Measurement of the fs laser-induced structural modification process inside transparent materials

Femtosecond laser pulses can cause structural modification when focused inside transparent material, which can be used in the formation of optical waveguides, 3D micromachining and other photonic machining applications. However, the structural modification process of the material is still unclear, which often occurs in the picosecond scale.
Here, we proposed a pump-probe based method to detect the laser-induced structural modification process inside transparent material. A fs laser pulse was acted as the pump pulse and focused inside fused silica, while a linear chirped pulse was adopted to probe the pump area. According to the mapping relationship between time and frequency of the chirped pulse, we have obtained the time dependent refractive index change process and the structural modification process in a single shot measurement. Results show that the volume and refractive index of the modified area gradually increase with time. Besides, we also observed the static structure after the experiment, and the microscopic image shows a donut-like structure was formed at the shock region, a void surrounded by a compressed shell, which is consistent with the micro-explosion theory.

[1] Gamaly, E. G. (2011). Physics Reports, 508(4-5), 91-243.