Designing laser-induced refractive index changes in “thermal” glasses

Abstract : Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials. In glasses characterized by high thermal expansion and slow relaxation (BK7) the refractive index change is negative, detrimental for waveguide writing. This relates to the formation of hot regions where material is quenched in a low-density phase. This presentation will focus on control mechanisms related to heat-source design which may be tuned by spatio-temporally shaped laser radiation. Programmable tailoring of laser intensity envelopes triggers transitions from thermal expansion to a regime of inelastic flow. Consequently, material compaction leads to a positive refractive index change and guiding structures may be created.