Optical Breakdown And Plasma Distribution Of Double Droplets By Femtosecond Laser

The interaction between laser and droplet has always been an important research content in the field of laser technology.In this paper,the optical breakdown of femtosecond laser and micron droplets and the influence of the double droplet structure on the optical breakdown and plasma morphology of the droplets are studied.First,based on the strong photoionization of the water medium and the nonlinear transmission equation of the laser light field,the transient coupling model of the laser and the droplet is established.The space-time evolution of a single droplet laser plasma and laser field is studied.The calculation results show that,due to the lens-like effect of the droplet and the cavity structure,the plasma forms a "W" shape distribution inside the droplet.The laser plasma in the droplet will move in the opposite direction of the laser transmission,and as the laser intensity increase,a shielding phenomenon occur on the incident surface of the droplet.The droplet energy loss ratio increase with the increase of laser intensity and eventually tend to be saturated,reaching 0.7 times the laser energy.These results are basically consistent with previous experimental results.The effect of double droplet structure on droplet optical breakdown,laser plasma and droplet nonlinear absorption are studied.The calculation results show that the breakdown threshold of double droplets of the same size is about 35% of that of single droplet under the same condition.As the second droplet gradually approache the focus area of the first droplet,the laser plasma continue to gather and generate nano-plasma jets.When the laser intensity is strong enough,the second droplet will be affected by the plasma shielding phenomenon of the first droplet.The double droplets can absorb less energy and breakdown by using the incident laser intensity of the double droplets and single droplet breakdown threshold respectively,which is about 3% of the single droplet.The absorption ratio of the second droplet to the laser energy will gradually approach 0.01 as the laser intensity increase,which is about 1.5% of the energy absorbed by the single droplet.The double droplet structure can make the droplet absorb weak laser energy and cause optical breakdown.The research results of this article can provide references for the laser transmission in the atmosphere,the precise control of the laser to droplets,and the laser eye surgery and other biomedical fields.