[INVITED] Tailoring the morphology of photowritten buried waveguides by helical trajectory in As2S3 glass

This paper deals with the control of the refractive index variation (Δn) profile induced by femtosecond laser irradiation in the bulk of As2S3 glass. The writing technique consists of an original laser inscription based on a helical translation of the sample parallel to the laser beam. Instead of inscribing the core of the waveguide as usually performed, the laser beam is used to induce a negative refractive index variation and consequently to write the cladding of the waveguide. However, it should be noticed that the matter displacement resulting from local heating leads to a densification at the center of the helix which induces a positive Δn relatively to the matrix. Therefore, the structure of Δn is complex, being composed of a positive core surrounded by a negative cladding. The influence of different parameters on the Δn profiles such as the sample translation velocity, the pitch and the radius of the helical displacement, and the pulse energy are analysed. This study demonstrates that both Δn and its diameter can be varied in a wide range of values and picked independently, allowing the design of single or multimode buried infrared waveguide.