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   Project Abstract

     The use of collective excitations of electron gases in metals, namely plasmonics, has proved to be a major breakthrough in the design of nano-systems in many fields, spanning from optics to biology. The huge local enhancement of the electrical field around a plasmonic object has been harvested to improve the emission yield of many light emitters or scattering cross-section of nano-objects. This has been fruitful for optical imaging (for biological labels, for instance) and has also paved the way to the design of many metamaterials and systems for transformation optics. The enhancement of the local electromagnetic field has further been harvested to improve the sensitivity of specific optical spectroscopies (such as Surface enhanced Raman Scattering).

     However, most of the afore-mentioned progresses have taken advantage of the plasmons in noble metal nanoparticles and more specifically those of silver and gold. The choice of these metals limits the interest of plasmonics to the visible range of electromagnetic waves. Another major challenge in plasmonics is the high loss of the metallic nanoparticles. Besides, noble metals such as gold and silver are hardly compatible with the conventional technologies of the Si industry.

     On the other hand, there is a major interest in expanding the spectacular properties of plasmonic nano-objects to the infra-red (IR) range. First, mid IR, corresponding to wavelengths of approximately 3 to 30 µm, is the range where most of the chemical molecules have their spectroscopic signature. Second, near IR, with wavelengths around 1.5 µm, corresponds to the telecommunication range. Thus, there is an obvious benefit to control the electromagnetic field amplitude and location at the nanoscale of these different waves: one could improve the coupling of light with molecular vibrations and telecom signal respectively and design highly sensitive optical detectors.
     A recent and timely issue concerns the development of nanomaterials that allow tuning the plasmon resonance through the electron gas concentration. Degenerate semiconductors are particularly suited for that purpose. Among them, Ga doped ZnO (GZO) appears a most interesting candidate.

     In the present project, we aim at developing GZO based nanostructures with tunable IR plasmons. If the demonstration of GZO potential for IR plasmonics has been done recently using thin films or nanoparticles, much remains to be done for systems with low dimensionality (nanowires or nanoparticles). Two main issues remain to be addressed and explained: first, what is the range over which the plasmon resonance can be tuned? In other words, what are the doping limits (both minimum and maximum) that can be sustained in nano-objects? Second, what are the factors that rule the losses? Indeed, it is observed on the one hand that not all the dopants contribute to the plasmon and on the other hand that the plasmon resonances are broader, thus more subject to losses, that what is expected. Our project aims at answering these two issues in order to design efficient tunable IR plasmonics nano-objects.

     This project, based on novel degenerate ZnO nanostructures, will allow the development tunable IR nano-plasmonics. For this wavelength range, there is no competitive alternative which allows tuning the plasmon resonance (through composition, size, shape…). The present project is not an incremental development of existing technologies. It rather presents a high breakthrough potential.