# The Quest for Low Loss High Refractive Index Dielectric Materials for UV Photonic Applications
## 1 Introduction  
Plasmonics has been an extremely active area of research in the last decade [1]. The potential of using metallic nanoparticles (NPs) as electromagnetic field enhancers has opened up new possibilities in fields like chemistry, surface-enhanced spectroscopies, nanosensing, and medicine [2]. Although, in general, metallic NPs exhibit good plasmonic performance [3; 4], their intrinsic Joule losses limit their use for many applications [5; 6]. To overcome this problem, a lot of attention has been paid to dielectric nanostructures made of High-Refractive-Index (HRI) materials, working in the Visible (VIS) and Near-infrared (NIR) ranges. These materials are characterized by a high real part \(n>2\) and a low imaginary part \(k\approx 0\) of their refractive indices (some examples can be found in ref. [7]). They show the \"ability\" of enhancing the electromagnetic field both in the near and far field regimes without much absorption [5; 6; 8; 9] in those spectral ranges. In addition, they support magnetic resonances originated from the excited displacement currents. Moreover, HRI dielectric NPs manifest interesting properties of scattering directionality arising from the coherent effects between the excited electric and magnetic dipolar components [9; 10; 11]. Under some specific conditions, scattered radiation from a single HRI dielectric spherical nanoparticle can be concentrated either in the backward or forward scattering regions. When the electric and magnetic dipoles oscillate in- or out-of-phase, two special situations appear, the so-called \"zero-backward condition\" (null scattering in the backward direction) and the \"minimum forward condition\" (minimum scattering in the forward direction), respectively. Up to now, most of the studies concerning these effects have been done in the visible (VIS) and near-infrared (NIR)ranges using common semiconductors such as Si [12; 13] or GaAs [14], because of their interest for optical communications applications. Unlike metals, when HRI dielectric materials show negligible absorption in the VIS-NIR spectral ranges (see ref. [7] for some examples), no localized heat appears. Consequently, neither the surrounding medium nor the nanostructure is heated, something that could have a clear negative effect. For instance, it could limit the performance of nan-antennas [15; 16] by altering the refractive index of both the nanostructure and surrounding medium through the thermo-optic effect [17]. Also, depending on the incident power, the nanoantenna could be reshaped, or even melted [18]. In addition, the heat generated by the nanoparticle can either vaporize the solvent media, affect or even destroy molecules and proteins. Heat can also affect the SERS signal, and has connection to problems or anomalies found in several processes including thermal annealing, modified adsorption/desorption kinetics of surface molecules, and changes in the dielectric properties of both, medium and particle [19]. In consequence, HRI dielectric materials have been proposed as an alternative to metals in surface-enhanced spectroscopies [5; 6]. Extending plasmonics into the UV (3-6 eV or 200 < wavelength < 400 nm) has triggered immense interest due to new challenges arising in specific areas [20]. For example, many biological compounds such as nucleotide bases, nucleic acids, or aromatic amino acids in cells have absorption bands in the ultraviolet (UV). By using the local electric field enhancement generated by nanoparticles when illuminated at resonant wavelength, surface enhanced spectroscopy techniques [21; 22; 23] (SERS, TERS, etc.) can be improved to ease the detection of biomolecules. Other biosensing techniques rely on monitoring the spectral shifts induced by some target analytes in the extinction peaks of the NPs acting as nanoantennas. Bioimaging can also benefit from using UV radiation, since the diffraction limit is proportional to the wavelength (the lower the wavelength the higher the resolution). Photocatalytic processes take also advantage of the UV radiation [24; 25; 26].