Experimental study of the effects of femtosecond laser power on the induced-plasma geometry, supercontinuum white light generation and conical emission in air

Abstract

This paper presents the experimental investigation of the changes in the geometrical shape of femtosecond laser-induced plasma in air under different laser power, and its effects on supercontinuum white light generation and conical emission. When a femtosecond laser is focused into a tiny spot in air, optical breakdown of air molecules occurs and this leads to the generation of plasma filament whose geometrical size and shape depend on laser power. This process is then followed by two light-emitting processes, namely supercontinuum white light generation and conical emission, both of which scatter light that reveals the characteristics of the plasma filament. Our experiment shows that the laser-induced plasma becomes thinner and longer at high average laser power but appears thick and round at lower laser power. At higher laser power, conical emission which scatters laser light in the forward direction dominates the scattering process while at lower laser power, it is the scattering of supercontinuum white light in all directions that plays a bigger role. The intricate rainbow-like pattern formed on a white screen located far away in the forward direction reveals sophisticated nonlinear optical processes that take place in conical emission which slowly diminishes as the laser power is gradually reduced.