Appendix A -Plasmons in general (2020) Ukhtary and Saito, 2020, are helping us about plasmons. "Plasmon is collective oscillation of charge density in a metal in which the oscillation propagates in the direction of the wavevector q. Here "collective oscillation" means that free electrons in the metal in the same direction and phase to reduce the Coulomb repulsion between two electrons. Volume plasmon in three-dimensional (3D) metal is longitudinal oscillation of charge density in which the direction of the electric field is parallel to q … Since the longitudinal oscillation cannot be excited by transverse oscillation of electric field, the volume plasmon cannot be excited by a photon. On the other hand, surface plasmon is defined by the oscillation of charge density that propagates on the surface of a metal. In particular, for the case of two-dimensional (2D) graphene or one-dimensional (1D) carbon nanotube (CNT), since all carbon atoms exist on the surface, the plasmons in graphene and CNT are surface plasmons. In the case of surface plasmon, since the electric field induced by the charge density can be perpendicular to the surface, the surface plasmon can be excited by a photon. In solid state physics, the surface plasmon at the surface of 3D metal is often called a surface plasmon polariton in which the surface plasmon is coupled with electric polarization of a surrounding dielectric material" (Ukhtary and Saito, 2020).