Monochromatic Conical IR Emission from Decaying KrF Laser Filaments in Xenon as Coherent Stimulated Four-Wave Mixing Process

We develop theoretical background for the new nonlinear optical phenomenon of narrowly directed monochromatic IR conical emission which has been recently observed when 248-nm UV filaments propagate in xenon (V. D. Zvorykin, et al., Laser Phys. Lett. 13, 125404 (2016)). We treat it as coherent stimulated four-wave mixing process in which two pump KrF laser photons are converted into the coupled pair of resonance IR(828 nm) and VUV (147 nm) photons through 5p5(2P3/2)6p[1/2]0→5p5(2P3/2)6s[3/2]1o and 5p5(2P3/2)6s[3/2]1o→1S0 transitions. We explore the coherent interaction regime which proceeds at a time scale shorter than transverse relaxation time T2. The momentum and energy conservation laws determine the characteristic angle of conical emission. We find that the threshold of this coherent process is determined by the KrF laser pump pulse area.

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

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.

Experimental investigation of unique color-changing property of multicolored sparkling of microbubbles formed due to femtosecond laser–water interaction

We present an experimental study on the interaction between femtosecond laser and water. When a beam of femtosecond laser is focused at a single point inside water, nonlinear interaction between the laser and water molecules gives rise to several interesting optical phenomena, starting with filamentation, supercontinuum white light generation and then the formation of cavitation microbubbles near the laser focus region. We observe drifting of the laser focus region and cavitation bubbles against the direction of laser propagation with increasing laser power. Due to conical emission, geometric scattering of white light on the bubble surface manifests itself as multicolored sparkling at the edge of the bubbles. Careful analysis of the video footage of the geometric scattering event reveals a unique color-changing property which occurs only within a tiny fraction of a second. This property can be explained as due to the hydrodynamical flow of the laser-induced plasma strings that leads to instability in the frequency of the emitted light.