Chiral Transformation of a Nanostructured Silver Film by Illumination with Circularly Polarized Light

TL;DR

This paper shows how circularly polarized light can change the chirality of silver nanostructures, enabling scalable fabrication of chiral plasmonic materials.

Contribution

A scalable method to transform racemic silver nanostructures into chiral states using circularly polarized light is introduced.

Findings

Low-power circularly polarized light promotes growth of nanoparticles matching the light's handedness.

High-power illumination causes inversion of enantiomeric excess through hot-carrier-driven processes.

Aged silver films with a thin Ag2O layer show enhanced chiroptical response with a g-factor of 1.2 × 10–2 at 532 nm.

Abstract

Chiral plasmonic nanoparticles combine interesting geometrical
and optical properties and hold great promise for applications in
polarization optics, light sources, and enantioselective sensing.
However, simple, low-cost, large-area, and reproducible routes to
fabricate such chiral plasmonic nanostructures are still limited.
Here, we demonstrate a scalable chiral optical imprinting of nanostructured
Ag films in which an initially racemic ensemble of nanoparticle enantiomers
is transformed into a chiral state under continuous-wave circularly
polarized illumination. The imprinting is driven by plasmon-induced
redox processes mediated by hot carriers and by surface diffusion
of Ag+ ions and leads to a robust, unchanged circular dichroism
upon flipping the sample, evidencing the formation of 3D chiral nanostructures.
We identified two different irradiation regimes depending on the power
density. At low-power densities, circularly polarized light creates
asymmetrical distribution of electric field around nanoparticles that
promotes the growth and stabilization of Ag nanoparticles with handedness
matching the incident light polarization. The microscopic origin of
the observed chiral growth was elucidated by numerical simulations
of the near-field response and hot-carrier generation, which show
that under circularly polarized illumination, size-asymmetric (achiral)
Ag dimers act as plasmonic nanoantennas, giving rise to a strongly
handed local hot-carrier generation profile. At high-power densities
(>5 W·cm–2), selective hot-carrier-driven
photo-oxidation
and thermally assisted coarsening of same-handed particles dominate,
giving rise to inversion of the enantiomeric excess. The described
transformations are observed only in the aged granular films, where
a thin Ag2O layer accelerates Ag atom/ion migration and
facilitates nanoparticle reshaping. The resulting films exhibit a
pronounced chiroptical response with a maximum g-factor
of 1.2 × 10–2 at 532 nm.

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