Visible region polarization spectroscopic studies of template-synthesized gold nanoparticles oriented in polyethylene

We have obtained visible range polarization spectra of template-synthesized gold nanoparticles oriented in polyethylene (PE). The plasmon resonance extinction bands observed with the incident electric field polarized parallel, perpendicular, and at intermediate angles to the direction of friction orientation are consistent with the long axis of the particles being aligned with the gross orientation axis. For all particle sizes considered (radii 16, 38, and 60 nm) the degree of linear dichroism increases with the amount of gold deposited in the template synthesis step prior to extraction and orientation. The experimental spectra agree with the predictions of the Rayleigh, Maxwell-Garnett, and dynamical Maxwell-Garnett theories only qualitatively. All of these treatments fail to predict the dependence of the spectral extinction intensities on the polarization angle θ. T-matrix scattering calculations suggest that the contribution from electric quadrupole modes cannot be ignored in the 38 and 60 nm radius particles. The calculated θ-dependence of the extinction intensity does not resemble experiment for 16 nm radius particles, but the theory - experiment comparison is more favorable for the larger radius systems. Some possible models for these observations are discussed.