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Initial process of the refractive index change induced by tightly focused femtosecond laser inside a transparent material

When a femtosecond laser pulse is moderately focused inside a glass using an objective lens, the density of the irradiated region increases. Although this phenomena has become one of the key technique to fabricate various three dimensional devices, the mechanism has not been well understood. In this study, the initial step of refractive index change of the irradiated region was investigated using the time-resolved transient lens method with a time resolution of sub-picosecond. The oscillating signal intensity with several GHz was observed, and it was interpreted in terms of the density change due to the thermoelastic relaxation after the photoexcitation. The calculation of the thermoelastic relaxation of the ultrafast heated material surrounded by the solid material indicates that the density at the center of the heated region is reduced, and the acoustic wave is generated and propagated. The oscillation in the transient lens signal can be almost perfectly simulated using the density change from the thermoelastic calculation. The temporal profile of the transient lens signal will be a good method to monitor the time dependence of the refractive index change at the laser light focal region.