Ultrafast laser-induced refractive index changesin Ge15As15S70 chalcogenide glass
Résumé
Understanding and controlling laser-induced refractive indexmodifications in bulk chalcogenide glasses is important for a range ofphotonics applications targeting the mid-infrared spectral region. We focushere on material engineering aspects and pulse spatio-temporal designcharacteristics that are able to induce and maintain positive refractive indexchanges in laser-irradiated Sulfur-based chalcogenide glass, mandatory for3D photonic design. Specifically we study the photoinscription process of aGe-doped Sulfur-based chalcogenide glass, Ge15As15S70, irradiated byfocused ultrafast near-infrared laser pulses where Ge doping plays adeterminant role in generating high-contrast positive index changes. Bymeans of aposteriori and real-time in situ observations we show thatpositive refractive index changes (type I) are the result of a restructuring ofthe glass matrix and a photo-induced contraction process initiated by twophotonelectronic excitation leading to bond softening, molecular mobility,structural changes and rearrangements. Oppositely, negative refractiveindex changes (type II) could be associated with two different processes:photo-expansion at higher intensities and hydrodynamic evolution initiatedby plasma generation and laser heating, with thermomechanical relaxationand stress unload. Alongside the role of Ge in setting various degrees of thematrix connectivity, the structural arrangement developed under differentthermal history schemes for glass preparation is equally important as itdefines to which extent further structural flexibility is possible. Thus weindicate the role of glass matrix metastability in generated high-contrastrefractive index changes and we show that a higher degree of relaxation isan impediment for contrasted positive index changes, while these aredeveloping in unrelaxed glasses, where several degrees of structuralflexibility exist. Alongside dynamic time-resolved imaging experimentsprobing the development and relaxation of excitation, we also show, viastatic Raman analysis of the modified regions, that significant structuralchanges are induced by laser irradiation and we discuss the potentialprocesses involved.