Excitation and Probing of Infrared Nanoantenna Modes under Oblique Illumination

We study near-field distributions of infrared nanoantennas under oblique illumination. We obtain the amplitude- and phase-resolved images of the near-fields on Au nanoantennas by use of the scattering-type scanning near-field optical microscopy. Resonant excitation of the 1st-order (dipole) mode and that of the 2nd-order (quadrupole) mode, and their asymmetric near-field distributions are experimentally visualized and numerically reproduced.