Spectral Degeneracy Breaking of the Plasmon Resonance of Single Metal Nanoparticles by Nanoscale Near-Field Photopolymerization

We report on controlled nanoscale photopolymerization triggered by enhanced near fields of silver nanoparticles excited close to their dipolar plasmon resonance. By anisotropic polymerization, symmetry of the refractive index of the surrounding medium was broken: $C\infty v$ symmetry turned to $C2v$ symmetry. This allowed for spectral degeneracy breaking in particles plasmon resonance whose apparent peak became continuously tunable with the incident polarization. From the spectral peak, we deduced the refractive-index ellipsoid fabricated around the particles. In addition to this control of optical properties of metal nanoparticles, this method opens new routes for nanoscale photochemistry and provides a new way of quantification of the magnitude of near fields of localized surface plasmons.

Figure 1

Determination of the threshold energy. (a) and (b) AFM image of polymer grating obtained after $20\mathrm{mJ}/{\mathrm{cm}}^2$ and $11.5\mathrm{mJ}/{\mathrm{cm}}^2$ exposures, respectively. (c) and (d) corresponding profiles. (e) Grating height as a function of incident energy density.

Figure 2

(a) and (b) AFM images recorded after irradiation and developing of the silver nanoparticles arrays covered with the photopolymerizable formulation. (c) Intensity distribution in the vicinity of an Ag particle embedded in the formulation as calculated by FDTD method ($\lambda =514\mathrm{nm}$). The white arrow represent the incident polarization used for exposure.

Figure 3

The extinction spectra from a silver nanoparticle array. Top: experimental data. (a) Array in air on a glass substrate. (b) Array in initial liquid polymer (before exposure). (c) on hybrid particles excited along the minor axis (d) on hybrid particles excited along the major axis. Bottom: corresponding theoretical spectra.

Figure 4

Polar diagrams: data as a function of the polarization angle around the hybrid nanoparticles: (a) SPR peak; (b) experimental configuration; (c) spectral FWHM; (d) effective refraction index.