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.