Photoinscription domains for ultrafast laser writing of refractive index changes in BK7 borosilicate crown optical glass

Abstract : Ultrashort laser pulses can induce structural modifications in BK7 borosilicate optical glass leading to refractive index variations. These changes and the related optical functions are determined by specific electronic and thermo-mechanical factors, namely network modifiers and high expansion coefficient. We investigate here laser-induced soft positive index changes underpinning photo-physical transformations involving electronic and polarizabillity changes in a narrow processing window (type I), as well as contrasted positive or negative index changes based on thermomechanical mechanisms, non-uniform material expansion, and density redistribution in dense and rarefied regions (type II). These index changes resemble apriori those of pure silica networks despite different thermo-mechanical behavior of BK7 glass upon heating and cooling. The associated structural changes are discussed on the basis of phase contrast microscopy, photoluminescence of defects, and Raman spectroscopy. Material influences are emphasized, pointing out the increasing role of defects as compared to structural reorganization in type I refractive index changes, and mechanical rarefaction and compaction in type II, departing thus from a pure silicate scenario. The resulting optical properties are discussed based on their ability to guide light.