title: 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.
authors.affiliation: JILA and Department of Physics, University of Colorado , Boulder, Colorado 80309, United States.
authors.forename: Daniel D
authors.initials: DD
authors.lastname: Hickstein
authors.forename: Franklin
authors.initials: F
authors.lastname: Dollar
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Ellis
authors.forename: Kyle J
authors.initials: KJ
authors.lastname: Schnitzenbaumer
authors.forename: K Ellen
authors.initials: KE
authors.lastname: Keister
authors.forename: George M
authors.initials: GM
authors.lastname: Petrov
authors.forename: Chengyuan
authors.initials: C
authors.lastname: Ding
authors.forename: Brett B
authors.initials: BB
authors.lastname: Palm
authors.forename: Jim A
authors.initials: JA
authors.lastname: Gaffney
authors.forename: Mark E
authors.initials: ME
authors.lastname: Foord
authors.forename: Stephen B
authors.initials: SB
authors.lastname: Libby
authors.forename: Gordana
authors.initials: G
authors.lastname: Dukovic
authors.forename: Jose L
authors.initials: JL
authors.lastname: Jimenez
authors.forename: Henry C
authors.initials: HC
authors.lastname: Kapteyn
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Murnane
authors.forename: Wei
authors.initials: W
authors.lastname: Xiong
keywords: femtosecond lasers; finite-difference time-domain; local field enhancement; photoion spectroscopy; plasmonics