X-ray preconditioning for enhancing refractive index contrast in femtosecond laser photoinscription of embedded waveguides in pure silica

We investigate femtosecond pulse laser-induced refractive index changes (∆n) in bulk fused silica (Corning 7980) subject to prior hard (40 keV) X-ray irradiation with accumulated doses up to 6 MGy. The preconditioning before photoinscription improves strongly the light transport of the laser-written embedded waveguides in a range of fluences and accumulation doses. Results show that the ultrafast laser-induced ∆n is higher in samples pre-irradiated with X-rays at similar laser photon dose. The effect lies in the kick-off generation of defects and precursors by energetic X-ray photons, enhancing a subsequent laser-induced densification process. The photoinscription efficiency increases up to a saturation point and the pre-treatment can be considered as an effective option for high contrast low loss index modifications and waveguiding structures in glasses.