Real-time observation of photothermal effect after photo-irradiation of femtosecond laser pulse inside a glass

Abstract
The initial processes of the refractive index change in a focal region of a femtosecond laser pulse inside a soda-lime glass is investigated by the transient lens (TrL) method with a time resolution of subpicosecond. The TrL signal showed a damped oscillation with about an 800 ps period. This oscillation was explained in terms of the thermoelastic response of a heated material by a short pulsed laser. The essential feature of the oscillation can be reproduced by the pressure wave creation and propagation to the outward direction from the irradiated region. The temporal evolution of the refractive index distribution inside a glass is retrieved from the probe beam deformation (TrL image) at various delay times between the pump and probe pulses. Two phases of the refractive index increase at the laser focal region were observed in a range of 1-50 ps and 500 - 700 ps. This is the first real time observation of the initial processes of the refractive index change in a focal region of a femtosecond laser pulse.