Probing ultrafast-laser inscribed waveguides in Er:LiYF4 via <i>μ</i> -spectroscopy for refractive-index engineering

Ultrafast-laser inscription enables the fabrication of three-dimensional photonic microstructures in laser-active dielectric crystals, yet the interplay between geometry and laser-induced refractive-index changes remains insufficiently quantified. We investigate depressed-cladding waveguides written in a uniaxial Er3+:LiYF4 fluoride crystal using spatially resolved confocal luminescence and Raman spectroscopy with sub-micrometric resolution. The measurements reveal partial amorphization within the irradiated zones, as well as a complex distribution of local stress fields—compressive within the cladding and tensile at the periphery of the core. These stress patterns, and the associated refractive-index modifications, are found to be highly sensitive to the cladding geometry. From the spectroscopic analysis, we estimate a maximum compressive stress of 1 GPa and a refractive-index change of −2.6 × 10−3 for the e-wave within the damage tracks. Complementary differential interference contrast (Nomarski) microscopy yields quantitative maps of stress-induced birefringence, on the order of 10−6—well below the intrinsic birefringence of LiYF4 and therefore not detrimental to the polarization properties of laser waveguides. Based on these findings, we provide design guidelines for optimizing mid-infrared waveguide structures.