Refractive index change measurement by quantitative microscopy phase imaging for femtosecond laser written structures

Quantitative phase imaging has been widely exploited and developed during the last decade. This technique is efficient, allowing for quantitative and qualitative spatially resolved measurements of the refractive index profiles. However, there are some measurement limitations for thick samples when exceeding the depth of field-of-view of the imaging system. In this paper, we present an approach based on the complete Rayleigh-Sommerfeld diffraction theory, which allows quantitative retrieve of the refractive index change from the measured optical path difference, especially in the case of thick samples. This approach is validated by a comparison with experimental measurements of refractive index changes of distinct series of cylindrically shaped volume modifications inscribed by femtosecond laser irradiation in silica glass. The results presented hereby, enable quantitative measurement of refractive index change for thick embedded 3D structures.

Automated summary

Quantitative phase imaging is an efficient technique for quantitatively and qualitatively measuring refractive index profiles spatially, but has limitations in measuring the depth for thick samples. This paper presents an approach based on the complete Rayleigh-Sommerfeld diffraction theory to quantitatively measure refractive index changes in thick samples, which is validated through experimental measurements on cylindrically shaped volume modifications inscribed in silica glass. The results enable quantitative measurement of refractive index changes in thick embedded 3D structures.