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

Abstract

Water droplets are omnipresent in biophysical environments and ecosystems, playing an important role in communication through the atmosphere (i.e., aerosols and contaminants, fog, mist, haze, drizzle, clouds) and being involved in a variety of physicochemical reactions and biological processes (respiratory droplets due to breathing, talking, coughing, etc., biological cells, bioaerosols, water clusters, surface adhesion, and wettability). Broad opportunities in analysis, detection, and control over droplet distributions are enabled by the application of powerful lasers used for laser breakdown, [1-3] electron photoemission [4-7] and cavity-enhanced droplet spectroscopy, [8,9] long-distance optical communications, [10-12] and high harmonic generation from water microdroplets. [13-15] The processes of ultrashort laser excitation and ionization of water droplets and aerosols remain poorly explored for a wide range of photon energies and wavelengths from visible to long-wave infrared (IR) range. Within this spectral range, the contributions of scattering and absorption in water, [16] photo-and avalanche ionization, [17,18] positive and negative transient refractive index modifications due to free carrier heating, [19,20] as well as diffraction, Kerr self-focusing, and plasma defocusing upon propagation in the atmosphere [21-23] play interchangeable roles, making indispensable a comprehensible broad-frequency study of laser-induced nonlinear processes and optical breakdowns in laser-droplet interactions. Our current study addresses aspects of high-intensity femtosecond laser interaction with a single water microdroplet in ambient air for a wide range of laser wavelengths. Depending on the droplet size and wavelength (in Rayleigh, Mie, or geometrical optics [GO] ranges [24]), different complex field patterns are induced inside the droplet, contributing to extreme energy confinement due to a nanofocusing effect. [25] Such strong intensities result in electron plasma generation inside the droplets due to photo-and avalanche ionization processes, [11,26-30] changing locally the transient optical properties of water, influencing pulse propagation, and contributing to strong absorption. The effect of nanoscale electron plasma confinement inside microdroplets is comparable to ultrashort laser-induced plasma formation in the vicinity of plasmonic nanoparticles, routinely used for cancer cell phototherapy and viral or bacterial inactivation by selective nanobubble cavitation in water. [31-33] In fact, the pronounced absorption at the nanoscale in both cases reduces significantly the threshold for permanent modification, i.e., phase explosion [34] and cavitation via shock waves, produced by temperature gradients. [12,31,35] Knowledge of the minimum energy required to damage the droplet is relevant for high-repetition rate optical communications (e.g., fog clearing), [11,12] for laser spectroscopy, [2,3] and for biomedical applications (e.g., surface cleaning or airborne virus detection and inactivation in the respiratory drople...

Ion imaging of spatially inhomogeneous nanoplasmas in NaCl particles

“…This approach follows previous work on laser ablation of solids 14,16 and more recently nano-sized particles. 4,6,17,20,26 Laser intensities and pulse durations (i.e. laser fluence) used in our work set us in a regime of relatively rare NPs (electron density n e < 10 20 cm −3 ).…”

“…The second possibility is that light absorption of quasi-free electrons becomes significant within the pulse duration and leads to decreased intensity at the exit side of the particle. In the future, self-consistent simulations 4,6,17,20,26 will be required to further understand this behaviour.…”

“…3 Condensed phase samples in particulate form offer additional opportunities as the light field can be "tailored" inside and around the particle. Recently, studies on dielectric particles illustrated such advantages by reporting subwavelength transient refractive index reconfiguration, 4,5 appearance of photonic nanojets, 6 controlled electron acceleration, [7][8][9][10] and spatially resolved mapping of photodissociation yields. 11,12 In this work, we investigate interactions of dielectric NaCl particles with intense, ultrashort pulses in the near-infrared (NIR) and ultraviolet (UV) regime.…”

The propagation loss of ultrashort laser pulses in water clouds

“…In this case, both forward and backscattering of droplets attenuate laser energy. However, Rayleigh scattering has a certain proportional relationship λ −4 in the visible and NIR wavelength [53], which makes its influence on the attenuation of laser light propagation very small. Therefore, the electric field distribution in the droplet is uniform and the local electric field in the backward air of the droplet is not significantly enhanced.…”

Characteristic parameters of photonic nanojets of single dielectric microspheres illuminated by focused broadband radiation

“…Few theoretical studies related to the PNJs of single dielectric microspheres under pulsed irradiation (or coherent broadband illumination) are also reported in the literature 60 , 61 . Under the pulsed irradiation, the PNJs appear to be non-stationary and these are found useful in micromachining beyond the diffraction limit 62 , microscopy 63 , and non-linear optics 64 . However, details of the characteristic parameters of the PNJs generated by focused polychromatic light (or broadband radiation) and the role of R s , n m, and ω 0 on the characteristic parameters of the PNJs generated by broadband radiation are not reported in the literature.…”