Nanoscale Switching of Near-Infrared Hot Spots in Plasmonic Oligomers Probed by Two-Photon Absorption in Photopolymers

Plasmonic oligomers are near-field-coupled assemblies of metallic nanoparticles. Both their scattering/absorption spectra and the spatial distribution of the electromagnetic field can be tailored through the hybridization of plasmonic modes hosted by individual particles. Such a control on the field distribution opens new routes to deliver light at a deep subwavelength scale in targeted locations (hot spots). However, active control of hot spots in plasmonic oligomers and their observation in the near field are highly challenging. Here, we propose using a two-photon absorption process in azopolymer in the near-infrared to imprint from the far field the near-field distribution around a trimer antenna. The trimer antenna comprises two nanogaps separated by a quarter of the wavelength in the polymer and is designed to allow for the switch on a single nanogap when illuminated at 900 nm wavelength by a collimated beam at an oblique incidence. The monitoring of the topographical depletions in the photopolymer proves that it is possible to address a single hot spot in the structure and to remotely switch its location in the two nanogaps on demand, simply by illuminating with an opposite oblique incidence. This work shows that bonding and antibonding gap modes can be selectively excited, resulting in controlled hot spot locations. Two-photon absorption by azobenzene-containing photopolymer turns out to be a reliable approach for probing and investigating confined plasmonic fields in the near-infrared with a 20 nm resolution.