Nanoplasmonics and surface enhanced spectroscopy

Shape-dependent field enhancement and plasmon resonance of Au, Al and Al/Al2O3 core/shell nanoparticles on Si surface
M. Kushlyk1, D. Slobodzian1, R. Lys1, B. Pavlyk1
1 Ivan Franko National University of Lviv, 107 Tarnavskoho str., Lviv 79017, Ukraine  
E-mail: kushlykmarik@gmail.com

Silicon-based light-emitting structures (LESs) are very promising light sources for optical communication technologies. However, their development are slowdowning by low efficiencies of radiative recombination. Efforts to improve the limitations of the LESs efficiency have, among others, made use of plasmonic nanostructures. Large efficiency enhancement of luminescence was achieved by placing metal nanoparticles close to light emitters. This has attracted intense research interests devoted to demonstrating such an enhancement in various media. Metallic nanostructures can manipulate light-matter interactions to induce absorption, scattering, and local field enhancement through their localized surface plasmon resonances. Field enhancement, especially near features with high curvature, is essential in many applications of plasmonic metal nanostructures, yet the potential of localised surface plasmon resonance for IR luminescence in Si crystals remains unknown. Here, we use the FDTD method (Lumerical FDTD Solution) to understand the dependence of field enhancement on size, shape, and metal type of the NP. In our simulation Au, Al and Al/Al2O3 core/shell NP with hemispherical, hemiellipsoidal, cylindrical, cubic, star like, etc. shape types were used.
Metal nanostructures can be designed to have infrared resonance, this has required the use of complex shapes such as nanoellipse or nanoshells and large particles (radius > 150 nm), which has been a major bottleneck for the development of infrared plasmonics. Our calculations predict that they can offer substantial near field enhancement up to 20 times.
We found that the plasmonic response depended strongly on the morphology of the nanoparticle. For all Al or Al/Al2O3 core/shell NPs, the absorption spectrum showed a single broad band ascribed to the dipole plasmon, whereas for Au NPs the LSPR band were much stronger and narrow. Also, the additional bands were assigned to the higher-order plasmon excitations were in the region of Si absorption. Finally, we showed that the near-field distribution was significantly perturbed and screened in the perturbed and concentrated in the presence of sharp angular NPs like an elongated ellipsoid, triangular or star.