Narrow Results

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.

Conical emission is investigated for Ti:sapphire femtosecond laser pulses propagating in water. The colored rings can be observed in the forward direction due to the constructive and destructive interference of transverse wavevector, which are induced by the spatio-temporal gradient of the free-electron density. With increasing input laser energy, due to filamentation and pulse splitting induced by the plasma created by multiphoton excitation of electrons from the valence band to the conduction band, the on-axis spectrum of the conical emission is widely broadened and strongly modulated with respect to input laser spectrum, and finally remains fairly constant at higher laser energy due to intensity clamping in the filaments.