# Multipolar Plasmonic Resonances of Aluminum Nanoantenna Tuned by Graphene
## 1 Introduction  
Aluminum plasmonics is a rapidly growing field of nanoscience due to interest in both its scientific research and its promising potential applications [1, 2, 3, 4]. Localized surface plasmon resonances (LSPR) [5, 6] of Al nanostructures in the ultraviolet (UV) and visible range have been reported in several geometries, including spheres [7, 8], disks [9, 10], rods [2, 11], and triangles [12, 13]. Due to a low-cost alternative to the traditional noble metals (gold and silver), applications of the Al plasmonic nanostructures include enhanced light-mater interactions [14], full color display [15], UV fluorescence [16], UV surface enhanced Raman spectroscopy [17], label-free biosensing [18, 19] and light harvesting [20, 21]. In recent years, the research on LSPRs of Al nanostructures has mainly been focused on the fundamental electric or magnetic dipole modes [2, 7, 8, 9, 22, 23, 24]. Nevertheless, these fundamental resonant modes of Al nanostructures with poor \(Q\)-factor suffer from both strong radiative and non-radiative losses in UV and visible range [25]. In contrast, multipolar plasmonic high-order resonance modes [2, 26, 27] of Al nanoantennas with high \(Q\)-factor due to minimal non-radiative losses have attracted great attention as optical ultrahigh-sensitive sensors [28], multipolar radiations of quantum emitters [29], exciton-plasmon coupling [30, 31] and nanolasing [32, 33, 34]. The multipolar high-order plasmonic modes have been experimentally investigated by a powerful tool of electron energy loss spectroscopy [1, 2, 10], revealing the spatial and spectral distributions. However, there are seldom reports on optical excitations of multipolar high-order plasmonic modes owing totheir low excitation efficiency in UV range [35], which hinders the practical applications. Therefore, finding a way to effectively tune multipolar high-order resonant modes of Al nanoantenna excited by light with strong absorbance and highQ-factor is a prerequisite for its widespread applications. Graphene, a flat monolayer of carbon atoms arranged in a honeycomb lattice, has been drawing tremendous attention owing to its electronic and optical properties for graphene-based photonics and optoelectronics devices in recent years [36; 37; 38; 39; 40; 41]. From mid-infrared to THz range, the monolayer graphene can support the dynamic control of its plasmonic modes by tuning Fermi levels, resulting in enhanced optical absorption of graphene-based metamaterials [36; 37; 38]. However, from UV to near-infrared range, a poor optical absorption with universal value of 2.3% [42] of a monolayer graphene at a normal angle of incidence limits the applications of graphene-based optoelectronics devices. It is due to graphene's optical response being dominated by interband transitions with no plasmonic response, acting as a passive lossy conductive surface. Recently, various mechanisms have been presented to enhance the absorption of graphene-based metasurfaces from UV to near-infrared range [28; 43; 44] by the strong interaction of the monolayer graphene with resonant metasurfaces. [PERSON] el. al. [44] and [PERSON] et al. [28] reported that graphene-dielectric/metal metamaterial could be designed to enhance the UV ultranarrow absorption of graphene by the optical resonance modes supported by the dielectric or metal metamaterial. On the other hand, it is also important to understand the monolayer graphene-mediated resonant metasurfaces since the passive response acts as a precursor for the calibration of the metamaterial and plasmonic device designs. [PERSON] et al. [45; 46] demonstrated that the changes in the transmission amplitude of the Fano resonance mode and the dipolar mode was observed owing to the interactions between the monolayer graphene and asymmetric spitting resonators in the terahertz range. Such prospect of switch-off effect of the resonance in metasurfaces is quite in demand for the potential applications based on light-matter interaction.