an increase in 340 cm−1 band intensity.
Fig. 5 Raman spectra of region O for waveguides inscribed with (a) different translational speeds at 0.21 µJ pulse energy. Inset graph shows the change in intensity of 340 cm−1 peak for different translation speeds (b) different pulse energies at 6 mm/s translational speed. Inset graph shows the change in intensity of 340 cm−1 peak for different pulse energies.
4. Summary
In summary, waveguides have been fabricated on Er2O3 doped GeGaS glass with different laser energies and translation speeds. The mechanical properties are studied at different places of the waveguides using the nanoindentation technique. It is found that in the photo modified region, the mechanical properties such as E and H are position dependent. The micro-Raman analysis has also been undertaken on the photo modified region. The nanoindentation results are correlated with variations in the Raman response of exposed glass which are found to be in good agreement with each other.
Acknowledgments
The authors gratefully acknowledge Prof. Ajoy K Kar and Dr. Airán Ródenas, Heriot-Watt University, Edinburgh, for their useful discussions and technical help.
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