Influence of bandgap and polarization on photo-ionization: Guidelines for ultrafast laser inscription [Invited]

An ultrafast laser was used to fabricate waveguides in Yb:QX phosphate glass and BK7 borosilicate glass using linearly polarized and circularly polarized beams. Circularly polarized pulses were found to induce a higher refractive index change in Yb:QX phosphate glass, while in BK7 borosilicate glass circularly and linearly polarized pulses were found to induce the same refractive index change. An explanation for these contrasting results is proposed based on the fundamental polarization-dependence of photo-ionization. This explanation reconciles observations made in this study and also in a previous study in fused silica glass.