Mapping Nanoscale Absorption of Femtosecond Laser Pulses Using Plasma Explosion Imaging

Abstract

We make direct observations of localized light absorption in a single nanostructure irradiated by a strong femtosecond laser field, by developing and applying a technique that we refer to as plasma explosion imaging. By imaging the photoion momentum distribution resulting from plasma formation in a laser-irradiated nanostructure, we map the spatial location of the highly localized plasma and thereby image the nanoscale light absorption. Our method probes individual, isolated nanoparticles in vacuum, which allows us to observe how small variations in the composition, shape, and orientation of the nanostructures lead to vastly different light absorption. Here, we study four different nanoparticle samples with overall dimensions of ∼100 nm and find that each sample exhibits distinct light absorption mechanisms despite their similar size. Specifically, we observe subwavelength focusing in single NaCl crystals, symmetric absorption in TiO2 aggregates, surface enhancement in dielectric particles containing a single gold nanoparticle, and interparticle hot spots in dielectric particles containing multiple smaller gold nanoparticles. These observations demonstrate how plasma explosion imaging directly reveals the diverse ways in which nanoparticles respond to strong laser fields, a process that is notoriously challenging to model because of the rapid evolution of materials properties that takes place on the femtosecond time scale as a solid nanostructure is transformed into a dense plasma.

Nanofocusing, shadowing, and electron mean free path in the photoemission from aerosol droplets

“…Ziemann and McMurry performed secondary electron yield measurements for probing organic films on aerosol particles [4] and recently Su et al used photoelectron spectroscopy to study solvation phenomena in droplets [5]. Plasma formation in clusters and different types of particles after exposure to intense femtosecond pulses was observed through mass spectrometry and ion imaging [6,7], while electron imaging was used to study directional emission from dielectric particles using intense few-cycle laser fields [8,9].…”

Femtosecond time-resolved X-ray absorption spectroscopy of anatase TiO2 nanoparticles using XFEL

“…On the basis of these estimations, we conclude that, even if the conduction band electrons are almost uniformly distributed within each primary nanoparticle just after photoexcitation, a large fraction of these excited electrons are in the vicinity of the surface. Furthermore, the near field in nanoparticles (the primary particle size of ∼7 nm in our case) may not be characterized by a simple attenuation length; a recent FDTD simulation revealed considerable enhancement of the electric field on the surface of 5 nm TiO 2 nanoparticles 44 . Similar field enhancement near the surface is expected to induce stronger UV excitation than in the bulk, which would lead to trapping of a large fraction of electrons at penta-coordinated Ti sites.…”

Photogenerated Free Carrier‐Induced Symmetry Breaking in Spherical Silicon Nanoparticle

“…Therefore, in order to manipulate the propagation angle of the transmitted light, it was proposed to use complicated nanostructures with reduced symmetry . On the other hand, plasma explosion imaging technique revealed in situ strongly asymmetrical electron–hole plasma (EHP) distribution in various dielectric NPs during their irradiation by femtosecond laser pulses. Therefore, local permittivity in the photoexcited NPs can be significantly inhomogeneous, and symmetry of nanoparticles can be reduced.…”