Femtosecond laser writing of photonic devices in borate glasses compositionally designed to be laser writable

The design and performance of borate glass samples compositionally pre-designed to be femtosecond laser writable via laser-induced ion migration is reported in this Letter. It is demonstrated that borate glasses modified on purpose with small amounts of La₂O₃ and Na₂O can be straightforwardly used to produce high-index contrast (Δn) waveguides by femtosecond-laser-assisted ion migration. The positive Δn of the waveguides is caused by the local enrichment of La₂O₃ in the guiding region with a slope of 8·10^-3(mol.%)^-1. The value of this is consistent with numerical aperture measurements of the waveguides and local compositional measurements at the guiding region performed by energy-dispersive x-ray micro-analysis. The maximum achievable Δn values can be controlled through the initial La₂O₃ content of the glass. Maximum Δn values >10^-2 for samples with just 5.5 mol. % of La₂O₃ have been produced. This compositional design approach could be potentially used to produce highly efficient femtosecond laser writeable glasses in other glass families.