Boosting infrared energy transfer in 3D nanoporous gold antennas

TL;DR

This paper demonstrates that nanoporous gold can be used as an effective plasmonic material in the mid-infrared range, enabling enhanced energy transfer to molecules within nanopores, which could improve applications in energy transfer and chemical reactions.

Contribution

The study introduces a novel nanofabrication method for creating nanoporous gold antennas resonant in the mid-infrared, showing improved light-matter coupling over bulk gold.

Findings

Enhanced electromagnetic field confinement in nanoporous gold at mid-infrared wavelengths.

Successful fabrication of resonant nanoantenna arrays using polymeric templates.

Increased optical energy transfer to molecular vibrations compared to bulk gold structures.

Abstract

The applications of plasmonics to energy transfer from free-space radiation to molecules are currently limited to the visible region of the electromagnetic spectrum due to the intrinsic optical properties of bulk noble metals that support strong electromagnetic field confinement only close to their plasma frequency in the visible/ultraviolet range. In this work, we show that nanoporous gold can be exploited as plasmonic material for the mid-infrared region to obtain strong electromagnetic field confinement, co-localized with target molecules into the nanopores and resonant with their vibrational frequency. The effective optical response of the nanoporous metal enables the penetration of optical fields deep into the nanopores, where molecules can be loaded thus achieving a more efficient light-matter coupling if compared to bulk gold. In order to realize plasmonic resonators made of nanoporous gold, we develop a nanofabrication method based on polymeric templates for metal deposition and we obtain antenna arrays resonating at mid-infrared wavelengths selected by design. We then coat the antennas with a thin (3 nm) silica layer acting as target dielectric layer for optical energy transfer. We study the strength of the light-matter coupling at the vibrational absorption frequency of silica at 1250 cm-1 through the analysis of the experimental Fano lineshape that is benchmarked against identical structures made of bulk gold. The boost of optical energy transfer from free-space mid-infrared radiation to molecular vibrations in nanoporous 3D nanoantenna arrays can open new application routes for plasmon-enhanced physical-chemical reactions.