Tunable Near‐ to Far‐Infrared Optical Breakdown in Nonlinear Interactions of Ultrashort Laser Pulses with Water Microdroplets in Ambient Air

Intense laser pulse propagation results in excitation and partial or complete ionization of aerosols and water microdroplets in the atmosphere. While the problems of light scattering by particles and laser‐induced breakdown in bulk water have been investigated in depth, nonlinear interaction of ultrashort laser pulses with microdroplets has been barely covered for a wide range of droplet distributions and wavelengths from visible to long‐wave infrared, relevant for laser breakdown spectroscopy, optical communications, and biomedical applications. Herein, detailed investigation of transient optical response and ionization processes is performed by coupling full‐vector nonlinear Maxwell and plasma kinetics equations in water and in ambient air. Such a self‐consistent approach unravels a variety of nonlinear processes occurring during laser‐droplet interactions such as extreme electron plasma confinement, nanofocusing and nanoshadowing inside water droplets, as well as plasma shielding and restriction on the maximum intensities attained in photonic nanojet and laser‐produced free carriers in the surrounding air. The numerical results provide and explain dependencies of the damage threshold on laser wavelengths and droplet sizes in a reasonable agreement with available experimental results.