Multiphoton Electron Photoemission Linear Polarizance/Circular Dichroism Dynamics for Achiral and Chiral Plasmonic Nanostructures

Plasmonic nanostructures exhibit intense (> 106 Å2) resonance absorption features that can be smoothly tuned by shape and elemental content from near UV to near IR regions of spectrum. This novel combination of high tunability and absorption intensity offers a powerful entry point into harnessing solar radiation to generate electron-hole pair (and possible chiral) excitations and enabling a variety of applications ranging from photocatalysis to photothermal cancer therapy. Our previous efforts have concentrated on developing Scanning Photoionization Microscopy (SPIM) as a technique to explore the temporal/3D multiphoton photoemission dynamics with correlated SEM methods, exploiting velocity map imaging (VMI) to probe both the scalar (kinetic energy) and vector (momentum) properties of the electrons photoemitted from single, well-characterized plasmonic nanostructures. Our recent work has now extended these capabilities in several new directions. First of all, we have rebuilt the SPIM instrument for automated polarization control of the Mueller matrix for laser light entering the SPIM microscope and thereby programmable modulation of the ultrafast pulse polarization state. This has permitted us to perform SEM correlated linear polarizance (LP) and circular dichroism (CD) measurements on multiphoton electron photoemission from single plasmonic nanostructures. Secondly, in collaboration with the Wang group1, we have access to selective chirality plasmonic nanostructures from halide assisted differential growth (HADG) templated with, for example, L vs D glutathione ligands. These chiral “triskelion” structures exhibit remarkably large circular polarization photoemission sensitivities in the SPIM apparatus, with CD factors for RCP vs LCP excitation of order 35% in 3-photon photoemission. Finally, in order to better understand the fundamental origins of such high CD activity, we have theoretically pursued detailed E&M modelling of the RCP vs LCP surface/volume photoemission dynamics as well as photoelectron forward/backward asymmetry (PECD) in the photoejection propensities. Though much of the chiral plasmonic “experimental phase space” remains to be explored, we find encouraging agreement between experimentally predicted CD values and theoretical modelling from correlated SEM measurements of the chiral nanostructures. Time permitting, this talk will report on progress in each of these areas.

1Zheng et al, Nat. Comm. 14, 3783 (2023).