Plasmonic Nanoparticles and Redshifting

As the simplest nanoantennas, plasmonic nanoparticles can be utilized to enhance the absorption within thin-film solar cells [243]. They couple incoming waves with the localized SPP field, have increased scattering cross-section and strongly localize electromagnetic field just in the thin active region of the detector. Fig. 2.62. The same principle is applicable for infrared detection [321]. This cannot be done with pure noble metal nanoparticles since their surface plasmon resonance is in ultraviolet or visible part of the spectrum. Because of that their response must be redshifted. In this part, two approaches to such redshifting are described. 

As Fig. 2.62 shows, the scattering cross-section of a plasmonic nanoparticle is greatly enhanced by plasma resonance. It can readily reach an order of magnitude value larger than the geometrical cross-section. The scattering cross-section for plasmonic nanoparticles at a wavelength X can be calculated as [322] 

Spherical nanoparticle Fig. 2.62 Increase of scattering by nanoparticies on detector surface 

Nanoparticles can be ordered at the detector surface or elsewhere using self-assembly techniques or pattering by top-down approach. They can be distributed in regular patterns, thus accurately controlling the interparticle distance or they can be randomly scattered. A possible way to implement nanoparticles for plasmonic enhancement is to arrange them in a quasicrystal pattern (for instance Penrose tiling), which ensures an isotropic photonic response of the strucmre [325]. 

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