# Plasmonics in the Ultraviolet with Aluminum, Gallium, Magnesium and Rhodium
## 1 Introduction  
In the last two decades, nanoplasmonics has experienced a huge impulse from both theoretical and experimental points of view. This branch of nanophotonics studies the distribution of the electromagnetic field, and its local charge resonances (Localized Surface Plasmon Resonances, LSPRs) in sub-wavelength metallic nanostructures. These SPRs localize and strongly enhance the incident field near the nanostructure at dimensions much smaller than its wavelength [1]. This phenomenon has been exploited for many applications in several scientific fields such as medicine [2], optical communications [3], spectroscopy [4], chemical sensing [5] or photocatalysis [6]. Many studies have analyzed how the LSPRs can be spectrally tuned by varying the shape, size and material of the nanoparticle, as well as its surrounding conditions [7]. Most of this research has been done in the visible and near-infrared (\(\lambda>400\) nm) where Au nanoparticles have played a leading role due to their oxide-free nature and biocompatibility. Extending nanoplasmonics to the UV range has become a topic of interest due to the new challenges arising in specific areas. For example, many biological compounds, such as nucleotide bases, nucleic acids or aromatic amino acids in cells, have absorption bands in the UV (below \(\lambda\) = 300 nm) [8; 9; 10]. By using hot spots excited by LSPRs in the UV, it is possible to improve surface enhanced spectroscopy techniques (SERS [4], TERS [11], etc.) that ease the detection and recognition of these kinds of biomolecules. Also, biosensing assisted by LSPRs monitors the spectrum around the plasma resonance looking for shifts produced by changes in the extinction curves related to some target analyte. Bioimaging is another area that benefits from using UV radiation since the diffraction limit is proportional to the wavelength. Another example is photocatalysis, an area of research which has experienced a great development in the last decade. It has been recently discovered that metallic nanoparticles (NPs) are photocatalytically active, driving chemical reactions with photo-generated hotcarriers originated from the LSPRs' decay [6; 12; 13]. These hot carriers and their subsequent transfer to adsorbates can affect reactions pathways. By tuning the photon and LSPR energies so that hot carriers are injected into specific orbitals of reaction intermediates, product selectivity can be achieved. However, not all metals can be used for these kind of applications, only those with good plasmonic and catalytic behaviors. As conventional metals such as Au or Ag cannot operate in the UV range due to the presence of interband transitions, several researches have analyzed possible new metallic alternatives in order to find those whose plasmonic performance could be suitable for the new challenges arising for photon energies above 3 eV [14; 15]. Among the studied metals are aluminum (Al), gallium (Ga), indium (In), rhodium (Rh), ruthenium (Ru), tungsten (W), titanium (Ti), chromium (Cr), palladium (Pd), copper (Cu), platinum (Pt), and magnesium (Mg). Figure 1 shows the dimensionless Faraday number, \(Fa\) (or field-enhancement factor) [16] for different materials including all the above mentioned plus gold (Au), silver (Ag) and bismuth (Bi). Very recent results have presented Bi as a promising candidate for plasmonics in the UV range [17]. \(Fa\) quantifies and allows comparison of the ability of a material to enhance the electromagnetic field in the proximity of its surface. This expression has been proposed as a refinement to the plasmonic efficiency used in previous studies [14; 15]. Figure 1 shows that Mg, Al, Ga and Rh present the highest value of \(Fa\) while having LSPRs in the UV. Although In also presents a value of \(Fa\) comparable to that of Ga, its oxidation process (5 nm vs 1 nm oxide shell thickness) prevents its application in the UV [14]. The plasmonic performance of these metals depends on the surrounding dielectric environment.