Experimental Evidence for Axial Anisotropy beyond the Diffraction Limit Induced with a Bias Voltage Plasmonic Nanoantenna and Longitudinal Optical Near-Fields in Photoreactive Polymer Thin Films

This work highlights a mechanism for inducing axial anisotropy in side-chain nitroazobenzene (NAB) polymer thin films based on the combined effect of both local dc electrical poling and the longitudinal optical near-field. We show that highly anisotropic NAB chromophores are effectively oriented in the glassy environment under optical pumping with 632.8 nm excitation wavelength, which is out of the absorption band of chromophores. Axial anisotropy across the polymer thin film and its non-centrosymmetric behavior beyond the diffraction limit are experimentally rendered with tip-enhanced Raman scattering microscopy and scanning Kelvin probe microscopy.