Investigation of the modifications properties in fused silica by the deep-focused femtosecond pulses

In this study, we demonstrate the elongated Type I modifications in fused silica with an axial length > 50 mu m. Such extended longitudinal dimensions were obtained by deep focusing radiation of a femtosecond laser inside fused silica at a depth of 2 mm. The transition from the Type II modification (nanogratings) to the Type I modification (refraction index change) was observed with increasing focusing depth at the constant pulse energy. The refractive index changes of similar to 1.5x10(-3) for a single pass and 2.4x10(-3) for multiple passes were demonstrated. The radial dimensions of the deep-focused modifications were confined to 0.5-1.5 mu m size. By overlapping the modifications in radial and axial directions, 1D phase grating in the depth range from 2 to 5 mm was recorded, allowing to split of the beam with a diffraction efficiency of > 96%. We demonstrate that the aberration-based recording with a Gaussian beam in fused silica is a simple tool for fabricating complex phase diffractive optical elements. (c) 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Automated summary

In this study, elongated Type I modifications in fused silica were achieved through deep focusing radiation of a femtosecond laser, resulting in axial lengths greater than 50 μm. The transition from Type II modification to Type I modification was observed as focusing depth increased. By overlapping modifications in the radial and axial directions, a 1D phase grating in the depth range of 2-5 mm was created, enabling efficient beam splitting.