Revisiting ultrafast laser inscribed waveguide formation in commercial alkali-free borosilicate glasses

Abstract

Alkali-free borosilicate glasses are one of the most used dielectric platforms for ultrafast laser inscribed integrated photonics. Femtosecond laser written waveguides in commercial Corning Eagle 2000, Corning Eagle XG and Schott AF32 glasses were analyzed. They were studied in depth to disclose the dynamics of waveguide formation. We believe that the findings presented in this paper will help bridge one of the major and important gaps in understanding the ultrafast light-matter interaction with alkali-free boroaluminosilicate glass. It was found that the waveguides are formed mainly due to structural and elemental reorganization upon laser inscription. Aluminum along with alkaline earth metals were found to be responsible for the densification and silicon being the exchanging element to form a rarefied zone. Strong affinity towards alkaline earth elements to form the densified zone for waveguides written with high feed rate (>200 mm/min) were identified and explained. Finally we propose a plausible solution to form positive refractive index change waveguides in different glasses based on current and previous reports.

Record-high positive refractive index change in bismuth germanate crystals through ultrafast laser enhanced polarizability

“…The BSE image rules out the possibility of refractive index increase through stress-induced localized density variation due to the rapid quench at high feed rates. If that was the case then the core would generate more backscattered electrons and a higher Z-contrast(psuedo) due to incident electron beam interacting with more nuclei (protons) of stress accumulated constituent atoms compared to the unirradiated zones 20 .The compositional maps (insets of Fig. 5 ) acquired using electron probe micro-analysis (EPMA) by utilising wavelength dispersive X-ray spectroscopy (WDS) clearly confirm elemental migration.…”

“…no. contrast) but the positive index in the core could not be explained as it shows as a rarefied zone (darker zones) 20 . The increase of refractive index in the core due to the migration of elements that exhibit a higher electronic polarizabiliy ( ), as for instance observed in the case of chalcogenide glass 25 , can be ruled out since the discrepancy of polarizability between Ge ( ) and Bi( ) is more than one order of magnitude 26 .…”

“…1 c,f formed by identical pulse energy (90 nJ) but at feed rates of 1,000 and 500 mm/min, respectively, exhibit significant differences. Quench times of 30 and 60 ms were calculated 20 from the time it takes to translate the sample by the average structure width of 5 um. Hence a 60 ms re-solidification window is too long to either produce a modification or provides enough time for the melt to re-attain its long range order.This suggests that in BGO the interplay between rapid thermal quenching and heat diffusion is critical for the formation of optical waveguides.…”

Photonic circuits written by femtosecond laser in glass: improved fabrication and recent progress in photonic devices

“…As the optical properties and photoresponse to fs lasers depend on the chemistry of glass, and the fs laser-induced refractive index change would be determined by the structural and elemental reorganization, composition engineering is a simple and promising alternative route to improve the performance of written WGs in glasses, especially in the thermal writing regime with high repetition rate lasers. [95][96][97][98] In the fs laser writing process, the local temperature would be higher than that of the softening and working point of glass. 67,98 As a result, the breaking of bonds linking the network modifiers or the network formers will occur in this high-temperature field, and the ions, including O 2− , would diffuse and modify the local compositions of glass, which usually leads to formation of two typical zones with positive and negative refractive index change, respectively.…”

Resonant opto-mechanical modulators and switches by femtosecond laser micromachining

“…Glass materials typically present wide transparency windows, from the near infrared up to the whole visible region, have low chemical reactivity and excellent bio-compatibility. Although silica waveguides can be fabricated by conventional photolithography on silicon substrates [6], high-quality waveguides can also be directly inscribed in the bulk of several different glass substrates by means of femtosecond laser micromachining [7][8][9]. The latter technology enables the realization of advanced devices, which combine refractive index modifications with complex microstructuring of the material [10,11].…”