Local Electric Polarization Vector Detection

Abstract

The function of nanophotonic devices, such as metamaterials for the visible range, photonic and plasmonic crystals, nanoscale waveguides, resonators, switches or optical antennas relies essentially on our ability to tailor and control electromagnetic fields on a subwavelength scale, in much the same way as the wavefunction of electronic nanostructures is localized on an atomic scale. Unlike matter waves, these localized electromagnetic waves are vectorial in nature, and their orientation and magnitude varies on a sub-wavelength scale. Therefore it is vital for a complete description of light in nano-scale devices to map the field vectors with subwavelength resolution. Experimental techniques aimed at probing the field intensity, e.g. aperture-less or aperture-based near-field microscopies, are rapidly improving, but the local orientation of the electromagnetic polarization vector, a fundamental property of the local electromagnetic field, could not be accessed experimentally. Yet, the orientation of the field vector is a key quantity in many thousands of theoretical studies on nano-optics, nanophotonic devices and optical sciences in general. Evidently, the ability to experimentally probe electromagnetic field vectors with nanometer resolution is of fundamental importance for understanding and improving nano-optical devices and for reconciling experiment and theory in the bourgeoning field of nanophotonics. In this chapter, we describe and demonstrate the local field polarization vector detection using the gold nanoparticle (GNP) attached tip as the local field scatterer acting as a nanometer-scaled polarizer. For a suitably small GNP, the far-field scattering is dominated by the electric dipole radiation. Dipole radiation conserve its polarization state into the farfield region enabling characterization of the dipole moment induced at the GNP by measuring the far-field polarization state. And also the dipole moment is determined by the local electric field via polarizability tensor of GNP. Therefore, by characterizing the polarizability tensor of GNP and the polarization state of far-field scattered light, the local electric field vector can be reconstructed. In doing that, the different scattering shape of GNP, the polarizability tensor, is carefully measured and considered to get a consistant result independently of the tip shape. Mapping the local polarization vector in the near-field demands a careful consideration of the surface effect. For example, far field detected light is a complex mixture of the scattered light from GNP and its reflected light at the sample surface which interfere each other strongly depending on the polarization. This so called image dipole effect should be taken into account to correctly address the reconstructed local field vectors in near-field region. In the last section of this chaper, we discuss the limitations of our method and give suggestions to improve the functionality of the polarization vector microscopy.