Form birefringence and negative index change created by femtosecond direct writing in transparent materials

TLDR

Femto‑laser micromachining of bulk transparent materials is widely used, yet the fundamental physics driving the process remain poorly understood. The study proposes a model that explains the three principal characteristics observed in femtosecond laser‑written features. By varying laser intensity, the authors produce three distinct feature types in glass and present a model accounting for these characteristics. In the intermediate regime, they observed a correlation between negative effective index change, anisotropic reflection, and birefringence, with local index changes reaching as high as 0.1.

Abstract

Although femtosecond lasers have proved to be of great utility for micromachining within bulk transparent materials, little is known about the fundamental physics that drive the process. Depending on the laser intensity delivered to the sample, any of three types of feature can be written into the glass. We observed that in the intermediate regime there is a correlation among the negative sign of the effective index change, the presence of anisotropic reflection, and birefringence. We propose a model that can explain all three principal characteristics. Results show that the local index change can be as high as 10(-1).