Designing laser-induced refractive index changes in "thermal" glasses
Résumé
Ultrafast lasers emerged as promising tools to process refractive index changes in band-gap materials, resulting in waveguiding functions. Positive refractive index changes were often reported in fused silica matrices. However, in glasses characterized by slow electronic relaxation and high thermal expansion, the refractive index change is usually negative, detrimental for waveguide writing. This relates to the formation of hot regions, where, due to thermal expansion, material is quenched in low-density phases. We discuss control mechanisms related to spatio-temporal heat-source design which may be tuned by temporally shaped laser radiation. Programmable temporal tailoring of pulse envelopes triggers transitions from thermal expansion to directional inelastic flow. Consequently, material compaction leads to a positive refractive index change and guiding structures may thus be created. From an application perspective, the structuring quality degrades with the focusing depth due to wavefront distortions generated at the air-dielectric interface inducing spatial energy dispersion. Spatial beam tailoring corrects beam propagation distortion, improving the structuring accuracy. The corrective process is becoming important when laser energy has to be transported without losses at arbitrary depths, with the purpose of triggering mechanisms of positive index change.