# Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium
## 3 Low Oxidation Tendency: Gallium and Rhodium  
### Gallium  
Gallium is a standard material in optoelectronics that is becoming attractive also for its plasmonic properties. It presents a broad plasmon tunability, it is stable along a wide range of temperatures and its deposition is simple even at room temperature [24]. Moreover, the new range of bottom-up fabrications techniques that allow the production of Ga NPs, make this material very appealing for practical purposes. These fabrication methods include molecular beam epitaxy (MBE) [24; 56], optically regulated self-assembly [57], thermal evaporation [58], and colloidal synthesis [59]. In addition, unlike either Al or Mg, its very low tendency to oxidation (1-2 nm oxide shell thickness), minimally affects its optical properties, keeping them stable over months or even years [23; 24]. Ga NPs form liquid droplets with a melting point of 30 \({}^{\circ}\)C, so when deposited on a substrate at room temperature, they produce a close-packed array of smooth truncated spheres whose mean radii and size distributions vary with growth conditions. [PERSON] et al. [56] presented the optical evolution of Ga NPs surface plasmon resonance during deposition by in situ ellipsometry to control and tune the plasmon resonance photon energy. This work reported both longitudinal and transverse modes for the hemispherical Ga NPs supported on a sapphire substrate. Similar results were obtained by [PERSON] et al. [60] through a numerical study based on the Discrete Dipole Approximation (DDA) [61]. Other methods used to study this type of nanoparticles include variable-angle spectroscopic and Mueller matrix ellipsometry [62] or hyperspectral cathodoluminescence imaging [63]. The utility of these Ga NPs seeded substrates has been demonstrated for UV surface-enhanced Raman spectroscopy, fluorescence, and photodegradation, following excitation by a HeCd laser operating at 325 nm [64]. Other studies have explored the possibility of alloying or coating these Ga NPs with other metals such as Mg. [PERSON] et al. [65] experimentally analyzed the plasmonic properties of this type of NPs. These bimetallic NPs allow a new degree of freedom for tuning the LSPRs since their optical constants are dependent on the mixture composition. The resulting Ga-Mg NPs exhibit a plasmonresonance tunable from the UV to the IR by playing with the particle size and the Ga/Mg ratio [50]. Electromagnetic simulations based on DDA numerically corroborated these experimental results [50]. Very recently, it was experimentally shown, through real-time ellipsometry, the stable coexistence of a solid-phase core and a liquid shell in substrate-supported Ga NPs [20]. Both liquid and solid phases present significantly different optical properties [20]. While the liquid phase presents a Drude-like dielectric function, the solid phase presents interband transitions in the 2.19-2.40 eV range. This fact is relevant for plasmonic applications, since the plasmonic performance of metallic NPs is very sensitive to their optical properties. [PERSON] et al. [66] proposed a new concept for active plasmonics that exploits light-induced nanoscale structural transformations in the waveguide material. Specifically, they showed that surface plasmon polaritons signals in a metal-on-dielectric waveguide containing a Ga section a few microns long can be effectively controlled by switching the structural phase of gallium. They were able to induced the phase change by thermal or optical excitation. Taking advantage of the gallium's solid-liquid phase transition, [PERSON] et al. [67] fabricated liquid Ga gratings which exhibited reversible and switchable phase-dependent plasmonic properties. The liquid phase showed higher surface plasmon polaritons coupling efficiencies and narrower resonance line widths compared to the solid phase. In order to predict the optical constants of liquid metals and metal alloys, [PERSON] et al.