Selective Excitation of Dark Plasmon Modes Using Cylindrical Vector Beams Studied by Microscopic Imaging of Nonlinear Photoluminescence

Noble metal nanostructures exhibit multiple plasmon modes with different spatial characteristics and resonance energies. Plasmons confine electromagnetic fields in the vicinity of nanostructures, and the confined field has been utilized for various applications, such as sensing and chemical reactions. The plasmon mode with no net polarization is optically dark, and thus, it is not accessible by plane wave excitation. The dark plasmon mode exhibits a long dephasing time due to suppression of the radiative decay process. As this feature is advantageous for applications, development of a novel excitation scheme for dark modes is indispensable. Optical selection rules of a plasmon mode are determined by the spatial symmetry of the plasmon mode and excitation field. Cylindrical vector beams possess unique spatial polarization characteristics that are different from the linearly and circularly polarized light and are capable of the excitation of the dark plasmon modes. This study examines the nonlinear photoluminescence properties of gold nanoplates excited by radially and azimuthally polarized beams and demonstrates that the spatial characteristics of the excitation images are strongly dependent on the excitation field. Electromagnetic simulations support the findings that selective excitation of the dark plasmon mode is feasible by the cylindrical vector beams